NEUROMUSCULAR JUNCTION: PARTS, FUNCTIONS AND PATHOLOGIES - GEOGRAFÍA - 2025

The neuromuscular junction or neuromuscular plate is the synapse between a motor neuron and a muscle. Thanks to the transmitted impulses, the muscle can contract or relax. Specifically, it is the connection between the terminal button of a neuron and the membrane of a muscle fiber.

The terminal buttons of neurons connect to the motor terminal plates. The latter refer to the membrane that receives nerve impulses from a neuromuscular junction.

This type of synapse is the most studied and the easiest to understand. To control a skeletal muscle, a motor neuron (motor neuron) synapses with a cell in this muscle.

Components of the neuromuscular junction

1. The action potential reaches the terminal axon. 2. The voltage-gated calcium channel opens, allowing calcium to enter the terminal axon. 3. Neurotransmitter vesicles fuse with the presynaptic membrane and acetylcholine is released into the synaptic space by exocytosis. 4. Acetylcholine binds to postsynaptic receptors in the sarcolemma. 5. This binding causes ion channels to open and allows sodium ions to flow through the membrane into the muscle cell. 6. The flux of sodium ions across the membrane into muscle cells generates an action potential that travels through myofibers and results in muscle contractions. A: Motor neuron axon. B: Terminal axon. C: Synaptic space. D: Muscle cell. E. Part of a myofibril. Source: User Elliejellybelly13 CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0) via Wikimedia Commons

The neuromuscular junction is made up of the following elements:

A motor neuron (motor neuron)

This neuron is called presynaptic because it emits nerve impulses or action potentials. Specifically, nerve impulses travel through the axon of this neuron to the terminal button that is located very close to the muscle. This termination has an oval shape about 32 microns wide.

In the terminal button are the mitochondria and other elements that allow the creation and storage of acetylcholine. Acetylcholine is the main neurotransmitter for muscle stimulation.

Many authors refer to this element as an alpha motor neuron, as it is a type of neuron whose axon synapses with extrafusal muscle fibers from a skeletal muscle. When activated, it releases acetylcholine, which causes muscle fibers to contract.

Synaptic cleft or synaptic space

The terminal button of the neuron and the muscle membrane are not in direct contact, there is a small space between them.

The motor union

It is made up of one or more muscle cells. These target cells make up a muscle fiber.

Types of muscle fibers

Neuromuscular junction or myoneural junction. Source: Doctor Jana CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0) via Wikimedia Commons

There are different types of muscle fibers. The muscle fibers that innervate at the neuromuscular junction are called extrafusal muscle fibers. They are controlled by alpha motor neurons and are responsible for the force that arises from the contraction of a skeletal muscle.

Unlike these, there are other types of muscle fibers that detect the stretching of a muscle and are parallel to the extrafusal fibers. These are called intrafusal muscle fibers.

A muscle fiber is made up of a bundle of myofibrils. Each myofibril is made up of overlapping filaments of actin and myosin, which are responsible for muscle contractions.

Actin and myosin are proteins that form the physiological basis for muscle contraction.

Myosin filaments have small protrusions called myosin cross-linking bridges. They are the intermediaries between myosin and actin filaments and are the mobile elements that produce muscle contractions.

The parts where actin and myosin filaments overlap are seen as dark bands or streaks. For this reason, skeletal muscles are often called striated muscles.

The myosin crosslinking bridges "row" along the actin filaments so that the muscle fiber shortens, contracting.

How does the neuromuscular junction work?

1. Ion channel receptor 2. Ions 3. Ligand (such as acetylcholine). This is an example of an ion channel receptor. On the left, the channel is closed, because the ligand (dark purple triangle) has not bound to the receptor. When the ligand binds to the receptor, the channel opens and the ions (orange circles) can flow freely through the membrane. Source: Isaac Webb CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0) via Wikimedia Commons

Neuromuscular junctions are located in the grooves across the surface of the muscle fibers. When an action potential or electrical impulse travels through the neuron, its terminal button releases a neurotransmitter called acetylcholine.

When a certain amount of acetylcholine accumulates, it produces the so-called end plate potential in which the muscle membrane depolarizes. This potential is much broader compared to that which occurs between two neurons.

The terminal binding potential always leads to the activation of the muscle fiber, expanding this potential throughout the entire fiber. This causes a contraction or jerk of the muscle fiber.

Depolarization

Depolarization is the reduction of the membrane potential of a cell. When a muscle fiber is depolarized, calcium channels begin to open, allowing calcium ions to penetrate into them. This phenomenon is what causes muscle contraction.

This is because calcium works as a cofactor, which helps the myofibrils extract energy from the ATP that is in the cytoplasm.

A single nerve impulse from a motor neuron results in a single contraction of a muscle fiber. The physical effects of these shocks are much longer than those of an action potential between two neurons.

This is due to the elasticity of the muscle and the time it takes to rid the cells of calcium. In addition, the physical effects of a set of nerve impulses can accumulate, leading to a prolonged contraction of the muscle fiber.

Muscle contraction is not an all or nothing phenomenon, as are the contractions of the muscle fibers that make up the muscle. Rather, the force of the shock is determined by the average discharge frequency of the different motor units.

If at a particular moment, many motor units discharge, the contraction will be more energetic, and if they discharge few, it will be weak.

Pathologies of the neuromuscular junction

Pathologies of the neuromuscular junction can affect the terminal button of the motor neuron, or the membrane of the muscle fibers. For example, botulism produces an alteration and inhibition in the release of acetylcholine, both in the skeletal muscles and in the autonomic nervous system.

It is acquired by consuming contaminated food, mainly. Within a few hours it produces a progressive and rapid muscle weakness.

On the other hand, myasthenia gravis, which is the best-known neuromuscular disease, appears due to the inflammation of acetylcholine receptors. It arises from antibodies that these patients have that attack these receptors.

Its main symptom is weakness of the voluntary skeletal muscles. It is seen mainly in the muscles involved in breathing, salivation, and swallowing; as well as on the eyelids.

Another example of pathology of the neuromuscular junction is Lambert-Eaton syndrome, which consists of an autoimmune disease in which the immune system mistakenly attacks the calcium channels of motor neurons.

This generates an alteration in the release of acetylcholine. Specifically, the propagation of the motor action potential is blocked. Muscle weakness is also observed, in addition to tumors.

References

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