Neuromuscular Junction

Synaptic Transmission: Neuromuscular Junction

Active zone

Schwann cell process

Acetylcholine receptor sites

Active zone

Schwann cell process

Acetylcholine receptor sites

Basement membrane

Sarcolemma

Nucleus of muscle cell

The Neuromuscular Junction Myofibril

Postsynaptic membrane

Figure 2.5 Structure of the Neuromuscular Junction

Basement membrane

Sarcolemma

Nucleus of muscle cell

Myofibrils Synaptic cleft

Postsynaptic membrane

Junctional fold Sarcoplasm Acetylcholine receptor sites

Figure 2.5 Structure of the Neuromuscular Junction

Motor axons that synapse on skeletal muscle form expanded terminals called neuromuscular junctions (motor endplates). The motor axon loses its myelin sheath and expands into a Schwann cell-invested synaptic terminal that resides within a trough in the muscle fiber. Acetylcholine-containing synaptic vesicles accumulate adjacent to the presynaptic membrane and, when appropri ately stimulated, release their neurotransmitter into the synaptic cleft. The transmitter then binds to receptors that mediate depolarization of the muscle sarcolemma and initiate a muscle action potential. A single muscle fiber has only one neuromuscular junction, but a motor axon can innervate multiple muscle fibers.

Synaptic Transmission: Visceral Efferent Endings

A. Smooth muscle

Serous cells

Parasympathetic terminal ending

Figure 2.6 Visceral Efferent Endings

Neuronal efferent endings on smooth muscle (A) and glands (B and C) exhibit unique endings unlike the presynaptic and postsynaptic terminals observed in neuronal and neuromuscular junction synapses. Rather, neurotransmitter substances are released into interstitial spaces (A and B) or into the bloodstream (C, neu-

neurophysiology

C. Neurosecretory (posteriorpituitary)

Schwann cell cap enclosing nerve axons

Varicosity

rosecretion) from expanded nerve terminal endings. This arrangement allows for the stimulation of numerous target cells over a wide area. Not all smooth muscle cells are innervated. They are connected to adjacent cells by gap junctions and can therefore contract together with the innervated cells.

B. Gland (submandibular)

Mucous cells

Schwann cell cap enclosing nerve axons

Sympathetic terminal ending

Varicosity

Varicosity

Varicosities Smooth Muscle

Serous cells

Axon

Smooth muscle cells (cut)

Schwann cell cap enclosing nerve axons nerve axons

Spinal Nerve
Schwann cell cap Smooth muscle cells

Varicosities

Terminal endings

Pituicyte processes

Axon

Axon

Axon

Axon

Axon

Neuromuscular Junction

Neurosecretory vesicles

Collagen space Basement membrane

Capillary Endothelium Mast cell

Neurosecretory vesicles

Collagen space Basement membrane

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