Oxytocin is a 9-residue secreted peptide. As a hormone, can it travel through gap junctions, assuming that it is stored in pre-synaptic neuronal vesicles?
Synaptic vesicles are on the order of tens of nanometers (for example Zhang et al give a mean diameter around 40 nm).
Gap junctions are much smaller, on the order of 1-2 nm. Maeda et al measure one at 1.4 nm.
Peptides the size of oxytocin present in the cytosol could theoretically travel through gap junctions. Vesicle-packaged peptides or molecules of any size cannot.
Maeda, S., Nakagawa, S., Suga, M., Yamashita, E., Oshima, A., Fujiyoshi, Y., & Tsukihara, T. (2009). Structure of the connexin 26 gap junction channel at 3.5 Å resolution. Nature, 458(7238), 597-602.
Zhang, B., Koh, Y. H., Beckstead, R. B., Budnik, V., Ganetzky, B., & Bellen, H. J. (1998). Synaptic vesicle size and number are regulated by a clathrin adaptor protein required for endocytosis. neuron, 21(6), 1465-1475.
Can oxytocin travel from one cell to another via gap junctions? - Biology
The extracellular matrix of animal cells holds cells together to form a tissue and allow tissues to communicate with each other.
Explain the role of the extracellular matrix in animal cells
- The extracellular matrix of animal cells is made up of proteins and carbohydrates.
- Cell communication within tissue and tissue formation are main functions of the extracellular matrix of animal cells.
- Tissue communication is kick-started when a molecule within the matrix binds a receptor the end results are conformational changes that induce chemical signals that ultimately change activities within the cell.
- collagen: Any of more than 28 types of glycoprotein that forms elongated fibers, usually found in the extracellular matrix of connective tissue.
- proteoglycan: Any of many glycoproteins that have heteropolysaccharide side chains
- extracellular matrix: All the connective tissues and fibres that are not part of a cell, but rather provide support.
Extracellular Matrix of Animal Cells
Most animal cells release materials into the extracellular space. The primary components of these materials are proteins. Collagen is the most abundant of the proteins. Its fibers are interwoven with carbohydrate-containing protein molecules called proteoglycans. Collectively, these materials are called the extracellular matrix. Not only does the extracellular matrix hold the cells together to form a tissue, but it also allows the cells within the tissue to communicate with each other.
The Extracellular Matrix: The extracellular matrix consists of a network of proteins and carbohydrates.
How does this cell communication occur? Cells have protein receptors on the extracellular surfaces of their plasma membranes. When a molecule within the matrix binds to the receptor, it changes the molecular structure of the receptor. The receptor, in turn, changes the conformation of the microfilaments positioned just inside the plasma membrane. These conformational changes induce chemical signals inside the cell that reach the nucleus and turn “on” or “off” the transcription of specific sections of DNA. This affects the production of associated proteins, thus changing the activities within the cell.
An example of the role of the extracellular matrix in cell communication can be seen in blood clotting. When the cells lining a blood vessel are damaged, they display a protein receptor called tissue factor. When a tissue factor binds with another factor in the extracellular matrix, it causes platelets to adhere to the wall of the damaged blood vessel and stimulates the adjacent smooth muscle cells in the blood vessel to contract (thus constricting the blood vessel). Subsequently, a series of steps are initiated which then prompt the platelets to produce clotting factors.
Types of Cell Junctions
1. Tight Junction
Among the different types of cell junctions, the Tight Junction directs the movement of solutes and water nestled between epithelia. This happens at that point where cells brush against each other.
The gap between cells is so very tight that nothing may pass through. The only way substances can travel is by passing through the cell itself.
Should the cell allow a certain amount of nutrients to pass through, it is the Tight Junction that acts as the doorway to the body of the cell. Tight Junctions also act as a link between cytoskeletons of individual cells, thus holding adjacent cells in place.
Tight Junctions can only be found in vertebrates. For invertebrates, junctions that match the functions of the Tight Junction are called Septate Junctions.
2. Adherens Junction
Otherwise known as Zonula Adherens, the Adherens Junction literally forms a continuous belt around a cell. The primary function of the Adherens Junction is to stick to an adjacent cell or surface. The junction is formed primarily with calcium.
In certain parts of the body, Adherens Junctions perform a very important function. They help in binding the structure of the heart, keeping the heart together even as is it expands and contracts to supply oxygen throughout our body.
Several proteins make up an Adherens Junction:
- Cadherin are floating adhesive substances located outside of the cell.
- There are anchor proteins inside the cell. They connect the cytoskeleton to Cadherin.
- The cytoskeleton of the cell itself, which has actin microfilaments.
Desmosomes are similar in function to Adherens Junctions. Desmosomes are attached to the cytoskeleton of a cell via the demosplakin.
From there, the adhesion protein of the Desmosomes extends toward the cellular membrane, passes through the protective membrane, and attaches itself to another Desmosomes from another adjacent cell.
The two Desmosomes intertwine to in an S, W, or A shaped manner. In muscular tissue, Desmosomes hold muscles together.
Hemidesmosomes are located in the basal lamina of a cell. They connect with other cells by extending filaments to reach other Hemidesmosomes of other adjacent cells.
Like Desmosomes, the Hemidesmosome acts as an anchor between adjacent cells.
The main difference lies in the fact that the Hemidesmosome is anchored to the basal lamina of the cell, while the Desmosome is anchored to the cytoskeleton of the cell.
5. Gap Junction
Gap Junctions are specialized connections between cells. The presence of the Gap Junction directly connects the cytoplasm of adjacent cells. Their primary function is to allow the regulated passage of electrical impulses, molecules and ions from one cell to another.
This communication is very important as Gap Junctions allow for complex functions to happen. For example, Gap Junctions perform a necessary function for the development of organs, embryos, and tissues.
In the heart, the electrical impulses that order the muscles to contract pass through Gap Junctions. The signal for cellular death also passes through the Gap Junction.
Sometimes, cells must die in order for tissue to evolve into its primary form and purpose. Gap Junctions also transmit “orders” for nearby and surrounding, healthy cells, to die, if and when a diseased cell is found to be dying.
Gap Junction type cells are found in almost every kind of tissue within the body. The only exceptions to this are mobile cells such as erythrocytes, and fully matured skeletal muscle. Gap Junctions are not found in life forms like slime molds, sponges, and other simple organisms.
Cells can also communicate with each other via direct contact, referred to as intercellular junctions. There are some differences in the ways that plant and animal cells do this. Plasmodesmata are junctions between plant cells, whereas animal cell contacts include tight junctions, gap junctions, and desmosomes.
In general, long stretches of the plasma membranes of neighboring plant cells cannot touch one another because they are separated by the cell wall that surrounds each cell. How then, can a plant transfer water and other soil nutrients from its roots, through its stems, and to its leaves? Such transport uses the vascular tissues (xylem and phloem) primarily. There also exist structural modifications called plasmodesmata (singular = plasmodesma), numerous channels that pass between cell walls of adjacent plant cells, connect their cytoplasm, and enable materials to be transported from cell to cell, and thus throughout the plant (Figure 2).
Figure 2. A plasmodesma is a channel between the cell walls of two adjacent plant cells. Plasmodesmata allow materials to pass from the cytoplasm of one plant cell to the cytoplasm of an adjacent cell.
A tight junction is a watertight seal between two adjacent animal cells (Figure 3). The cells are held tightly against each other by proteins (predominantly two proteins called claudins and occludins).
Figure 3. Tight junctions form watertight connections between adjacent animal cells. Proteins create tight junction adherence.
This tight adherence prevents materials from leaking between the cells tight junctions are typically found in epithelial tissues that line internal organs and cavities, and comprise most of the skin. For example, the tight junctions of the epithelial cells lining your urinary bladder prevent urine from leaking out into the extracellular space.
Also found only in animal cells are desmosomes, which act like spot welds between adjacent epithelial cells (Figure 4). Short proteins called cadherins in the plasma membrane connect to intermediate filaments to create desmosomes. The cadherins join two adjacent cells together and maintain the cells in a sheet-like formation in organs and tissues that stretch, like the skin, heart, and muscles.
Figure 4. A desmosome forms a very strong spot weld between cells. Linking cadherins and intermediate filaments create it.
Gap junctions in animal cells are like plasmodesmata in plant cells in that they are channels between adjacent cells that allow for the transport of ions, nutrients, and other substances that enable cells to communicate (Figure 5). Structurally, however, gap junctions and plasmodesmata differ.
Figure 5. A gap junction is a protein-lined pore that allows water and small molecules to pass between adjacent animal cells.
Gap junctions develop when a set of six proteins (called connexins) in the plasma membrane arrange themselves in an elongated donut-like configuration called a connexon. When the pores (“doughnut holes”) of connexons in adjacent animal cells align, a channel between the two cells forms. Gap junctions are particularly important in cardiac muscle: The electrical signal for the muscle to contract is passed efficiently through gap junctions, allowing the heart muscle cells to contract in tandem.
In Summary: Cell Junctions
Animal cells communicate via their extracellular matrices and are connected to each other via tight junctions, desmosomes, and gap junctions. Plant cells are connected and communicate with each other via plasmodesmata.
