muscles: (skeletal)

Muscles: (skeletal)

Muscles have three functions. They are motion, maintenance of posture and heat production

The fascia is the sheet or broad band of fibrous tissue beneath the skin or around muscles and other organs. Superficial fascia is immediately deep to the skin. It is made up of adipose tissue and loose connective tissue. It serves as a storehouse for water and fat, it forms a layer of insulation, protection from mechanical blows, and provides a pathway for blood vessels and nerves. Deep fascia is the dense connective tissue that lines the body wall and extremities, and binds them into functioning groups. Functionally, the deep fascia allows free movement of muscles, carries nerves and blood vessels, and sometimes provides the origin of a muscle

The entire muscle is wrapped in a sheet of connective tissue called the epimysium, an extension of the deep fascia. The epimysium divides in the muscle into sheets of perimysium, binding muscle fiber together into groups. Each cells is covered by and endomysium. All still part of the deep fascia. All three blend together at the end of muscles to form strings, rope-like tendons. When a tendon is flat and sheet-like, it is referred to as an aponeurosis, such as the galea aponeurotica. The aponeurosis' sometimes serve at origins for muscles. Certain tendons, especially those of the hands and feet, are unclosed by tendon sheaths, much like the bursae of synovial articulations

Skeletal muscles are well supplied with nerves and blood vessels. This innervation and vascularization is directly related to the muscles contraction. Generally, an artery and one or two veins accompany each nerve that penetrates the muscle. Capillaries distribute blood evenly throughout the muscle

Muscles fibers, or myofibers, are the individual, cylindrical muscle cells. They are wrapped with a specialized plasma membrane, the sarcolemma. The cells range from 10 to 100 microns in diameter. Some reach lengths of a foot. The cytoplasm of the myofiber contains many nuclei, myofibrils, flattened, elongated mitochondria, and a sarcoplasmic reticulum, comparable to a smooth endoplasmic reticulum. Running transversely to the sarcoplasmic reticulum are transverse tubules (T-tubules). The tubules are extensions of the sarcolemma which opens to the outside of the myofiber. A triad consists of a T-tubule and the segments of sarcoplasmic reticulum on either side

Mentioned above are the myofibrils, the contractile fibers with in the myofiber. Myofibrils are about one or two microns in diameter. Even smaller fibers make up the myofibril, called myofilaments. The thick (myosin) fibers are about 16 nanometers across, and the thin (actin) are about 6 nanometers

Myofilaments do not stretch across the entire length of the myofibril. They are instead arranged into segments of the fibril called sarcomeres. Sarcomeres are separated from one another by dark, dense bands called Z lines. Within the sarcomere, there are several definable areas and structures. A dark, dense area called the anisotrphic or A band is the length of the myosin filament. At either end of the A band, the thick and thin filaments overlap. A greater degree of contraction causes more overlapping. There is also a light colored band, the isotrophic or I band. This area contains only actin filaments, split in the middle by the Z line where two sets of actin filaments terminates and join the sarcomeres. In the middle of the the A band is the H zone, where there is no overlapping of filaments. In the middle of the H zone is the M line, a series of fine threads that appear to connect the middles of the myosin threads

The thin filaments are composed of two actin strands entwined helically. Each actin molecule contains a myosin-binding site that interacts with with a cross bridge of a myosin molecule. The filaments also contains tropomyosin and troponin. The tropomyosin is arranged in strands that are loosely attached to the actin helices. Troponin is located at regular intervals on the surface of the tropomyosin. Troponin is made up of three subunits: troponin I which binds to actin, troponin C which binds to calcium ions, and troponin T which binds to tropomyosin. Together, the tropomyosin and the troponin are referred to as tropomyosin-troponin complex

Myosin molecules makeup mot of the thick filament. The myosin is shapes like a golf club with the tails arranges parallel to on another. The heads face outward, referred to as cross bridges and containing actin-binding sites and atp-binding sites on each one

During contraction (sliding filament theory), thin myofilaments slide towards the H zone. The sarcomere shortens, but the length if the myofilaments remains the same. The cross bridges of the thick filaments connect with the actin portion of the thin. The myosin heads move like oars and the filaments slide over one another. The H zone narrows as they come together, sometimes disappearing all together

