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 even the slightest movement you have to start a lot of processes, from planning the movement to its realization and requiring the involvement of a large part of the central nervous system.

The last step after the nerve impulse in order to be able to produce movement is to transmit the information sent by the neurons to the muscle, so that it occurs in the so-called neuromuscular plaque or junction. In this article, we will briefly review and analyze what this table is and how it works.

    Neuromuscular junction: definition and main elements

    We understand by neuromuscular plaque 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 at the junction of several elements that make up a functional unit. Within these elements, three main defined parts mainly stand out.

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

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

    In this junction, the main neurotransmitter involved, which causes receptors on the muscle plate to activate so that the muscles contract, is acetylcholine. Its main receptors are muscarinic and nicotinic, the latter being the most common in 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 as follows. First, the nerve impulse that passed through the nervous system to the motor neuron he reaches the terminal buttons of his axon.

    Once there, the electrical signal generates the activation of voltage-gated calcium channels, entering said calcium into the neuron and allowing it to release and secrete acetylcholine 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 generates depolarization in the membrane ** which will eventually cause the muscle cells to open channels for calcium.

    This calcium allows the activation of proteins that are part of the muscles, Like actin and myosin, which move on top of each other (actin slides over myosin) causing muscle contraction.

      Disorders and problems resulting from alterations of the neuromuscular plate

      The process that muscles go through to contract and relax is essential for the movement of the body. However, sometimes we may find that the neuromuscular plaque may be damaged or affected by different circumstances, causing various engine control difficulties. Some of the main disorders derived from this fact are as follows.

      1. Heavy myasthenia gravis

      Myasthenia gravis 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 which significantly hinders the ability to perform movements, also decreasing the ability to contract muscles and the strength with which this is done. This disorder affects all types of muscles, it can affect the ability to chew or even breathe. The ability to move worsens with physical activity.

      2. Botulism

      Botulism is another relevant disorder in which symptoms are largely due to problems with neuromuscular plaque. 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) which prevents acetylcholine from adhering to other substances that allow its excretion from the presynaptic membrane.

      This way, acetylcholine cannot come out, 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 the direction of facial flow. It can lead to 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 blockage and hindrance of the release of acetylcholine into the synaptic space, which ultimately generates a high level of fatigue and muscle weakness, both voluntary and vegetative. Strength level improves with physical activity, And changes such as hypotension may occur.

      4. Paraneoplastic syndromes

      Other disorders related to the neuromuscular junction (although in this case it is not something specific) are among some of the paraneoplastic syndromes, a collection of disorders derived from the presence of a certain type of cancer. The presence of tumor cells it can cause the components of the neuromuscular junction to degenerate and die, resulting in a weakened ability to use muscles. Among them, necrotizing myopathy.

      Bibliographical 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. and Pedroza, A. (2013). Diseases of the neuromuscular plaque. University of the Rosary.
      • Rosich-Estragó, M. (2000). Paraneoplastic diseases of the motor plate and muscle. Tower. Neurol., 31: 1225-1228.
      • Sanders, D and Howard, J (2011). Neuromuscular transmission disorders; In Bradley, W. (2011). Neurology in Clinical Practice, Chapter 82.

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