When protein receptors on the surface of the plasma membrane of an animal cell bind to a substance in the extracellular matrix, a chain of reactions begins that changes activities taking place within the cell. Plasmodesmata are channels between adjacent plant cells, while gap junctions are channels between adjacent animal cells. However, their structures are quite different. A tight junction is a watertight seal between two adjacent cells, while a desmosome acts like a spot weld.
In vertebrates, gap junction hemichannels are primarily homo- or hetero-hexamers of connexin proteins. Invertebrate gap junctions comprise proteins from the innexin family. Innexins have no significant sequence homology with connexins.  Though differing in sequence to connexins, innexins are similar enough to connexins to state that innexins form gap junctions in vivo in the same way connexins do.    The recently characterized pannexin family,  which was originally thought to form inter-cellular channels (with an amino acid sequence similar to innexins  ), in fact functions as a single-membrane channel that communicates with the extracellular environment, and has been shown to pass calcium and ATP. 
At gap junctions, the intercellular space is between 2 and 4 nm  and unit connexons in the membrane of each cell are aligned with one another. 
Gap junction channels formed from two identical hemichannels are called homotypic, while those with differing hemichannels are heterotypic. In turn, hemichannels of uniform connexin composition are called homomeric, while those with differing connexins are heteromeric. Channel composition is thought to influence the function of gap junction channels.
Before innexins and pannexins were well characterized, the genes coding for connexin gap junction channels were classified in one of three groups, based on gene mapping and sequence similarity: A, B and C (for example, GJA1, GJC1).    However, connexin genes do not code directly for the expression of gap junction channels genes can produce only the proteins that make up gap junction channels. An alternative naming system based on this protein's molecular weight is also popular (for example: connexin43=GJA1, connexin30.3=GJB4).
- DNA to RNA to Connexin protein.
- One connexin protein has four transmembranedomains
- 6 Connexins create one Connexon (hemichannel). When different connexins join together to form one connexon, it is called a heteromeric connexon
- Two hemichannels, joined together across a cell membrane comprise a Gap Junction channel.
When two identical connexons come together to form a Gap junction channel, it is called a homotypic GJ channel. When one homomeric connexon and one heteromeric connexon come together, it is called a heterotypic gap junction channel. When two heteromeric connexons join, it is also called a heterotypic Gap Junction channel.
- Several gap junction channels (hundreds) assemble within a macromolecular complex called a gap junction plaque.
- Allows for direct electrical communication between cells, although different connexin subunits can impart different single channel conductances, from about 30 pS to 500 pS.
- Allows for chemical communication between cells, through the transmission of small second messengers, such as inositol triphosphate ( IP
3 ) and calcium ( Ca 2+
),  although different connexin subunits can impart different selectivity for particular small molecules.
- In general, allows transmembrane movement of molecules smaller than 485 Daltons (1,100 Daltons through invertebrate gap junctions  ), although different connexin subunits may impart different pore sizes and different charge selectivity. Large biomolecules, for example, nucleic acid and protein, are precluded from cytoplasmic transfer between cells through gap junction connexin channels.
- Ensures that molecules and current passing through the gap junction do not leak into the intercellular space.
To date, five different functions have been ascribed to gap junction protein:
- Electrical and metabolic coupling between cells
- Electrical and metabolic exchange through hemichannels
- Tumor suppressor genes (Cx43, Cx32 and Cx36)
- Adhesive function independent of conductive gap junction channel (neural migration in neocortex)
- Role of carboxyl-terminal in signaling cytoplasmic pathways (Cx43)
Gap Junctions have been observed in various animal organs and tissues where cells contact each other. From the 1950s to 1970s they were detected in crayfish nerves,  rat pancreas, liver, adrenal cortex, epididymis, duodenum, muscle,  Daphnia hepatic caecum,  Hydra muscle,  monkey retina,  rabbit cornea,  fish blastoderm,  frog embryos,  rabbit ovary,  re-aggregating cells,   cockroach hemocyte capsules,  rabbit skin,  chick embryos,  human islet of Langerhans,  goldfish and hamster pressure sensing acoustico-vestibular receptors,  lamprey and tunicate heart,   rat seminiferous tubules,  myometrium,  eye lens  and cephalopod digestive epithelium.  Since the 1970s gap junctions have continued to be found in nearly all animal cells that touch each other. By the 1990s new technology such as confocal microscopy allowed more rapid survey of large areas of tissue. Since the 1970s even tissues that were traditionally considered to possibly have isolated cells such as bone showed that the cells were still connected with gap junctions, however tenuously.  Gap junctions appear to be in all animal organs and tissues and it will be interesting to find exceptions to this other than cells not normally in contact with neighboring cells. Adult skeletal muscle is a possible exception. It may be argued that if present in skeletal muscle, gap junctions might propagate contractions in an arbitrary way among cells making up the muscle. At least in some cases this may not be the case as shown in other muscle types that do have gap junctions.  An indication of what results from reduction or absence of gap junctions may be indicated by analysis of cancers    or the aging process. 
Gap junctions may be seen to function at the simplest level as a direct cell to cell pathway for electrical currents, small molecules and ions. The control of this communication allows complex downstream effects on multicellular organisms as described below.
Embryonic, organ and tissue development Edit
In the 1980s, more subtle but no less important roles of gap junction communication have been investigated. It was discovered that gap junction communication could be disrupted by adding anti-connexin antibodies into embryonic cells.   Embryos with areas of blocked gap junctions failed to develop normally. The mechanism by which antibodies blocked the gap junctions was unclear but systematic studies were undertaken to elucidate the mechanism.   Refinement of these studies showed that gap junctions appeared to be key to development of cell polarity  and the left/right symmetry/asymmetry in animals.   While signaling that determines the position of body organs appears to rely on gap junctions so does the more fundamental differentiation of cells at later stages of embryonic development.      Gap junctions were also found to be responsible for the transmission of signals required for drugs to have an effect  and conversely some drugs were shown to block gap junction channels. 
Gap junctions and the "bystander effect" Edit
Cell death Edit
The "bystander effect" with its connotations of the innocent bystander being killed is also mediated by gap junctions. When cells are compromised due to disease or injury and start to die messages are transmitted to neighboring cells connected to the dying cell by gap junctions. This can cause the otherwise unaffected healthy bystander cells to also die.  The bystander effect is, therefore, important to consider in diseased cells, which opened an avenue for more funding and a flourish of research.          Later the bystander effect was also researched with regard to cells damaged by radiation or mechanical injury and therefore wound healing.      Disease also seems to have an effect on the ability of gap junctions to fulfill their roles in wound healing.  
Tissue restructuring Edit
While there has been a tendency to focus on the bystander effect in disease due to the possibility of therapeutic avenues there is evidence that there is a more central role in normal development of tissues. Death of some cells and their surrounding matrix may be required for a tissue to reach its final configuration and gap junctions also appear essential to this process.   There are also more complex studies that try to combine our understanding of the simultaneous roles of gap junctions in both wound healing and tissue development.   
Areas of electrical coupling Edit
Gap junctions electrically and chemically couple cells throughout the body of most animals. Electrical coupling can be relatively fast acting. Tissues in this section have well known functions observed to be coordinated by gap junctions with inter-cellular signaling happening in time frames of micro-seconds or less.
Gap junctions are particularly important in cardiac muscle: the signal to contract is passed efficiently through gap junctions, allowing the heart muscle cells to contract in unison.
A gap junction located in neurons is often referred to as an electrical synapse. The electrical synapse was discovered using electrical measurements before the gap junction structure was described. Electrical synapses are present throughout the central nervous system and have been studied specifically in the neocortex, hippocampus, vestibular nucleus, thalamic reticular nucleus, locus coeruleus, inferior olivary nucleus, mesencephalic nucleus of the trigeminal nerve, ventral tegmental area, olfactory bulb, retina and spinal cord of vertebrates. 
There has been some observation of weak neuron to glial cell coupling in the locus coeruleus, and in the cerebellum between Purkinje neurons and Bergmann glial cells. It appears that astrocytes are coupled by gap junctions, both to other astrocytes and to oligodendrocytes.  Moreover, mutations in the gap junction genes Cx43 and Cx56.6 cause white matter degeneration similar to that observed in Pelizaeus–Merzbacher disease and multiple sclerosis.
Connexin proteins expressed in neuronal gap junctions include:
with mRNAs for at least five other connexins (mCx26, mCx30.2, mCx32, mCx43, mCx47) detected but without immunocytochemical evidence for the corresponding protein within ultrastructurally-defined gap junctions. Those mRNAs appear to be down-regulated or destroyed by micro interfering RNAs ( miRNAs ) that are cell-type and cell-lineage specific.
Neurons within the retina show extensive coupling, both within populations of one cell type, and between different cell types. 
Gap junctions were so named because of the "gap" shown to be present at these special junctions between two cells.  With the increased resolution of the transmission electron microscope (TEM) gap junction structures were first able to be seen and described in around 1953.
The term "gap junction" appeared to be coined about 16 years later circa 1969.    A similar narrow regular gap was not demonstrated in other intercellular junctions photographed using the TEM at the time.
Form an indicator of function Edit
Well before the demonstration of the "gap" in gap junctions they were seen at the junction of neighboring nerve cells. The close proximity of the neighboring cell membranes at the gap junction lead researchers to speculate that they had a role in intercellular communication, in particular the transmission of electrical signals.    Gap junctions were also proven to be electrically rectifying and referred to as an electrical synapse.   Later it was found that chemicals could also be transported between cells through gap junctions. 
