15.7: The Peripheral Nervous System - Biology

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Learning Objectives

• Describe the structures found in the PNS
• Distinguish between somatic and autonomic structures, including the special peripheral structures of the enteric nervous system
• Name the twelve cranial nerves and explain the functions associated with each
• Describe the sensory and motor components of spinal nerves and the plexuses that they pass through

The PNS is not as contained as the CNS because it is defined as everything that is not the CNS. Some peripheral structures are incorporated into the other organs of the body. In describing the anatomy of the PNS, it is necessary to describe the common structures, the nerves and the ganglia, as they are found in various parts of the body. Many of the neural structures that are incorporated into other organs are features of the digestive system; these structures are known as the enteric nervous system and are a special subset of the PNS.

Ganglia

A ganglion is a group of neuron cell bodies in the periphery. Ganglia can be categorized, for the most part, as either sensory ganglia or autonomic ganglia, referring to their primary functions. The most common type of sensory ganglion is a dorsal (posterior) root ganglion. These ganglia are the cell bodies of neurons with axons that are sensory endings in the periphery, such as in the skin, and that extend into the CNS through the dorsal nerve root. The ganglion is an enlargement of the nerve root. Under microscopic inspection, it can be seen to include the cell bodies of the neurons, as well as bundles of fibers that are the posterior nerve root (Figure 1). The cells of the dorsal root ganglion are unipolar cells, classifying them by shape. Also, the small round nuclei of satellite cells can be seen surrounding—as if they were orbiting—the neuron cell bodies.

Another type of sensory ganglion is a cranial nerve ganglion. This is analogous to the dorsal root ganglion, except that it is associated with a cranial nerve instead of a spinal nerve. The roots of cranial nerves are within the cranium, whereas the ganglia are outside the skull. For example, the trigeminal ganglion is superficial to the temporal bone whereas its associated nerve is attached to the mid-pons region of the brain stem. The neurons of cranial nerve ganglia are also unipolar in shape with associated satellite cells. The other major category of ganglia are those of the autonomic nervous system, which is divided into the sympathetic and parasympathetic nervous systems.

The sympathetic chain ganglia constitute a row of ganglia along the vertebral column that receive central input from the lateral horn of the thoracic and upper lumbar spinal cord. Superior to the chain ganglia are three paravertebral ganglia in the cervical region. Three other autonomic ganglia that are related to the sympathetic chain are the prevertebral ganglia, which are located outside of the chain but have similar functions. They are referred to as prevertebral because they are anterior to the vertebral column. The neurons of these autonomic ganglia are multipolar in shape, with dendrites radiating out around the cell body where synapses from the spinal cord neurons are made. The neurons of the chain, paravertebral, and prevertebral ganglia then project to organs in the head and neck, thoracic, abdominal, and pelvic cavities to regulate the sympathetic aspect of homeostatic mechanisms.

Another group of autonomic ganglia are the terminal ganglia that receive input from cranial nerves or sacral spinal nerves and are responsible for regulating the parasympathetic aspect of homeostatic mechanisms. These two sets of ganglia, sympathetic and parasympathetic, often project to the same organs—one input from the chain ganglia and one input from a terminal ganglion—to regulate the overall function of an organ. For example, the heart receives two inputs such as these; one increases heart rate, and the other decreases it.

The terminal ganglia that receive input from cranial nerves are found in the head and neck, as well as the thoracic and upper abdominal cavities, whereas the terminal ganglia that receive sacral input are in the lower abdominal and pelvic cavities. Terminal ganglia below the head and neck are often incorporated into the wall of the target organ as a plexus. A plexus, in a general sense, is a network of fibers or vessels. This can apply to nervous tissue (as in this instance) or structures containing blood vessels (such as a choroid plexus). For example, the enteric plexus is the extensive network of axons and neurons in the wall of the small and large intestines. The enteric plexus is actually part of the enteric nervous system, along with the gastric plexuses and the esophageal plexus. Though the enteric nervous system receives input originating from central neurons of the autonomic nervous system, it does not require CNS input to function. In fact, it operates independently to regulate the digestive system.

View the University of Michigan WebScope to explore the tissue sample in greater detail. If you zoom in on the dorsal root ganglion, you can see smaller satellite glial cells surrounding the large cell bodies of the sensory neurons. From what structure do satellite cells derive during embryologic development?

Nerves

Bundles of axons in the PNS are referred to as nerves. These structures in the periphery are different than the central counterpart, called a tract. Nerves are composed of more than just nervous tissue. They have connective tissues invested in their structure, as well as blood vessels supplying the tissues with nourishment. The outer surface of a nerve is a surrounding layer of fibrous connective tissue called the epineurium. Within the nerve, axons are further bundled into fascicles, which are each surrounded by their own layer of fibrous connective tissue called perineurium. Finally, individual axons are surrounded by loose connective tissue called the endoneurium (Figure 3).

These three layers are similar to the connective tissue sheaths for muscles. Nerves are associated with the region of the CNS to which they are connected, either as cranial nerves connected to the brain or spinal nerves connected to the spinal cord.

View the University of Michigan WebScope to explore the tissue sample in greater detail. With what structures in a skeletal muscle are the endoneurium, perineurium, and epineurium comparable?

Cranial Nerves

The nerves attached to the brain are the cranial nerves, which are primarily responsible for the sensory and motor functions of the head and neck (one of these nerves targets organs in the thoracic and abdominal cavities as part of the parasympathetic nervous system). There are twelve cranial nerves, which are designated CNI through CNXII for “Cranial Nerve,” using Roman numerals for 1 through 12. They can be classified as sensory nerves, motor nerves, or a combination of both, meaning that the axons in these nerves originate out of sensory ganglia external to the cranium or motor nuclei within the brain stem. Sensory axons enter the brain to synapse in a nucleus. Motor axons connect to skeletal muscles of the head or neck. Three of the nerves are solely composed of sensory fibers; five are strictly motor; and the remaining four are mixed nerves.

Learning the cranial nerves is a tradition in anatomy courses, and students have always used mnemonic devices to remember the nerve names. A traditional mnemonic is the rhyming couplet, “On Old Olympus’ Towering Tops/A Finn And German Viewed Some Hops,” in which the initial letter of each word corresponds to the initial letter in the name of each nerve. The names of the nerves have changed over the years to reflect current usage and more accurate naming. An exercise to help learn this sort of information is to generate a mnemonic using words that have personal significance. The names of the cranial nerves are listed in Table 1 along with a brief description of their function, their source (sensory ganglion or motor nucleus), and their target (sensory nucleus or skeletal muscle).

Figure 5 shows where the nerves are located in the brain. The olfactory nerve and optic nerve are responsible for the sense of smell and vision, respectively. The oculomotor nerve is responsible for eye movements by controlling four of the extraocular muscles. It is also responsible for lifting the upper eyelid when the eyes point up, and for pupillary constriction. The trochlear nerve and the abducens nerve are both responsible for eye movement, but do so by controlling different extraocular muscles. The trigeminal nerve is responsible for cutaneous sensations of the face and controlling the muscles of mastication. The facial nerve is responsible for the muscles involved in facial expressions, as well as part of the sense of taste and the production of saliva. The vestibulocochlear nerve is responsible for the senses of hearing and balance. The glossopharyngeal nerve is responsible for controlling muscles in the oral cavity and upper throat, as well as part of the sense of taste and the production of saliva. The vagus nerve is responsible for contributing to homeostatic control of the organs of the thoracic and upper abdominal cavities. The spinal accessory nerve is responsible for controlling the muscles of the neck, along with cervical spinal nerves. The hypoglossal nerve is responsible for controlling the muscles of the lower throat and tongue.

Table 1. Cranial Nerves
Mnemonic	#	Name	Function (S/M/B)	Central connection (nuclei)	Peripheral connection (ganglion or muscle)
On	I	Olfactory	Smell (S)	Olfactory bulb	Olfactory epithelium
Old	II	Optic	Vision (S)	Hypothalamus/thalamus/midbrain	Retina (retinal ganglion cells)
Olympus	III	Oculomotor	Eye movements (M)	Oculomotor nucleus	Extraocular muscles (other 4), levator palpebrae superioris, ciliary ganglion (autonomic)
Towering	IV	Trochlear	Eye movements (M)	Trochlear nucleus	Superior oblique muscle
Tops	V	Trigeminal	Sensory/motor—face (B)	Trigeminal nuclei in the midbrain, pons, and medulla	Trigemal
A	VI	Abducens	Eye movements (M)	Abducens nucleus	Lateral rectus muscle
Finn	VII	Facial	Motor—face, Taste (B)	Facial nucleus, solitary nucleus, superior salivatory nucleus	Facial muscles, Geniculate ganglion, Pterygopalatine ganglion (autonomic)
And	VIII	Auditory (Vestibulocochlear)	Hearing/balance (S)	Cochlearn nucleus, Vestibular nucleus/cerebellum	Spiral ganglion (hearing), Vestibular ganglion (balance)
German	IX	Glossopharyngeal	Motor—throat Taste (B)	Solitary nucleus, inferior salivatory nucleus, nucleus ambiguus	Pharyngeal muscles, Geniculate ganglion, Otic ganglion (autonomic)
Viewed	X	Vagus	Motor/sensory—viscera (autonomic)	Medulla	Terminal ganglia serving thoracic and upper abdominal organs (heart and small intestines)
Some	XI	Spinal Accessory	Motor—head and neck (M)	Spinal accessory nucleus	Neck muscles
Hops	XII	Hypoglossal	Motor—lower throat (M)	Hypoglossal nucleus	Muscles of the larynx and lower pharynx

Three of the cranial nerves also contain autonomic fibers, and a fourth is almost purely a component of the autonomic system. The oculomotor, facial, and glossopharyngeal nerves contain fibers that contact autonomic ganglia. The oculomotor fibers initiate pupillary constriction, whereas the facial and glossopharyngeal fibers both initiate salivation. The vagus nerve primarily targets autonomic ganglia in the thoracic and upper abdominal cavities.

