The muscular system is the body's network of tissues for both conscious and unconscious movement. Movement is generated through the contraction and relaxation of specific muscles. Some muscles, like those in the arms and legs, areinvolved in voluntary movements such as raising a hand or flexing the foot. Other muscles are involuntary and function without conscious effort. Voluntarymuscles include skeletal muscles and total about 650 in the whole human body. Skeletal muscles are controlled by the somatic nervous system; whereas the autonomic nervous system controls involuntary muscles. Involuntary muscles include muscles that line internal organs. These smooth muscles are called visceral muscles, and they perform tasks not generally associated with voluntary activity throughout the body even when it is asleep. Smooth muscles control several automatic physiological responses such as pupil constriction when iris muscles contract in bright light and blood vessel dilation when smooth muscles around them relax, or lengthen. In addition to skeletal and smoothmuscle which are considered voluntary and involuntary, respectively, cardiacmuscle exists which is considered neither. Cardiac muscle is not under conscious control, and it can also function without external nervous system regulation.
Smooth muscles derive their name from their appearance when viewed in 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 the 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 the striated appearance. This microscopic view of muscle reveals some hint of how muscles alter their shape to induce movement. Because muscle cells tend to be elongated, they are often called muscle fibers. Muscle cells are distinct from other cells in the body in shape, protein composition, and in the fact that they are multi-nucleated (have more than one nucleus per cell).
Skeletal muscles are probably the must familiar type of muscle to people. Skeletal muscles are the ones that ache when someone goes for that first outdoorrun in the spring after not running much during the winter. And skeletal muscles are heavily used when someone carries in the grocery bags. Exercise may increase muscle fiber size, but muscle fiber number generally remainsconstant. Skeletal muscles take up about 40% of the body's mass, or weight. They also use a great deal of oxygen and nutrients from the blood supply. Multiple levels of skeletal muscle tissue receive their own blood supplies.
Like all muscles, skeletal muscles can be studied at both a macroscopic and amicroscopic level. At the macroscopic level, skeletal muscles usually originate at one point of attachment to a tendon and terminate at another tendon atthe other end of an adjoining bone. Tendons are rich in the protein collagenwhich is arranged in a wavy way so that it can stretch out and provide additional length at the muscular-bone junction.
Skeletal muscles act in pairs where the flexing (shortening) of one muscle isbalanced by a lengthening (relaxation) of its paired muscle or a group of muscles. These antagonistic (opposite) muscles can open and close joints such as the elbow or knee. Muscles which contract and cause a joint to close are called flexor muscles, and those which contract to cause a joint to stretch outare called extensors. Skeletal muscle which support the skull, backbone, andrib cage are called axial skeletal muscles; whereas, skeletal muscles of thelimbs are called distal. These muscles attach to bones via strong, thick connective tissue called tendons. Several skeletal muscles work in a highly coordinated manner in activities such as locomotion, walking.
Skeletal muscles are organized into extrafusal and intrafusal fibers. Extrafusal fibers are the strong, outer layers of muscle. This type of muscle fiberis the most common. Intrafusal fibers which make up the central region of themuscle are weaker than extrafusal fibers. Skeletal muscles fibers are additionally characterized as "fast" or "slow" based on their activity patterns. Fast, also called "white," muscle fibers contract rapidly, have poor blood supply, operate anaerobically, and fatigue rapidly. Slow, also called "red," muscle fibers contract more slowly, have better blood supplies, operate aerobically, and do not fatigue as easily. Slow muscle fibers are used in movements which are sustained such as maintaining posture.
Skeletal muscles are enclosed in a dense sheath of connective tissue called the epimysium. Within the epimysium, muscles are sectioned into columns of muscle fiber bundles, called primary bundles or fasciculi, which are each covered by connective tissue called the perimysium. An average skeletal muscle mayhave 20 - 40 fasciculi which are further subdivided into several muscle fibers. Each muscle fiber (cell) is covered by connective tissue called endomysium. Both the epimysium and the perimysium contain blood and lymph vessels to supply the muscle with nutrients and oxygen and remove waste products, respectively. The endomysium has an extensive network of capillaries that supply individual muscle fibers. Individual muscle fibers vary in diameter from 10-60 micrometers and in length from a few millimeters to about 12 in (30 cm) in thesartorius muscle of the thigh.
