Muscle fiber recruitment refers to the process of muscle fiber filaments sliding over each other

The myosin and actin filaments overlap and slide past each other when muscles contract and relax. The Z line separates one sarcomere from the next and is also where actin filaments attach. Other areas of the sarcomere are defined by the presence or absence of acting and/or myosin. The actin-only area (including the Z line) is known as the I band, the myosin-only area is the H zone, and the area where myosin is present as well as where it overlaps with actin is the A band. The A band includes the H zone. The midline of the myosin fibers is the M line.

Myosin and actin filaments are laid side by side to form the cylindrical sarcomere. Sarcomeres are positioned end to end to form a myofibril. Each myofibril is surrounded by the sarcoplasmic reticulum, the specialized endoplasmic reticulum of the myocyte. The lumen of the sarcoplasmic reticulum is filled with Ca2+ ions. Lodged between the myofibrils are the mitochondria which create the ATP required for muscular contraction. Each myocyte is multinucleate. Sarcolemma, a modified cellular plasma membrane, wraps several myofibrils together to form the myocyte. Many muscle fibers are further bound into a fasiculus and many fasiculi form a single muscle.

The contraction of skeletal muscle is controlled by the somatic nervous system. A motor neuron attaches to a myocyte at a motor end plate, forming a neuromuscular junction. The motor end plate is a region of highly excitable muscle while the neuromuscular junction is the synapse between the motor neuron and the motor end plate. The action potential of the neuron releases acetylcholine into the synaptic cleft. The acetylcholine binds membrane-bound receptors on the motor end plate, which activates ion channels and creates action potential that propagates along the sarcolemma. The action potential moves deep into the muscle cell via small infoldings of the sarcolemma called T-tubules. T-tubules facilitate the uniform contraction of the muscle by allowing the action potential to spread through the myocyte more quickly. The action potential is transferred to the sarcoplasmic reticulum, causing voltage-gated channels on the sarcoplasmic reticulum to open. As a result, the sarcoplasmic reticulum becomes more permeable to Ca2+ , which is released around the sarcomere. The presence of Ca2+ allows the sarcomere’s myosin and actin fibers to slide across each other, causing the contraction of the muscle fiber.

When a muscle contracts, the actin is pulled along myosin toward the center of the sarcomere until the actin and myosin filaments are completely overlapped. In other words, for a muscle cell to contract, the sarcomere must shorten. However, thick and thin filaments—the components of sarcomeres—do not shorten. Instead, they slide by one another, causing the sarcomere to shorten while the filaments remain the same length. The sliding filament theory of muscle contraction was developed to fit the differences observed in the named bands on the sarcomere at different degrees of muscle contraction and relaxation. The mechanism of contraction is the binding of myosin to actin, forming cross-bridges that generate filament movement.

When a sarcomere shortens, some regions shorten whereas others stay the same length. A sarcomere is defined as the distance between two consecutive Z discs or Z lines; when a muscle contracts, the distance between the Z discs is reduced. The H zone—the central region of the A zone—contains only thick filaments (myosin) and is shortened during contraction. The H zone becomes smaller and smaller due to the increasing overlap of actin and myosin filaments, and the muscle shortens. Thus when the muscle is fully contracted, the H zone is no longer visible. The I band contains only thin filaments and also shortens. The A band does not shorten—it remains the same length—but A bands of different sarcomeres move closer together during contraction, eventually disappearing. Thin filaments are pulled by the thick filaments toward the center of the sarcomere until the Z discs approach the thick filaments. The zone of overlap, in which thin filaments and thick filaments occupy the same area, increases as the thin filaments move inward. Note that the actin and myosin filaments themselves do not change length, but instead slide past each other.

A motor unit consists of a single alpha motor neuron and all of the corresponding muscle fibers it innervates; all of these fibers will be of the same type. When a motor unit is activated, all of its fibers contract. Groups of motor units often work together to coordinate the contractions of a single muscle. All of the motor units that subserve a single muscle are considered a motor unit pool.

The number of muscle fibers within each unit can vary. In general, the number of muscle fibers innervated by a motor unit is a function of a muscle’s need for refined motion. The smaller the motor unit, the more precise the action of the muscle. Muscles requiring more refined motion are innervated by motor units that synapse with fewer muscle fibers. Motor unit recruitment is a measure of how many motor neurons are activated in a particular muscle; the higher the recruitment, the stronger the muscle contraction will be. Motor units are generally recruited in order of smallest to largest (from fewest fibers to most fibers) as contraction increases. This is known as Henneman’s Size Principle.

Muscle fiber types differ in their contractile velocity (speed of contraction), maximum force production, and resistance to fatigue. Type I fibers, or slow twitch fibers, have slow contractile velocity and produce a low amount of force. However, they also have the advantage that they are slow to fatigue and can be employed for long periods of time. The large number of mitochondria and high myoglobin content of slow twitch fibers provide them with the ATP and oxygen needed to operate for long periods without fatigue. There are two types of Type II, or fast twitch, fibers: Type II A and Type II B. Type II A fibers have a fast contractile velocity and are resistant to fatigue. Type II B fibers have a low myoglobin content, contract rapidly, are able to generate great force, and fatigue quickly.