Visit this site to read about a man who wakes with a headache and a loss of vision. His regular doctor sent him to an ophthalmologist to the vision loss. The ophthalmologist recognizes a greater problem and immediately sends him to the emergency room. Once there, the patient undergoes a large battery of tests, but a definite cause cannot be found. A specialist recognizes the problem as meningitis, but the question is what caused it originally. How can that be cured? The loss of vision comes from swelling around the optic nerve, which probably presented as a bulge on the inside of the eye. Why is swelling related to meningitis going to push on the optic nerve?

Another important aspect of the cranial nerves that lends itself to a mnemonic is the functional role each nerve plays. The nerves fall into one of three basic groups. They are sensory, motor, or both (see Table 1). The sentence, “Some Say Marry Money But My Brother Says Brains Beauty Matter More,” corresponds to the basic function of each nerve.

The first, second, and eighth nerves are purely sensory: the olfactory (CNI), optic (CNII), and vestibulocochlear (CNVIII) nerves. The three eye-movement nerves are all motor: the oculomotor (CNIII), trochlear (CNIV), and abducens (CNVI). The spinal accessory (CNXI) and hypoglossal (CNXII) nerves are also strictly motor. The remainder of the nerves contain both sensory and motor fibers. They are the trigeminal (CNV), facial (CNVII), glossopharyngeal (CNIX), and vagus (CNX) nerves.

The nerves that convey both are often related to each other. The trigeminal and facial nerves both concern the face; one concerns the sensations and the other concerns the muscle movements. The facial and glossopharyngeal nerves are both responsible for conveying gustatory, or taste, sensations as well as controlling salivary glands. The vagus nerve is involved in visceral responses to taste, namely the gag reflex. This is not an exhaustive list of what these combination nerves do, but there is a thread of relation between them.

Spinal Nerves

The nerves connected to the spinal cord are the spinal nerves. The arrangement of these nerves is much more regular than that of the cranial nerves. All of the spinal nerves are combined sensory and motor axons that separate into two nerve roots. The sensory axons enter the spinal cord as the dorsal nerve root. The motor fibers, both somatic and autonomic, emerge as the ventral nerve root. The dorsal root ganglion for each nerve is an enlargement of the spinal nerve.

There are 31 spinal nerves, named for the level of the spinal cord at which each one emerges. There are eight pairs of cervical nerves designated C1 to C8, twelve thoracic nerves designated T1 to T12, five pairs of lumbar nerves designated L1 to L5, five pairs of sacral nerves designated S1 to S5, and one pair of coccygeal nerves. The nerves are numbered from the superior to inferior positions, and each emerges from the vertebral column through the intervertebral foramen at its level. The first nerve, C1, emerges between the first cervical vertebra and the occipital bone. The second nerve, C2, emerges between the first and second cervical vertebrae. The same occurs for C3 to C7, but C8 emerges between the seventh cervical vertebra and the first thoracic vertebra. For the thoracic and lumbar nerves, each one emerges between the vertebra that has the same designation and the next vertebra in the column. The sacral nerves emerge from the sacral foramina along the length of that unique vertebra.

Spinal nerves extend outward from the vertebral column to enervate the periphery. The nerves in the periphery are not straight continuations of the spinal nerves, but rather the reorganization of the axons in those nerves to follow different courses. Axons from different spinal nerves will come together into a systemic nerve. This occurs at four places along the length of the vertebral column, each identified as a nerve plexus, whereas the other spinal nerves directly correspond to nerves at their respective levels. In this instance, the word plexus is used to describe networks of nerve fibers with no associated cell bodies. Of the four nerve plexuses, two are found at the cervical level, one at the lumbar level, and one at the sacral level (Figure 6).

The cervical plexus is composed of axons from spinal nerves C1 through C5 and branches into nerves in the posterior neck and head, as well as the phrenic nerve, which connects to the diaphragm at the base of the thoracic cavity. The other plexus from the cervical level is the brachial plexus.

Spinal nerves C4 through T1 reorganize through this plexus to give rise to the nerves of the arms, as the name brachial suggests. A large nerve from this plexus is the radial nerve from which the axillary nerve branches to go to the armpit region. The radial nerve continues through the arm and is paralleled by the ulnar nerve and the median nerve. The lumbar plexus arises from all the lumbar spinal nerves and gives rise to nerves enervating the pelvic region and the anterior leg. The femoral nerve is one of the major nerves from this plexus, which gives rise to the saphenous nerve as a branch that extends through the anterior lower leg.

The sacral plexus comes from the lower lumbar nerves L4 and L5 and the sacral nerves S1 to S4. The most significant systemic nerve to come from this plexus is the sciatic nerve, which is a combination of the tibial nerve and the fibular nerve. The sciatic nerve extends across the hip joint and is most commonly associated with the condition sciatica, which is the result of compression or irritation of the nerve or any of the spinal nerves giving rise to it.

These plexuses are described as arising from spinal nerves and giving rise to certain systemic nerves, but they contain fibers that serve sensory functions or fibers that serve motor functions. This means that some fibers extend from cutaneous or other peripheral sensory surfaces and send action potentials into the CNS. Those are axons of sensory neurons in the dorsal root ganglia that enter the spinal cord through the dorsal nerve root. Other fibers are the axons of motor neurons of the anterior horn of the spinal cord, which emerge in the ventral nerve root and send action potentials to cause skeletal muscles to contract in their target regions. For example, the radial nerve contains fibers of cutaneous sensation in the arm, as well as motor fibers that move muscles in the arm. Spinal nerves of the thoracic region, T2 through T11, are not part of the plexuses but rather emerge and give rise to the intercostal nerves found between the ribs, which articulate with the vertebrae surrounding the spinal nerve.

Try It

Anosmia is the loss of the sense of smell. It is often the result of the olfactory nerve being severed, usually because of blunt force trauma to the head. The sensory neurons of the olfactory epithelium have a limited lifespan of approximately one to four months, and new ones are made on a regular basis. The new neurons extend their axons into the CNS by growing along the existing fibers of the olfactory nerve. The ability of these neurons to be replaced is lost with age. Age-related anosmia is not the result of impact trauma to the head, but rather a slow loss of the sensory neurons with no new neurons born to replace them.

Smell is an important sense, especially for the enjoyment of food. There are only five tastes sensed by the tongue, and two of them are generally thought of as unpleasant tastes (sour and bitter). The rich sensory experience of food is the result of odor molecules associated with the food, both as food is moved into the mouth, and therefore passes under the nose, and when it is chewed and molecules are released to move up the pharynx into the posterior nasal cavity.

Anosmia results in a loss of the enjoyment of food. As the replacement of olfactory neurons declines with age, anosmia can set in. Without the sense of smell, many sufferers complain of food tasting bland. Often, the only way to enjoy food is to add seasoning that can be sensed on the tongue, which usually means adding table salt. The problem with this solution, however, is that this increases sodium intake, which can lead to cardiovascular problems through water retention and the associated increase in blood pressure.

Adrenomedullin: an important participant in neurological diseases

Adrenomedullin, a peptide with multiple physiological functions in nervous system injury and disease, has aroused the interest of researchers. This review summarizes the role of adrenomedullin in neuropathological disorders, including pathological pain, brain injury and nerve regeneration, and their treatment. As a newly characterized pronociceptive mediator, adrenomedullin has been shown to act as an upstream factor in the transmission of noxious information for various types of pathological pain including acute and chronic inflammatory pain, cancer pain, neuropathic pain induced by spinal nerve injury and diabetic neuropathy. Initiation of glia-neuron signaling networks in the peripheral and central nervous system by adrenomedullin is involved in the formation and maintenance of morphine tolerance. Adrenomedullin has been shown to exert a facilitated or neuroprotective effect against brain injury including hemorrhagic or ischemic stroke and traumatic brain injury. Additionally, adrenomedullin can serve as a regulator to promote nerve regeneration in pathological conditions. Therefore, adrenomedullin is an important participant in nervous system diseases.

Keywords: adrenomedullin brain injury glia mechanism morphine tolerance neural regeneration neuroprotective effect pathological pain regeneration sensitization target.

Conflict of interest statement

Figures

Summary schematic of the involvement…

Summary schematic of the involvement of adrenomedullin in pathological pain. Peripheral noxious stimuli…

Summary schematic of the involvement…

Summary schematic of the involvement of adrenomedullin in morphine tolerance. Chronic morphine administration…

Summary schematic of potential applications…

Summary schematic of potential applications of adrenomedullin for neuropathological diseases. Potential applications of…

Social Recognition

Notable Examples

Clonal Recognition in Anemones

Sea anemones compete for space on rocks or large shells, engaging in territorial battles using their stinging cells. Because asexual reproduction by fission is common in anemones, adjacent anemones are likely to be clonemates. Aggression is reduced or absent among clonemates, and intense between unrelated anemones. This self/nonself recognition system seems to function analogously to the vertebrate MHC system: internal identification of tissues is extended to an external phenotype that shapes aggressive interactions.

Vertebrates

A fascinating thread that runs through the vertebrate kin recognition literature is in the intimate linkage between MHC variation and recognition phenotypes. This association between MHC and recognition draws an obvious inference that the recognition of self extends from the internal immune identification of tissue and organs to the external environment. Given this inference, it is unsurprising that MHC differences among animals correlate with urinary odorants.

