The muscular system
Skeletal muscle is the tissue of the muscular system, and cardiac and smooth muscle, which differ from it in structure and in how they contract, belong to other systems. Skeletal muscle moves the body by shortening and pulling on bone, some of its contractions hold a posture without moving anything, and muscle contraction produces most of the heat that maintains body temperature. A muscle fibre shortens as its actin and myosin filaments slide toward each other, and its contraction begins with a nerve impulse from a motor neuron.
Three kinds of muscle and how they differ
Muscle tissue accounts for approximately 50% of the body's weight, and it is characterised by contractility, extensibility and elasticity. Its three major types are skeletal muscle, which is striated and voluntary, cardiac muscle, which is striated and involuntary, and smooth muscle, which is nonstriated and involuntary. Of the three, only skeletal muscle is the tissue of the muscular system. Cardiac and smooth muscle are muscle tissues of other systems, among them the cardiac, respiratory, digestive, urinary and reproductive systems.
The three types differ in several ways. Skeletal muscle is usually attached to bones, and its long cylindrical fibres, 1 to 40 mm long, are arranged in bundles. Each fibre has a distinct double-layered sarcolemma, limited sarcoplasm with numerous organelles, striated myofibrils that fill the cell, and multiple flattened nuclei scattered over the cell periphery just under the sarcolemma. Skeletal muscle has a good blood supply and motor and sensory nerve endings, and its contractions are voluntary, often rigorous and short, with each fibre contracting independently.
Cardiac muscle lies in the heart wall, and its short fibres, 0.05 to 0.1 mm long, form a branched network. Its sarcolemma forms intercalated discs, it has a large amount of sarcoplasm with fewer organelles, its myofibrils are striated and fewer than in skeletal muscle, and it usually has one oval nucleus, sometimes two, at the centre of the cell. Cardiac muscle has dense blood capillary beds and motor and sensory nerve endings, and its contractions are involuntary and rhythmic, coordinated across its fibres.
Smooth muscle lies in the walls of hollow internal organs, and its elongated tapering fibres, 0.015 to 0.5 mm long, are arranged in bundles or sheets. Its delicate sarcolemma is fused with adjacent fibres, it has a small amount of sarcoplasm with numerous organelles, its nonstriated myofibrils fill the cell, and it has one oval or rod-shaped nucleus at the centre of the cell. Smooth muscle has a fair blood supply and motor and sensory nerve endings, and its contractions are involuntary and rhythmic, coordinated across its fibres.
Movement, posture and heat
Muscles of the muscular system function only by shortening and pulling on bone. Movement occurs at a joint between the stationary bone, the origin, and the moving bone, the insertion. The rest of the muscle is its body, or sometimes its belly. Tendons anchor muscles firmly to bones and are made of dense fibrous connective tissue, and they have great strength but are flexible. Tendons differ from ligaments, which connect bone to bone. Muscle can move some body parts in several directions and others in only two. The type of movement depends largely on the shape of the bones and the type of articulation.
Not all muscle contractions produce movement. Isometric contractions increase tension in muscles without producing movement. Tonic contractions, or muscle tone, produce no movement either, but they increase the firmness or tension of muscle that maintains posture and holds body parts in position. Because relatively few of a muscle's fibres shorten at any one time in a tonic contraction, the muscle as a whole does not shorten.
Heat production is a further function of muscle tissue. The contraction of muscle fibres produces most of the heat required to maintain body temperature. Shivering is an involuntary muscle contraction that produces heat.
How a muscle fibre shortens
Muscle tissue consists of specialised multinucleated contractile cells, which are sometimes called muscle fibres because they are long and slender. A muscle cell has a sarcolemma, sarcoplasm and a sarcoplasmic reticulum, which are its specialised plasmalemma, cytoplasm and endoplasmic reticulum. Muscle cells contain myofibrils, and each myofibril contains myofilaments organised into sarcomeres. A sarcomere contains the overlapping protein myofilaments actin, the thin filaments, and myosin, the thick filaments. Sarcomeres are repeating units, delimited by the Z bands along the length of the myofibril, and the sarcomere is the structural unit of a skeletal muscle cell. Actin is a protein in muscle cells involved in muscle contraction.
The A band has overlapping thick and thin myofilaments, the H zone has only thick myosin filaments, and the I band has only thin actin filaments. Within the zone of overlap, thick and thin filaments can interact to produce a contraction. During contraction, ATP allows the two types of myofilament to slide toward each other and shorten the sarcomere and, eventually, the whole muscle. Actin filaments have sites that are covered by troponin and tropomyosin when they are not involved in contraction. Myosin molecules have cross-bridges that extend outward from the surface of the thick filament. During a contraction, the cross-bridges attach, pivot and detach, sliding the thin filaments toward the centre of the sarcomere.
How a nerve impulse starts a contraction
Muscle cells produce movement only after they have been stimulated by a nervous impulse, which enters the muscle fibre through a motor neuron at the myoneural junction. When the impulse passes through this junction, neurotransmitters that cause the muscle to contract are released. The myoneural junction is the site where a motor nerve fibre meets the skeletal muscle fibre it innervates, and no direct contact is made there.
Living cells normally have an excess of negative ions inside their cell membranes and an excess of positive ions outside, which establishes the transmembrane potential. The extracellular fluid contains sodium and chloride ions, whereas the sarcoplasm contains potassium ions and negatively charged proteins. Sodium channels do not let sodium into the cell as fast as potassium channels let potassium out. Proteins cannot cross the membrane at all, but chloride can diffuse into the cell. The resting potential remains stable because, at that level of transmembrane potential, the sodium–potassium exchange pumps can keep pace with sodium entry and potassium loss.
A motor neuron provides the stimulus for an action potential in a skeletal muscle cell. Acetylcholine released by the synaptic knob diffuses across the synaptic cleft to reach the opposing surface of the sarcolemma. The stimulus alters the resting potential by changing the membrane's permeability to sodium or potassium. The sodium channels open, and sodium rushes into the cell. In a skeletal muscle cell, the transmembrane potential in that portion of the membrane changes from −65 mV to +30 mV. The sodium channels then close, the potassium channels open, potassium leaves the cell, and repolarisation begins. The ion exchange pumps restore the normal distribution of sodium and potassium, and the resting potential reappears.
An action potential that sweeps over the sarcolemma travels along the sarcoplasmic reticulum. The membrane of the sarcoplasmic reticulum normally removes calcium from the sarcoplasm and stores it in the cisternae, the hollow swellings of its tubules. After a single action potential has passed, the sarcoplasmic reticulum becomes very permeable to calcium, which interacts with the troponin and tropomyosin along the thin filaments and uncovers the active sites. Calcium gives the cross-bridges the ability to split ATP and release the energy needed for contraction. Once started, the contraction proceeds until the activities of the sarcoplasmic reticulum reduce the calcium concentration.