Implicit or explicit in most of the early studies is that the area of the gap junction was different in structure to the surrounding membranes in a way that made it look different. The gap junction had been shown to create a micro-environment between the two cells in the extra-cellular space or "gap". This portion of extra-cellular space was somewhat isolated from the surrounding space and also bridged by what we now call connexon pairs which form even more tightly sealed bridges that cross the gap junction gap between two cells. When viewed in the plane of the membrane by freeze-fracture techniques, higher-resolution distribution of connexons within the gap junction plaque is possible. 
Connexin free islands are observed in some junctions. The observation was largely without explanation until vesicles were shown by Peracchia using TEM thin sections to be systematically associated with gap junction plaques.  Peracchia's study was probably also the first study to describe paired connexon structures, which he called somewhat simply a "globule". Studies showing vesicles associated with gap junctions and proposing the vesicle contents may move across the junction plaques between two cells were rare, as most studies focused on the connexons rather than vesicles. A later study using a combination of microscopy techniques confirmed the early evidence of a probable function for gap junctions in intercellular vesicle transfer. Areas of vesicle transfer were associated with connexin free islands within gap junction plaques. 
Electrical and chemical nerve synapses Edit
Because of the widespread occurrence of gap junctions in cell types other than nerve cells the term gap junction became more generally used than terms such as electrical synapse or nexus. Another dimension in the relationship between nerve cells and gap junctions was revealed by studying chemical synapse formation and gap junction presence. By tracing nerve development in leeches with gap junction expression suppressed it was shown that the bidirectional gap junction (electrical nerve synapse) needs to form between two cells before they can grow to form a unidirectional "chemical nerve synapse".  The chemical nerve synapse is the synapse most often truncated to the more ambiguous term "nerve synapse".
The purification   of the intercellular gap junction plaques enriched in the channel forming protein (connexin) showed a protein forming hexagonal arrays in x-ray diffraction. Now systematic study and identification of the predominant gap junction protein  became possible. Refined ultrastructural studies by TEM   showed protein occurred in a complementary fashion in both cells participating in a gap junction plaque. The gap junction plaque is a relatively large area of membrane observed in TEM thin section and freeze fracture (FF) seen filled with trans-membrane proteins in both tissues and more gently treated gap junction preparations. With the apparent ability for one protein alone to enable intercellular communication seen in gap junctions  the term gap junction tended to become synonymous with a group of assembled connexins though this was not shown in vivo. Biochemical analysis of gap junction rich isolates from various tissues demonstrated a family of connexins.   
Ultrastructure and biochemistry of isolated gap junctions already referenced had indicated the connexins preferentially group in gap junction plaques or domains and connexins were the best characterized constituent. It has been noted that the organisation of proteins into arrays with a gap junction plaque may be significant.   It is likely this early work was already reflecting the presence of more than just connexins in gap junctions. Combining the emerging fields of freeze-fracture to see inside membranes and immunocytochemistry to label cell components (Freeze-fracture replica immunolabelling or FRIL and thin section immunolabelling) showed gap junction plaques in vivo contained the connexin protein.   Later studies using immunofluorescence microscopy of larger areas of tissue clarified diversity in earlier results. Gap junction plaques were confirmed to have variable composition being home to connexon and non-connexin proteins as well making the modern usage of the terms "gap junction" and "gap junction plaque" non-interchangeable.  In other words, the commonly used term "gap junction" always refers to a structure that contains connexins while a gap junction plaque may also contain other structural features that will define it.
The "plaque" or "formation plaque" Edit
Early descriptions of "gap junctions" and "connexons" did not refer to them as such and many other terms were used. It is likely that "synaptic disks"  were an accurate reference to gap junction plaques. While the detailed structure and function of the connexon was described in a limited way at the time the gross "disk" structure was relatively large and easily seen by various TEM techniques. Disks allowed researchers using TEM to easily locate the connexons contained within the disk like patches in vivo and in vitro. The disk or "plaque" appeared to have structural properties different from those imparted by the connexons alone.  It was thought that if the area of membrane in the plaque transmitted signals the area of membrane would have to be sealed in some way to prevent leakage.  Later studies showed gap junction plaques are home to non-connexin proteins making the modern usage of the terms "gap junction" and "gap junction plaque" non-interchangeable as the area of the gap junction plaque may contain proteins other than connexins.   Just as connexins do not always occupy the entire area of the plaque the other components described in the literature may be only long term or short term residents. 
Studies allowing views inside the plane of the membrane of gap junctions during formation indicated that a "formation plaque" formed between two cells prior to the connexins moving in. They were particle free areas when observed by TEM FF indicating very small or no transmembrane proteins were likely present. Little is known about what structures make up the formation plaque or how the formation plaque's structure changes when connexins and other components move in or out. One of the earlier studies of the formation of small gap junctions describes rows of particles and particle free halos.  With larger gap junctions they were described as formation plaques with connexins moving into them. The particulate gap junctions were thought to form 4–6 hours after the formation plaques appeared.  How the connexins may be transported to the plaques using tubulin is becoming clearer.  
The formation plaque and non-connexin part of the classical gap junction plaque have been difficult for early researchers to analyse. It appears in TEM FF and thin section to be a lipid membrane domain that can somehow form a comparatively rigid barrier to other lipids and proteins. There has been indirect evidence for certain lipids being preferentially involved with the formation plaque but this cannot be considered definitive.   It is difficult to envisage breaking up the membrane to analyse membrane plaques without affecting their composition. By study of connexins still in membranes lipids associated with the connexins have been studied.  It was found that specific connexins tended to associate preferentially with specific phospholipids. As formation plaques precede connexins these results still give no certainty as to what is unique about the composition of plaques themselves. Other findings show connexins associate with protein scaffolds used in another junction, the zonula occludens ZO1.  While this helps us understand how connexins may be moved into a gap junction formation plaque the composition of the plaque itself is still somewhat sketchy. Some headway on the in vivo composition of the gap junction plaque is being made using TEM FRIL.  
Oxytocin Signaling Pathway
The oxytocin signaling pathway refers to signaling pathway proteins including oxytocin, oxytocin receptors, and related regulatory factors. Oxytocin is a peptide hormone secreted by the posterior pituitary. Synthesized from the hypothalamic paraventricular nucleus and supraoptic nucleus, it consists of 9 amino acids. It is transported to the pituitary gland at a rate of 2 mm to 3 mm per day. The methionine residues at the "1" and "6" positions form a cyclic structure of a 6-peptide in the form of a disulfide bond. The physiological role of the oxytocin signaling pathway is mainly to stimulate the breast to secrete milk, promote the contraction of the uterine smooth muscle during childbirth, and promote the role of maternal love. In addition, it can reduce the level of stress hormones such as adrenal ketones in the body to lower blood pressure. It is not a woman's patent, and both men and women can secrete it. Its role is mainly in lactating mammary glands to continuously secrete milk under the action of prolactin and store it in the mammary gland. Oxytocin can contract the myoepithelial cells around the breast acinus and promote breast milk with lactation function oxytocin has a strong promoting contraction effect on the uterus, but it is sensitive to the pregnant uterus.
The oxytocin signaling pathway initiates a subsequent response by the binding of oxytocin and the receptor. Oxytocin is composed of 9 amino acids, of which 2 cysteines form a disulfide bond at 1,6 positions with a relative molecular weight of 1,007, which exists as a free peptide in the blood circulation. The biological half-life of oxytocin is only 3 to 10 minutes, and the half-life is shorter at high concentrations. The metabolic clearance rate of the mother during pregnancy is 19-21 ml per kilogram of body weight, which is mainly cleared by the liver and discharged from the kidney in an inactive form. The oxytocin is also degraded by oxytocin in the uterus during pregnancy. Oxytocin is mainly synthesized in the large cells of the suprachiasmatic nucleus and paraventricular nucleus of the hypothalamus, and a small amount is synthesized in peripheral organs. The regulation mechanism is still not clear, and the regulatory sequences in the oxytocin gene are also unknown. The special anatomical structure of neurons makes oxytocin have a dual role of hormones and neurotransmitters. Neurosecretory granules containing oxytocin and pituitary vasopressin are widely distributed in Purkinje fibers and distributed along neurons. In addition, oxytocin is widely distributed in organs such as uterus, ovary, testis, adrenal gland, thymus, and pancreas, and has functions of autocrine and paracrine. The Oxytocin Receptor (OTR) belongs to the type A G-protein coupled receptor (GPCR) family and contains seven transmembrane alpha helices consisting of 389 amino acid residues. Oxytocin receptors can be coupled to subunits such as Gq, Gi1, Gi2, Gi3, GoA, and GoB, causing an increase in cytosolic calcium concentration (coupling to the Gq subunit) or inhibition of adenylate cyclase activity (coupling with the Gi subunit). The oxytocin receptor gene is located on human chromosome 3p25 and is about 19 kb in length, containing 3 introns and 4 exons. Because oxytocin has high sequence homology with another neuropeptide (vasopressin, AVP, also known as arginine vasopressin), when studying novel agonists and antagonists of the oxytocin system, vasopressin receptors (ie, V1a receptor and V2 receptor) are usually used as controls for examining whether the affinity of the novel ligand to the oxytocin receptor is significantly stronger than its affinity for the vasopressin receptor. The major region that mediates the binding of the oxytocin receptor and the vasopressin receptor to the corresponding ligand is in the third transmembrane region of the receptor, while the oxytocin receptor 5 and 6 transmembrane regions are specifically recognized by oxytocin. There are three different affinity oxytocin binding sites in the myometrium of rats. Among them, the binding site of the intermediate affinity is the oxytocin receptor, and the other two roles are still unclear. The low-affinity binding site has no obvious pharmacological effects. The high-affinity binding site is believed to interact with the central affinity binding site to affect the uterus. Upon binding to oxytocin or its agonist or antagonist, the receptor undergoes a change in affinity. Therefore, although the receptor density does not change, if the affinity of the receptor changes, the biological function and biological activity of the uterus can be affected. The human mesenteric oxytocin receptor has a relative molecular mass of about 43,000 and consists of 388 amino acids, presumably containing seven transmembrane peptides, like the G protein-binding receptor.
