Chapters 3- Neurophysiology basics 06. The neuromuscular junction

06. The neuromuscular junction

yellow-warning1Article automatically translated : We'll be reviewing and optimising it soon ...



Body movements are done through the muscles. By contracting, the muscle is reduced in length and thus comes closer to the two bones to which it is inserted. However, it is the nervous system that controls the muscle contraction by nerves.

Each nerve contains thousands of nerve fibers [ 51 , ​​75 ] organized in dendrites that convey sensory information and sensory axons (extensions of neurons) that convey motor impulses.

1. The power unit:

jnm2 Each motor neuron innervates several muscle fibers, the association set called (a [power unit 41 , 54 , 107 , 109 ]).

In general, there is less muscle fibers in a motor unit is more precise movement. In the temporal muscle for example, there are about 1,000 muscle fibers per motor unit [ 1 , 3 ], whereas for external ocular muscles there are only five, reflecting the degree of accuracy eye movements [ 4 , 41 ].

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

2. The neuromuscular junction:

A neuron gives several endings that sometimes scatter throughout the thickness of a muscle, every ending is intended to stimulate a single muscle fiber in a specific place: the neuromuscular junction [ 2 , 4 , 54 ].

2.1. The button terminal:

Just before the axon terminal, the neuron loses its myelin sheath and forms a terminal button. It contains many mitochondria providing energy intake and several synaptic vesicles. Each vesicle contains about 10,000 molecules of acetylcholine [ 4 , 100 , 136 ] (exclusive neurotransmitter of the neuromuscular junction).


2.2. Endplate:

jnm4 Side of the muscle fiber, we find (The driving plate) which is the area directly opposite the terminal button. Although these two regions (synaptic button and driving plate) are very close to each other, there is no real contact between the two.

Endplate [ 39 , 109 , 135 ], thick and electrically non-excitable, form junctional folds that increase the surface area of synaptic contact.

3. Process:

jnm5 Once in the nerve terminal, the motor impulse causes the opening of calcium channels, which triggers a massive influx of calcium ions inside the cell. Calcium promotes the fusion of the vesicles with the cell membrane acetylcholine [ 39 , 57 ] thereby releasing their entire content in this neurotransmitter in the synaptic cleft.

The molecules then diffuse acetylcholine across substantially accumulated at the folds cholinergic receptors.

The binding of two molecules of ACh to a receiver [ 5 , 100 , 113 , 136 ] causes the opening of the sodium channel which favors the entry of sodium ions within the muscle fiber and depolarizer the postsynaptic membrane and creating a potential plate [ 4 ].

jnm6 Depending on the number of activated receptors, this potential may exceed a threshold value and thus trigger a muscle action potential which will diffuse to the entire muscle membrane and cause contraction of the muscle fiber.

There may be a small spontaneous release of ACh by exocytosis into the synaptic space without any nerve stimulation. However, the number of activated receptors is far and trigger a muscle action potential [ 136 ].

4. Elimination of Acetylcholine:


The molecules of acetylcholine are rapidly destroyed by the enzyme (acetylcholinesterase [ 1 , 12 ]) present at the synaptic cleft. This lysis will give two molecules: acetate, and choline will reach the nerve endings to form new molecules of acetylcholine.

The rapid destruction of acetylcholine and prevents the extension of the muscle contraction.

Chapters 3- Neurophysiology basics 06. The neuromuscular junction