Stimulus must be applied for a muscle fiber to contract. A neuron which stimulates muscle tissue is called a motor neuron. A motor unit is made up of a single neuron and all of the myofibers it attaches to, sometime up to 500 myofibers to a single neuron. More precise controls are only 10 to one ratios. The attachment points are the neuromuscular (myoneural) junctions or motor end plates. Closer inspection of the neuron reveals that the axon terminals expand into bulb-like structures known as synaptic end bulbs. These bulbs contain membrane-enclosed sacs, synaptic vesicles, which store neurotransmitters. The depressed area in which the synaptic bulbs nestle are called synaptic gutters. The space between them is the synaptic cleft. The synaptic vesicles contain what is known as acetylcholine, or ACh. Upon it's release, ACh diffuses across the synaptic cleft and combines with receptor sites on the sarcolemma of the muscle fiber, resulting in a nerve impulse which travels along the sarcolemma. The total tension of a muscle contraction is controlled by the number of motor units stimulated, a process called recruitment. Asynchronous firing of the synapses allows for rest of inactive units and thus helps prevent muscle fatigue

When a myofiber is at rest, the concentration of calcium ions (Ca2+) in the sarcoplasm is low; the ions are stored in the sarcoplasmic reticulum. The atp concentration is high, and the atp is attached to the binding sites of the myosin cross bridges. The cross bridges are prevented from combining with the actin by the tropomyosin-troponin complex and the atp-binding sites. When stimulated, acetylcholine alters the sarcolemma into the transverse tubules. ACh travels through the T-tubules and into the sarcoplasmic reticulum. Calcium ions are released and combine with troponin, causing it to change shape. The actin's myosin-binding sites are then exposed and the thin and thick fibers lock together. The contraction requires energy, supplied by atp. When the nerve impulse stimulates a myofiber, an atpase enzyme in the myosin cross bridges splits the atp into adp and p. Energy is released and activates the cross bridges. The stored energy in the myosin cross bridges causes them to move towards the H zone, like oars of a boat. Accordingly, the movement is called the power stroke. Once this stroke is completed, atp combines with it's binding sites on the cross bridges and the actin filament is released. ACh is is rendered inactive by acetylcholinesterase (AChE) and calcium ions are transported back into the sarcoplasmic reticulum by calsequestrin and calcium atpase, using energy from the atp breakdown. The low calcium concentration allows the tropomyosin-troponin complex to regain it's usual form. ADP is resynthesized into atp and the cell is relaxed

Unlike most other cells of the body, myofibers function in a, to say the least, discontinuous manner. They lie in virtual inactivity, then are fired to using atp up in a manner of seconds. The cells have several basic mechanisms for generating atp continuously. The first involves a high-energy molecule named phosphocreatine, found in concentrations about five times as great as that of atp. When needed, it is broken down into creatine, phosphate, and energy. The released energy is used to turn adp back into atp in a fraction of a second. There is about enough of this in the cell for 15 seconds of contraction. After that, glycogen is broken down into glucose. This will last for several minutes. After that, the cells break down fats

Twitches are rapid, jerky movements resulting from a single stimulus. When cells are not able to relax at between stimuli, the result is a sustained contraction, or tetanus. At lower stimuli speeds, this tetanus is 'unfused', but at rates at or exceeding 35 to 50 impulses/second, it is 'fused'

There are two main types of muscle contractions. The ones most often user are isotonic. There the contractions in which the muscle moves, as in opening a door or kicking a soccer ball. The other type is isometric. This results when we push against an unmovable object. The muscles strain, but do not noticeably contract

Muscles are never at complete rest. Alternating motor units contract, giving what is known as muscle tone

Not all myofibers are alike. For one, they vary in color depending upon how much myoglobin the cells contain. Myoglobin is red in color, similar to hemoglobin in the blood. It stores oxygen until it is needed by the cells mitochondria. Myofibers with high amounts of myoglobin are referred to as red muscle fibers. Those with lower contents are white muscle fibers. Red myofibers are smaller in diameter and have more mitochondria in their cytoplasm, while white fibers have a more extensive sarcoplasmic reticulum. Based on their speed of contraction, fibers are placed into three categories: slow-twitch red fibers, fast-twitch red fibers, and fast-twitch white fibers. Endurance sports cause the slow change of fast-twitch white into fast-twitch red fibers

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