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How are cardiac cells connected by intercalated discs?


I understand that intercalated discs run transversely across the longitudinal axis of the muscle. But what I don't understand is how this same disc binds the cardiac cells together. Are the cells joined end-to-end (in series) or side-to-side (in parallel) by the disc?


Cardiac Muscle

Cardiac muscle, also known as heart muscle, is the layer of muscle tissue which lies between the endocardium and epicardium. These inner and outer layers of the heart, respectively, surround the cardiac muscle tissue and separate it from the blood and other organs. Cardiac muscle is made from sheets of cardiac muscle cells. These cells, unlike skeletal muscle cells, are typically unicellular and connect to one another through special intercalated discs. These specialized cell junction and the arrangement of muscle cells enables cardiac muscle to contract quickly and repeatedly, forcing blood throughout the body.


Cardiac Muscle Structure and Cardiac Muscle Function

Let us look at the Cardiac Muscle Function and Cardiac Muscle Structure in detail, here.

Gross Anatomy

Cardiac muscle tissue is also called the myocardium, and forms the heart's bulk. A thick layer of myocardium is sandwiched between the outer epicardium (also known as visceral pericardium) and the inner endocardium, forming the heart wall. The inner endocardium lines the cardiac chambers, which cover the cardiac joins and valves, with the endothelium, which lines the blood vessels that connect to the heart. Whereas, on the outer aspect of the myocardium is the epicardium that forms part of the pericardium, which is the sack that protects, surrounds, and lubricates the heart.

Cardiac Muscle Cells

Cardiac muscle cells or the cardiomyocytes are given as the contracting cells, which allow the heart to pump. Every cardiomyocyte needs to contract in coordination with its neighbouring cells - called a functional syncytium that is working to efficiently pump blood from the heart. If this coordination breaks down, then, despite the individual cells contracting, the heart may not pump at all, such as can take place during abnormal heart rhythms like ventricular fibrillation.

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T-Tubules

T-tubules are the microscopic tubes, which run from the surface of the cell to deep within the cell. These are continuous with the cell membrane and are composed of a similar phospholipid bilayer. They are open at the cell's extracellular fluid surface that surrounds the cell. T-tubules present in the cardiac muscle are wider and bigger than the ones in skeletal muscle, but some in number. In the cell's centre, they join together by running into and along with the cell as a transverse-axial network. They lie close to the cell's internal calcium store inside the cell, the sarcoplasmic reticulum. A single tubule is paired with a terminal cisterna from the sarcoplasmic reticulum in a diad combination.

Intercalated Discs

The cardiac syncytium is a network of cardiomyocytes linked by intercalated discs that allow for the rapid transmission of electrical impulses across a network by allowing the syncytium to participate in the synchronised contraction of the myocardium. There are a ventricular syncytium and an atrial syncytium, which are connected by cardiac connection fibres.


Why cardiomyocytes are important?

Cardiovascular disease is a leading cause of death worldwide. Nearly 2,400 Americans die of cardiac causes each day, one death every 37 seconds.

As the chief cell type of the heart, cardiac muscle cells primarily dedicate to the contractile function of the heart and enable the pumping of blood around the body. If anything goes wrong in the heart, it can lead to a catastrophic outcome. A myocardial infarction (MI), commonly known as a heart attack, occurs when blood flow ceases to a part of the heart, causing massive cardiomyocyte death in that area. Severe cases can, ultimately, lead to heart failure and death.

[In this figure] The progress of myocardial infarction or heart attack. At time post-infarction:

0-12 hours: Beginning of necrotic coagulation due to the blockage of coronary arteries – Cardiomyocytes suffer the lack of oxygen (hypoxia)

12-72 hours: Culmination of necrotic coagulation – Neutrophils infiltrate by an inflammatory response.

