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Neuromuscular Junction and Neuromuscular Transmission



Anatomy of Neuromuscular Junction

  • Neuromuscular junction is a junction between a motor neuron and a muscle.


The cell body of a motor neuron is located in the ventral horn of the spinal cord

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Near the muscle, its axon branches into multiple processes

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Each axon process innervates a separate muscle fiber midway along the length of the fiber

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The junction between the axon process and a muscle fiber is called the neuromuscular junction or motor endplate.

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Here the axon process further branches like a tree into multiple bulb-shaped presynaptic terminals called buttons

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The buttons finally contact the muscle.

  • Motor Unit: The assembly of all muscle fibers innervated by one motor neuron is called the motor unit.

Presynaptic Terminal

  • is the nerve terminal of the motor neuron.

  • Contains large number of synaptic vesicles → store neurotransmitter acetylcholine.

  • Also contains abundant mitochondria → supply ATP required for synthesis of acetylcholine.

  • The membrane contains voltage-gated Ca channels.

Postsynaptic Membrane

  • is the membrane of muscle fiber.

  • Contains acetylcholine receptors which are basically ligand-gated Na channels.

  • have extensive foldings called postjunctional folds → increase surface area of postsynaptic region → more receptors can be accommodated in small area.

Synaptic Cleft

  • is the space between the presynaptic nerve terminal and the postsynaptic membrane.

  • is filled with meshwork of proteins and proteoglycans.

  • contains an enzyme acetylcholinesterase → destroy acetylcholine.


Physiology of Neuromuscular Transmission

Synthesis of Acetylcholine

Substrates for synthesis of acetylcholine are: acetyl CoA and choline

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An enzyme choline acetyltransferase catalyzes transfers of acetyl group from CoA to choline

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acetylcholine is formed in the cytosol

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Acetylcholine-Hydrogen Exchanger transports the acetylcholine from cytosol into the synaptic vesicle

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Acetylcholine is stored inside the vesicle till nerve terminal receives impulse.


Neurotransmission

Impulse opens the voltage-gated Ca channel on presynaptic membrane

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Ca ions diffuse into the nerve terminal

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Causes exocytosis of synaptic vesicles

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Acetylcholine is released into the synaptic cleft

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It binds with the acetylcholine receptor (a type of Na channel)

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Opens the Na channel

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Na ions diffuse into the muscle fiber

↓ Generation of action potential on postsynaptic membrane

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Muscle contraction.


Termination of Acetylcholine Action

This can occur through two mechanisms:

1. Acetylcholinesterase enzyme breaks acetylcholine into acetate and choline → choline is transported back into the nerve terminal → reutilized for synthesis of acetylcholine.

2. Diffusion of acetylcholine out of the synaptic cleft

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Termination of acetylcholine action

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Prevents continued stimulation of the receptors

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Muscle relaxes.

 

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