Young salmonid fish swim in schools that are predominantly full sibs. Later in life, territorial interactions among adults can occur between highly related fish that have settled on adjacent territories. The cues used in maintaining sib associations are MHC related. The prevailing argument in studies of these fish is that MHC-related phenotype matching allows territorial fish to modulate their aggression to close relatives, resulting in inclusive fitness benefits to the fish.

One of the earliest areas of exploration of kin associations was schooling tadpoles. Larvae of many species of amphibia, including frogs, toads, and salamanders, preferentially school with close kin. The underlying ecological and evolutionary benefit of kin schooling in amphibia has been elusive. As in salmonid fish, kin associations may lead to modulation of competitive interactions among related animals. Alternatively, kin selection benefits from shared vigilance and predation risk may be important. In salamanders that develop cannibalistic morphs under food deprivation, cannibals avoid consuming close relatives. This is a clear example of kin recognition facilitating a kin-selected evolutionary response.

Rodent studies, using a wide variety of species, have provided key clues to the mechanisms and function of social recognition in mammals. Classic studies in mice link the cues used in phenotypic matching to the MHC complex. Mice that differ only at MHC loci can be discriminated using phenotypic matching mechanisms. Generally speaking, rodents show strong abilities to discriminate kin from nonkin using phenotype matching. This ability is important as many rodent populations may contain previously unmet close relatives both nepotistic (aid-giving) and mating decisions can reflect information concerning kinship. Enough is known about rodent kin recognition to reveal that expression of phenotype matching as a source of kinship information in behavioral decisions is very much affected by life history. A 2003 review of rodent kin recognition by Mateo shows that while recognition by familiarity (presumably individual recognition) is nearly universal in rodents, phenotypic matching is more restricted. Hypothetically, phenotype matching is used by those species the life histories of which are likely to bring previously unmet relatives together ( Figure 2 ).

Figure 2 . Mammals, such as this coati (Nasua narica), often have variable color patterns that might be used in making individual discriminations within social groups. Their highly developed olfactory senses could provide a platform for social recognition, as well. While individual recognition is often assumed with mammalian social groups, rigorous tests of this hypothesis are only available for a few species.

Human social biology is largely structured around individual recognition and classification. Familial similarity, at least in visual cues, provides at best weak evidence of relatedness, with the notable exception of monozygotic twins. The hypothesis that kin information, including possible phenotypic matching, operates at subconscious levels and shapes human social interactions is relatively unexplored.

Unlike all of the other animals discussed in this article, birds appear not to use olfactory information in social recognition. Studies of cooperatively breeding and communally nesting birds demonstrate that social recognition via distinctive calls, such as the contact call of the long-tailed tit, carry individually distinctive information that can be learned. Thus, kin are identified by association, but as yet there is no evidence for kin recognition by phenotypic matching in birds.

Eusocial Insects

Nestmate recognition in eusocial insects usually relies on phenotype matching. Colony-specific cues are learned by young workers and that information is used in social interactions, particularly in excluding nonnestmates from the colony. Transfers among colonies of larvae or newly emerged adults are usually fully tolerated, suggesting that the cues are acquired from the surrounding environment rather than distinct productions of each worker. A key early finding was that recognition cues in social wasps are usually transmitted among workers via the colony’s nesting material. In honeybees, fatty acids that serve as strengtheners in the comb wax give all workers in the colony the same odor because of contact of the workers with the comb. Some ants use compound sequestration of hydrocarbons in the postpharyngeal gland as a method of establishing a colony-level recognition odor. These collective, or gestalt, labels simplify the recognition process in colonies that may have hundreds or thousands of members.

In eusocial insects, significant progress has been made in identifying the chemical compounds composing the recognition phenotype. Numerous studies show that young workers learn phenotypes, typically odors, of other workers in the colony. Most often the odors are metabolic products of the insects themselves as discussed earlier, the products of colony members typically combine to form a colony-level recognition signature. The most commonly used compounds are waxy materials secreted as waterproofing on the surface of the insect. These compounds are typically alkanes, methyalkanes, and alkenes, with chain lengths from 21 to 37 carbons. In some species, though, odors acquired from the environment contribute to nestmate recognition cues. Chemical analyses of surface extracts of insects often yield 20 or more compounds, but the presence of a compound does not automatically translate in the use of that compound in recognition. Experimental studies suggest that alkanes are used less as signals, perhaps because alkanes lack an easily perceived functional group or conformational feature. Multivariate analyses of gas chromatographic results can easily separate colonies, but offer no proof of which compounds are used in discriminations by the insects. Bioassays of putative recognition cues have implicated alkenes and methylalkanes (in ants and wasps), fatty acids (in honeybees), and macrocyclic lactones (in halictid bees) as recognition cues.

Phenotypic variation among colonies in the relative proportion of these compounds (correlated with genotypic variation among colonies) provides the information needed to make discriminations. In most species, young workers learn the phenotype of their colony and use this to make nestmate/nonnestmate discriminations. Young workers are flexible enough to learn the phenotype of the colony in which they emerge, even if they are not genetic members of the colony. This extends to an ability to integrate into colonies of other species: a mechanism that facilitates slave-making in ants and other types of social parasitism.

A notable exception to the use of odors in nestmate recognition is the discovery that some eusocial wasps use interindividual variation in surface markings in individual recognition and as the basis for social classifications.

Explorations of differential nepotism within colonies have generally yielded negative results. The most intensely explored question is whether honeybee queen larvae, which may be either full- or half-sisters to the workers that tend them, are preferentially treated by full-sisters. While some experiments suggest the existence of such preferential treatment, others show no such effect. The opportunity for differential nepotism exists in many species of eusocial insect, either because the colony’s queen mates several times, as in the honeybee, or because the colony has several queens. Worker policing mechanisms, in which competition among worker subgroups counterbalances possible nepotism, may prevent the emergence of measurable nepotistic biases.

List of Changes

Many thanks to reviewers of this text for their comments, suggestions, and corrections, most of which were incorporated throughout this new edition of Human Biology.

A thorough copy edit has improved the overall quality of the entire text.

As in the previous edition, the contributions of each organ system to maintaining homeostasis are emphasized throughout. A new homeostasis icon (scale) is used to identify homeostatic functions in the systems chapters, chapter 4 through chapter 16.

All statistics have been updated for this edition.

New Bioethical Focus readings present pros and cons on particular bioethical issues. Students are challenged to develop and defend his or her own opinions on the issues.

Chapter 4 - New title: Organization and Regulation of Body Systems. This was chapter 3, Introduction to Homeostasis, in the previous edition. The title was changed to better reflect the content of the chapter. Homeostasis was expanded and rewritten to provide better coverage of this topic.

Chapter 13: Nervous System has been extensively reorganized and rewritten. The discussion of the central nervous system now precedes that of the peripheral nervous system.

Chapter 19: Chromosomal Inheritance (previously chapter 18). This chapter has been reorganized. The human life cycle, including mitosis and meiosis, now begins the chapter. The chapter ends with a discussion of chromosomal inheritance abnormalities.

Chapter 23: Human Evolution (previously chapter 22: Evolution) has been completely rewritten and expanded. More detailed information on the origin of life and human evolution is given. This chapter contains many new, interesting, and helpful illustrations and photographs.

Chapter 24: Ecosystems and Human Interferences (previously chapter 23: Ecosystems). This chapter was rewritten and reorganized, and combines the material previously found in chapters 23 and 24.

Chapter 25: Conservation of Biodiversity is a completely new chapter, which discusses the current biodiversity crises including why we should care, the root causes, and how to preserve species and prevent extinctions.

e-Learning Connection is new to this edition, and gives access information to new learning technologies.

Chapter 1: A Human Perspective

This was the Introduction chapter in the previous edition.

1.2 The Process of Science includes an expanded explanation and summary of the scientific method.

New Bioethical Focus: Animals in the Laboratory

1.5 Flow diagram for the scientific method

Chapter 2: Chemistry of Life

This was chapter 1 in the previous edition.

2.6 Lipids. The discussion of soap was replaced by a discussion of emulsifiers.

New Bioethical Focus: Organic Pollutants

2.12 The pH scale 2.18 Glycogen structure and function

Chapter 3: Cell Structure and Function

This was chapter 2 in the previous edition.

3.3 Cellular Metabolism. The discussion of cellular respiration has been simplified. The phrase aerobic cellular respiration has been changed to cellular respiration for clarity.

New Bioethical Focus: Stem Cells

3.3 Animal cell 3.5 Tonicity 3.7 The nucleus and the nuclear envelope 3.9 The Golgi apparatus 3.12 Sperm cells 3.14 Cellular respiration

Chapter 4: Organization and Regulation of Body Systems

This was chapter 3, Introduction to Homeostasis, in the previous edition. The title was changed to better reflect the content of the chapter. Homeostasis was expanded and rewritten to provide better coverage of this topic.

4.1 Types of Tissues. As in the previous edition, this section covers the tissues, cavities, membranes, and organ systems of the human body. The term fiber (with regard to nerves) is explained. The phrase neuroglial cell has been changed to neuroglia throughout.

4.3 Organ Systems. This section has been reorganized so that the discussions of the Integumentary System and Regions of the Skin are kept together. The Working Together box has been moved to section 4.4 Homeostasis.

4.4 Homeostasis. The entire section has been rewritten and reorganized to give more emphasis on this topic. Negative and positive feedback mechanisms are more clearly explained in this edition. Regulation of Body Temperature has been moved to this section and rewritten. Homeostasis and Body Systems is new to this section.

4.2 Epithelial tissue 4.4 Connective tissue examples 4.6 Muscular tissue 4.11 Homeostasis 4.12 Negative feedback 4.13 Homeostasis and body temperature regulation 4.14 Regulation of tissue fluid composition

Part 2: Maintenance of the Human Body

Chapter 5: Digestive System and Nutrition

This was chapter 4 in the previous edition.

