Cardiac muscle is a type of involuntary striated muscle found only in the walls of the heart. This is a specialized muscle that, while similar in some fundamental ways to smooth muscle and skeletal muscle, has a unique structure and with an ability not possessed by muscle tissue elsewhere in the body. Cardiac muscle, like other muscles, can contract, but it can also carry an action potential (i.e. conduct electricity), like the neurons that constitute nerves. Furthermore, some of the cells have the ability to generate an action potential, known as cardiac muscle automaticity.
As the muscle contracts, it propels blood into the heart and through the blood vessels of the circulatory system. For a human being, the heart beats about once a second for the entire life of the person, without any opportunity to rest (Ward 2001). It can adjust quickly to the body's needs, increasing output from five liters of blood per minute to more than 25 liters per minute (Ward 2001). The muscles that contract the heart can do so without external stimulation from hormones or nerves, and it does not fatigue or stop contracting if supplied with sufficient oxygen and nutrients.
The actions of cardiac muscle reflect on the remarkable harmony within a body and the underlying principle that individual entities in nature provide a larger function. In order for the heart to work properly, and have the necessary waves of contraction to pump blood, the cardiac cells must fire in intricate coordination with each other. In doing so, each cell provides a larger function for the sake of the body, allowing the heart to beat properly, while in turn being provided essential nutrients by the body. The coordination of the cardiac cells is essential. Should the cells fire randomly, the heart would not be able to contract in a synchronized manner and pump blood, and the body (and thus the cell) would die.
The muscular tissue of the heart is known as myocardium. The myocardium is composed of specialized cardiac muscle, which consists of bundles of muscle cells, technically known as myocytes. A myocyte, or muscle fiber, is a single cell of a muscle. These muscle fibers contain many myofibrils, the contractile units of muscles. Myofibrils run from one end of the cell to the other and are alternating bundles of thin filaments, comprising primarily actin, and thick filaments, comprising primarily the protein myosin. Like smooth and skeletal muscle, cardiac muscle contracts based on a rise of calcium inside the muscle cell, allowing interaction of actin and myosin.
Cardiac and skeletal muscle are similar in that both appear to be "striated" in that they contain sarcomeres. In striated muscle, such as skeletal and cardiac muscle, the actin and myosin filaments each have a specific and constant length on the order of a few micrometers, far less than the length of the elongated muscle cell (a few millimeters in the case of human skeletal muscle cells). The filaments are organized into repeated subunits along the length. These subunits are called sarcomeres. The sarcomeres are what give skeletal and cardiac muscles their striated appearance of narrow dark and light bands, because of the parallel arrangement of the actin and myosin filaments. The myofibrils of smooth muscle cells are not arranged into sarcomeres. Striated muscle (cardiac and skeletal) contracts and relaxes in short, intense bursts, whereas smooth muscle sustains longer or even near-permanent contractions.
However, cardiac muscle has unique features relative to skeletal muscle. For one, the myocytes are much shorter and are narrower than the skeletal muscle cells, being about 0.1 millimeters long and 0.02 millimeters wide (Ward 2001). Furthermore, while skeletal muscles are arranged in regular, parallel bundles, cardiac muscle connects at branching, irregular angles. Anatomically, the muscle fibers are typically branched like a tree branch. In addition, cardiac muscle fibers connect to other cardiac muscle fibers through intercalcated discs and form the appearance of a syncytium (continuous cellular material). These intercalcated discs, which appear as irregularly-spaced dark bands between myocytes, are a unique and prominent feature of cardiac muscle (Ward 2001).
Cardiac muscle also shares many properties with smooth muscle, including control by the autonomic nervous system and spontaneous (automatic) contractions.
Intercalated disks are a unique, prominent, and important feature of cardiac muscle. An intercalated disc is an undulating double membrane separating adjacent cells in cardiac muscle fibers. They have two essential functions. For one, they act as a glue to hold myocytes together so that they do not separate when the heart contracts. Secondly, they allow an electrical connection between the cells, supporting synchronized contraction of cardiac tissue. They can easily be visualized by a longitudinal section of the tissue.
Three types of membrane junctions exist within an intercalated disc: fascia adherens, macula adherens, and gap junctions. Fascia adherens are anchoring sites for actin, and connect to the closest sarcomere. Macula adherens stop separation during contraction by binding intermediate filaments joining the cells together, also called a desmosome. Gap junctions contain pores and allow action potentials to spread between cardiac cells by permitting the passage of ions between cells, producing depolarization of the heart muscle.
When observing cardiac tissue through a microscope, intercalated discs are an identifying feature of cardiac muscle
Striations. Cardiac muscle exhibits cross striations formed by alternation segments of thick and thin protein filaments, which are anchored by segments called T-lines. The primary structural proteins of cardiac muscle are actin and myosin. The actin filaments are thin causing the lighter appearance of the I bands in muscle, while myosin is thicker and darker lending a darker appearance to the alternating A bands in cardiac muscle as observed by a light enhanced microscope.
T-Tubules. Another histological difference between cardiac muscle and skeletal muscle is that the T-tubules in cardiac muscle are larger, broader, and run along the Z-Discs. There are fewer T-tubules in comparison with skeletal muscle. Additionally, cardiac muscle forms dyads instead of the triads formed between the T-tubules and the sarcoplasmic reticulum in skeletal muscle.
