The Muscular System

A skeletal muscle consists of numerous muscle cells called muscle fibers. Muscle fibers have special terminology and distinguishing characteristics, as follows:

1. The sarcolemma, or plasma membrane of the muscle cell, is highly invaginated by transverse tubules (or T tubules) that permeate the cell.

2. The sarcoplasm, or cytoplasm of the muscle cell, contains calcium-storing sarcoplasmic reticulum, the specialized endoplasmic reticulum of a muscle cell.

3. Skeletal muscle cells are multinucleate. The nuclei lie along the periphery of the cell, forming swellings visible through the sarcolemma.

4. Nearly the entire volume of the muscle cell is filled with numerous, long myofibrils. Myofibrils consist of two types of filaments:

• Thin filaments consist of two strands of the globular protein actin arranged in a double helix. Along the length of the helix are troponin and tropomyosin molecules that cover special binding sites on the actin.

• Thick filaments consist of groups of the filamentous protein myosin. Each myosin filament forms a protruding head at one end. An array of myosin filaments possesses protruding heads at numerous positions at both ends.

Within a myofibril, actin and myosin filaments are parallel and arranged side by side. The overlapping filaments produce a repeating pattern that gives skeletal muscle a striated appearance. Each repeating unit of the pattern, called a sarcomere, is separated by a border, or Z-line, to which the actin filaments are attached. The myosin filaments, with their protruding heads, are located between the actin, unattached to the Z-line.

Muscle contraction is described by the sliding-filament model, as follows:

1. ATP binds to a myosin head and forms ADP + Pi. When ATP binds to a myosin head, it is converted to ADP and Pi, which remain attached to the myosin head.

2. Ca2+ exposes the binding sites on the actin filaments. Ca2+ binds to the troponin molecule causing tropomyosin to expose positions on the actin filament for the attachment of myosin heads.

3. Cross bridges between myosin heads and actin filaments form. When attachment sites on the actin are exposed, the myosin heads bind to actin to form cross bridges.

4. ADP and Pi are released and sliding motion of actin results. The attachment of cross bridges between myosin and actin causes the release of ADP and Pi. This, in turn, causes a change in shape of the myosin head, which generates a sliding movement of the actin toward the center of the sarcomere. This pulls the two Z-lines together, effectively contracting the muscle fiber.

5. ATP causes the cross bridges to unbind. When a new ATP molecule attaches to the myosin head, the cross bridge between the actin and myosin breaks, returning the myosin head to its unattached position. Without the addition of a new ATP molecule, the cross bridges remain attached to the actin filaments. This is why corpses are stiff (new ATP molecules are unavailable). Neurons form specialized synapses with muscles called neuromuscular junctions. Muscle contraction is stimulated through the following steps:

1. Action potential generates release of acetylcholine. When an action potential of a neuron reaches the neuromuscular junction, the neuron secretes the neurotransmitter acetylcholine, which diffuses across the synaptic cleft.

2. Action potential is generated on sarcolemma and throughout the T-tubules. Receptors on the sarcolemma initiate a depolarization event and action potential. The action potential travels along the sarcolemma throughout the transverse system of tubules.

3. Sarcoplasmic reticulum releases Ca2+ . As a result of the action potential throughout the transverse system of tubules, the sarcoplasmic reticulum releases Ca2+.

4. Myosin cross bridges form. The Ca2+ released by the sarcoplasmic reticulum binds to troponin molecules on the actin helix, prompting tropomyosin molecules to expose binding sites for myosin cross-bridge formation. If ATP is available, muscle contraction begins. Humans and other vertebrates have three kinds of muscles:

1. Skeletal muscle is attached to bones and causes movements of the body.

2. Smooth muscle lines the walls of blood vessels and the digestive tract where it serves to advance the movement of substances. Due to its arrangement of actin and myosin filaments, smooth muscle does not have the striated appearance of skeletal muscle. In addition, the sarcolemma does not form a system of transverse tubules, and as a result, contraction is controlled and relatively slow, properties appropriate for its function.

3. Cardiac muscle is responsible for the rhythmic contractions of the heart. Although striated, cardiac muscle differs from skeletal muscle in that it is highly branched with cells connected by gap junctions. In addition, cardiac muscle generates its own action potential, which spreads rapidly throughout muscle tissue by electrical synapses across the gap junctions.