5.5 Nutrition. In the discussion of calcium, the usefulness of vitamin D and other vitamins in preventing osteoporosis is presented. The Health Focus "Weight Loss the Healthy Way" has been revised to improve clarity.

5.3 Swallowing 5.7 Hormonal control of digestive gland secretions

Chapter 6: Composition and Function of Blood

This was chapter 5 in the previous edition.

6.2 The White Blood Cells. Colony-stimulating factors (CSFs) are introduced.

6.3 Blood Clotting has been reorganized and rewritten.

6.5 Action of erythropoietin 6.8 Capillary exchange

Chapter 7: Cardiovascular System

This was chapter 6 in the previous edition. Throughout the chapter and entire text, the terms "O2-rich" and "O2-poor" replace the phrases "high in oxygen" and oxygenated" and "low in oxygen" or "deoxygenated."

7.4 The Vascular Pathways. The path of blood to and from the lower legs has been corrected and now includes the femoral artery, lower leg capillaries, and femoral vein.

7.6 Homeostasis. The end of the chapter has been repaged so that The Working Together page does not interrupt the end matter.

7.5 Internal view of the heart 7.6 Stages in the cardiac cycle 7.7 Conduction system of the heart

Chapter 8: Lymphatic and Immune Systems

This was chapter 7, Lymphatic System and Immunity, in the previous edition. The introductory story was revised to better introduce the immune system and its functions.

8.4 Induced Immunity. The immunization schedule for infants and young children has been updated to contain the latest requirements. In Cytokines and Immunity, the explanation of the technique to activate cytotoxic T cells to destroy cancer cells has been clarified. The explanation of the delayed allergic response has been simplified.

8.6 Clonal selection theory as it applies to B cells 8.8 Clonal selection theory as it applies to T cells 8.10b (updated Immunization table)

8.1 immunization table in Figure 8.10

Chapter 9: Respiratory System

This was chapter 8 in the previous edition.

9.3 The introductory paragraph was rewritten to emphasize the contribution of gas exchange to homeostasis.

9.5 Homeostasis has been rewritten and clearly explains how the respiratory system regulates pH and immunity.

9.1 The path of air(caption) 9.2 The respiratory tract 9.6 Vital capacity 9.8 Inspiration and expiration

Chapter 10: Urinary System and Excretion

This was chapter 9 in the previous edition.

10.1 has been revised to introduce the urinary system and the path of urine right away, before discussing the urinary organs. Some reorganization of heads allows the discussion of the role of kidneys in maintaining homeostasis to logically lead to a discussion of salt-water balance and acid-base balance.

10.7 Problems with Kidney Function. Replacing a kidney is a new topic to this edition.

New Bioethical Focus: Organ Transplants

10.1 Taking a drink of water 10.5 Nephron anatomy 10.7 Steps in urine formation 10.11 An artificial kidney machine

Part 3: Movement and Support in Humans

Chapter 11: Skeletal System

This was chapter 10 in the previous edition.

11.1 Tissues of the Skeletal System. The opening paragraph now introduces bone, cartilage, and connective tissues before discussing each in depth.

11.3 Bones of the Skeleton. The discussions of the pectoral girdle and arm have been rewritten, and the rotator cuff is mentioned.

11.4 Articulations has been revised - the discussion of arthritis has been expanded and was moved to the end of the section. The text for Figure 11.12 Joint Movements now more closely follows the illustration. The Working Together illustration now follows 11.5 Homeostasis, so it does not break up the text.

11.7 The vertebral column 11.8 Thoracic vertebrae and the rib cage 11.9 Bones of the pectoral girdle and arm 11.12 Joint movements, 11.13 Hip prosthesis

Chapter 12: Muscular System

This was chapter 11 in the previous edition.

12.4 Energy for Muscle Contraction introductory paragraphs have been rewritten for clarity. The discussion entitled Muscular Disorders is completely new and discusses muscle spasms and cramps, tendonitis, tetanus, muscular dystrophy, and myasthenia gravis.

12.7 Neuromuscular junction 12.12 Myasthenia gravis.

Part 4: Integration and Coordination in Humans

Chapter 13: Nervous System

This was chapter 12 in the previous edition. This chapter has been extensively reorganized. Many sections and topics have been rewritten. The central nervous system, limbic system, memory, language, and speech are discussed before the peripheral nervous system. Homeostasis ends the chapter.

13.1 Nervous Tissue was previously entitled Neurons and How They Work. Neuron Structure and Myelin Sheath have been rewritten. Synaptic Integration now follows the discussion of transmission across a synapse.

13.2 The Central Nervous System is discussed next in the logical sequence of spinal cord and brain. Functions of the Spinal Cord has been rewritten and now discusses the role the spinal cord plays in regulating internal organs in addition to the skeletal muscles. Parts of the brain are discussed in more depth.

13.3 The Limbic System and Higher Mental Functions contains discussions of the limbic system, memory and learning, and language and speech. (The discussion of Alzheimer disease has been moved to the end of the chapter).

13.4 The Peripheral System. The organization and content of this section remains essentially the same as in the last edition.

13.6 Homeostasis has been expanded to include discussions of two degenerative nervous system diseases, Alzheimer disease and Parkinson disease. The Alzheimer disease discussion has been updated with the newest information, and the Parkinson disease discussion is new to this chapter.

13.1 Organization of the nervous system 13.3 Myelin sheath 13.4 Resting and action potential 13.5 Synapse structure and function 13.6 Integration 13.7 Organization of the nervous system 13.9 The human brain 13.10 The cerebral cortex 13.12 The limbic system 13.13 Long-term memory circuits 13.15 Cranial and spinal nerves 13.16 A reflex arc 13.18 Drug actions at a synapse 13.19 Drug use 13.20 Alzheimer disease.

This was chapter 13 in the previous edition.

14.1 Sensory Receptors. Table 14.1 Exteroceptors is new and replaces Table 13.1 Special Sense Organs. Discussions of sensory receptors have been revised. How Sensation Occurs has been revised to include the influence of the reticular activating system, and how sensory receptors contribute to homeostasis.

14.2 Proprioceptors and Cutaneous Receptors. New A head title identifies and focuses the discussion of these topics. The topics Cutaneous Receptors and Pain Receptors were revised.

14.6 Sense of Equilibrium. Terminology has been changed. The term dynamic equilibrium has been changed to rotational equilibrium, and the term static equilibrium has been changed to gravitational equilibrium. The Health Focus reading Protecting Vision and Hearing now follows the discussion of hearing and is found at end of the chapter.

14.2 Sensation 14.10 Structure and function of the retina 14.15 Mechanoreceptors for equilibrium

14.1 Exteroceptors is new and replaces Table 13.1 Special Sense Organs

Chapter 15: Endocrine System

This was chapter 14 in the previous edition. The chapter has been reorganized, and some heads have changed. The chapter now ends with Chemical Signals (previously called Environmental Signals), instead of beginning with it. The introductory story is new. Terminology change: contrary hormone has been changed to antagonistic hormone. As before, each gland is discussed in turn with an emphasis on medical disorders caused by too much or too little hormones.

15.1 Endocrine Glands introduces and defines endocrine glands and hormones in general, and discusses the contribution of hormones to homeostasis. Table 15.1 logically ends this section.

15.4 Adrenal Glands. Glucocorticoids has been revised it now precedes the discussion of mineralocorticoids.

15.7 Chemical Signals. The information in this section has been reorganized and rewritten, and includes the discussion of steroid and peptide hormones. Hormonal versus Neural Signals includes the material formerly discussed in Environmental Signals.

New Health Focus: melatonin

New Bioethical Focus: Fertility Drugs

15.1 Puberty 15.9 Adrenal glands 15.14 Glucose tolerance test 15A Melatonin production 15.16 Cellular activity of hormones 15.17 Chemical signals 15B Higher-order multiple births

Part 5: Reproduction in Humans

The AIDS supplement and chapter 17 regarding STDs have been rewritten to include the latest research, techniques, and information.

Chapter 16: Reproductive System

This was chapter 15 in the previous edition. Development of Male and Female Sex Organs has been moved to chapter 18 Development and Agin.

16.1 Male Reproduction System. The discussion of sperm production and movement has been rewritten.

16.2 Female Reproduction System. The discussions of external genitals and orgasm in females has been rewritten.

16.3 Female Hormone Levels. The discussion of follicle development has been rewritten.

16.4 Control of Reproduction. Information about the "male pill" has been updated. Infertility is redefined. New to this section are discussions of fertility drugs, higher-order births, and vasectomy reversals. The terminology Assisted Reproductive replaces the terminology Alternative Methods of Reproduction in the previous edition. A new procedure called Intracytoplasmic Sperm Injection (ICSI) is covered in this section.

New Bioethical Focus: Assisted Reproductive Technologies.

16.3 Testis and sperm 16.9 Female hormone levels 16.10 Implantation 16.12 In vitro fertilization 16B Couples and children

Chapter 17: Sexually Transmitted Diseases

This was chapter 16 in the previous edition. The chapter has been revised to include non-sexually transmitted infectious diseases caused by viruses, bacteria, fungi, and other animals. Statistics of new cases of AIDS and other STDs have been updated to reflect the most current information from the Centers of Disease Control.

17.1 Viral Infectious Diseases (previously Viral in Origin) has been rewritten and the discussion of the typical DNA animal virus life cycle has been simplified for better understanding Figure 17.3 illustrating this life cycle has also been simplified. The discussion of HIV infections summarized and identifies types and subtypes of HIV found in Africa and in the United States. HIV infections and AIDS are covered in detail in the AIDS supplement.