Intercalated discs. Under light microscopy, intercalated discs appear as thin, typically dark-staining lines dividing adjacent cardiac muscle cells. The intercalated discs run perpendicular to the direction of muscle fibers. Under electron microscopy, an intercalated disc's path appears more complex. At low magnification, this may appear as a convoluted electron dense structure overlying the location of the obscured Z-line. At high magnification, the intercalated disc's path appears even more convoluted, with both longitudinal and transverse areas appearing in longitudinal section. Gap junctions (or nexus junctions) fascia adherens (resembling the zonula adherens), and desmosomes are visible. In transverse section, the intercalated disk's appearance is labyrinthine and may include isolated interdigitations.
When the resting membrane potential (the voltage across the membrane) of a cardiac myocyte is reduced sufficiently to initiate an action potential, the cardiac myocyte will contract. For most parts of the heart, this contraction is caused by an action potential in an adjacent myocyte being transmitted through the gap junctions. As neighboring cells are stimulated, a wave of activation, and thus contraction, continues through the heart; the result is a synchronization of contraction throughout the heart. Some specialized myoctyes, in the pacemaker region of the heart, are responsible for initiating the heartbeat (Ward 2001).
A single cardiac muscle cell, if left without input, will contract rhythmically at a steady rate; if two cardiac muscle cells are in contact, whichever one contracts first will stimulate the other to contract, and so on. This inherent contractile activity is heavily regulated by the autonomic nervous system. If synchronization of cardiac muscle contraction is disrupted for some reason (for example, in a heart attack), uncoordinated contraction known as fibrillation can result. In severe cases of the loss of synchronization, such as in ventricular fibrillation, the heart cannot pump at all and has been compared to a "bag of (writhing) worms" (Ward 2001).
The sinoatrial node or pacemaker region in the right atrium contains myocytes with a specialized function (Ward 2001). Unlike skeletal muscle, which contracts in response to nerve stimulation, the specialized pacemaker cells at the entrance of the right atrium, the sinoatrial node, display the phenomenon of automaticity and are myogenic, meaning that they are self-excitable without a requisite electrical impulse coming from the central nervous system. The rest of the myocardium conducts these action potentials by way of electrical synapses called gap junctions. It is because of this automaticity that an individual's heart does not stop when a neuromuscular blocker (such as succinylcholine or rocuronium) is administered, such as during general anesthesia.
The atria and ventricles in the heart are separated by a non-conducting area except at the atrio-ventricular node, which consists of small myocytes that conduct but delay the impulse from the pacemaker, allowing the atria to contract before the ventricles (Ward 2001).
Cardiac muscle is adapted to be highly resistant to fatigue: it has a large number of mitochondria, enabling continuous aerobic respiration, numerous myoglobins (oxygen-storing pigment), and a good blood supply, which provides nutrients and oxygen. The heart is so tuned to aerobic metabolism that it is unable to pump sufficiently in ischaemic conditions. At basal metabolic rates, about one percent of energy is derived from anaerobic metabolism. This can increase to ten percent under moderately hypoxic conditions, but, under more severe hypoxic conditions, not enough energy can be liberated by lactate production to sustain ventricular contractions (Ganong 2005).
Under basal aerobic conditions, 60 percent of energy comes from fat (free fatty acids and triacylglycerols/triglycerides), 35 percent from carbohydrates, and five percent from amino acids and ketone bodies. However, these proportions vary widely according to nutritional state. For example, during starvation, lactate can be recycled by the heart. This is very energy efficient, because one NAD+ is reduced to NADH and H+ (equal to 2.5 or 3 ATP) when lactate is oxidized to pyruvate, which can then be burned aerobically in the TCA cycle, liberating much more energy (ca 14 ATP per cycle).
In the condition of diabetes, more fat and less carbohydrate is used due to the reduced induction of GLUT4 glucose transporters to the cell surfaces. However, contraction itself plays a part in bringing GLUT4 transporters to the surface (Lund et al. 1995). This is true of skeletal muscle, but relevant in particular to cardiac muscle, since it is always contracting.
Specialized pacemaker cells in the sinoatrial node normally determine the overall rate of contractions, with an average resting pulse of 72 beats per minute.
The central nervous system does not directly create the impulses to contract the heart, but only sends signals to speed up or slow down the heart rate through the autonomic nervous system using two opposing kinds of modulation:
Since cardiac muscle is myogenic, the pacemaker serves only to modulate and coordinate contractions. The cardiac muscle cells would still fire in the absence of a functioning SA node pacemaker, albeit in a disordered and ineffective manner. Note that the heart can still beat properly even if its connections to the central nervous system are completely severed.
In contrast to skeletal muscle, cardiac muscle cannot contract in the absence of extracellular calcium ions as well as extracellular sodium ions. In this sense, it is intermediate between smooth muscle, which has a poorly developed sarcoplasmic reticulum and derives its calcium across the sarcolemma, and skeletal muscle, which is activated by calcium stored in the sarcoplasmic reticulum (SR).
The reason for the calcium dependence is due to the mechanism of calcium-induced calcium release (CICR) from the SR that must occur under normal excitation-contraction (EC) coupling to cause contraction.
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