cardiac muscle

cardiac muscle 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.

Jeremy Ward

See also heart; pacemaker.