v Cardiac or heart muscle resembles skeletal muscle in some ways: it is striated and each cell contains sarcomeres with sliding filaments of actin and myosin. Each cell usually has one centrally-located nucleus. However, cardiac muscle has a number of unique features that reflect its function of pumping blood.
1. The myofibrils of each cell (and cardiac muscle is made of single cells - each with a single nucleus) are branched. The branches interlock with those of adjacent fibers by adherens junctions (intercalated disks) composed of desmosomes and gap junctions. These strong junctions enable the heart to contract forcefully without ripping the fibers apart.1
2. The action potential that triggers the heartbeat is generated within the heart itself. Motor nerves (of the autonomic nervous system) do run to the heart, but their effect is simply to modulate - increase or decrease - the intrinsic rate and the strength of the heartbeat. Even if the nerves are destroyed (as they are in a transplanted heart), the heart continues to beat.
3. The action potential that drives contraction of the heart passes from fiber to fiber through gap junctions. Significance: All the fibers contract in a synchronous wave that sweeps from the atria down through the ventricles and pumps blood out of the heart. Anything that interferes with this synchronous wave (such as damage to part of the heart muscle from a heart attack) may cause the fibers of the heart to beat at random - called fibrillation. Fibrillation is the ultimate cause of most deaths and its reversal is the function of defibrillators.
- The refractory period in heart muscle is longer than the period it takes for the muscle to contract (systole) and relax (diastole).
- Cardiac muscle has a much richer supply of mitochondria than skeletal muscle. This reflects its greater dependence on cellular respiration for ATP.
- Cardiac muscle has little glycogen and gets little benefit from glycolysis when the supply of oxygen is limited. Thus anything that interrupts the flow of oxygenated blood to the heart leads quickly to damage - even death - of the affected part. This is what happens in heart attacks.