1-3 weeks: Disintegration of death myocytes and formation of granulation tissue (collagenous fibers, macrophages, and fibroblasts)

> 1 month: Formation of fibrous scar (fewer cells with an abundance of collagenous fibers)


Cardiac muscle tissue is only found in the heart. Highly coordinated contractions of cardiac muscle pump blood into the vessels of the circulatory system. Similar to skeletal muscle, cardiac muscle is striated and organized into sarcomeres, possessing the same banding organization as skeletal muscle.

However, cardiac muscle fibers are shorter than skeletal muscle fibers and usually contain only one nucleus, which is located in the central region of the cell. Cardiac muscle fibers also possess many mitochondria and myoglobin, as ATP is produced primarily through aerobic metabolism. Cardiac muscle fibers cells also are extensively branched and are connected to one another at their ends by intercalated discs. An intercalated disc allows the cardiac muscle cells to contract in a wave-like pattern so that the heart can work as a pump.

Intercalated discs are part of the sarcolemma and contain two structures important in cardiac muscle contraction: gap junctions and desmosomes. A gap junction forms channels between adjacent cardiac muscle fibers that allow the depolarizing current produced by cations to flow from one cardiac muscle cell to the next. This joining is called electric coupling, and in cardiac muscle it allows the quick transmission of action potentials and the coordinated contraction of the entire heart. This network of electrically connected cardiac muscle cells creates a functional unit of contraction called a syncytium. The remainder of the intercalated disc is composed of desmosomes. A desmosome is a cell structure that anchors the ends of cardiac muscle fibers together so the cells do not pull apart during the stress of individual fibers contracting (Figure 4).

Figure 4. Cardiac Muscle. Intercalated discs are part of the cardiac muscle sarcolemma and they contain gap junctions and desmosomes.

Contractions of the heart (heartbeats) are controlled by specialized cardiac muscle cells called pacemaker cells that directly control heart rate. Although cardiac muscle cannot be consciously controlled, the pacemaker cells respond to signals from the autonomic nervous system (ANS) to speed up or slow down the heart rate. The pacemaker cells can also respond to various hormones that modulate heart rate to control blood pressure.

The wave of contraction that allows the heart to work as a unit, called a functional syncytium, begins with the pacemaker cells. This group of cells is self-excitable and able to depolarize to threshold and fire action potentials on their own, a feature called autorhythmicity they do this at set intervals which determine heart rate. Because they are connected with gap junctions to surrounding muscle fibers and the specialized fibers of the heart’s conduction system, the pacemaker cells are able to transfer the depolarization to the other cardiac muscle fibers in a manner that allows the heart to contract in a coordinated manner.


Cardiac Muscle Intercalated Discs

The heart of the human beings is made up of cardiac muscles which are different from smooth and skeletal muscles. Inside the cardiac muscles is the intercalated discs which is known to join various adjacent cells together. There are a number of functions of intercalated discs. It is only when it functions properly that the heart can work effectively.

The human heart is made up of cardiac muscle. Since the muscles primarily contract so they lead the muscle cells to shorten up. In function the cardiac muscles are said to be similar to the smooth muscles. But in anatomy, they tend to be quite different as in this regard cardiac muscles are said to be more similar to the skeletal muscles. So the cardiac muscles can be said to be a combination of the two other muscle types. In fact in terms of appearance, structure, metabolism there are various similarities to be found between these muscles.

What are Cardiac Muscles

Cardiac muscles are said to be the striated muscles which are found in the walls of the heart. They are also called as cardiac myocytes. When these muscles function effectively only then the heart too can perform all its functions properly. It is only when the cardiac muscles contracts accordingly that it enables the proper circulation of the blood from the atria and ventricles to the circulatory system of the body. These muscles also help in the removal of waste products from the body like carbon dioxide.

Intercalated Discs and their Functions

Intercalated discs are known to be located inside the cardiac muscles. They are considered to be very important as they work as a connecting link between the two adjacent cardiac cells. Their aim is to help a number of cardiac cells in contracting since when it is done, the heart can function effectively so it is very much essential. When the cardiac muscles are stretched a bit, they are known to perform even slightly better.

