Skeletal muscles are probably the most familiar type of muscle to people. Skeletal muscles are the ones that ache when someone goes for that first outdoor run 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 remains constant. 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 a microscopic level. At the macroscopic level, skeletal muscles usually originate at one point of attachment to a tendon and terminate at another tendon at the other end of an adjoining bone. Tendons are rich in the protein collagen, which 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 is balanced 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 that contract and cause a joint to close are called flexor muscles, and those that contract to cause a joint to stretch out are called extensors. Skeletal muscle that supports the skull, backbone, and rib cage are called axial skeletal muscles; whereas, skeletal muscles of the limbs 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 walking.
Skeletal muscles are organized into extrafusal and intrafusal fibers. Extrafusal fibers are the strong, outer layers of muscle. This type of muscle fiber is the most common. Intrafusal fibers, which make up the central region of the muscle, 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 that 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 may have 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 the sartorius muscle of the thigh.
At the microscopic level, a single muscle cell has several hundred nuclei and a striped appearance derived from the pattern of myofilaments. Long, cylindrical muscle fibers are formed from several myoblasts in fetal development. Multiple nuclei are important in muscle cells because of the tremendous amount of activity in muscle. The myofilaments, actin and myosin, overlap one another in a very specific arrangement. Myosin is a thick protein with two globular head regions. Each myosin filament is surrounded by six actin (thin) filaments. These filaments run longitudinally along the length of the cell in parallel. Multiple hexagonal arrays of actin and myosin exist in each skeletal muscle cell.
Each actin filament slides along adjacent myosin filaments with the help of other proteins and ions present in the cell. Tropomyosin and troponin are two proteins attached to the actin filaments that enable the globular heads on myosin to instantaneously attach to the myosin strands. The attachment and rapid release of this bond induces the sliding motion of these filaments which result in muscle contraction. In addition, calcium ions and ATP (cellular energy) are required by the muscle cell to process this reaction. Numerous mitochondria are present in muscle fibers to supply the extensive ATP required by the cell.
The system of myofilaments within muscle fibers are divided into units called sarcomeres. Each skeletal muscle cell has several myofibrils, long cylindrical columns of myofilaments. Each myofibril is composed of the myofilaments that interdigitate to form the striated sarcomere units. The thick myosin filaments of the sarcomere provide the dark, striped appearance in striated muscle, and the thin actin filaments provide the lighter sarcomere regions between the dark areas. A sarcomere can induce muscle contraction the way a paper towel roll holder can be pushed together before inserting it into a dispenser. The actin and myosin filaments slide over one another like the outer and inner layers of the roll holder. Muscle contraction creates an enlarged center region in the whole muscle. The flexing of a bicep makes this region anatomically visible. This large center is called the belly of the muscle.
Skeletal muscles function as the link between the somatic nervous system and the 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 carry out 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 way to 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 of obtaining 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 the brain. Then signals travel down the spinal cord to the alpha motor neuron to result 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 number of 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.