17.2 Bacterial Infectious Diseases (previously Bacterial in Origin). All statistics have been updated.

17.3 Other Infectious Diseases (previously Other Sexually Transmitted Diseases) has been rewritten and includes a more detailed introduction to kingdoms Protista, Fungi, and Animalia, and how these organisms transmit infectious diseases. Several diseases caused by protozoa are discussed. There is a new topic on infectious diseases caused by fungi. The topic on infectious diseases caused by animals discusses head lice and parasitic worms, as well as pubic lice.

New Bioethical Focus: HIV Vaccine Testing in Africa

17.3 Life cycle of an animal DNA virus 17.4 Genital warts 17.5 Genital herpes 17.8 Chlamydial infection 17.10 Gonorrhea 17.12 Syphilis 17A AIDS in Africa 17.13 Organisms that cause vaginitis 17.14 Sexually transmitted animal

17.1 Infectious Diseases Caused by Viruses (revised) 17.2 Infectious Diseases Caused by Bacteria (revised) 17.3 Infectious Diseases Caused by Protozoa, Fungi, and Animals (new).

All sections of The AIDS Supplement have been rewritten and updated with the latest research, information, and statistics. The new introduction identifies the types and subtypes of HIV. The prevalence of AIDS in Africa and other less-developed countries is presented in the introductory story and reinforced in Figure S.2 and in Section S.1, which has been extensively rewritten.

S.2 Phases of an HIV Infection identifies HIV-1B as the prevalent subtype in the U.S. The definitions of the three categories remain the same. The discussions of the HIV structure and life cycle have been simplified for better understanding Figure S.5 illustrating the reproduction of HIV has also been simplified. The discussion of drug therapy and vaccines have been revised, reflecting the latest information on therapies now in use, in trials, and undergoing research.

A new Health Focus reading, Preventing Transmission of HIV, gives more emphasis to this information, which was contained in section S.4 in the previous edition.

S.2 Global HIV prevalence rates in adults at the end of 1999 S.5 Reproduction of HIV.

Chapter 18: Development and Aging

This was chapter 17 in the previous edition.

18.1 Fertilization has been completely rewritten and clearly shows the steps of fertilization. Figure 18.2 has been corrected.

18.2 Development Before Birth. The topic Gastrulation has been reorganized, rewritten, and clarified. The difference between embryonic development and fetal development is made clear in the discussion of embryonic development. New to this section, the discussion of the first month of embryonic development introduces stem cells and the controversy over using embryonic stem cells to cure human conditions. Much of the information in the First Month has been rewritten. Includes 18.3 Development of Male and Female Sex Organs (previously in the reproduction chapter).

18.4 Birth has been rewritten and explains the positive feedback mechanism in relation to the onset and continuation of labor.

There is a new discussion of the benefits of breast feeding to the mother and child under female breast and lactation.

New Bioethical Focus: Maternal Health Habits

18.2 Fertilization 18.3 Human development before implantation 18.4 Early developmental stages in cross section 18.7 Fetal circulation and the placenta 18.9 Human embryo at five weeks 18.10 A three- to four-month-old fetus 18.11 A six- to seven-month-old fetus 18B Health habits

Part 6: Human Genetics

Chapter 19: Chromosomal Inheritance

This was chapter 18 in the previous edition. The chapter has been reorganized. The human life cycle, including mitosis and meiosis, now begins the chapter. The chapter ends with a discussion of chromosomal inheritance abnormalities.

19.2 Mitosis contains a new topic Cytokinesis, which discusses cytokinesis and formation of a cleavage furrow.

19.4 Chromosomal Inheritance. The discussion of nondisjunction now precedes an expanded explanation of nondisjunction, how it occurs, and its resulting chromosomal abnormalities. Down syndrome and other syndromes caused by abnormalities in chromosome makeup follow the discussion of nondisjunction. The term triplo-X syndrome has been changed to poly-X syndrome.

New Bioethical Focus: Cloning in Humans

19.1 Life cycle of humans 19.8 Spermatogenesis and oogenesis 19.9 Human karyotype preparation

19.1 Meiosis I Versus Mitosis 19.2 Meiosis II Versus Mitosis These new tables help summarize the information given in the chapter.

Chapter 20: Genes and Medical Genetics

This was chapter 19 in the previous edition. This chapter has been fewer A heads. The new section 20.3 Beyond Simple Inheritance Patterns includes polygenic inheritance, multiple allelic traits, and incompletely dominant traits. Four sets of Practice Problems have been added.

20.2 Dominant/Recessive Traits. Recessive Disorders are now discussed before dominant disorders. Pedigree Charts makes it clear that with recessive genetic disorders, when both parents are affected, all children are affected (and why) and with dominant genetic disorders, two affected parents can have an unaffected child (and why). This information will help the student be able to understand and successfully answer the related practice problems.

20.3 Beyond Simple Inheritance Patterns includes polygenic inheritance, multiple allelic traits, and incompletely dominant traits.

New Bioethical Focus: Genetic Profiling

20.2 Genetic inheritance 20.9 Autosomal recessive pedigree chart 20.10 Autosomal dominant pedigree chart 20.12 Inheritance of blood type 20.13 Incomplete dominance 20.14 Cross involving an X-linked allele 20.15 X-linked recessive pedigree chart 20A Genetic profiling

Chapter 21: DNA and Biotechnology

This was chapter 20 in the previous edition. Most main sections and topics were rewritten for clarity.

21.1 DNA and RNA Structure and Function. Most topics in this section were rewritten for clarity.

21.2 Gene Expression. The DNA Code and Transcription topics were rewritten for clarity.

21.3 Biotechnology. Polymerase Chain Reaction was rewritten for clarity. Cloning of Transgenic Animals was updated, and the diagram (Fig. 21.18) that illustrates this procedure has been simplified for better understanding. The Human Genome Project discussion was updated to include recent achievements in that area. Gene sequencing of diseases or afflictions found on chromosome 17 is illustrated in new Figure 21.19. The Gene Therapy discussion has been updated and greatly expanded. It gives new information on gene therapy treatments for cystic fibrosis and for children with SCID using bone marrow stem cells. It also discussed the possibilities for the use of gene therapy to treat other illnesses, such as hemophilia, AIDS, cancer, and heart disease.

New Health Focus: Organs for Transplant

New Bioethical Focus: Transgenic Plants

21.2 DNA location and structure 21.9 Function of introns 21.16 Polymerase chain reaction 21.18 Genetically engineered animals 21.19 Genetic map of chromosome 17 Colors have been made consistent in all DNA/RNA illustrations.

21.2 Some DNA Codes and RNA Codons has been expanded.

This was chapter 21 in the previous edition. Statistics have been updated.

22.2 Origin of Cancer. Regulation of the Cell Cycle has been reorganized and rewritten for better understanding of the stimulatory and inhibitory pathways involved in the action of proto-oncogenes and tumor-suppressor genes. Apoptosis has been rewritten and contains new information on caspases and how they work to bring about apoptosis.

22.4 Diagnosis and Treatment. Future Therapies, which ends the section and the chapter has been updated and includes new information and a new illustration regarding cancer vaccine therapy and inhibitory drug therapy (previously called chemoprevention).

The Health Focus and Bioethical Focus readings have been moved to the end of the chapter so text is not interrupted.

New Bioethical Focus: Tobacco and Alcohol Use

22.3 Origin of cancer 22.4 Function of p53 22.5 Industrial chemicals 22.7 Treatment of cancer 22.8 Cancer vaccine

Part 7: Human Evolution and Ecology

Part 7 contains a new part introduction. Chapter 25 Conservation of Biodiversity is a completely new chapter.

Chapter 23: Human Evolution

This was chapter 22, Evolution, in the previous edition. The entire chapter has been completely rewritten and expanded to include more detailed information on the origin of life and human evolutionary events. This chapter contains many new, interesting, and helpful illustrations and photographs. The chapter has a new introductory story.

23.1 Origin of Life (previously 22.3 Organic Evolution). This section has been rewritten in more detail and Miller's experiment is explained. Taxonomy has been moved to 23.3 Humans are Primates. Only the classification of humans is examined.

23.2 Biological Evolution includes evidences of evolution - common descent and natural selection. The entire section has been rewritten. Each topic goes into more detail than previously.

23.3 Humans are Primates. This section has been completely rewritten. Characteristics of primates and the primate evolutionary tree are examined.

23.4 Evolution of Australopithecines. This new section gives details about the discoveries of australopithecine fossils in Southern and Eastern Africa.

23.5 Evolution of Humans. This entire section has been rewritten and has much more information and detail than in the previous edition.

New Bioethical Focus: The Theory of Evolution.

23.1 Chemical evolution 23.2 Fossils 23.3 Mechanism of evolution 23.4 Primate evolutionary tree 23.5 Australopithecus africanus 23.6 Human evolution 23.7 Homo erectus 23.8 Origin of modern humans 23.9 Neanderthals 23.10 Cro-Magnons 23A Australopithecus africanus skull

23.1 Evolution and Classification of Humans

Chapter 24: Ecosystems and Human Interferences

This was chapter 23 Ecosystems in the previous edition. This chapter has been rewritten and reorganized, and combines the material previously found in chapters 23 and 24.

24.2 Energy Flow and Chemical Cycling is now a main head, which emphasizes its importance. The content is the same as in the previous edition.

24.3 Global Biogeochemical Cycles. The order of the cycles has been changed to this: water cycle, carbon cycle, nitrogen cycle, and phosphorus cycle.

The discussion of the carbon cycle has been reorganized and rewritten. The topic Carbon Dioxide and Global Warming is new and contains information and statistics on global warming.

The discussion of the nitrogen cycle has been reorganized and rewritten. The topic Nitrogen and Air Pollution is new and contains information about acid rain, smog, and thermal inversions.

