Muscular System Health Article

Definition

The muscular system is the body's network of tissues for both voluntary and involuntary movements. Muscle cells are specialized for contraction.

Description

Body movements are generated through the contraction and relaxation of specific muscles. Some muscles, like those in the arms and legs, bring about such voluntary movements as raising a hand or flexing the foot. Other muscles are involuntary and function without conscious effort. Voluntary muscles include the skeletal muscles, of which there are about 650 in the human body. Skeletal muscles are controlled by the somatic nervous system; whereas the autonomic nervous system controls the involuntary muscles. Involuntary muscles include muscles that line the internal organs and the blood vessels. These smooth muscles are called visceral and vascular smooth muscles, and they perform tasks not generally associated with voluntary activity. Smooth muscles control several automatic physiological responses such as pupil constriction, which occurs when the muscles of the iris contract in bright light. Another example is the dilation of blood vessels, which occurs when the smooth muscles surrunding the vessels relax or lengthen. In addition to the categories of skeletal (voluntary) and smooth (involuntary) muscle, there is a third category, namely cardiac muscle, which is neither voluntary nor involuntary. Cardiac muscle is not under conscious

control, and it can also function without regulation from the external nervous system.

Smooth muscles derive their name from their appearance under polarized light microscopy. In contrast to cardiac and skeletal muscles, which have striations (appearance of parallel bands or lines), smooth muscle is unstriated. Striations result from the pattern of myofilaments, which are very fine threads of protein. There are two types of myofilaments, actin and myosin, which line the myofibrils within each muscle cell. When many myofilaments align along the length of a muscle cell, light and dark regions create a striated appearance. This microscopic view of muscle reveals that muscles alter their shape to produce movement. Because muscle cells are usually elongated, they are often called muscle fibers. Compared to other cells in the body, striated muscle cells are distinctive in shape, protein composition, and multinucleated structure.

Skeletal muscles

Skeletal muscles function as the link between the somatic nervous system and the skeletal system. Skeletal muscles carry out instructions from the brain related to voluntary movement or action. For instance, when a person decides to eat a piece of cake, the brain tells the forearm muscle to contract, allowing it to flex and position the hand to lift a forkful of cake to the mouth. But the muscle alone cannot support the weight of the fork; the sturdy bones of the forearm assist the muscles in completing the task of moving the bite of cake. Hence, the skeletal and muscular systems work together as a lever system, with the joints acting as a fulcrum to carry out instructions from the nervous system.

The somatic nervous system controls skeletal muscle movement through motor neurons. Alpha motor neurons extend from the spinal cord and terminate on individual muscle fibers. The axon, or signal-sending end, of the alpha neuron branches to innervate multiple muscle fibers. The nerve terminal forms a synapse, or junction, with the muscle to create a neuromuscular junction. The neurotransmitter acetylcholine (ACh) is released from the axon terminal into the synapse. From the synapse, the ACh binds to receptors on the muscle surface that trigger events leading to muscle contraction. While alpha motor neurons innervate extrafusal fibers, intrafusal fibers are innervated by gamma motor neurons.

Voluntary skeletal muscle movements are initiated by the motor cortex in the brain. Signals travel down the spinal cord to the alpha motor neuron to result in contraction. Not all movement of skeletal muscles is voluntary, however. Certain reflexes occur in response to such dangerous stimuli as extreme heat or the edge of a sharp object. Reflexive skeletal muscular movement is controlled at the level of the spinal cord and does not require higher brain initiation. Reflexive movements are

Cardiac muscle

Cardiac heart muscle is responsible for more than two billion beats in the course of a human lifetime of average length. Cardiac muscle cells are surrounded by endomysium like the skeletal muscle cells. The autonomic nerves to the heart, however, do not form any special junctions like those found in skeletal muscle. Instead, the branching structure and extensive interconnectedness of cardiac muscle fibers allows for stimulation of the heart to spread into neighboring myocardial cells. This feature does not require the individual fibers to be stimulated. Although external nervous stimuli can enhance or diminish cardiac muscle contraction, heart muscles can also contract spontaneously. Like skeletal muscle cells, cardiac muscle fibers can increase in size with physical conditioning, but they rarely increase in number