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The specialized striated muscle tissue of the heart; the myocardium.

Your heart beats about once a second for the whole of your life, and of course has no opportunity to rest. Its output must adjust rapidly to meet the needs of the body, and can increase from about 5 litres of blood/min at rest to more than 25 litres/min in heavy exercise. The special requirements of the heart call for a special type of muscle, cardiac muscle, which is not found anywhere else in the body. Cardiac muscle is in some ways similar to skeletal and smooth muscle. For example, all three contract when a rise in calcium inside the muscle cell allows interaction between actin and myosin filaments. However, cardiac muscle has a unique structure, and differs in the way that contraction is initiated and regulated.

Structure

Under the microscope, cardiac muscle is seen to consist of interlacing bundles of cardiac myocytes (muscle cells). Like skeletal muscle it is striated with narrow dark and light bands, due to the parallel arrangement of actin and myosin filaments that extend from end to end of each myocyte. However, cardiac myocytes are narrower and much shorter than skeletal muscle cells, being about 0.02 mm wide and 0.1 mm long, and are more rectangular than smooth muscle cells, which are normally spindle-shaped. They are often branched, and contain one nucleus but many mitochondria, which provide the energy required for contraction. A prominent and unique feature of cardiac muscle is the presence of irregularly-spaced dark bands between myocytes. These are known as intercalated discs, and are due to areas where the membranes of adjacent myocytes come very close together. The intercalated discs have two important functions: one is to ‘glue’ the myocytes together so that they do not pull apart when the heart contracts; the other is to allow an electrical connection between the cells, which, as we will see, is vital to the function of the heart as a whole. The electrical connection is made via special junctions (gap junctions) between adjoining myocytes, containing pores through which small ions and therefore electrical current can pass. As the myocytes are electrically connected, cardiac muscle is often referred to as a functional syncytium (continuous cellular material).

Mechanism of contraction

Cardiac myocytes contract when the voltage across the membrane, the resting membrane potential, is reduced sufficiently to initiate an action potential. In most parts of the heart this is caused by an action potential in an adjacent myocyte being transmitted through the gap junctions. The action potential starts with a very rapid reduction in voltage toward zero, which is due to sodium ions entering the myocyte. This phase of the action potential is also seen in skeletal muscle and nerves. In cardiac muscle, however, the membrane potential then remains close to zero for about 0.3 sec — the plateau phase, which is largely due to entry of calcium ions. It is this entry of calcium that leads to contraction. At the end of the plateau phase the membrane potential returns to resting levels. The plateau means that cardiac muscle action potentials last much longer than those in skeletal muscle or nerves, where calcium does not enter the cell and there is therefore no plateau phase.

When an action potential is initiated in one myocyte, it causes an electrical current to pass through gap junctions in the intercalated discs to its neighbours. This current initiates action potentials in these cells, which in turn stimulate their neighbours. As a result, a wave of activation, and therefore contraction, passes through the heart. This process allows synchronization of contraction throughout the heart, and is vital for proper function. When it is disrupted, as in some types of heart disease, the myocytes may lose synchronization. In severe cases, such as ventricular fibrillation, the heart cannot pump at all, and is said to look like a ‘bag of (writhing) worms’.

The amount of calcium entering the myocyte during an action potential is not enough to cause contraction. However, its entry causes more calcium to be released from stores in the sarcoplasmic reticulum, a membranous structure within the myocyte. This is known as calcium-induced calcium release. The amount of calcium released depends on the amount that enters during the action potential, so that contractile force can therefore be regulated by controlling calcium entry. This is increased by adrenaline and the autonomic nervous system. At the end of the beat, calcium is rapidly taken back into the sarcoplasmic reticulum, causing relaxation. Excess calcium — the amount that entered during the action potential — is expelled from the myocyte during the interval between beats by pumps in the membrane. If the heart rate increases there is less time to remove this calcium. As a result there is more calcium in the myocyte for the next beat, and so the force developed increases. This staircase effect allows the heart to expel blood more rapidly when the heart rate is increased. Drugs that inhibit removal of calcium from the myocyte can similarly increase cardiac muscle force. An example is digitalis, which was originally derived from the foxglove and has been used for treating heart disease for centuries.

Special types of cardiac muscle

Some areas of the heart contain myocytes that have specialized functions. One is the sino-atrial node or pacemaker region in the right atrium, where modified myocytes generate action potentials automatically, and are responsible for initiating the heartbeat. Although nervous activity is not required for the heart to beat, the autonomic nervous system can modulate the activity of the pacemaker, and hence heart rate. The atria and ventricles are separated by a non-conducting band except at the atrio-ventricular node. This node consists of small myocytes that do conduct, but delay the impulse from the pacemaker, thus allowing the atria to contract before the ventricles. From here the impulse is distributed rapidly around the ventricles via bundles of specialized large myocytes called Purkinje fibres. Defects in any part of this conduction system can lead to a disordered heartbeat.