Oxytocin signaling pathway
- Oxytocin signaling pathway cascade
Oxytocin promotes uterine contraction through the activation of calcium channels associated with receptors and the release of sarcoplasmic reticulum calcium. Oxytocin binds to the receptor and is mediated by a second messenger, which is regulated by voltage or hormone regulation on the muscle cell membrane and by contractor-mediated extracellular calcium influx. Oxytocin increases the production of inositol 1,4,5-triphosphate, and the mobilization of 5-trisphosphate inositol stores intracellular calcium release in the endoplasmic reticulum and sarcoplasmic reticulum. In addition, oxytocin also causes cells to produce inward currents through receptor-activated, non-selective cation channels that depolarize cell membranes, producing action potentials and muscle contractions. In vitro experiments on human uterine myocytes are derived from a term that pregnancy indicates that oxytocin affects uterine contractility through membrane potential depolarization but does not affect the interaction of muscle cells through gap junctions, indicating uterine muscle contraction and myocyte coordination. The mechanism of action is different. At present, the role of the second messenger cAMP and cGMP is not yet clear. In vitro studies have observed that cAMP is involved in an exponential increase in the number of amniotic oxytocin receptors during rabbit pregnancy. Oxytocin increases the activity of mitogen-activated protein kinases through the mediation of G-protein. Oxytocin also increases the activity of alanine aminotransferase and phospholipase C through the interaction of the oxytocin receptor with the Gα q and Gα 11 chains of the binding protein.
The genetic structure and genomic composition between oxytocin and vasopressin (VP) in the hypothalamus are closely related. They all have the same origin, all of which are small fragments of up to 2500 base pairs. The oxytocin gene consists of three exons and two inserts. Exon A includes a signal peptide, an oxytocin sequence portion, and an N-terminal portion of the vasopressin carrier protein. Exon B is the middle portion of the pituitary vasopressin, and exon C is the pituitrin, and carries the C-terminal portion of the protein. The human oxytocin and VP genes are located at the same position on chromosome 20, but the direction of transcription is reversed. The sequence between the two genes is 9 kb. The non-coding region at the 5' end of the transcriptional origin of the oxytocin gene in human, rat and female cattle is highly variable, with only a few pairs of nucleotide sequences being similar. The difference between the 5' end of the oxytocin and the VP gene is also large. There is also an oxytocin-specific enhancer in or near the VP gene, because the oxytocin gene can be expressed only when a small fragment is ligated to the VP gene. Estrogen increases the expression of the oxytocin gene in rat uterus and human amniotic, chorionic, and decidual cultures. The promoter region of the rat oxytocin gene contains two estrogen-responsive fragments, and the human gene contains only one gene. The reaction fragment requires only one gene fragment of -49 to +36 located in the 5' flanking region of the gene. However, the direct stimulatory effect of estrogen on the oxytocin gene may be limited to a subset of oxytocin neurons that bind to estrogen.
In recent years, studies on oxytocin and receptor genes and depression have provided new directions for the study of neurological mechanisms and clinical research of mental disorders such as depression. Moreover, oxytocin can improve the patient's depressive symptoms by regulating the therapeutic targets such as HPA axis and hippocampal neurogenesis, so oxytocin is likely to become a new drug for target treatment of depressed patients.
Autism is a widespread neurodevelopmental disorder of unknown causes, and there is no effective treatment to date. As one of the characteristic symptoms of autism, social disorders seriously affect the physical and mental health and quality of life of patients. Studies have shown that oxytocin plays an important role in social interactions, and oxytocin deficiency or underutilization may be associated with social disorders in autistic patients.
A study by Rubin et al. (2010) found that the higher the level of oxytocin in the peripheral nervous system of women with mental illness, the less clinical symptoms they have. Goldman, Marlow-O'Connor, Torres, and Carter (2008) collected oxytocin levels in the blood of normal and schizophrenia patients and found that oxytocin levels were positively correlated with facial expression recognition, whereas patients with low-sodium schizophrenia Oxytocin levels were significantly lower than normal sodium patients with schizophrenia and normal people. More interestingly, the researchers found that the levels of oxytocin in the blood of the normal group increased before and after the trust interaction of sharing secrets, while the schizophrenic group did not change.
|Sodium||Na +||135 - 145||10||14:1|
|Potassium||K +||3.5 - 5.0||155||1:30|
|Chloride||Cl −||95 - 110||10 - 20||4:1|
|Calcium||Ca 2+||2||10 −4||2 x 10 4 :1|
|Although intracellular Ca 2+ content is about 2 mM, most of this is bound or sequestered in intracellular organelles (mitochondria and sarcoplasmic reticulum). |
Similar to skeletal muscle, the resting membrane potential (voltage when the cell is not electrically excited) of ventricular cells, is around -90 millivolts (mV 1 mV = 0.001 V) i.e. the inside of the membrane is more negative than the outside. The main ions found outside the cell at rest are sodium (Na + ), and chloride (Cl − ), whereas inside the cell it is mainly potassium (K + ). 
The action potential begins with the voltage becoming more positive this is known as depolarization and is mainly due to the opening of sodium channels that allow Na + to flow into the cell. After a delay (known as the absolute refractory period see below), termination of the action potential then occurs, as potassium channels open, allowing K + to leave the cell and causing the membrane potential to return to negative, this is known as repolarization. Another important ion is calcium (Ca 2+ ), which can be found outside of the cell as well as inside the cell, in a calcium store known as the sarcoplasmic reticulum (SR). Release of Ca 2+ from the SR, via a process called calcium-induced calcium release, is vital for the plateau phase of the action potential (see phase 2, below) and is a fundamental step in cardiac excitation-contraction coupling. 
There are important physiological differences between the cells that spontaneously generate the action potential (pacemaker cells e.g. SAN) and those that simply conduct it (non-pacemaker cells e.g. ventricular myocytes). The specific differences in the types of ion channels expressed and mechanisms by which they are activated results in differences in the configuration of the action potential waveform, as shown in figure 2.
The standard model used to understand the cardiac action potential is that of the ventricular myocyte. Outlined below are the five phases of the ventricular myocyte action potential, with reference also to the SAN action potential.
Phase 4 Edit
In the ventricular myocyte, phase 4 occurs when the cell is at rest, in a period known as diastole. In the standard non-pacemaker cell the voltage during this phase is more or less constant, at roughly -90 mV.  The resting membrane potential results from the flux of ions having flowed into the cell (e.g. sodium and calcium) and the ions having flowed out of the cell (e.g. potassium, chloride and bicarbonate) being perfectly balanced.
The leakage of these ions across the membrane is maintained by the activity of pumps which serve to keep the intracellular concentration more or less constant, so for example, the sodium (Na + ) and potassium (K + ) ions are maintained by the sodium-potassium pump which uses energy (in the form of adenosine triphosphate (ATP)) to move three Na + out of the cell and two K + into the cell. Another example is the sodium-calcium exchanger which removes one Ca 2+ from the cell for three Na + into the cell. 
During this phase the membrane is most permeable to K + , which can travel into or out of cell through leak channels, including the inwardly rectifying potassium channel.  Therefore, the resting membrane potential is mainly determined by K + equilibrium potential and can be calculated using the Goldman-Hodgkin-Katz voltage equation.
However, pacemaker cells are never at rest. In these cells, phase 4 is also known as the pacemaker potential. During this phase, the membrane potential slowly becomes more positive, until it reaches a set value (around -40 mV known as the threshold potential) or until it is depolarized by another action potential, coming from a neighboring cell.
The pacemaker potential is thought to be due to a group of channels, referred to as HCN channels (Hyperpolarization-activated cyclic nucleotide-gated). These channels open at very negative voltages (i.e. immediately after phase 3 of the previous action potential see below) and allow the passage of both K + and Na + into the cell. Due to their unusual property of being activated by very negative membrane potentials, the movement of ions through the HCN channels is referred to as the funny current (see below). 
Another hypothesis regarding the pacemaker potential is the ‘calcium clock’. Here, calcium is released from the sarcoplasmic reticulum, within the cell. This calcium then increases activation of the sodium-calcium exchanger resulting in the increase in membrane potential (as a +3 charge is being brought into the cell (by the 3Na + ) but only a +2 charge is leaving the cell (by the Ca 2+ ) therefore there is a net charge of +1 entering the cell). This calcium is then pumped back into the cell and back into the SR via calcium pumps (including the SERCA). 
