Page 306 - Atlas of Histology with Functional Correlations

vesicles to fuse with the plasma membrane of the axon and release the

acetylcholine into the synaptic cleft, a small gap between the axon terminal
and cell membrane (sarcolemma) of the muscle fiber. The neurotransmitter
then diffuses across the synaptic cleft, combines with acetylcholine
receptors on the cell membrane of the muscle fiber, and stimulates the
muscle to contract. An enzyme called acetylcholinesterase, located in the

basal lamina of the synaptic cleft, inactivates or neutralizes the released and
excess acetylcholine. Inactivation of acetylcholine is necessary in order to
prevent further muscle stimulation and muscle contraction until the next

impulse arrives at the axon terminal.

Contraction of Skeletal Muscles

Before the arrival of the nerve stimulus to the muscle, the muscle is
relaxed, and calcium ions are stored in the cisternae of the sarcoplasmic
reticulum bound to protein calsequestrin. Muscle contractions depend on
the availability of calcium ions. After the arrival of the nerve stimulus and

the release of the neurotransmitter at the motor endplates, the sarcolemma is
depolarized, or activated. The stimulus signal (action potential) is propagated
along the entire length of the sarcolemma and rapidly transmitted deep to

every myofiber by the network of the T tubules that surround each
sarcomere at the A–I junction. Expanded terminal cisternae of the
sarcoplasmic reticulum and T tubules form triads. At each triad, the action
potential is transmitted from the T tubules to every myofiber and myofibril as
well as the sarcoplasmic reticulum membrane. After stimulation, cisternae of

the sarcoplasmic reticulum in each myofibril release calcium ions into the
individual sarcomeres and the overlapping thick and thin myofilaments of the
myofibril. Calcium ions activate binding between actin and myosin, which

results in their sliding past each other, causing muscle contraction and muscle
shortening. When the stimulus subsides and the membrane is no longer
stimulated, calcium ions are actively transported back into and stored in the
cisternae of the sarcoplasmic reticulum, causing muscle relaxation.

Nearly all skeletal muscles contain sensitive stretch receptors called

neuromuscular spindles. These spindles consist of a connective tissue
capsule that contains modified muscle fibers called intrafusal fibers and
numerous nerve endings, surrounded by a fluid-filled space. The muscles
surrounding the neuromuscular spindles are called the extrafusal fibers. The

neuromuscular spindles monitor the changes (distension) in muscle length

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