Diagrammatic section of cardiac muscle

— Jeremy Ward

See also heart; pacemaker.

Special muscle found only within the heart. It can contract rhythmically without any external stimulation from nerves or hormones, as long as it is supplied with sufficient nutrients and oxygen. Unlike other types of muscle, cardiac muscle does not fatigue. However, it will stop contracting if its oxygen supply is interrupted.

Muscle found only in the heart. The cells are striated, contain a single nucleus and branch, so that they fit together tightly at junctions called intercalated discs. Although cardiac muscle is myogenic, it has a contractile mechanism similar to that of striped muscle (see sliding-filament theory). However, cardiac muscle does not fatigue and it cannot tolerate lack of oxygen.

Cardiac muscle

Cardiac muscle

'Cardiac muscle' is a type of involuntary striated muscle found within the heart. Its function is to "pump" blood through the circulatory system by contracting.

Metabolism

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 metabolic substrate 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 1% of energy is derived from anaerobic metabolism. This can increase to 10% under moderately hypoxic conditions, but under more severe hypoxic conditions, not enough energy can be liberated by lactate production to sustain ventricular contractions. ^[1]

Under basal aerobic conditions, 60% of energy comes from fat (free fatty acids and triacylglycerides), 35% from carbohydrates, and 5% from amino acids and ketone bodies. However, these proportions vary widely according to nutritional state. E.g., in starvation, lactate can be recycled by the heart. There is a cost to lactate recycling, since one NAD+ is reduced to get pyruvate from lacate, but the pyruvate can then be burnt aerobically in the TCA cycle, liberating much more energy.

In 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. ^[2] This is true of skeletal muscle, but relevant in particular to cardiac muscle, since it is always contracting.

Contractions

Initiation

Unlike skeletal muscle, which contracts in response to nerve stimulation, and like single unit smooth muscle, cardiac muscle is myogenic, meaning that it is self-excitable stimulating contraction without a requisite electrical impulse coming from the central nervous system.

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.

This transmission of impulses makes cardiac muscle tissue similar to nerve tissue, although cardiac muscle cells are notably connected to each other by intercalated discs. Intercalated discs conduct electrochemical potentials directly between the cytoplasms of adjacent cells via gap junctions. In contrast to the chemical synapses used by neurons, electrical synapses, in the case of cardiac muscle, are created by ions flowing from cell to cell, known as an action potential.

Intercalated disc

An intercalated disc is an undulating double membrane separating adjacent cells in cardiac muscle fibers. Intercalated discs support 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 connects to the closest sarcomere. Macula adherens stop separation during contraction by binding intermediate filaments joining the cells together, also called a desmosome. Gap junctions 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

Rate

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:

• (1) sympathetic nervous system (fight or flight response)
• (2) parasympathetic nervous system (rest and repose)

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 chaotic and ineffective manner. This condition is known as fibrillation. Note that the heart can still beat properly even if its connections to the central nervous system are completely severed.

Role of calcium

In contrast to skeletal muscle, cardiac muscle cannot contract in the absence of extracellular calcium ions as well as extracellular potassium 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.

Appearance

Striation

Cardiac muscle exhibits cross striations formed by alternation segments of thick and thin protein filaments which are anchored by segments called Z-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 shorter, 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.^[3] 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.

References

1. ^ Ganong, Review of Medical Physiology, 22nd Edition. p81
2. ^ S Lund, GD Holman, O Schmitz, and O Pedersen. Contraction Stimulates Translocation of Glucose Transporter GLUT4 in Skeletal Muscle Through a Mechanism Distinct from that of Insulin. PNAS 92: 5817-5821.
3. ^ Histology at BU 22501loa

• Indiana State University, Muscle action
• [1]

External links

• Physiology at MCG 2/2ch7/2ch7line

See also

• Heart
• Circulatory system
• Cardiac action potential
• Calcium sparks

Muscular system
Topics	Muscular tissue • Muscle contraction • Muscles of the human body
Types of muscles	Cardiac muscle • Skeletal muscle • Smooth muscle

Histology: muscle tissue
skeletal muscle/general	epimysium, fascicle, perimysium, endomysium, muscle fiber (intrafusal, extrafusal), myofibril sarcomere (a, i, and h bands; z and m lines), myofilaments (thin filament/actin, thick filament/myosin, elastic filament/titin, nebulin), tropomyosin, troponin (T, C, I) costamere (dystrophin, α,β-dystrobrevin, syncoilin, synemin/desmuslin, dysbindin, sarcoglycan, dystroglycan, sarcospan), desmin neuromuscular junction, motor unit, muscle spindle, excitation-contraction coupling, sliding filament mechanism myoblast, satellite cell, sarcoplasm, sarcolemma, sarcoplasmic reticulum, T-tubule
cardiac muscle	myocardium, intercalated disc, nebulette
smooth muscle	calmodulin, vascular smooth muscle

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