Phase 0 Edit
This phase consists of a rapid, positive change in voltage across the cell membrane (depolarization) lasting less than 2 ms, in ventricular cells and 10/20 ms in SAN cells.  This occurs due to a net flow of positive charge into the cell.
In non-pacemaker cells (i.e. ventricular cells), this is produced predominantly by the activation of Na + channels, which increases the membrane conductance (flow) of Na + (gNa). These channels are activated when an action potential arrives from a neighbouring cell, through gap junctions. When this happens, the voltage within the cell increases slightly. If this increased voltage reaches a certain value (threshold potential
-70 mV) it causes the Na + channels to open. This produces a larger influx of sodium into the cell, rapidly increasing the voltage further (to
+50 mV  i.e. towards the Na + equilibrium potential). However, if the initial stimulus is not strong enough, and the threshold potential is not reached, the rapid sodium channels will not be activated and an action potential will not be produced this is known as the all-or-none law.   The influx of calcium ions (Ca 2+ ) through L-type calcium channels also constitutes a minor part of the depolarisation effect.  The slope of phase 0 on the action potential waveform (see figure 2) represents the maximum rate of voltage change, of the cardiac action potential and is known as dV/dtmax.
In pacemaker cells (e.g. sinoatrial node cells), however, the increase in membrane voltage is mainly due to activation of L-type calcium channels. These channels are also activated by an increase in voltage, however this time it is either due to the pacemaker potential (phase 4) or an oncoming action potential. The L-type calcium channels activate towards the end of the pacemaker potential (and therefore contribute to the latter stages of the pacemaker potential). The L-type calcium channels are activated more slowly than the sodium channels, in the ventricular cell, therefore, the depolarization slope in the pacemaker action potential waveform is less steep than that in the non-pacemaker action potential waveform.  
Phase 1 Edit
This phase begins with the rapid inactivation of the Na + channels by the inner gate (inactivation gate), reducing the movement of sodium into the cell. At the same time potassium channels (called Ito1) open and close rapidly, allowing for a brief flow of potassium ions out of the cell, making the membrane potential slightly more negative. This is referred to as a ‘notch’ on the action potential waveform. 
There is no obvious phase 1 present in pacemaker cells.
Phase 2 Edit
This phase is also known as the "plateau" phase due to the membrane potential remaining almost constant, as the membrane slowly begins to repolarize. This is due to the near balance of charge moving into and out of the cell. During this phase delayed rectifier potassium channels allow potassium to leave the cell while L-type calcium channels (activated by the flow of sodium during phase 0), allow the movement of calcium ions into the cell. These calcium ions bind to and open more calcium channels (called ryanodine receptors) located on the sarcoplasmic reticulum within the cell, allowing the flow of calcium out of the SR. These calcium ions are responsible for the contraction of the heart. Calcium also activates chloride channels called Ito2, which allow Cl − to enter the cell. The movement of Ca 2+ opposes the repolarizing voltage change caused by K + and Cl − [ citation needed ] . As well as this the increased calcium concentration increases the activity of the sodium-calcium exchanger, and the increase in sodium entering the cell increases activity of the sodium-potassium pump. The movement of all of these ions results in the membrane potential remaining relatively constant.   This phase is responsible for the large duration of the action potential and is important in preventing irregular heartbeat (cardiac arrhythmia).
There is no plateau phase present in pacemaker action potentials.
Phase 3 Edit
During phase 3 (the "rapid repolarization" phase) of the action potential, the L-type Ca 2+ channels close, while the slow delayed rectifier (IKs) K + channels remain open as more potassium leak channels open. This ensures a net outward positive current, corresponding to negative change in membrane potential, thus allowing more types of K + channels to open. These are primarily the rapid delayed rectifier K + channels (IKr) and the inwardly rectifying K + current, IK1. This net outward, positive current (equal to loss of positive charge from the cell) causes the cell to repolarize. The delayed rectifier K + channels close when the membrane potential is restored to about -85 to -90 mV, while IK1 remains conducting throughout phase 4, which helps to set the resting membrane potential 
Ionic pumps as discussed above, like the sodium-calcium exchanger and the sodium-potassium pump restore ion concentrations back to balanced states pre-action potential. This means that the intracellular calcium is pumped out, which was responsible for cardiac myocyte contraction. Once this is lost the contraction stops and myocytic cells relax, which in turn relaxes the heart muscle.
During this phase, the action potential fatefully commits to repolarisation. This begins with the closing of the L-type Ca 2+ channels, while the K + channels (from phase 2) remain open. The main potassium channels involved in repolarization are the delayed rectifiers (IKr) and (IKs) as well as the inward rectifier (IK1). Overall there is a net outward positive current, that produces negative change in membrane potential.  The delayed rectifier channels close when the membrane potential is restored to resting potential, whereas the inward rectifier channels and the ion pumps remain active throughout phase 4, resetting the resting ion concentrations. This means that the calcium used for muscle contraction, is pumped out of the cell, resulting in muscle relaxation.
In the sinoatrial node, this phase is also due to the closure of the L-type calcium channels, preventing inward flux of Ca 2+ and the opening of the rapid delayed rectifier potassium channels (IKr). 
Cardiac cells have two refractory periods, the first from the beginning of phase 0 until part way through phase 3 this is known as the absolute refractory period during which it is impossible for the cell to produce another action potential. This is immediately followed, until the end of phase 3, by a relative refractory period, during which a stronger-than-usual stimulus is required to produce another action potential.  
These two refractory periods are caused by changes in the states of sodium and potassium channels. The rapid depolarization of the cell, during phase 0, causes the membrane potential to approach sodium's equilibrium potential (i.e. the membrane potential at which sodium is no longer drawn into or out of the cell). As the membrane potential becomes more positive, the sodium channels then close and lock, this is known as the "inactivated" state. During this state the channels cannot be opened regardless of the strength of the excitatory stimulus—this gives rise to the absolute refractory period. The relative refractory period is due to the leaking of potassium ions, which makes the membrane potential more negative (i.e. it is hyperpolarised), this resets the sodium channels opening the inactivation gate, but still leaving the channel closed. This means that it is possible to initiate an action potential, but a stronger stimulus than normal is required. 
Gap junctions allow the action potential to be transferred from one cell to the next (they are said to electrically couple neighbouring cardiac cells). They are made from the connexin family of proteins, that form a pore through which ions (including Na + , Ca 2+ and K + ) can pass. As potassium is highest within the cell, it is mainly potassium that passes through. This increased potassium in the neighbour cell causes the membrane potential to increase slightly, activating the sodium channels and initiating an action potential in this cell. (A brief chemical gradient driven efflux of Na+ through the connexon at peak depolarization causes the conduction of cell to cell depolarization, not potassium.)  These connections allow for the rapid conduction of the action potential throughout the heart and are responsible for allowing all of the cells in the atria to contract together as well as all of the cells in the ventricles.  Uncoordinated contraction of heart muscles is the basis for arrhythmia and heart failure. 
|Current (I)||α subunit protein||α subunit gene||Phase / role|
|Na +||INa||NaV1.5||SCN5A ||0|
|Ca 2+||ICa(L)||CaV1.2||CACNA1C ||0-2|
|K +||Ito1||KV4.2/4.3||KCND2/KCND3||1, notch|
|K +||IKr||KV11.1 (hERG)||KCNH2||3|
|Na + , Ca 2+||INaCa||3Na + -1Ca 2+ -exchanger||NCX1 (SLC8A1)||ion homeostasis|
|Na + , K +||INaK||3Na + -2K + -ATPase||ATP1A||ion homeostasis|
|Ca 2+||IpCa||Ca 2+ -transporting ATPase||ATP1B||ion homeostasis|
Ion channels are proteins, that change shape in response to different stimuli to either allow or prevent the movement of specific ions across a membrane (they are said to be selectively permeable). Stimuli, which can either come from outside the cell or from within the cell, can include the binding of a specific molecule to a receptor on the channel (also known as ligand-gated ion channels) or a change in membrane potential around the channel, detected by a sensor (also known as voltage-gated ion channels) and can act to open or close the channel. The pore formed by an ion channel is aqueous (water filled) and allows the ion to rapidly travel across the membrane.  Ion channels can be selective for specific ions, so there are Na + , K + , Ca 2+ , and Cl − specific channels. They can also be specific for a certain charge of ions (i.e. positive or negative). 
Each channel is coded by a set of DNA instructions that tell the cell how to make it. These instructions are known as a gene. Figure 3 shows the important ion channels involved in the cardiac action potential, the current (ions) that flows through the channels, their main protein subunits (building blocks of the channel), some of their controlling genes that code for their structure and the phases they are active during the cardiac action potential. Some of the most important ion channels involved in the cardiac action potential are described briefly below.
Hyperpolarisation activated cyclic nucleotide gated (HCN) channels Edit
Located mainly in pacemaker cells, these channels become active at very negative membrane potentials and allow for the passage of both Na + and K + into the cell (this movement is known as a funny current, If). These poorly selective, cation (positively charged ions) channels conduct more current as the membrane potential becomes more negative (hyperpolarised). The activity of these channels in the SAN cells causes the membrane potential to depolarise slowly and so they are thought to be responsible for the pacemaker potential. Sympathetic nerves directly affect these channels, resulting in an increased heart rate (see below).  