In the intercalated discs the various cardiac cells are said to be intertwined to each other followed by gap junctions. This position helps in the stabilisation of the positions of the cells as well as helps to maintain the three dimensional structure of the cells. The intercalated disc is made up of double membrane. It is easy to identify them owing to the longitudinal section of the tissue of which they are made up.

Basically, the intercalated disc consists of three types of membrane junctions- these include Fascia Adherens, Macual Adherens, Gap Junctions. These three membrane junctions have their own functions and in fact help in the effective working of the intercalated disc.

The good thing is that with the help of a powerful microscope you can easily recognise the intercalated disc in the cardiac muscles.


Do intercalated discs have gap junctions?

Click to explore further. Regarding this, what type of muscle has gap junctions?

Skeletal muscle does not have any cell-cell junctions. Smooth muscle contains gap junctions, to allow a rapid spread of depolarisation, as in cardiac muscle.

Beside above, where are gap junctions found in cardiac muscle? Intercellular junctions and the cardiac intercalated disk. Severs NJ. Cardiac muscle cells are equipped with three distinct types of intercellular junction--gap junctions, "spot" desmosomes, and "sheet" desmosomes (or fasciae adherentes)--located in a specialized portion of the plasma membrane, the intercalated disk.

Herein, do pacemaker cells have gap junctions?

The pacemaker cells are connected to neighboring contractile cells via gap junctions, which enable them to locally depolarize adjacent cells. Having cardiomyocytes connected via gap junctions allow all contractile cells of the heart to act in a coordinated fashion and contract as a unit.


What to know about cardiac muscle tissue

Cardiac muscle tissue, or myocardium, is a specialized type of muscle tissue that forms the heart. This muscle tissue, which contracts and releases involuntarily, is responsible for keeping the heart pumping blood around the body.

The human body contains three different kinds of muscle tissue: skeletal, smooth, and cardiac. Only cardiac muscle tissue, comprising cells called myocytes, is present in the heart.

In this article, we discuss the structure and function of cardiac muscle tissue. We also cover medical conditions that can affect cardiac muscle tissue and tips for keeping it healthy.

Share on Pinterest A person can strengthen cardiac muscle tissue by doing regular exercise.

Muscle is fibrous tissue that contracts to produce movement. There are three types of muscle tissue in the body: skeletal, smooth, and cardiac. Cardiac muscle is highly organized and contains many types of cell, including fibroblasts, smooth muscle cells, and cardiomyocytes.

Cardiac muscle only exists in the heart. It contains cardiac muscle cells, which perform highly coordinated actions that keep the heart pumping and blood circulating throughout the body.

Unlike skeletal muscle tissue, such as that which is present in the arms and legs, the movements that cardiac muscle tissue produces are involuntary. This means that they are automatic, and that a person cannot control them.

The heart also contains specialized types of cardiac tissue containing “pacemaker” cells. These contract and expand in response to electrical impulses from the nervous system.

Pacemaker cells generate electrical impulses, or action potentials, that tell cardiac muscle cells to contract and relax. The pacemaker cells control heart rate and determine how fast the heart pumps blood.

Cardiac muscle tissue gets its strength and flexibility from its interconnected cardiac muscle cells, or fibers.

Most cardiac muscle cells contain one nucleus, but some have two. The nucleus houses all of the cell’s genetic material.

Cardiac muscle cells also contain mitochondria, which many people call “the powerhouses of the cells.” These are organelles that convert oxygen and glucose into energy in the form of adenosine triphosphate (ATP).

Cardiac muscle cells appear striated or striped under a microscope. These stripes occur due to alternating filaments that comprise myosin and actin proteins. The dark stripes indicate thick filaments that comprise myosin proteins. The thin, lighter filaments contain actin.

When a cardiac muscle cell contracts, the myosin filament pulls the actin filaments toward each other, which causes the cell to shrink. The cell uses ATP to power this contraction.

A single myosin filament connects to two actin filaments on either side. This forms a single unit of muscle tissue, called a sarcomere.

