Neuromuscular junction: the bridge between neuron and muscle

Something apparently as simple as moving a hand or a foot may not seem like a big deal, but the truth is that to be able to make the slightest movement it is necessary to put it on. a large number of processes are underway, ranging from movement planning to carrying it out and that require the involvement of a large part of the nervous system central.

The last step that the nerve impulse follows to be able to produce a movement is to transmit the information sent by the neurons to the muscle, a fact that occurs in the so-called neuromuscular junction or plaque. In this article we are going to see and briefly analyze what this board is and how it works.

• Related article: "Parts of the Nervous System: anatomical structures and functions"

Neuromuscular junction: definition and main elements

We understand by neuromuscular plate the connection established between muscle fibers (usually skeletal) and neurons that innervate them.

Also known as the neuromuscular junction, the neuromuscular plate is not a single structure but is considered as such as the union of various elements that make up a functional unit. Within these elements, three main defined parts stand out.

First we find the motor neuron, from the spinal cord, through which the information and bioelectric signals from the nervous system will arrive.

The second major element is the muscle union, formed by one or more muscle fibers whose membrane or sarcolemma has receptors that are affected by different substances and that will react to the neural signal by contracting. Finally, among them we find the synaptic space, through which the substances secreted by the motor neuron will travel to the muscle in order to stimulate it.

In this union, the main neurotransmitter involved, the one that causes the receptors of the muscle plate to be activated in such a way that the muscles contract, is the acetylcholine. Its main receptors are muscarinic and nicotinic, the latter being the most frequent at the neuromuscular junction.

Basic functioning: muscle contraction

The process by which a muscle contracts or relaxes, once at the level of the neuromuscular plate, is the following. First, the nerve impulse that has traveled through the nervous system to the motor neuron reaches the terminal buttons of the axon of this.

Once there, the electrical signal generates the activation of voltage-gated calcium channels, entering said calcium in the neuron and allowing acetylcholine to be released and secreted into space through exocytosis synaptic.

This acetylcholine will be captured by the nicotinic receptors present in the sarcolemma of the muscle fiber, which in turn generates the opening of ionic channels. Through these channels, a large amount of sodium ions enter the muscle membrane, ** which generate a membrane depolarization ** which will ultimately cause muscle cells to open channels for the calcium.

This calcium allows the activation of proteins that are part of the muscles, like actin and myosin, that move over each other (actin glides over myosin) causing muscle contraction.

• You may be interested: "What is neuronal depolarization and how does it work?"

Disorders and problems derived from alterations in the neuromuscular plate

The process that muscles follow to contract and relax is essential in allowing the body to move. However, sometimes we can find that the neuromuscular plate can be damaged or affected by different circumstances, generating different difficulties in motor control. Some of the main disorders derived from this fact are the following.

1. Myasthenia gravis

Myasthenia is a disorder in which the immune system itself attacks the neuromuscular junction, causing inflammation of the postsynaptic acetylcholine receptors.

Its main symptom is the presence of muscle weakness that greatly hinders the possibility of perform movements, also decreasing the ability to contract the muscles and the force with which this is makes. This disorder affects all types of muscles, can affect the ability to chew or even breathe. The ability to move worsens with physical activity.

2. Botulism

Another relevant disorder in which symptoms are largely due to neuromuscular plaque problems is botulism. In this disease an alteration is generated due to the presence of botulinum toxin (which is usually introduced into the body through the consumption of food in poor condition) that prevents acetylcholine from attaching itself to other substances that allow its excretion from the membrane presynaptic.

In this way, acetylcholine cannot get out, which prevents its action in the muscle. The symptoms of this disease are the progressive weakening of the muscles of the body, generally in the face-caudal direction. It can cause death if it is not treated in time.

3. Lambert-Eaton syndrome

A disease in which the immune system affects the calcium pathways present in motor neurons. This causes it to block and hinder the emission of acetylcholine in the synaptic space, which ends up generating a high level of fatigue and muscle weakness, both voluntary and neurovegetative. Strength level improves with physical activity, and alterations such as hypotension may appear.

4. Paraneoplastic syndromes

Other disorders related to the neuromuscular junction (although in this case it is not something specific to it) We find among some of the paraneoplastic syndromes, a set of disorders derived from the presence of some type Of cancer. The presence of tumor cells It can cause the components of the neuromuscular junction to degenerate and die, causing a weakening of the ability to use muscles. Among them, necrotizing myopathy stands out.

Bibliographic references:

• Díaz-Manera J, Rojas R, Illa I. (2008). Neuromuscular junction disorders. In: Pascual J (Ed), Treatise on clinical neurology, (pp 879-909). Barcelona: Ars Medica.
• Rodríguez, J. & Pedroza, A. (2013). Neuromuscular plaque diseases. Rosario University.
• Rosich-Estragó, M. (2000). Paraneoplastic diseases of the motor endplate and muscle. Rev. Neurol., 31: 1225-1228.
• Sanders, D & Howard, J (2011). Disorders of Neuromuscular Transmission; In Bradley, W. (2011). Neurology in Clinical Practice, Chapter 82.