The fast Na + channel Edit
These sodium channels are voltage-dependent, opening rapidly due to depolarization of the membrane, which usually occurs from neighboring cells, through gap junctions. They allow for a rapid flow of sodium into the cell, depolarizing the membrane completely and initiating an action potential. As the membrane potential increases, these channels then close and lock (become inactive). Due to the rapid influx sodium ions (steep phase 0 in action potential waveform) activation and inactivation of these channels happens almost at exactly the same time. During the inactivation state, Na + cannot pass through (absolute refractory period). However they begin to recover from inactivation as the membrane potential becomes more negative (relative refractory period).
Potassium channels Edit
The two main types of potassium channels in cardiac cells are inward rectifiers and voltage-gated potassium channels.
Inwardly rectifying potassium channels (Kir) favour the flow of K + into the cell. This influx of potassium, however, is larger when the membrane potential is more negative than the equilibrium potential for K + (
-90 mV). As the membrane potential becomes more positive (i.e. during cell stimulation from a neighbouring cell), the flow of potassium into the cell via the Kir decreases. Therefore, Kir is responsible for maintaining the resting membrane potential and initiating the depolarization phase. However, as the membrane potential continues to become more positive, the channel begins to allow the passage of K + out of the cell. This outward flow of potassium ions at the more positive membrane potentials means that the Kir can also aid the final stages of repolarisation.  
The voltage-gated potassium channels (Kv) are activated by depolarization. The currents produced by these channels include the transient out potassium current Ito1. This current has two components. Both components activate rapidly, but Ito,fast inactivates more rapidly than Ito, slow. These currents contribute to the early repolarization phase (phase 1) of the action potential.
Another form of voltage-gated potassium channels are the delayed rectifier potassium channels. These channels carry potassium currents which are responsible for the plateau phase of the action potential, and are named based on the speed at which they activate: slowly activating IKs, rapidly activating IKr and ultra-rapidly activating IKur. 
Calcium channels Edit
There are two voltage-gated calcium channels within cardiac muscle: L-type calcium channels ('L' for Long-lasting) and T-type calcium channels ('T' for Transient, i.e. short). L-type channels are more common and are most densely populated within the t-tubule membrane of ventricular cells, whereas the T-type channels are found mainly within atrial and pacemaker cells, but still to a lesser degree than L-type channels.
These channels respond to voltage changes across the membrane differently: L-type channels are activated by more positive membrane potentials, take longer to open and remain open longer than T-type channels. This means that the T-type channels contribute more to depolarization (phase 0) whereas L-type channels contribute to the plateau (phase 2). 
Electrical activity that originates from the sinoatrial node is propagated via the His-Purkinje network, the fastest conduction pathway within the heart. The electrical signal travels from the sinoatrial node (SAN), which stimulates the atria to contract, to the atrioventricular node (AVN) which slows down conduction of the action potential, from the atria to the ventricles. This delay allows the ventricles to fully fill with blood before contraction. The signal then passes down through a bundle of fibres called the bundle of His, located between the ventricles, and then to the purkinje fibers at the bottom (apex) of the heart, causing ventricular contraction. This is known as the electrical conduction system of the heart, see figure 4.
Other than the SAN, the AVN and purkinje fibres also have pacemaker activity and can therefore spontaneously generate an action potential. However, these cells usually do not depolarize spontaneously, simply because, action potential production in the SAN is faster. This means that before the AVN or purkinje fibres reach the threshold potential for an action potential, they are depolarized by the oncoming impulse from the SAN  This is called "overdrive suppression".  Pacemaker activity of these cells is vital, as it means that if the SAN were to fail, then the heart could continue to beat, albeit at a lower rate (AVN= 40-60 beats per minute, purkinje fibres = 20-40 beats per minute). These pacemakers will keep a patient alive until the emergency team arrives.
An example of premature ventricular contraction, is the classic athletic heart syndrome. Sustained training of athletes causes a cardiac adaptation where the resting SAN rate is lower (sometimes around 40 beats per minute). This can lead to atrioventricular block, where the signal from the SAN is impaired in its path to the ventricles. This leads to uncoordinated contractions between the atria and ventricles, without the correct delay in between and in severe cases can result in sudden death. 
Regulation by the autonomic nervous system Edit
The speed of action potential production in pacemaker cells is affected, but not controlled by the autonomic nervous system.
The sympathetic nervous system (nerves dominant during the bodies fight or flight response) increase heart rate (positive chronotropy), by decreasing the time to produce an action potential in the SAN. Nerves from the spinal cord release a molecule called noradrenaline, which binds to and activates receptors on the pacemaker cell membrane called β1 adrenoceptors. This activates a protein, called a Gs-protein (s for stimulatory). Activation of this G-protein leads to increased levels of cAMP in the cell (via the cAMP pathway). cAMP binds to the HCN channels (see above), increasing the funny current and therefore increasing the rate of depolarization, during the pacemaker potential. The increased cAMP also increases the opening time of L -type calcium channels, increasing the Ca 2+ current through the channel, speeding up phase 0. 
The parasympathetic nervous system (nerves dominant while the body is resting and digesting) decreases heart rate (negative chronotropy), by increasing the time taken to produce an action potential in the SAN. A nerve called the vagus nerve, that begins in the brain and travels to the sinoatrial node, releases a molecule called acetylcholine (ACh) which binds to a receptor located on the outside of the pacemaker cell, called an M2 muscarinic receptor. This activates a Gi-protein (I for inhibitory), which is made up of 3 subunits (α, β and γ) which, when activated, separate from the receptor. The β and γ subunits activate a special set of potassium channels, increasing potassium flow out of the cell and decreasing membrane potential, meaning that the pacemaker cells take longer to reach their threshold value.  The Gi-protein also inhibits the cAMP pathway therefore reducing the sympathetic effects caused by the spinal nerves. 
Neural Circuits Underlying Escape Behavior in Drosophila
P. Phelan , . J.M. Blagburn , in Network Functions and Plasticity , 2017
3.3 Some GFS Synapses Are Rectifying Junctions
Gap junctions are assemblies of intercellular channels each of which is composed of two multimeric hemichannels. The terms homotypic and heterotypic refer to channels in which the subunit composition of the docked hemichannels is identical or different, respectively. Both channel types are possible and probable in the GFS as a consequence of the cell-specific expression of shakB splice variants ( Fig. 2.2 ). shakB(neural+16) is expressed in the GF ( Zhang et al., 1999 Phelan et al., 2008 ) and is necessary and sufficient to couple this cell to the GCIs in the brain and the TTMn and PSI in the thoracic ganglion ( Phelan et al., 2008 ). As the GCIs also express shakB(neural+16) ( Zhang et al., 1999 Phelan et al., 2008 ), the gap-junction channels at GF-GCI synapses are suggested to be homotypic. Channels at the GF-TTMn (and possibly the GF-PSI) synapses, on the other hand, are predicted to be heterotypic, with pre- and postsynaptic hemichannels composed of ShakB(Neural+16) and ShakB(Lethal), respectively ( Phelan et al., 2008 Jacobs et al., 2000 ).
How does the molecular composition of the GFS synapses influence their functional properties? This question has not yet been addressed ex vivo in the Drosophila nervous system. It would involve direct recording of transmission at individual synapses, which remains a technical challenge in the diminutive fruit fly. Modeling the synapses in a paired cell expression system, however, has been instructive ( Phelan et al., 2008 , Fig. 2.5 ). Cell pairs in which both cells expressed ShakB(Neural+16), as at the GF-GCI synapses, formed simple homotypic channels ( Fig. 2.5A, B and D ) that transmitted currents bidirectionally, a property that fits with a possible role for the GCIs in synchronizing the activity of right and left GFs. Cell pairs in which one cell expressed ShakB(Neural+16) and the other, ShakB(Lethal), mimicking the GF-TTMn synapse, formed heterotypic channels that displayed rectification, ie, they transmitted depolarizing signals in one direction from the ShakB(Neural+16)-expressing cell (“presynaptic GF”) to the ShakB(Lethal)-expressing cell (“postsynaptic TTMn”). This unidirectionality was a result of differential voltage sensitivity of the molecularly distinct hemichannels channels opened when the presynaptic side was positive relative to the postsynaptic side and vice versa ( Phelan et al., 2008 , Fig. 2.5A, B, E and F ). Structure–function studies ( Marks and Skerrett, 2014 ) suggested that the ShakB N-terminus mediates voltage gating. Rectification at electrical synapses classically is associated with speed and reliability. The phenomenon was first described in the crayfish (where the GF-giant motorneuron synapse can be recorded in situ Furshpan and Potter, 1959 ) and shown to depend on a differential response to voltage of the apposing hemichannels ( Jaslove and Brink, 1986 Giaume et al., 1987 ). The work in Drosophila (albeit based in part on artificial synapses) confirmed and extended this by demonstrating that asymmetric voltage gating is underpinned by molecular asymmetry ( Phelan et al., 2008 ).