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Key Points

• The smallest functional unit of the contractile apparatus in skeletal muscle is the sarcomere. The sarcomere is comprised of myosin and actin.

• The myosin and actin filaments overlap and slide past each other when muscles contract and relax.

• The contraction of skeletal muscle is controlled by the somatic nervous system. The action potential of the neuron propagates along the sarcolemma and moves deep into the muscle cell via T-tubules. The action potential is transferred to the sarcoplasmic reticulum, causing voltage-gated channels on the sarcoplasmic reticulum to open. The sarcomere’s myosin and actin fibers to slide across each other, causing the contraction of the muscle fiber.

• When a muscle contracts, the actin is pulled along myosin toward the center of the sarcomere until the actin and myosin filaments are completely overlapped.

• The sliding filament theory of muscle contraction was developed to fit the differences observed in the named bands on the sarcomere at different degrees of muscle contraction and relaxation.

• Motor units contain muscle fibers of all the same type; these may be many muscle fibers (as in the case of quadriceps) or a few muscle fibers (as in the case of the muscles that control eye movement).

• Groups of motor units often work together to coordinate the contractions of a single muscle; all of the motor units that subserve a single muscle are considered a motor unit pool.

• Motor units are generally recruited in order of smallest to largest (from fewest fibers to most fibers) as contraction increases. This is known as Henneman’s Size Principle.

• The smaller the motor unit, the more precise the action of the muscle.

• Muscle fiber types differ in their contractile velocity (speed of contraction), maximum force production, and resistance to fatigue.

• Type I fibers, or slow twitch fibers, have slow contractile velocity and produce a low amount of force. They are also slow to fatigue and can be employed for long periods of time.

• There are two types of Type II, or fast twitch, fibers: Type II A and Type II B. Type II A fibers have a fast contractile velocity and are resistant to fatigue. Type II B fibers have a low myoglobin content, contract rapidly, are able to generate great force, and fatigue quickly.

Key Terms

Contractile apparatus: Comprised of the sarcomere, muscle fiber, fasciculus, and (ultimately) muscle.

Myosin: A fibrous protein that forms (together with actin) the contractile filaments of muscle cells and is also involved in motion in other types of cells.

Actin: A protein that forms (together with myosin) the contractile filaments of muscle cells, and is also involved in motion in other types of cells.

Myosin head: Binds to thin filamentous actin, and uses ATP hydrolysis to generate force and “walk” along the thin filament.

Troponin: A complex of three regulatory proteins that is integral to muscle contraction in skeletal and cardiac muscle, or any member of this complex.

Tropomyosin: A protein involved in skeletal muscle contraction and that wraps around actin and prevents myosin from grabbing it.

Z line: Neighbouring, parallel lines that define a sarcomere.

I band: The area adjacent to the Z-line, where actin myofilaments are not superimposed by myosin myofilaments.

A band: The length of a myosin myofilament within a sarcomere.

H zone: The area adjacent to the M-line, where myosin myofilaments are not superimposed by actin myofilaments.

M line: The disc in the middle of the sarcomere, inside the H-zone.

Myofibril: Any of the elongated contractile threads found in striated muscle cells.

Myocyte: A muscle cell.

Sarcoplasm: The cytoplasm of a myocyte.

Sarcoplasmic reticulum: The equivalent of the smooth endoplasmic reticulum in a myocyte.

Sarcolemma: The cell membrane of a myocyte.

Sarcomere: The functional contractile unit of the myofibril of a striated muscle.

Multinucleate: Eukaryotic cells that have more than one nucleus per cell

Fasiculus: A slender bundle of anatomical fiber.

Motor neuron: A nerve cell forming part of a pathway along which impulses pass from the brain or spinal cord to a muscle or gland.

Motor end plate: The point of junction of a motor nerve fiber and a muscle fiber.

Neuromuscular junction: A chemical synapse between a motor neuron and a muscle fiber.

Acetylcholine: A compound which occurs throughout the nervous system, in which it functions as a neurotransmitter.

T-tubules (transverse tubules): Extensions of the cell membrane that penetrate into the center of skeletal and cardiac muscle cells.

Sliding filament theory: A proposed mechanism of muscle contraction in which the actin and myosin filaments of striated muscle slide over each other to shorten the length of the muscle fibers.

Cross-bridge: Refers to the attachment of myosin with actin within the muscle cell.

Motor unit: Made up of a motor neuron and the skeletal muscle fibers innervated by that motor neuron’s axonal terminals.

Motor unit pool: Consists of all individual motor neurons that innervate a single muscle.

Henneman’s Size Principle: Motor units are generally recruited in order of smallest to largest (from fewest fibers to most fibers) as contraction increases.

Contractile velocity

Type I/Slow Twitch fibers: Of, relating to, or being muscle fiber that contracts slowly especially during sustained physical activity requiring endurance.

Type II/Fast Twitch fibers: Of, relating to, or being muscle fiber that contracts quickly especially during brief high-intensity physical activity requiring strength.

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