University:
Kentucky State University
Course:
BIO 303 | Human Physiology
Academic year:

2020
Views:

64

Pages:

33
Author:

Tarime9wn

Physiology 1 Muscle contraction 15-11-2021 Synapse • A synapse is a region where communication occurs between two neurons or between a neuron and an effector cell (muscle cell or glandular cell). • The term presynaptic neuron refers to a nerve cell that carries a nerve impulse toward a synapse. It is the cell that sends a signal. Synapse • A postsynaptic cell is the cell that receives a signal. It may be a nerve cell called a postsynaptic neuron that carries a nerve impulse away from a synapse or an effector cell that responds to the impulse at the synapse. Neuromuscular junction • Neuromuscular junction (NMJ): the synapse between a somatic motor neuron and a skeletal muscle fiber. • At most synapses a small gap, called the synaptic cleft, separates the two cells. Neuromuscular junction Neuromuscular junction • Because the cells do not physically touch, the action potential cannot “jump the gap” from one cell to another. Instead, the first cell communicates with the second by releasing a chemical messenger called a neurotransmitter. • Acetylcholine (ACh) is the neurotransmitter released at the NMJ. Excitation-contraction coupling • An increase in Ca2+ concentration in the sarcoplasm starts muscle contraction, and a decrease stops it. • A muscle action potential propagates to cause the release of Ca2+ from the SR into the sarcoplasm and this triggers muscle contraction. • The sequence of events that links excitation (a muscle action potential) to contraction (sliding of the filaments) is referred to as excitation–contraction coupling. • In a relaxed muscle, myosin is blocked from binding to actin because strands of tropomyosin cover the myosin- binding sites on actin. • The tropomyosin strands in turn are held in place by troponin molecules. Excitaton-contraction coupling - The sarcoplasmic reticulum releases calcium ions (Ca+2) into the sarcoplasm. - Ca+2 binds to troponin which undergoes a conformational change; this change moves tropomyosin away from the myosin-binding sites on actin. - Myosin head binds actin. Sliding filament mechanism • Muscle contraction occurs because myosin heads attach to and “walk” along the thin filaments at both ends of a sarcomere, progressively pulling the thin filaments toward the M line. • As a result, the thin filaments slide inward and meet at the center of a sarcomere. Sliding filament mechanism • However, the individual lengths of the thick and thin filaments remain unchanged. • Shortening of the sarcomeres causes shortening of the whole muscle fiber, which in turn leads to shortening of the entire muscle. Contraction cycle ATP production Muscle fibers have three ways to produce ATP: • (1) from creatine phosphate (unique to muscle fibers). • (2) by anaerobic glycolysis. • (3) by aerobic respiration. Motor unit • Even though each skeletal muscle fiber has only a single neuromuscular junction, the axon of a somatic motor neuron branches out and forms neuromuscular junctions with many different muscle fibers. • A motor unit consists of a somatic motor neuron plus all of the skeletal muscle fibers it stimulates. • Whole muscles that control precise movements consist of many small motor units. Twitch contraction • A twitch contraction is the brief contraction of all muscle fibers in a motor unit in response to a single action potential in its motor neuron. Twitch contraction • A brief delay occurs between application of the stimulus and the beginning of contraction is termed the latent period. • During the latent period, the muscle action potential sweeps over the sarcolemma and calcium ions are released from the sarcoplasmic reticulum. Twitch contraction • The second phase, the contraction period. During this time, Ca2+ binds to troponin, myosin-binding sites on actin are exposed, and cross-bridges form. Peak tension develops in the muscle fiber. • During the third phase, the relaxation period, Ca2+ is actively transported back into the sarcoplasmic reticulum, myosin-binding sites are covered by tropomyosin, myosin heads detach from actin, and tension in the muscle fiber decreases. Control of muscle tension The total force or tension that a single muscle fiber can produce depends on: • The rate at which nerve impulses arrive at NMJ. The number of impulses per second is the frequency of stimulation. • The amount of stretch before contraction. • Nutrient and oxygen availability. Refractory period • A muscle fiber temporarily loses its excitability and cannot respond for a time. This period is called the refractory period, it is a characteristic of all muscle and nerve cells. • The duration of the refractory period varies with the muscle involved. Wave summation • When a second stimulus occurs after the refractory period of the first stimulus is over, but before the skeletal muscle fiber has relaxed, the second contraction will actually be stronger than the first. • This phenomenon, in which stimuli arriving at different times cause larger contractions, is called wave summation. Tetanus • When a skeletal muscle fiber is stimulated at a rate of 20 to 30 times per second, it can only partially relax between stimuli. The result is a sustained but wavering contraction called unfused (incomplete) tetanus. • When a skeletal muscle fiber is stimulated at a higher rate of 80 to 100 times per second, it does not relax at all. The result is fused (complete) tetanus, a sustained contraction in which individual twitches cannot be detected. Tetanus • Wave summation and both kinds of tetanus occur when additional Ca2+ is released from the sarcoplasmic reticulum by subsequent stimuli while the levels of Ca2+ in the sarcoplasm are still elevated from the first stimulus. Muscle fatigue • The inability of a muscle to maintain force of contraction after prolonged activity is called muscle fatigue. • Fatigue results mainly from changes within muscle fibers. Even before actual muscle fatigue occurs, a person may have feelings of tiredness and the desire to cease activity; this response, called central fatigue, is caused by changes in the CNS. Muscle fatigue Although the precise mechanisms that cause muscle fatigue are still not clear, several factors are thought to contribute: • Inadequate release of calcium ions from the SR, resulting in a decline of Ca2+ concentration in the sarcoplasm. • Depletion of creatine phosphate also is associated with fatigue. Muscle fatigue • Insufficient oxygen. • Depletion of glycogen and other nutrients. • Buildup of lactic acid and ADP. • Failure of action potentials in the motor neuron to release enough acetylcholine. Muscle tone • Even at rest, a skeletal muscle exhibits muscle tone. • To sustain muscle tone, small groups of motor units are alternatively active and inactive in a constantly shifting pattern. • Muscle tone keeps skeletal muscles firm, but it does not result in a force strong enough to produce movement.

Muscle Contraction

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