Something apparently as simple as moving a hand or a foot may not seem like much, but the truth is that to be able to perform even the slightest movement it is necessary to set in motion a large number of processes, which range from planning the movement to its execution and which require the involvement of a large part of the central nervous system.
The last step that follows the nerve impulse in order to produce a movement is to transmit the information sent by the neurons to the muscle, a fact that occurs in the so-called plate or neuromuscular junction . In this article we are going to see and briefly analyse what this plate is and how it works.
Neuromuscular junction: definition and main elements
We understand by neuromuscular plate the connection established between the muscle fibres (generally skeletal) and the neurons that innervate them.
Also known as neuromuscular junction , the neuromuscular plate is not a single structure but is considered as such the union of various elements that make up a functional unit. Within these elements there are three main defined parts.
In the first place we find the motor neuron , coming from the spinal cord, through which the information and bioelectric signals coming from the nervous system will arrive.
The second major element is the muscle bond, 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 neuronal 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 junction the main neurotransmitter involved, the one that activates the muscle plate receptors so that the muscles contract, is acetylcholine. Its main receptors are muscarinic and nicotinic, the latter being the most frequent in the neuromuscular junction.
Basic function: muscle contraction
The process by which a muscle contracts or relaxes, once at the level of neuromuscular plaque, is what follows. First, the nerve impulse that has travelled through the nervous system to the motor neuron reaches the axon end buttons of the motor neuron .
Once there, the electrical signal generates the activation of voltage-dependent calcium channels, bringing the calcium into the neuron and allowing the exocytosis to release and secrete acetylcholine into the synaptic space.
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 enters the muscle membrane,** which generates a depolarization in the membrane** that will ultimately cause the muscle cells to open channels for calcium.
This calcium allows the activation of proteins that are part of the muscles , such as actin and myosin, which move over each other (actin slides over myosin) causing muscle contraction.
Disorders and problems derived from alterations in neuromuscular plaque
The process the muscles follow to contract and relax is fundamental 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, generating an inflammation of the post-synaptic acetylcholine receptors.
Its main symptom is the presence of muscle weakness that makes it very difficult to move, and also reduces the ability to contract the muscles and the strength with which this is done. This disorder affects all types of muscles, and can affect the ability to chew or even breathe . The ability to move worsens with physical activity.
Another relevant disorder in which the symptoms are largely due to problems in the neuromuscular plaque 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 adhering to other substances that allow its excretion from the presynaptic membrane.
In this way, acetylcholine cannot escape, which prevents it from acting on the muscle . The symptoms of this disease are the progressive weakening of the muscles of the body, usually in a face-to-face direction. It can cause death if not treated in time.
3. Lambert-Eaton syndrome
A disease in which the immune system affects the calcium pathways present in motor neurons. This leads to the blocking and impeding of the release of acetylcholine in the synaptic space, which ends up generating a high level of voluntary and neurovegetative muscle fatigue and weakness. The level of strength improves with physical activity , and alterations such as hypotension may appear.
4. Paraneoplastic syndromes
Other disorders linked to the neuromuscular junction (although in this case it is not specific to it) are found among some of the paraneoplastic syndromes, a group of disorders derived from the presence of some type of cancer. The presence of tumour cells can cause the components of the neuromuscular junction to degenerate and die, leading to a weakening of the ability to use the muscles. Necrotizing myopathy is one of these.
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- Sanders, D & Howard, J (2011). Disorders of Neuromuscular Transmission; In Bradley, W. (2011). Neurology in Clinical Practice, Chapter 82.