In the discussion of the phosphorus cycle, the Phosphorus and Water Pollution is new and contains information on eutrophication, biological magnification, and pollution of coastal regions and the seas.

New Health Focus: Stratospheric Ozone Depletion Threatens the Biosphere.

New Bioethical Focus: Preserving Ecosystems Abroad

24.2 Example of primary succession 24.5 Nature of an ecosystem 24A Ozone shield depletion 24B preserving ecosystems

Chapter 25: Conservation of Biodiversity

Chapter 25 Conservation of Biodiversity is a completely new chapter, which discusses the current biodiversity crises including why we should care, the root causes, and how to preserve species and prevent extinctions.

Transcriptional regulation of the peripheral nervous system in Ciona intestinalis

The formation of the sensory organs and cells that make up the peripheral nervous system (PNS) relies on the activity of transcription factors encoded by proneural genes (PNGs). Although PNGs have been identified in the nervous systems of both vertebrates and invertebrates, the complexity of their interactions has complicated efforts to understand their function in the context of their underlying regulatory networks. To gain insight into the regulatory network of PNG activity in chordates, we investigated the roles played by PNG homologs in regulating PNS development of the invertebrate chordate Ciona intestinalis. We discovered that in Ciona, MyT1, Pou4, Atonal, and NeuroD-like are expressed in a sequential regulatory cascade in the developing epidermal sensory neurons (ESNs) of the PNS and act downstream of Notch signaling, which negatively regulates these genes and the number of ESNs along the tail midlines. Transgenic embryos mis-expressing any of these proneural genes in the epidermis produced ectopic midline ESNs. In transgenic embryos mis-expressing Pou4, and MyT1 to a lesser extent, numerous ESNs were produced outside of the embryonic midlines. In addition we found that the microRNA miR-124, which inhibits Notch signaling in ESNs, is activated downstream of all the proneural factors we tested, suggesting that these genes operate collectively in a regulatory network. Interestingly, these factors are encoded by the same genes that have recently been demonstrated to convert fibroblasts into neurons. Our findings suggest the ascidian PNS can serve as an in vivo model to study the underlying regulatory mechanisms that enable the conversion of cells into sensory neurons.

Riveting hammer vibration damages mechanosensory nerve endings

Danny A. Riley, PhD, 13701 Evergreen Way, Austin, TX 78737.

Cell Biology, Neurobiology and Anatomy, Medical College of Wisconsin, Milwaukee, Wisconsin, USA

Cell Biology, Neurobiology and Anatomy, Medical College of Wisconsin, Milwaukee, Wisconsin, USA

Plastic Surgery, Medical College of Wisconsin, Milwaukee, Wisconsin, USA

Material Manufacturing, Swerea IVF, Mölndal, Sweden

Material Manufacturing, Swerea IVF, Mölndal, Sweden

Cell Biology, Neurobiology and Anatomy, Medical College of Wisconsin, Milwaukee, Wisconsin, USA

Danny A. Riley, PhD, 13701 Evergreen Way, Austin, TX 78737.

Part of the research material was presented in abstract form at the 7th American Conference on Human Vibration, Seattle, WA on June 13, 2018.

Funding information: National Institute for Occupational Safety and Health, Grant/Award Number: R01OH003493 Sweden's Innovation Agency Plastic Surgery Foundation Wisconsin Space Grant Consortium Research Fellowship

Abstract

Hand-arm vibration syndrome (HAVS) is an irreversible neurodegenerative, vasospastic, and musculoskeletal occupational disease of workers who use powered hand tools. The etiology is poorly understood. Neurological symptoms include numbness, tingling, and pain. This study examines impact hammer vibration-induced injury and recoverability of hair mechanosensory innervation. Rat tails were vibrated 12 min/d for 5 weeks followed by 5 week recovery with synchronous non-vibrated controls. Nerve fibers were PGP9.5 immunostained. Lanceolate complex innervation was compared quantitatively in vibrated vs sham. Vibration peak acceleration magnitudes were characterized by frequency power spectral analysis. Average magnitude (2515 m/s 2 , root mean squared) in kHz frequencies was 109 times that (23 m/s 2 ) in low Hz. Percentage of hairs innervated by lanceolate complexes was 69.1% in 5-week sham and 53.4% in 5-week vibration generating a denervation difference of 15.7% higher in vibration. Hair innervation was 76.9% in 5-weeks recovery sham and 62.0% in 5-week recovery vibration producing a denervation difference 14.9% higher in recovery vibration. Lanceolate number per complex (18.4 ± 0.2) after vibration remained near sham (19.3 ± 0.3), but 44.9% of lanceolate complexes were abnormal in 5 weeks vibrated compared to 18.8% in sham. The largest vibration energies are peak kHz accelerations (approximately 100 000 m/s 2 ) from shock waves. The existing ISO 5349-1 standard excludes kHz vibrations, seriously underestimating vibration injury risk. The present study validates the rat tail, impact hammer vibration as a model for investigating irreversible nerve damage. Persistence of higher denervation difference after 5-week recovery suggests repeated vibration injury destroys the capability of lanceolate nerve endings to regenerate.

15.7: The Peripheral Nervous System - Biology

The Peripheral Nervous System

Peripheral nervous system overview: The PNS is the communication network between the CNS and the rest of the body.
Organization and function: The peripheral nervous system (PNS) includes all neural tissue excluding the brain and the spinal cord.

• PNS specific neurons: Unipolar Sensory Neurons: large myelinated neurons with the cell body off to one side of the single dendritic-axon process. Multipolar Motor Neurons: large myelinated neurons that have many dendrites off the cell body and an axon that may branch to effect many effectors.
• Signal transmission: electrical signals are transmitted in 3 steps: (1) Neurotransmitters released from one neuron bind to and activate the dendrites of the next neuron. (2) If the signal is strong enough, an action potential is propagated down the axon. (3) Which causes the release of neurotransmitters from that neuron.
• Action potential: When another neuron sends a sufficiently strong signal to the next neuron, the neuron excites to a threshold potential. Transporters on the cell membrane let positive ions into the cell, causing a change in potential, which spreads down the axon. This electrical propagation is called the action potential.

Glial cells of the PNS

• Satellite Cells: The cell bodies of several sensory neurons form structures called Ganglia. Satellite cells are the glial cells that surround each ganglion.
• Schwann Cells: Like Oligodendrocytes in the CNS, Schwann cells wrap themselves around neurons in the PNS to form the myelin sheath. Unlike Oligodendrocytes, which myelinate several neurons, a single Schwann cell forms a segment of myelin sheath.

Proprioception: involve sensors that keep track of where the body is in space. The five senses: The sensory nervous system includes sensory organs, which receive information from the environment, and sends it to the CNS.

• Skin: detects temperature, touch, and painful stimuli. Three separate kinds of nerves detect sensation on the skin
  • Mechanoreceptors: Detect pressure and tension on the skin
  • Thermoreceptors: Detect the temperature of the
    stimulus
  • Nociceptors: Detect painful stimuli.
  • Spinal Nerve Anatomy: There are 31 nerves exiting the spinal cord, dorsal connections bring sensory information to the CNS, ventral motor connections send commands to the periphery.
  • Reflexes: For painful stimuli, involuntary withdrawal (like a hand from a flame) occurs without input from the brain. This very simple nervous pathway is called a reflex arc.
  • Autonomic nervous system: directly controls automatic body functions (involuntary movements). The autonomic system has two opposing parts: the sympathetic and parasympathetic nervous systems.
  • This tutorial is all about the peripheral nervous system and its functions. Specific details about the signal transmission through the peripheral nervous system are discussed. The cell types unique to the peripheral nervous system will be presented and their function discussed.
    
  • The afferent and efferent neurons that transmit the initial information to the spinal cord and then transmit the information from the brain will also be presented.

  Specific Tutorial Features:

  Animated diagrams showing the five sensory organs and their mode of actions.

  • Concept map showing inter-connections of new concepts in this tutorial and those previously introduced.
    
  • Definition slides introduce terms as they are needed.
    
  • Visual representation of concepts
    
  • Examples given throughout to illustrate how the concepts apply.
    
  • A concise summary is given at the conclusion of the tutorial.

  Peripheral nervous system overview

  Organization and function
  PNS specific neurons
  Signal transmission
  Action potential

  Proprioception
  The five senses:

  Spinal Nerve Anatomy
  Reflexes
  Autonomic nervous system

  See all 24 lessons in Anatomy and Physiology, including concept tutorials, problem drills and cheat sheets: Teach Yourself Anatomy and Physiology Visually in 24 Hours

  
  

  What is Peripheral Nervous System

  The peripheral nervous system (PNS) is the other part of the nervous system in vertebrates, which send sensory signals to the CNS and response of the body to the effector organs. The PNS is composed of neurons and neuron clusters called ganglia. The PNS can be divided into two as somatic nervous system and autonomic nervous system.

  Somatic Nervous System

  The somatic nervous system (SONS) controls actions of the body via voluntary movements and reflexes. The afferent fibers of the PNS carry sensory signals from the external stimuli. The sensory organs, which are connected by the afferent nerve fibers are eye, nose, tongue, ear, and skin. The efferent nerve fibers carry instructions from the CNS to the effector organs. The reflexes have no integration with the CNS for the response. The monosynaptic reflexes contain a single synapse between sensory and motor neuron and polysynaptic reflexes contain as least a single interneuron between the sensory and motor neurons.

