Neuromuscular junction

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Neuromuscular junction

Skeletal or striated muscle cells contract in response to nerve stimulation. The motor neuron that carries information from the central nervous system to the muscle contracts a muscle cell in a structure called the motor endplate. This endplate consists of:

1. Presynaptic terminal (of a motor neuron).
2. Postsynaptic membrane (muscle fiber innervated by the motor neuron) where neurotransmitter receptors are clustered on the crests of the folds.
3. Synaptic space separating the presynaptic terminal of the muscle fiber.

The plasma membrane of the neuron is polarized, because the ions are distributed differently on both sides of its sides. Inside there are K ⁺ ions, but also large proteins negatively charged so the plasma membrane is negatively charged. On the other hand, the outer part of the membrane is positively charged by the predominance of Na ⁺ ions.

The potential difference between the inner and the outer part of the membrane is called resting potential and equals to -70 mV. This variation between the exterior and interior is achieved by the operation of the pump of sodium/potassium (Na ⁺ /K ⁺ ) that ejects 3 sodium ions into the neuron and introduces two potassium ions from the surroundings. Sodium ions can not re-enter the neuron, because the membrane is impermeable to sodium.

When the nerve impulse reaches a neuron, the membrane is depolarized resulting on the opening of sodium channels. The interior becomes electropositive with respect to the outer part and whether the potential change is sufficient (+110mV) it turns out that the potential action which creates the necessary conditions for the release of the neurotransmitter into the synaptic gap is reached. The nature of nerve impulses is electrochemical since in most animals, neurons do not contact each other physically, nor with the muscle fibers, hence the need for a chemical transmitter.

The neurotransmitter will be responsible for depolarizing the membrane of the next neuron or, as in our case, the muscle fiber membrane or sarcolemma, opening the channels for sodium that remained closed.

Once the neuron emits a nerve impulse, it must return to the initial resting potential. In this case, sodium channels are closed, the neurotransmitter is destroyed by enzymatic action and the resting potential is reached by the action of the Na ⁺ / K ⁺ pump.

The muscle cell is a very specialized cell. The nomenclature of its parts is also specific:

• Plasma membrane: sarcolemma
• Cytoplasm: sarcoplasm
• Set of mitochondria: Sarcosome
• Smooth endoplasmic reticulum: sarcoplasmic reticulum
• Sarcomere: microscopic contractile element of the muscle fiber having sets of thin or actin fibers and thick or myosin fibers.

Etymology Sarc-/- sarc- comes from Greek and means meat.

There are three types of muscle tissue that differ in the structure of their cells, their location in the body, function, and how are activated for contraction.

1. Striated or skeletal muscle tissue: is attached to bones by tendons (although it also appears in certain organs such as the tongue and the eyeball). They are cells with several nuclei located in the periphery and have a cross-striations in their cytoplasm. It is a tissue that contracts quickly and voluntarily (Figure 1a).
2. Cardiac muscle tissue: it forms the heart walls. They have a unique central core and are bifurcated cells. They also have transverse striations. It is a tissue that contracts quickly and involuntarily (Figure 1b).
3. Smooth muscle tissue: long, spindle-shaped cells with a nucleus in central position. It lacks striations in the cytoplasm hence to be described as smooth. It has slow, involuntary contraction. It is found in the body structures that do not require voluntary movements such as the digestive system, some glands, blood vessels, uterus… (Figure 1c).