The heart is a hollow organ with walls made from specialized muscle called cardiac muscle. When excited, these muscles shorten, thicken, and squeeze on the hollow cavities of the heart forcing blood to flow in directions permitted by the heart valve. Cardiac and skeletal muscles are similiar in many ways: both contain actin and myosin filaments which interdigitate and slide closer together during contraction: both can be electrically excited: and both show action potentials that propagate along the surface membrane, carrying excitation to all parts of the muscle. However, there are also significant differences.
1. The duration of the action potential is very brief in skeletal muscle; in cardiac muscle, it is 100 times longer lasting throughout the contraction of the muscle.
2. The long retractory period associated with the prolonged action potential also lasts throughout the contraction. This implies that:
3. Cariac muscle contractions are always brief twitches in skeletal muscle contractions resulting from rapid repetitive stimulation can summate or “fuse” to provide smooth, sustained contractions. This cannot happen in cardiac muscle because the long refractory period “cancels” any stimulus that occurs before the heart has a chance to relax. Relaxation between eacch beat is essential for the heart to fill with blood to be pumped at the next beat.
4. Cardiac muscles are interconnected by gap junctions. These are channels that allow action potentials to pass from one cell to the next and ensure that the entire heart participates in each contraction. The heartbeat is all or none. In contrast, skeletal muscle cells are electrically isolated; one cell may contract while its neighbor remains relaxed.
5. Normally, skeletal muscle will contract only if it receives a nerve impulse. Cardiac muscle excites itself. Nerves that carry impulses to the heart influence the rate and strength of contraction, but they do not initiate the primitive heartbeat. When these nerves are destroyed, the heart continues to beat without any external prompt. In contrast, when nerves to sekletal muscle are destroyed, the muscle is paralyzed.
Exercise helps strengthen the walls of the heart to allow for better regulation of blood pressure. When exercising, the heart has to pump blood to the working systems faster. During the pumping action of the heart, the arteries, and veins are contracting more effieciently. The pumping blood helps strengthen the walls of the veins and arteries. Improving the cardio-respiratory and muscular systems of the body helps improve blood pressure.
Daryl Conant, M.Ed.