  Autonomic Nervous System

  The autonomic nervous system (ANS) controls the unconscious or involuntary muscular movements. The ANS controls the functioning of the internal organs, breathing, heartbeat, and digestion. The two complementary parts of the ANS are sympathetic and parasympathetic nervous systems. The sympathetic nervous system prepares the body for fight-or-flight response under stressful conditions by raising the heartbeat, blood pressure, and dilating the pupil. The parasympathetic nervous system keeps the body at rest. The secretion and digestion are stimulated by the parasympathetic nervous system. The third component of the ANS is the enteric nervous system, which is capable of directly controlling the digestive system of the body. The nervous system of the body in humans is shown in figure 2.

  

  Figure 2: Nervous System in Humans

  
  

  Peripheral nervous system

  

  Our nervous system is divided in two components: the central nervous system (CNS), which includes the brain and spinal cord, and the peripheral nervous system (PNS), which encompasses nerves outside the brain and spinal cord. These two components cooperate at all times to ensure our lively functions: we are nothing without our nervous system!

  Unlike the brain and the spinal cord of the central nervous system that are protected by the vertebrae and the skull, the nerves and cells of the peripheral nervous system are not enclosed by bones, and therefore are more susceptible to trauma.

  If we consider the entire nervous system as an electric grid, the central nervous system would represent the powerhouse, whereas the peripheral nervous system would represent long cables that connect the powerhouse to the outlying cities (limbs, glands and organs) to bring them electricity and send information back about their status.

  
  Image credit: Alessandra Donato

  Basically, signals from the brain and spinal cord are relayed to the periphery by motor nerves, to tell the body to move or to conduct resting functions (like breathing, salivating and digesting), for example. The peripheral nervous system sends back the status report to the brain by relaying information via sensory nerves (see above image).

  As with the central nervous system, the basic cell units of the peripheral central nervous system are neurons. Each neuron has a long process, known as the axon, which transmits the electrochemical signals through which neurons communicate.

  Axons of the peripheral nervous system run together in bundles called fibres, and multiple fibres form the nerve, the cable of the electric circuit. The nerves, which also contain connective tissue and blood vessels, reach out to the muscles, glands and organs in the entire body

  Nerves of the peripheral nervous system are classified based on the types of neurons they contain - sensory, motor or mixed nerves (if they contain both sensory and motor neurons), as well as the direction of information flow – towards or away from the brain.

  The afferent nerves, from the Latin "afferre" that means "to bring towards", contain neurons that bring information to the central nervous system. In this case, the afferent are sensory neurons, which have the role of receiving a sensory input – hearing, vision, smell, taste and touch - and pass the signal to the CNS to encode the appropriate sensation.

  The afferent neurons have also another important subconscious function. In this case, the peripheral nervous system brings information to the central nervous system about the inner state of the organs (homeostasis), providing feedback on their conditions, without the need for us to be consciously aware. For example, afferent nerves communicate to the brain the level of energy intake of various organs.

  The efferent nerves, from the Latin "efferre" that means "to bring away from", contain efferent neurons that transmit the signals originating in the central nervous system to the organs and muscles, and put into action the orders from the brain. For example, motor neurons (efferent neurons) contact the skeletal muscles to execute the voluntary movement of raising your arm and wiggling your hand about.

  Peripheral nervous system nerves often extend a great length from the central nervous system to reach the periphery of the body. The longest nerve in the human body, the sciatic nerve, originates around the lumbar region of the spine and its branches reach until the tip of the toes, measuring a meter or more in an average adult.

  Importantly, injuries can occur at any point in peripheral nerves and could break the connection between the "powerhouse" and its "cities", resulting in a loss of function of the parts of the body that nerves reach into. So, it of great interest for scientists to understand how the nerves, or even how the axonal structure within the nerves, are protected from the constant mechanical stresses exerted on them. Work in this area of biology is carried out by Dr. Sean Coakley, in the laboratory of A/Prof Massimo Hilliard.

  The peripheral nervous system can be divided into somatic, autonomic and enteric nervous systems, determined by the function of the parts of the body they connect to.

  Author: Alessandra Donato from the Hilliard Lab
  Top image credit: OpenStax Anatomy and Physiology / Wikimedia

  
  

  Journal of the Peripheral Nervous System

  The Journal of the Peripheral Nervous System is the official journal of the Peripheral Nerve Society. Founded in 1996, it is the scientific journal of choice for clinicians, clinical scientists and basic neuroscientists interested in all aspects of biology and clinical research of peripheral nervous system disorders. The Journal of the Peripheral Nervous System is a peer-reviewed journal that publishes high quality articles on cell and molecular biology, genomics, neuropathic pain, clinical research, trials, and unique case reports on inherited and acquired peripheral neuropathies. Original articles are organized according to the topic in one of four specific areas: Mechanisms of Disease, Genetics, Clinical Research, and Clinical Trials. The journal also publishes regular review papers on hot topics and Special Issues on basic, clinical, or assembled research in the field of peripheral nervous system disorders. Authors interested in contributing a review-type article or a Special Issue should contact the Editorial Office to discuss the scope of the proposed article with the Editor-in-Chief. Join the conversation about this journal

  The set of journals have been ranked according to their SJR and divided into four equal groups, four quartiles. Q1 (green) comprises the quarter of the journals with the highest values, Q2 (yellow) the second highest values, Q3 (orange) the third highest values and Q4 (red) the lowest values.

  Category	Year	Quartile
  Medicine (miscellaneous)	1999	Q1
  Medicine (miscellaneous)	2000	Q2
  Medicine (miscellaneous)	2001	Q1
  Medicine (miscellaneous)	2002	Q1
  Medicine (miscellaneous)	2003	Q1
  Medicine (miscellaneous)	2004	Q1
  Medicine (miscellaneous)	2005	Q1
  Medicine (miscellaneous)	2006	Q1
  Medicine (miscellaneous)	2007	Q1
  Medicine (miscellaneous)	2008	Q1
  Medicine (miscellaneous)	2009	Q1
  Medicine (miscellaneous)	2010	Q1
  Medicine (miscellaneous)	2011	Q1
  Medicine (miscellaneous)	2012	Q1
  Medicine (miscellaneous)	2013	Q1
  Medicine (miscellaneous)	2014	Q1
  Medicine (miscellaneous)	2015	Q1
  Medicine (miscellaneous)	2016	Q1
  Medicine (miscellaneous)	2017	Q1
  Medicine (miscellaneous)	2018	Q1
  Medicine (miscellaneous)	2019	Q1
  Medicine (miscellaneous)	2020	Q1
  Neurology (clinical)	1999	Q2
  Neurology (clinical)	2000	Q3
  Neurology (clinical)	2001	Q2
  Neurology (clinical)	2002	Q2
  Neurology (clinical)	2003	Q1
  Neurology (clinical)	2004	Q1
  Neurology (clinical)	2005	Q2
  Neurology (clinical)	2006	Q2
  Neurology (clinical)	2007	Q2
  Neurology (clinical)	2008	Q1
  Neurology (clinical)	2009	Q1
  Neurology (clinical)	2010	Q2
  Neurology (clinical)	2011	Q2
  Neurology (clinical)	2012	Q1
  Neurology (clinical)	2013	Q1
  Neurology (clinical)	2014	Q2
  Neurology (clinical)	2015	Q2
  Neurology (clinical)	2016	Q2
  Neurology (clinical)	2017	Q2
  Neurology (clinical)	2018	Q2
  Neurology (clinical)	2019	Q2
  Neurology (clinical)	2020	Q2
  Neuroscience (miscellaneous)	1999	Q3
  Neuroscience (miscellaneous)	2000	Q3
  Neuroscience (miscellaneous)	2001	Q3
  Neuroscience (miscellaneous)	2002	Q3
  Neuroscience (miscellaneous)	2003	Q2
  Neuroscience (miscellaneous)	2004	Q2
  Neuroscience (miscellaneous)	2005	Q2
  Neuroscience (miscellaneous)	2006	Q2
  Neuroscience (miscellaneous)	2007	Q2
  Neuroscience (miscellaneous)	2008	Q2
  Neuroscience (miscellaneous)	2009	Q2
  Neuroscience (miscellaneous)	2010	Q2
  Neuroscience (miscellaneous)	2011	Q2
  Neuroscience (miscellaneous)	2012	Q2
  Neuroscience (miscellaneous)	2013	Q2
  Neuroscience (miscellaneous)	2014	Q2
  Neuroscience (miscellaneous)	2015	Q2
  Neuroscience (miscellaneous)	2016	Q2
  Neuroscience (miscellaneous)	2017	Q2
  Neuroscience (miscellaneous)	2018	Q2
  Neuroscience (miscellaneous)	2019	Q2
  Neuroscience (miscellaneous)	2020	Q2

  The SJR is a size-independent prestige indicator that ranks journals by their 'average prestige per article'. It is based on the idea that 'all citations are not created equal'. SJR is a measure of scientific influence of journals that accounts for both the number of citations received by a journal and the importance or prestige of the journals where such citations come from It measures the scientific influence of the average article in a journal, it expresses how central to the global scientific discussion an average article of the journal is.

  Year	SJR
  1999	0.558
  2000	0.347
  2001	0.429
  2002	0.505
  2003	0.821
  2004	1.063
  2005	0.679
  2006	0.807
  2007	0.930
  2008	1.201
  2009	1.183
  2010	0.968
  2011	1.029
  2012	1.142
  2013	1.357
  2014	1.079
  2015	1.158
  2016	0.967
  2017	1.092
  2018	1.079
  2019	1.075
  2020	1.000

  Evolution of the number of published documents. All types of documents are considered, including citable and non citable documents.