Intercalated discs connect cardiac muscle cells. Gap junctions inside the intercalated discs relay electrical impulses from one cardiac muscle cell to another.

Desmosomes are other structures present within intercalated discs. These help hold cardiac muscle fibers together.

Cardiomyopathy refers to a group of medical conditions that affect cardiac muscle tissue and impair the heart’s ability to pump blood or relax normally.

Some common symptoms of cardiomyopathy include:

  • difficulty breathing or shortness of breath
  • swelling of the legs, ankles, and feet in the abdomen or neck
  • irregular heartbeat
  • heart murmurs
  • dizziness or lightheadedness

Factors that can increase a person’s risk of cardiomyopathy include:

A heart attack due to a blocked artery can cut off the blood supply to certain areas of the heart. Eventually, the cardiac muscle tissue in these areas will start to die.

The death of cardiac muscle tissue can also occur when the heart’s oxygen demand exceeds the oxygen supply. This causes the release of cardiac proteins such as troponin into the bloodstream.

Some examples of cardiomyopathy include:

Dilated cardiomyopathy

Dilated cardiomyopathy causes the cardiac muscle tissue of the left ventricle to stretch and the heart’s chambers to dilate.

Hypertrophic cardiomyopathy

Hypertrophic cardiomyopathy (HCM) is a genetic condition in which the cardiomyocytes are not arranged in a coordinated fashion and are instead disorganized. HCM can interrupt blood flow out of the ventricles, cause arrhythmias (abnormal electrical rhythms), or lead to congestive heart failure.

Restrictive cardiomyopathy

Restrictive cardiomyopathy (RCM) refers to when the walls of the ventricles become stiff. When this happens, the ventricles cannot relax enough to fill with an adequate amount of blood.

Arrhythmogenic right ventricular dysplasia

This rare form of cardiomyopathy causes fatty infiltration in cardiac muscle tissue in the right ventricle.

Transthyretin amyloid cardiomyopathy

Transthyretin amyloid cardiomyopathy (ATTR-CM) develops when amyloid proteins collect and form deposits in the walls of the left ventricle. The amyloid deposits cause the ventricle’s walls to stiffen, which prevents the ventricle from filling with blood and reduces its ability to pump blood out of the heart. This is a form of RCM.


Cardiac muscle

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Cardiac muscle, also called myocardium, in vertebrates, one of three major muscle types, found only in the heart. Cardiac muscle is similar to skeletal muscle, another major muscle type, in that it possesses contractile units known as sarcomeres this feature, however, also distinguishes it from smooth muscle, the third muscle type. Cardiac muscle differs from skeletal muscle in that it exhibits rhythmic contractions and is not under voluntary control. The rhythmic contraction of cardiac muscle is regulated by the sinoatrial node of the heart, which serves as the heart’s pacemaker.

The heart consists mostly of cardiac muscle cells (or myocardium). The outstanding characteristics of the action of the heart are its contractility, which is the basis for its pumping action, and the rhythmicity of the contraction. The amount of blood pumped by the heart per minute (the cardiac output) varies to meet the metabolic needs of peripheral tissues, particularly the skeletal muscles, kidneys, brain, skin, liver, heart, and gastrointestinal tract. The cardiac output is determined by the contractile force developed by the cardiac muscle cells, as well as by the frequency at which they are activated (rhythmicity). The factors affecting the frequency and force of heart muscle contraction are critical in determining the normal pumping performance of the heart and its response to changes in demand.

Cardiac muscle cells form a highly branched cellular network in the heart. They are connected end to end by intercalated disks and are organized into layers of myocardial tissue that are wrapped around the chambers of the heart. The contraction of individual cardiac muscle cells produces force and shortening in these bands of muscle, with a resultant decrease in the heart chamber size and the consequent ejection of the blood into the pulmonary and systemic vessels. Important components of each cardiac muscle cell involved in excitation and metabolic recovery processes are the plasma membrane and transverse tubules in registration with the Z lines, the longitudinal sarcoplasmic reticulum and terminal cisternae, and the mitochondria. The thick (myosin) and thin (actin, troponin, and tropomyosin) protein filaments are arranged into contractile units, with the sarcomere extending from Z line to Z line, that have a characteristic cross-striated pattern similar to that seen in skeletal muscle.

