Читать книгу Musculoskeletal Disorders - Sean Gallagher - Страница 77

Skeletal muscle is considered as a voluntary muscle because it can be made to contract by conscious controls. In order for a skeletal muscle fiber to contact, it must first be stimulated by an impulse (an action potential) from a motor nerve cell. The neurons that stimulate muscles to contract are somatic (of the body) motor neurons and have cell bodies that originate in the spinal cord or brainstem and have nerve processes (axons) that extend into the periphery to muscles (see Chapter 4). In skeletal muscle, myelinated somatic motor nerves branch into the perimysium and divide into several terminal smaller branches that innervate the muscle fibers. At the site of innervation, the axon loses its myelin sheath and forms a dilated terminal situated within a trough on the muscle cell surface, the neuromuscular junction (also known as a motor end plate) (Figure 3.11). Within this axon terminal are numerous synaptic vesicles containing the neurotransmitter acetylcholine. When a somatic neuron fires and the action potential reaches the motor end plate, acetylcholine is released into a synaptic cleft (a space between the axonal terminal and the muscle). The synaptic cleft is a highly folded region of the sarcolemma that allows for more surface area. There are numerous mitochondria, ribosomes, and glycogen granules at this site. When acetylcholine binds to its receptor, Na⁺ channels open and membrane depolarization results. Excess acetylcholine in the synaptic cleft is hydrolyzed by the enzyme cholinesterase to avoid prolonged contact of the neurotransmitter with its receptors. The depolarization is then propagated deep into the myofiber by the previously discussed transverse tubule system. At each triad (a T‐tubule and two cisterna), the depolarization signal is passed to the sarcolemma reticulum. This results in a release of Ca²⁺ and initiation of the muscle cells’s contraction cycle. When the depolarization ceases, the Ca²⁺ is actively transported back into the cisterna for storage, and the muscle cell relaxes.

Figure 3.10 The sliding‐filament mechanism.

Tortora, G. J., & Derrickson, B. H. (Eds.), (2010). Muscle. In Introduction to the human body, 11th ed., Wiley.