Neuromuscular Junction: The Bridge Between Neuron And Muscle
Something seemingly as simple as moving a hand or a foot may not seem like a big deal, but the truth is that in order to make the slightest movement, it is necessary to implement a large number of processes, ranging from planning the movement to its realization and that require the involvement of a large part of the central nervous system.
The last step that the nervous impulse follows 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 neuromuscular plate or junction In this article we are going to see and briefly analyze what this board is and how it works.
Neuromuscular junction: definition and main elements
We understand a neuromuscular plate the connection established between muscle fibers (usually skeletal) and 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, three large defined parts mainly stand out.
First we find the motor neuron coming from the spinal cord, through which the information and bioelectric signals from the nervous system will arrive.
The second major element is the muscular union, formed by one or several 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 union, the main neurotransmitter involved, the one that causes the muscle plate receptors to be activated in such a way that the muscles contract, is acetylcholine. Its main receptors are muscarinic and nicotinic, the latter being the most frequent at the neuromuscular junction.
Basic operation: muscle contraction
The process by which a muscle contracts or relaxes, once at the level of the neuromuscular plate, is the following. First, the nervous 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-dependent calcium channels, entering said calcium into the neuron and allowing acetylcholine to be released and secreted into the synaptic space through exocytosis.
This acetylcholine will be captured by the nicotinic receptors present in the sarcolemma of the muscle fiber, which in turn generates the opening of ion channels. Through these channels, a large amount of sodium ions enter the muscle membrane,** which generate 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 the neuromuscular plate
The process that muscles follow to contract and relax is essential when it comes to 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.
Myasthenia is a disorder in which the immune system itself attacks the neuromuscular junction, generating inflammation of the postsynaptic acetylcholine receptors.
Its main symptom is the presence of muscle weakness that greatly hinders the ability to perform movements, also reducing the ability to contract the muscles and the force with which this is done. This disorder affects all types of muscles, which can affect the ability to chew or even breathe The ability to move worsens with physical activity.
2. Botulism
Another relevant disorder in which the symptoms are largely due to problems in the neuromuscular plate 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 spoiled food) that prevents acetylcholine from adhering to other substances that allow its excretion from the presynaptic membrane.
Thus, acetylcholine cannot leave, 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 rostro-caudal 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 causes the emission of acetylcholine in the synaptic space to be blocked and difficult, 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 linked to the neuromuscular junction (although in this case it is not something 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 tumor cells It can cause components of the neuromuscular junction to degenerate and die, leading to a weakening of the ability to use the muscles. Among them, necrotizing myopathy stands out.