The rate at which the heart contracts and the synchronization of atrial and ventricular contraction required for the efficient pumping of blood depend on the electrical properties of the cardiac muscle cells and on the conduction of electrical information from one region of the heart to another. The action potential (activation of the muscle) is divided into five phases. Each of the phases of the action potential is caused by time-dependent changes in the permeability of the plasma membrane to potassium ions (K + ), sodium ions (Na + ), and calcium ions (Ca 2+ ).


The Cardiac Cycle

The main purpose of the heart is to pump blood through the body it does so in a repeating sequence called the cardiac cycle. The cardiac cycle is the coordination of the filling and emptying of blood by electrical signals that cause the heart muscles to contract and relax. The human heart beats over 100,000 times per day. In each cardiac cycle, the heart contracts (systole), pushing out the blood and pumping it through the body. This is followed by a relaxation phase (diastole), where the heart fills with blood. The atria contract at the same time, forcing blood through the atrioventricular valves into the ventricles. Closing of the atrioventricular valves produces a monosyllabic &ldquolup&rdquo sound. Following a brief delay, the ventricles contract at the same time forcing blood through the semilunar valves into the aorta and the pulmonary artery (which transports blood to the lungs). Closing of the semilunar valves produces a monosyllabic &ldquodup&rdquo sound.

Figure (PageIndex<1>): Diastole and systole: (a) During cardiac diastole, the heart muscle is relaxed and blood flows into the heart. (b) During atrial systole, the atria contract, pushing blood into the ventricles. (c) During atrial diastole, the ventricles contract, forcing blood out of the heart.

The pumping of the heart is a function of the cardiac muscle cells, or cardiomyocytes, that comprise the heart muscle. Cardiomyocytes are distinctive muscle cells that are striated like skeletal muscle, but pump rhythmically and involuntarily like smooth muscle they are connected by intercalated disks exclusive to cardiac muscle. Cardiomyocytes are self-stimulated for a period of time isolated cardiomyocytes will beat if given the correct balance of nutrients and electrolytes.

Figure (PageIndex<1>): Cardiomyocytes: Cardiomyocytes are striated muscle cells found in cardiac tissue.

The autonomous beating of cardiac muscle cells is regulated by the heart&rsquos internal pacemaker that uses electrical signals to time the beating of the heart. The electrical signals and mechanical actions are intimately intertwined. The internal pacemaker starts at the sinoatrial (SA) node, which is located near the wall of the right atrium. Electrical charges spontaneously pulse from the SA node, causing the two atria to contract in unison. The pulse reaches a second node, the atrioventricular (AV) node, between the right atrium and right ventricle, where it pauses for approximately 0.1 seconds before spreading to the walls of the ventricles. This pause allows the blood in the atria to empty completely into the ventricles before the ventricles pump out the blood. From the AV node, the electrical impulse enters the bundle of His, then to the left and right bundle branches extending through the interventricular septum. Finally, the Purkinje fibers conduct the impulse from the apex of the heart up the ventricular myocardium, causing the ventricles to contract. The electrical impulses in the heart produce electrical currents that flow through the body and can be measured on the skin using electrodes. This information can be observed as an electrocardiogram (ECG): a recording of the electrical impulses of the cardiac muscle.

Figure (PageIndex<1>): Electrical signals: The beating of the heart is regulated by an electrical impulse that causes the characteristic reading of an ECG. The signal is initiated at the sinoatrial valve. The signal then (a) spreads to the atria, causing them to contract. The signal is (b) delayed at the atrioventricular node before it is passed on to the (c) heart apex. The delay allows the atria to relax before the (d) ventricles contract. The final part of the ECG cycle prepares the heart for the next beat.