  Year	Documents
  1999	23
  2000	27
  2001	23
  2002	28
  2003	31
  2004	36
  2005	53
  2006	49
  2007	37
  2008	40
  2009	40
  2010	43
  2011	67
  2012	73
  2013	47
  2014	56
  2015	36
  2016	33
  2017	41
  2018	40
  2019	62
  2020	58

  This indicator counts the number of citations received by documents from a journal and divides them by the total number of documents published in that journal. The chart shows the evolution of the average number of times documents published in a journal in the past two, three and four years have been cited in the current year. The two years line is equivalent to journal impact factor ™ (Thomson Reuters) metric.

  Cites per document	Year	Value
  Cites / Doc. (4 years)	1999	1.229
  Cites / Doc. (4 years)	2000	0.953
  Cites / Doc. (4 years)	2001	1.110
  Cites / Doc. (4 years)	2002	1.127
  Cites / Doc. (4 years)	2003	1.822
  Cites / Doc. (4 years)	2004	2.330
  Cites / Doc. (4 years)	2005	2.356
  Cites / Doc. (4 years)	2006	2.216
  Cites / Doc. (4 years)	2007	2.124
  Cites / Doc. (4 years)	2008	2.469
  Cites / Doc. (4 years)	2009	2.855
  Cites / Doc. (4 years)	2010	2.464
  Cites / Doc. (4 years)	2011	2.850
  Cites / Doc. (4 years)	2012	2.974
  Cites / Doc. (4 years)	2013	3.193
  Cites / Doc. (4 years)	2014	3.491
  Cites / Doc. (4 years)	2015	2.560
  Cites / Doc. (4 years)	2016	2.198
  Cites / Doc. (4 years)	2017	2.314
  Cites / Doc. (4 years)	2018	2.289
  Cites / Doc. (4 years)	2019	2.533
  Cites / Doc. (4 years)	2020	2.636
  Cites / Doc. (3 years)	1999	1.229
  Cites / Doc. (3 years)	2000	0.951
  Cites / Doc. (3 years)	2001	1.013
  Cites / Doc. (3 years)	2002	1.205
  Cites / Doc. (3 years)	2003	1.923
  Cites / Doc. (3 years)	2004	2.561
  Cites / Doc. (3 years)	2005	2.105
  Cites / Doc. (3 years)	2006	1.908
  Cites / Doc. (3 years)	2007	2.130
  Cites / Doc. (3 years)	2008	2.626
  Cites / Doc. (3 years)	2009	2.587
  Cites / Doc. (3 years)	2010	2.496
  Cites / Doc. (3 years)	2011	2.707
  Cites / Doc. (3 years)	2012	2.920
  Cites / Doc. (3 years)	2013	3.202
  Cites / Doc. (3 years)	2014	2.701
  Cites / Doc. (3 years)	2015	2.517
  Cites / Doc. (3 years)	2016	2.338
  Cites / Doc. (3 years)	2017	2.416
  Cites / Doc. (3 years)	2018	2.236
  Cites / Doc. (3 years)	2019	2.491
  Cites / Doc. (3 years)	2020	2.601
  Cites / Doc. (2 years)	1999	1.322
  Cites / Doc. (2 years)	2000	1.058
  Cites / Doc. (2 years)	2001	1.060
  Cites / Doc. (2 years)	2002	1.240
  Cites / Doc. (2 years)	2003	2.235
  Cites / Doc. (2 years)	2004	2.407
  Cites / Doc. (2 years)	2005	1.493
  Cites / Doc. (2 years)	2006	1.876
  Cites / Doc. (2 years)	2007	2.402
  Cites / Doc. (2 years)	2008	2.081
  Cites / Doc. (2 years)	2009	2.675
  Cites / Doc. (2 years)	2010	2.350
  Cites / Doc. (2 years)	2011	2.675
  Cites / Doc. (2 years)	2012	2.818
  Cites / Doc. (2 years)	2013	2.500
  Cites / Doc. (2 years)	2014	2.367
  Cites / Doc. (2 years)	2015	2.350
  Cites / Doc. (2 years)	2016	2.293
  Cites / Doc. (2 years)	2017	2.232
  Cites / Doc. (2 years)	2018	2.108
  Cites / Doc. (2 years)	2019	2.309
  Cites / Doc. (2 years)	2020	2.539

  Evolution of the total number of citations and journal's self-citations received by a journal's published documents during the three previous years.
  Journal Self-citation is defined as the number of citation from a journal citing article to articles published by the same journal.

  Cites	Year	Value
  Self Cites	1999	4
  Self Cites	2000	4
  Self Cites	2001	2
  Self Cites	2002	7
  Self Cites	2003	9
  Self Cites	2004	11
  Self Cites	2005	11
  Self Cites	2006	16
  Self Cites	2007	14
  Self Cites	2008	22
  Self Cites	2009	13
  Self Cites	2010	21
  Self Cites	2011	27
  Self Cites	2012	18
  Self Cites	2013	29
  Self Cites	2014	26
  Self Cites	2015	14
  Self Cites	2016	14
  Self Cites	2017	19
  Self Cites	2018	18
  Self Cites	2019	27
  Self Cites	2020	30
  Total Cites	1999	102
  Total Cites	2000	78
  Total Cites	2001	80
  Total Cites	2002	88
  Total Cites	2003	150
  Total Cites	2004	210
  Total Cites	2005	200
  Total Cites	2006	229
  Total Cites	2007	294
  Total Cites	2008	365
  Total Cites	2009	326
  Total Cites	2010	292
  Total Cites	2011	333
  Total Cites	2012	438
  Total Cites	2013	586
  Total Cites	2014	505
  Total Cites	2015	443
  Total Cites	2016	325
  Total Cites	2017	302
  Total Cites	2018	246
  Total Cites	2019	284
  Total Cites	2020	372

  Evolution of the number of total citation per document and external citation per document (i.e. journal self-citations removed) received by a journal's published documents during the three previous years. External citations are calculated by subtracting the number of self-citations from the total number of citations received by the journal’s documents.

  Cites	Year	Value
  External Cites per document	1999	1.195
  External Cites per document	2000	0.925
  External Cites per document	2001	1.054
  External Cites per document	2002	1.174
  External Cites per document	2003	1.905
  External Cites per document	2004	2.653
  External Cites per document	2005	2.392
  External Cites per document	2006	2.420
  External Cites per document	2007	2.800
  External Cites per document	2008	3.500
  External Cites per document	2009	3.440
  External Cites per document	2010	3.151
  External Cites per document	2011	3.290
  External Cites per document	2012	3.415
  External Cites per document	2013	3.594
  External Cites per document	2014	2.957
  External Cites per document	2015	2.768
  External Cites per document	2016	2.528
  External Cites per document	2017	2.504
  External Cites per document	2018	2.257
  External Cites per document	2019	2.425
  External Cites per document	2020	2.591
  Cites per document	1999	1.229
  Cites per document	2000	0.951
  Cites per document	2001	1.013
  Cites per document	2002	1.205
  Cites per document	2003	1.923
  Cites per document	2004	2.561
  Cites per document	2005	2.105
  Cites per document	2006	1.908
  Cites per document	2007	2.130
  Cites per document	2008	2.626
  Cites per document	2009	2.587
  Cites per document	2010	2.496
  Cites per document	2011	2.707
  Cites per document	2012	2.920
  Cites per document	2013	3.202
  Cites per document	2014	2.701
  Cites per document	2015	2.517
  Cites per document	2016	2.338
  Cites per document	2017	2.416
  Cites per document	2018	2.236
  Cites per document	2019	2.491
  Cites per document	2020	2.601

  International Collaboration accounts for the articles that have been produced by researchers from several countries. The chart shows the ratio of a journal's documents signed by researchers from more than one country that is including more than one country address.

  Year	International Collaboration
  1999	26.09
  2000	14.81
  2001	13.04
  2002	25.00
  2003	19.35
  2004	2.78
  2005	7.55
  2006	24.49
  2007	10.81
  2008	20.00
  2009	27.50
  2010	23.26
  2011	19.40
  2012	15.07
  2013	19.15
  2014	19.64
  2015	61.11
  2016	30.30
  2017	21.95
  2018	35.00
  2019	41.94
  2020	36.21

  Not every article in a journal is considered primary research and therefore "citable", this chart shows the ratio of a journal's articles including substantial research (research articles, conference papers and reviews) in three year windows vs. those documents other than research articles, reviews and conference papers.

  Documents	Year	Value
  Non-citable documents	1999	1
  Non-citable documents	2000	2
  Non-citable documents	2001	5
  Non-citable documents	2002	4
  Non-citable documents	2003	4
  Non-citable documents	2004	7
  Non-citable documents	2005	16
  Non-citable documents	2006	32
  Non-citable documents	2007	38
  Non-citable documents	2008	41
  Non-citable documents	2009	35
  Non-citable documents	2010	31
  Non-citable documents	2011	30
  Non-citable documents	2012	27
  Non-citable documents	2013	28
  Non-citable documents	2014	25
  Non-citable documents	2015	21
  Non-citable documents	2016	16
  Non-citable documents	2017	12
  Non-citable documents	2018	9
  Non-citable documents	2019	8
  Non-citable documents	2020	11
  Citable documents	1999	82
  Citable documents	2000	80
  Citable documents	2001	74
  Citable documents	2002	69
  Citable documents	2003	74
  Citable documents	2004	75
  Citable documents	2005	79
  Citable documents	2006	88
  Citable documents	2007	100
  Citable documents	2008	98
  Citable documents	2009	91
  Citable documents	2010	86
  Citable documents	2011	93
  Citable documents	2012	123
  Citable documents	2013	155
  Citable documents	2014	162
  Citable documents	2015	155
  Citable documents	2016	123
  Citable documents	2017	113
  Citable documents	2018	101
  Citable documents	2019	106
  Citable documents	2020	132

  Ratio of a journal's items, grouped in three years windows, that have been cited at least once vs. those not cited during the following year.

  
  

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