The neuromuscular junction

Body movements are performed by muscles. By contracting, a muscle shortens and thus manages to bring the two bones to which it is attached closer together. However, it is the nervous system that controls this muscular contraction via the nerves.

Each nerve contains thousands of nerve fibers ^{[51, 75]} organized into sensory axons (peripheral processes) that carry sensory information and motor axons that convey motor impulses

The motor unit :

Each motor neuron innervates several muscle fibers; this association defines what is called a motor unit ^{[41, 54, 107, 109]}.

In general, the fewer muscle fibers there are in a motor unit, the more precise the movement. For example, in the temporal muscle, there are about 1,000 muscle fibers per motor unit ^{[1, 3]}, whereas in the extraocular muscles, there are only five, which reflects the high degree of precision in eye movements ^{[4, 41]}.

As for the intensity of the muscle contraction, it is proportional to the number of motor units recruited.

The neuromuscular junction :

A motor neuron gives off several branches that sometimes spread throughout the entire thickness of a muscle. Each ending is intended to stimulate a single muscle fiber at a very specific location: the neuromuscular junction ^{[2, 4, 54]}.

The terminal button :

Just before the axonal termination, the motor neuron loses its myelin sheath and forms a terminal button. The latter contains many mitochondria, to ensure an energy supply, and several synaptic vesicles. Each vesicle contains approximately 10,000 molecules of acetylcholine (the exclusive neurotransmitter of the neuromuscular junction) ^{[4, 100, 136]}.

The motor end plate :

On the muscle fiber side, there is the motor end plate, which is the area directly opposite the terminal button. Even though these two regions (synaptic knob and motor end plate) are very close to each other, there is no actual physical contact between them.

The motor end plate ^{[39, 109, 135]} , which is thick and electrically non-excitable, forms junctional folds that increase the synaptic contact surface area.

Process :

Upon reaching the nerve ending, the motor impulse triggers the opening of calcium channels, leading to a massive influx of calcium ions into the cell. Calcium promotes the fusion of acetylcholine vesicles with the cell membrane ^{[39, 57]} , releasing their entire neurotransmitter content into the synaptic cleft.

The acetylcholine molecules then diffuse across to the cholinergic receptors, which are primarily concentrated within the folds.

The binding of two ACh molecules to a receptor ^{[5, 100, 113, 136]} triggers the opening of a sodium channel, facilitating the entry of sodium ions into the muscle fiber, thereby depolarizing the postsynaptic membrane and creating an end-plate potential ^[4].

Depending on the number of activated receptors, this potential can exceed a threshold value and thus trigger a muscle action potential that will spread across the entire muscle membrane and cause the muscle fiber to contract.

There may be a minimal release of ACh by spontaneous exocytosis into the synaptic space in the absence of any nerve stimulation. However, the number of receptors activated in this way is far from sufficient to trigger a muscle action potential ^[136].

Elimination of acetylcholine :

Acetylcholine molecules are rapidly broken down by an enzyme (acetylcholinesterase ^{[1, 12]}) present in the synaptic space. This degradation produces two molecules: acetate and choline, the latter of which is taken up to the nerve ending to form new acetylcholine molecules.

The rapid destruction of acetylcholine thus prevents prolonged muscle contraction.