Interestingly, both fly and crayfish rectifying synapses also exhibit morphological asymmetry in the form of a single row of large (∼40–80 nm) juxtamembrane vesicles in the GF, but not in the postsynaptic motorneuron ( Hanna et al., 1978 Leitch, 1992 Blagburn et al., 1999 , Fig. 2.3B ). By contrast, nonrectifying junctions in the crayfish have vesicles on both sides of the synapse ( Peracchia, 1973 Leitch, 1992 ). These vesicles are clearly linked to gap junctions. In Drosophila, they are absent in shakB mutants that lack electrical synapses ( Blagburn et al., 1999 ). In crayfish (at least at nonrectifying septate synapses), the vesicles are anchored to the junctional membrane by filament-like structures ( Ohta et al., 2011 ). Although the presence of these vesicles has been widely documented, their function is unknown. Ohta et al. (2011) showed that they express the vesicular nucleotide transporter and speculate that they may store ATP, which is known to be released by innexin and pannexin unpaired channels ( Dahl and Muller, 2014 ). If so, this would provide another means of signaling between the GF and its targets.
Definition of Synapse
Synapses or neuronal junction refers to a region that helps in the conjugation and coordination of signal transmission activity between the two adjoining neurons. It forms a neuronal network to coordinate the tasks performed by the central nervous system and peripheral effector cells. Synapse has the following elements:
- Presynaptic terminal: It contains synaptic vesicles encapsulated around the neurotransmitter substance.
- Synaptic cleft: It refers to the 20 nm wide synaptic gap, which separates the two adjacent neurons.
- Postsynaptic terminal: It possesses receptor sites for the binding of neurotransmitters, which can either inhibit or promote the passage of nerve signal from one cell to the next.
Types of Synapses
Synapse is generally classified on the basis of two attributes, one is the attachment of nerve fibers and the other is the presence of neuroreceptors and neurotransmitters.
Based on the Attachment of Neurons
- Axodendritic: In this type, a synaptic junction is in between the axon endings of one neuron and the dendritic spines of another neuron.
- Axosomatic: Here, synapses occur between the axon terminal of one nerve fibre with the soma or cell body of an adjacent neuron.
- Axoaxonic: In this kind, synapses occur in between axon’s terminal endings of the neighbouring neurons.
Based on the Type of Neurotransmitters and Neuroreceptors in Neurons
- Excitatory Ion Channel Synapses: It contains neuroreceptor in the form of sodium ion channels, which influx the electropositive sodium ions into the cytosol. The movement of sodium ions will depolarize the membrane potential inside the cytosol and generate the action potential respective to the stimulus.
For the conduction of an action potential, a stimulus should reach the maximum threshold. When an action potential reaches the presynaptic terminal, it will excite the synaptic vesicles (having neurotransmitters like acetylcholine, glutamate or aspartate) to fuse with the plasma membrane. The fusion facilitates the diffusion of neurotransmitters and stimulation of the adjacent neurons.
- Inhibitory Ion Channel Synapses: It contains the neuroreceptor in the form of chloride channels, which influx the chloride ions into the cell cytoplasm. The movement of chloride ions will bring out hyperpolarization of the membrane potential and slow down the conduction of an action potential. Here, the diffusion of neurotransmitters from presynaptic neurons inhibit the postsynaptic neurons. It includes glycine or GABA as inhibitory neurotransmitters.
- Non-Channel Synapses: They possess neuroreceptors in the form of membrane-bound enzymes, instead of ion channels. When the membrane-bound enzymes get activated by the neurotransmitters, they release the chemical messengers. The significant role of chemical messengers is to mediate long-lasting responses (learning and memory). They comprise neurotransmitters like acetylcholine, epinephrine, and dopamine etc.
They produces nerve impulse, which can freely travel between the adjacent cells through gap junctions, without any carrier molecules. Electrical synapses facilitate faster conduction of nerve signals.
It does not require neurotransmitters, and it can conduct the transmission of information bidirectionally. Here, the ions move vigorously through the tiny apertures between gap junctions.
The movement of ions occurs in a synchronized manner, without collision of ions. The gap between electrical synapses is approximately 3.5 nm. Electrical signals are generally excitatory.
Destruction in signal strength occurs when it travels between the neighbouring neurons. To overcome the loss, it requires a big presynaptic neuron to excite much smaller postsynaptic neuron.
Here, the transmission occurs via synaptic knob containing synaptic vesicles. The vesicles store chemical messengers (neurotransmitters). The fusion of vesicles release neurotransmitters outside the neuron.
The nerve impulse along with carrier molecules or neurotransmitters cross the membrane through the voltage-gated calcium channels. An opening of voltage-gated calcium channels allows rapid influx of calcium ions.
As a result, the concentration of calcium ions in the presynaptic neuron increases, which eventually cause fusion of presynaptic vesicles with the plasma membrane.
It consequences release of chemical messengers out of the neuron. Chemical synapse is further classifies into the excitatory and inhibitory synapse, based on its effect on nerve signal.
Excitatory Chemical Synapses
It promotes the propagation or conduction of an action potential. The binding of neurotransmitters to excitatory synapses leads into an opening of non-voltage gated channels.
It allow an influx of sodium or sometimes both sodium and potassium ions into the plasma membrane. The opening of the channel facilitates depolarization of the presynaptic plasma membrane, which generates an action potential.
Inhibitory Chemical Synapses
It inhibits the propagation or conduction of an action potential. The binding of neurotransmitter with an inhibitory synapse results into an opening of potassium and chloride channels. Ultimately, it leads to the hyperpolarization of the postsynaptic membrane that ceases the further movement of an action potential.
The transmission of a nerve signal between the adjoining neurons is not a simple process. The nerve impulse must excite the synaptic vesicles to integrate them with the axon’s membrane. The dissolution of synaptic vesicles ensures the propagation of neurotransmitter molecules through exocytosis.
The chemical messengers or neurotransmitters further carry the nerve signal from the presynaptic terminal, beyond the synaptic gap. Afterwards, the neurotransmitters bind with the specific cell receptors of the postsynaptic neuron or target cell. The action of neurotransmitter can be either inhibitory or excitatory, i.e. it may either excite or inhibit the neuron, to which they bind with.
Here, we could take a reference of neurotransmitter with the key and cell receptors with a lock. Therefore, neurotransmitter functions as a key that can open or close the cell receptors. The specific binding of neurotransmitter with the cell receptor will initiate the further movement of the nerve signal.
Therefore, we can conclude that the synapses serve as the functional links between the neural network. It facilitates the transfer of nerve impulse from the nerve ending of pre-synaptic neuron to the post-synaptic neuron. The transmission of nerve impulse generally occurs via electrical and chemical synapses.
Question: Question 15: Which Of The Following Statements Regarding Oxytocin Is False? A. Oxytocin Is A Hormone That Also Acts As A Neurotransmitter In The Brain. B. Oxytocin Is A Biogenic Amine C. Oxytocin Is Involved In Social Recognition And Bonding D. Oxytocin Receptors Are Expressed By Neurons In Many Parts Of The Brain Question 16: Name 5 Different Hormones .
Which of the following statements regarding Oxytocin is false?
A. Oxytocin is a hormone that also acts as a neurotransmitter in the brain.
B. Oxytocin is a biogenic amine
C. Oxytocin is involved in social recognition and bonding
D. Oxytocin receptors are expressed by neurons in many parts of the brain
Name 5 different hormones released by the pituitary gland (5 marks)
Proteins that are fully translated in the cytosol and lack a sorting signal will end up in ____.
C. the interior of the nucleus.
How do proteins travel from one cisterna to the next in the Golgi apparatus?
A. By transport vesicles that bud off from one cisterna and fuse with the next cisterna
B. By physical connections between two cisternae
C. Through pores in the membranes of cisternae
D. Through passive diffusion
A protein sorting signal would be best described as a:
A. short sequence of bases in a mRNA molecule that is not translated on the ribosome.
B. small protein that binds to another protein as it passes through a membrane.
C. special receptor protein that occurs on the surface of membranes.
D. short sequence of amino acids located on the N-terminus of a protein
A. Nuclear localisation Site
B. Nuclear localisation Specifier
C. Nuclear localisation Signal
D. Nuclear localisation System
In a eukaryotic cell, where are most of the proteins for the electron transport chain located?
A. In the plasma membrane
B. In the mitochondrial inner membrane
C. In the mitochondrial outer membrane
D. In the ER membrane
In the electron transport chain, what provides the main reservoir for protons that are pumped across the membrane?
Activation of ATP Synthase drives ATP synthesis leading to___
Which of the following statements about the cell cycle is false?
A. Once a cell decides to enter the cell cycle, the time from start to finish is the same in all eukaryotic cells.
B. An unfavorable environment can cause cells to arrest in G1.
C. A cell has more DNA during G2 than it did in G1.
D. The cleavage divisions that occur in an early embryo have short G1 and G2 phases
Cells in the G0 state ________________.
B. cannot re-enter the cell cycle.
C. have entered this arrest state from either G1 or G2.
D. have duplicated their DNA.
The principal microtubule-organizing center in animal cells is the ____________.
A. binds to desmosomes of epithelial cells.
B. support epithelial cells to the basal lamina
C. allow ions and small molecules to pass from one cell to another.
D. link cadherin to the actin cytoskeleton.
A. four cells that are genetically identical and contain half as many chromosomes as the original
parent germ-line cell.
B. two genetically identical daughter cells.
C. two cells that are genetically identical and contain half as many chromosomes as the original
parent germ-line cell.
D. four cells that are genetically dissimilar and contain half as many chromosomes as the original
When does recombination (crossing-over) occur?