Skeletal muscles function as the link between the somatic nervous system andthe skeletal system. One does not move a skeletal muscle for the sake of moving the muscle unless one is a bodybuilder. Skeletal muscles are used to carryout instructions from the brain so that someone can accomplish something. For instance, someone decides that they would like a bite of cake. Unless the cake will come to the mouth by itself, the person needs to figure out some wayto get that cake to their mouth. The brain tells the muscle to contract in the forearm allowing it to flex so that the hand is in position to get a forkful of cake. But the muscle alone cannot support the weight of a fork; it is the sturdy bones of the forearm that allow the muscles to complete the task ofobtaining the cake. Hence, the skeletal and muscular systems work together as a lever system with 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 neurons branch 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 which triggers 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 thebrain. Then signals travel down the spinal cord to the alpha motor neuron toresult in contraction. However, not all movement of skeletal muscles is voluntary. Certain reflexes occur in response to dangerous stimuli, such as extreme heat. Reflexive skeletal muscular movement is controlled at the level of the spinal cord and does not require higher brain initiation. Reflexive movements are processed at this level to minimize the amount of time necessary to implement a response.
In addition to motor neuron activity in skeletal muscular activity, a numberof sensory nerves carry information to the brain to regulate muscle tension and contraction to optimize muscle action. Muscles function at peak performance when they are not overstretched or overcontracted. Sensory neurons within the muscle send feedback to the brain with regard to muscle length and state of contraction.
Cardiac muscles, as is evident from their name, make up the muscular portionof the heart. While almost all cardiac muscle is confined to the heart, someof these cells extend for a short distance into cardiac vessels before tapering off completely. The heart muscle is also called the myocardium. The heartmuscle is responsible for more than two billion beats in a lifetime. The myocardium has some properties similar to skeletal muscle tissue, but it is alsounique. Like skeletal muscles, myocardium is striated; however, the cardiac muscle fibers are smaller and shorter than skeletal muscle fibers averaging 5-15 micrometers in diameter and 20-30 micrometers in length. In addition, cardiac muscles align lengthwise more than side-by-side compared to skeletal muscle fibers. The microscopic structure of cardiac muscle is also unique in thatthese cells are branched such that they can simultaneously communicate withmultiple cardiac muscle fibers.
Cardiac muscle cells are surrounded by an endomysium like the skeletal musclecells. But innervation of autonomic nerves to the heart do not form any special junction like that 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 doesnot require the individual fibers to be stimulated. Although external nervousstimuli can enhance or diminish cardiac muscle contraction, heart muscles can also contract spontaneously making them myogenic. Like skeletal muscle cells, cardiac muscle fibers can increase in size with physical conditioning, butthey rarely increase in number.
Smooth muscle falls into two general categories, visceral smooth muscle and multi-unit smooth muscle. Visceral smooth muscle fibers line internal organs such as the intestines, stomach, and uterus. They also facilitate the movementof substances through tubular areas such as blood vessels and the small intestines. Multi-unit smooth muscles function in a highly localized way in areassuch as the iris of the eye. Contrary to contractions in visceral smooth muscle, contractions in multi-unit smooth muscle fibers do not readily spread toneighboring muscle cells.
Smooth muscle is unstriated with innervations from both sympathetic (flight or fight) and parasympathetic (more relaxed) nerves of the autonomic nervous system. Smooth muscle appears unstriated under a polarized light microscope, because the myofilaments inside are less organized. Smooth muscle fibers contain actin and myosin myofilaments which are more haphazardly arranged than they are in skeletal muscles. The sympathetic neurotransmitter, Ach, and parasympathetic neurotransmitter, norepinephrine, activate this type of muscle tissue.
The concentric arrangement of some smooth muscle fibers enables them to control dilation and constriction in the intestines, blood vessels, and other areas. While innervation of these cells is not individual, excitation from one cell can spread to adjacent cells through nexuses which join neighbor cells. Smooth muscle cells have a small diameter of about 5-15 micrometers and are long, typically 15-500 micrometers. They are also wider in the center than at their ends. Gap junctions connect small bundles of cells which are, in turn, arranged in sheets.
Within hollow organs, such as the uterus, smooth muscle cells are arranged into two layers. The outer layer is usually arranged in a longitudinal fashionsurrounding the inner layer which is arranged in a circular orientation. Manysmooth muscles are regulated by hormones in addition to the neurotransmitters of the autonomic nervous system. In addition, contraction of some smooth muscles are myogenic or triggered by stretching as in the uterus and gastrointestinal tract.
Smooth muscle differs from skeletal and cardiac muscle in its energy utilization as well. Smooth muscles are not as dependent on oxygen availability as cardiac and skeletal muscles are. Smooth muscle uses glycolysis to generate much of its metabolic energy.