Motor neurons: definition, types and pathologies

Our brain controls and allows our movements. While this may sound like a very reductive description, it is still real. Our nervous system, in which the brain is located, is responsible for sending signals to all the muscles in our body to move.

To be more exact, these signals are sent by motor neurons or motor neurons. Through what we can walk, breathe, sneeze or have our heart can.

    What are motor neurons?

    Motor neurons, also called motor neurons, are a collection of neurons in the central nervous system whose main task is to send a series of nerve impulses to muscles or glands. these neurons they are found in the brains of all vertebrate species. In humans, if they are located in particular in the spinal cord and in Brodman zone 4.

    Motor neurons are considered efferent neurons because they are responsible for sending information from these regions to other muscles in the body; unlike afferent or sensory neurons which perform the opposite path, sending information from the muscles to the rest of the nervous system.

    The purpose of this transmission of nerve impulses is to exert control over the skeletal muscles and smooth muscles that make up the organs and glands. In other words, thanks to motor neurons, we are able to perform any type of movement, just as our organs are able to function.

    However, to perform these functions, motor neurons need information sent to them by sensory or efferent neurons. Because to be able perform muscle movements appropriate to the situationOur brain must receive information from the outside. Hence the need for both types of neurons to function accordingly.

    In this way, our nervous system integrates information from both types of neurons and allows us to move and react according to the demands and circumstances of our external context.

    Although passive neurons have traditionally been viewed as channels for transmitting information in motor neurons, some results obtained in recent studies suggest that these nerve cells have a much more complex operating dynamic, To be able to produce behaviors or motor models by themselves.

      Motor neurons and motor units

      Instead of each neuron, they aim to activate a specific muscle fiber in order to perform a certain movement, each of these junctions is called motor units. These functional units can be divided into several types:

      1. Slow motor units (S or slow)

      In this type of motor unit, neurons stimulate small muscle fibers, also called red fibers, which make very slow contraction movements.

      These types of fibers tolerate fatigue and fatigue very well, so they are particularly suitable for maintaining muscle contraction or posture without getting tired. For example, they help us get up without getting tired.

      2. Motor units of rapid fatigue (FF or rapid fatigue)

      In this second case, the fibers involved are the white fibers, which are responsible for the innervation of larger muscle groups. Compared to slow motor units, fast fatigue motor units have very short reaction times but deplete their energy faster and therefore tire much sooner.

      These motor units are extremely efficient at performing movements that require rapid bursts of energy, like jumping or running.

        3. Fast Fatigue Resistant Motors

        Finally, these latter types of motor units are halfway between the two previous groups. Although they perform their function on muscles of average size, its reaction time is slower than in FF units and have the ability to tolerate fatigue longer.

        Types of motor neurons

        As mentioned above, each neuron plays a key role in activating a particular fiber or tissue; thus a classification of the different types of neurons can be made according to the tissue on which they exert their influence.

        1. Somatic motor neurons

        These types of motor neurons act on skeletal muscles, so that they have a transcendental role in locomotor skills.

        These skeletal muscles are made up of striated fibers, which make up the bulk of body mass and stand out from the rest by being muscles that can be moved at will.

        In addition, in this group of somatic motor neurons we can find two other subgroups. The first of these subgroups is used to classify neurons according to their position, while the second divides them according to the fibers to which they connect.

        Classification by position

        • Superior motor neuron: These neurons are located along the entire cerebral cortex and their nerve endings are arranged to form a pyramidal pathway connected to the spinal cord.
        • Lower motor neuron: In this case, the neurons are arranged forming circuits, located in the anterior horn of the spinal cord, which intervene in the reflex movements and the involuntary movements.
        Classification according to fibers
        • Alpha motor neurons: These are the largest motor neurons and their main function is to activate the extrafusal fibers. In other words, all of those fibers that make up skeletal muscles. Thanks to them, we can generate the force necessary to contract and move our muscles.
        • Beta motor neurons: These neurons connect to both skeletal muscle fibers and fibers outside the muscle spindle (intrafusal) and are responsible for receiving sensory information.
        • Gamma motor neurons: Finally, gamma motor neurons are only responsible for the innervation of intrafusal fibers; regulating the sensitivity to contraction and helping to maintain muscle tone.

        2. Visceral motor neurons

        Visceral motor neurons are responsible for the innervation of all those muscle fibers that we cannot move voluntarily; that is, smooth musculature. This musculature controls, for example, the movements of our heart, viscera and intestines, etc.

        In order to fulfill their function, visceral motor neurons also synchronize with neurons in the ganglia of the autonomic nervous system, send signals to the organ concerned and innervate the visceral musculature.

        3. Special visceral motor neurons

        The sole mission of this latter group of neurons is to activate muscles in the face and neck, called gill muscles.

        associated pathologies

        There are a number of diseases or pathologies of neurological origin which are distinguished by a progressive degeneration of motor neurons, with different symptoms depending on whether the affected neurons are higher or lower.

        Diseases in which degeneration of the upper motor neurons is experienced are characterized by general muscle weakness. When the affected motor neurons are lower, the person may experience muscle tension, stiffness, and overactive reflexes that cause involuntary muscle contractions.

        Some of the diseases related to motor neuron degeneration are:

        • Progressive bulbar palsy.
        • Pseudobulbar paralysis.
        • Amyotrophic lateral sclerosis (IT HAPPENED).
        • Primary lateral sclerosis.
        • Progressive muscle wasting.
        • Spinal muscular atrophy.
        • Postpolio syndrome.

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