A. Meiosis I
B. Meiosis II
C. Mitosis S Phase
D. Mitosis M Phase
Which epithelial cell junctions serve to seal neighboring cells together so that water-soluble molecules cannot easily leak between them?
B. Gap junctions
C. Tight junctions
D. Adherens junctions
In which group do pathway similarities suggest that ancestral signaling molecules first evolved in?
Which statement is correct?
A. Wnt stands for Wildtype Notch Transcriptor.
B. The Wnt signalling pathway is not conserved between animal species.
C. Wnt signalling is involved in a large number of diverse biological processes.
D. The APC/GSK3/Axin complex directly binds DNA to regulate target gene expression
The main finding of this study is that after exposing the mice to a visual environment enriched in upward motion for a prolonged time right after eye-opening, gap junction connections among upward preferring DSGCs (V-DSGCs) are significantly potentiated, while early exposure to downward motion has the opposite effect. This outcome is tightly linked to the level of synchronized firing in V-DSGCs during the visual training. The change is long-term it persists three months after the training stimulus has been removed. Our results demonstrate that gap junction connections in the retina are involved in experience-dependent plasticity during early development.
Connection numbers, connection strength, and potential mechanism
The results presented in this report indicate that both the number of gap junction–connected V-DSGCs and the connection strength are enhanced by upward VME training, which then lead to an elevated level of spike synchrony between these cells (Fig 4). However, the increase in the proportion of connected cells likely accounts for a major part of the observed enhancement in spike synchrony, while the strengthened gap junction connections seem to play a slightly more minor role (Fig 4B vs 4D). Our MEA recording can record V-DSGC pairs whose soma-to-soma distances range from 50 to 500 μm. Because the strength of gap junction connections decreases with increasing distance between cells, one needs to heed the distribution of pair distance within a population of V-DSGCs when comparing average cross-correlations between different populations. To restrict the influence of the distance, we focused much of our comparison of correlation on pairs that are within 250 μm of each other, thus having a significant proportion of overlapping dendritic fields and, consequently, stronger coupling. However, even if we compare correlation for pairs that are further apart than 250 μm, both the proportion of correlated pairs and the strength of the correlation are similarly impacted by VME training. These observations suggest that V-DSGCs in the upward VME group are much more likely to connect with V-DSGCs via gap junctions, and with stronger connection strength as well. Within the limit set by our recording method, distance between cell pairs does not seem to be a limiting factor on whether VME training may influence their gap junction connections.
When inferring gap junction connection strength from spike time correlation, one should be mindful that weak connections might go undetected due to the limited number of spikes used in the correlation analysis. Taking into account this limitation, we propose that most V-DSGCs within the contact range of their dendrites form gap junction connections. In the control retina, after normal development, many of these gap junction connections are weak or inactive only a portion of V-DSGC pairs, mostly those that are close to each other, with large overlap of dendritic fields, have enough active gap junction connections to exhibit measurable spike synchrony. When the mice are exposed to a visual environment such as those dominated by downward moving dots or vertical moving dots from eye-opening, activity synchronization through these gap junction connections is minimal or possibly suppressed (Fig 3A), while general activity level is normal or even higher (vertical dots). Through a Hebbian-like mechanism, this may lead to gradual depression of the gap junction connections, to the extent that no measurable coupling can be detected via spike synchrony post-development (Fig 2C). Conversely, with upward motion training, gap junction–mediated activity correlation is elevated in all V-DSGCs for a prolonged period, resulting in a general enhancement of gap junction connection strength among V-DSGCs. This manifests as more coupled cells, as well as stronger connections between. For pairs that are farther apart, with very small overlap of dendritic fields, and that do not show measurable connections under normal circumstances, potentiated connection may put the pair into the “connected” category with a very low measurement of total connection strength. This hypothesis considers the effects on connection numbers and connection strength being not fundamentally different from each other, that potentiated connection naturally leads to a higher percentage of pairs being considered connected. More studies such as the time course of the change in gap junction connections and the expression and localization of gap junction proteins during training are needed to test if the hypothesis is correct and to elucidate the mechanism behind this experience-dependent gap junction plasticity during development.
There are discrepancies between our gap junction coupling strength measurements (Fig 6) and those from previous studies [23,32]. Our measurements tend to be weaker: In control retina, we see on average 2–3 tracer coupled cells, and coupling strength varies from 0 to 0.08, with a mean value at approximately 0.02, whereas Trenholm and colleagues reported 5–7 coupled cells on average, and the coupling strength can be as high as 0.2. Potential explanations for these discrepancies include the adaptational state of the retina  and the distance between recorded pairs. We dark-adapted the animal before experiments to preserve the light response, while Trenholm and colleagues light-adapted the animal to enhance gap junction coupling. This and differences in tracer coupling protocols may account for the discrepancy in our tracer coupling results. In addition, different transgenic animals were used: The distances between our recorded V-DSGC pairs vary from 70 to 250 μm using Cdh6-CreER × Thy1-stop-YFP animals, while pair distances in the Hb9::eGFP retina Trenholm and colleagues recorded from appear to be more uniform, and much smaller, thus potentially have systematically stronger coupling. In our experiments, we recorded from both control and VME retinas under exactly the same condition, and oftentimes on the same day using the same reagents. Thus, despite these discrepancies in absolute values of correlation strength, we believe the direction of change we observed between VME and control groups is consistent and valid.
On closer examination of the response properties of V-DSGCs after VME training, it appears that first-order statistics such as absolute firing rates and autocorrelation vary between different VME groups (S1 Table and S5 Fig). In part, this may simply reflect variations in recording quality and noise. Nevertheless, there is a tendency for groups with higher cross-correlation to also have higher firing rates. We normalized the cross-correlation to the product of the spike numbers, so that the correlation value is independent of absolute activity levels. VME-induced changes in cross-correlation were still observed. Thus, the change in correlation is not caused by the change in firing rates. The question becomes whether the reverse is true, that enhanced coupling leads to elevated activity levels. Due to the nature of the electrical coupling, a network of neurons with enhanced homologous coupling among them has the potential to show increased activity level. More detailed study using gap junction blocker or gap junction protein knockout animals is needed to definitively answer this question.
Why V-DSGCs, upward motion, and not others
In the visual cortex, experience-dependent plasticity takes on a slightly different form. Motion training using any direction of motion can induce rapid emergence of DS columns for the trained direction . Similarly, early experience with any restricted orientation results in overrepresentation of that orientation in V1 . There does not appear to be a bias for neurons preferring certain directions or orientations to be affected more strongly than the others, whereas from the results in this report, only gap junction connections between V-DSGCs are affected. For gap junctions in the other subtypes of ooDSGCs, they are still present at the beginning of VME training, yet they disappear after eye-opening, with or without VME training. Thus, developmentally regulated expression of gap junctions in these other ooDSGCs are not impacted only gap junction connection strength among V-DSGCs is affected by VME. It is worth noting, however, that cortical neurons differ in their sensitivity to early visual experience too: In ferret V1, DS can be rapidly induced by motion experience, but orientation selectivity (OS) only shows much weaker enhancement . In the mouse visual cortex, OS is plastic to changes induced by altered visual experience, whereas DS is already mature at eye-opening [34,35]. One obvious explanation for such differences is that the developmental processes of the corresponding neural circuits are not completely synchronized, and visual experience may have stronger impact on those circuits that develop at a slower pace and thus more plastic during later stages of development.
Our results demonstrated a tight correlation between the level of correlated activity during training and the change in gap junction connection strength. The stronger the correlation during training, the more potentiation. Conversely, if the training decorrelates the V-DSGCs, then the gap junction connections are weakened. This strongly suggests that correlated activity in the connected V-DSGCs directly leads to changes in the gap junction connection. Direct examination of gap junction connections in V-DSGCs around eye-opening should provide more insights into this type of plasticity. Even if we answer the “how” question, the “why” question remains: Why is V-DSGC the only subtype of ooDSGCs that keep gap junction connections among them? These connections can even be modulated by visual experience during early development, implying a level of functional importance. Is there a functional or ethological reason that mice treat upward motion–preferring ooDSGCs, or upward motion in visual inputs, differently than the others? This is a question that also warrants attention.
Gap junction connections among neurons play critical roles during development. These connections are prominent before P12 in the mouse brain but decline afterward with the emergence of chemical synapses . It is proposed that the early gap junction connections among neurons might help to establish a developmental blueprint, affecting the formation of the mature neural circuits . Our work presented here is an example of how electrical synapses, through regulation of synchronized firing between neurons, may influence the neural circuits that harbor these synapses, potentially also exert impacts on the chemical synapses within the circuits, and consequently modulate the circuit functions.
Electrical synapses exhibit extensive plasticity, and the vertebrate retina is a popular place to study it [38,39]. In this work, we present another example of gap junction plasticity. Activity through the gap junction connections among ooDSGCs play an important role in inducing changes in the ooDSGC circuit. At the same time, these gap junction connections themselves are also potentiated or depressed by the process. These changes are stable and long-term. We also tested whether VME-induced change occurred in mature animals, and the answer is no. For nursing mothers that have been put into the same training environment, no change in their ooDSGCs has been found (S6 Fig). Our results implicate the involvement of gap junction in both the cause and the effect of experience-dependent plasticity in the retina this may provide a new angle to study neural plasticity during the development of the visual system.