Motor neurons: definition, types and pathologies

These cells are responsible for our ability to move and maintain our vital signs.

Our brain controls and enables our movements. Although this may seem a very reductionist description, it is nonetheless true. Our nervous system, which includes the brain, is responsible for sending signals to all the muscles in our body so that they can move.

To be more precise, these signals are sent by motor neurons or motoneurons.. Thanks to which we can walk, breathe, sneeze or get our Heart to beat.

What are motor neurons?

Motor neurons, also known as motoneurons, are a group of neurons of the central nervous system whose main mission is to send a series of nerve impulses to the muscles or glands. These neurons are found in the brains of all vertebrate species.. In the human species, they are especially located in the spinal cord and in Brodman's area 4.

Motor neurons are considered efferent neurons, since they are responsible for sending information from these regions to the rest of the body's muscles, as opposed to afferent or sensory neurons, which carry out the opposite route, sending information from the muscles to the rest of the nervous system.

The purpose of this transmission of nerve impulses is to exercise control over the skeletal musculature and the smooth musculature that constitutes the organs and glands. That is, thanks to motor neurons we are able to perform any type of movement, just as our organs are able to function correctly.

However, in order to carry out these functions, the motor neurons need the information sent to them by the sensory or efferent neurons. Since in order to be able to to be able to perform the muscle movements appropriate to the situation, our brain must receive information from the outside.our brain must receive information from the outside. Hence the need for both types of neurons to work in harmony.

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

Although motor neurons have traditionally been considered passive channels for the transmission of information, some results obtained in recent studies point to the idea that these nerve cells have much more complex functioning dynamics, being able to produce motor behaviors or patterns by themselves.being able to produce behaviors or motor patterns by themselves.

Motor neurons and motor units

Since the purpose of each neuron is to activate a specific muscle fiber in order to carry out 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 units, neurons stimulate small muscle fibers, also called red fibers, which perform very slow contraction movements.

These fibers tolerate fatigue and tiredness very well and are therefore particularly suitable for maintaining a muscular contraction or posture without becoming fatigued. For example, help us stand upright without tiring us out.

2. Fast fatiguing motor units (FF or fast fatiguing)

In this second case, the fibers involved are the white fibers, which are responsible for innervating larger muscle groups. Compared to slow motor units, fast fatiguing motor units have very short reaction times but use up their energy more quickly and therefore tire much sooner.

These motor units are extremely efficient for performing movements that require rapid bursts of energy, such as jumping or running..

3. Fatigue-resistant fast motor units

Finally, this last type of motor units is halfway between the two previous groups. Although they exercise their function on medium-sized muscles, their reaction time is slower, their reaction time is slower than in FF units and have the ability to tolerate fatigue for a longer time.

Types of motor neurons

As mentioned above, each neuron has a fundamental role in the activation of a specific fiber or tissue; therefore, a classification of different types of neurons can be made according to the tissue on which they exert their influence.

1. Somatic motor neurons

This type of motor neurons act on the skeletal musculature, so that they play a transcendental role in have a transcendental role in locomotor skills..

These skeletal muscles are formed by striated fibers, which make up most of the body mass and are distinguished from the rest because they are muscles that we can move at will.

In addition, within this group of somatic motor neurons we can find two more subgroups. The first of these subgroups serves to classify neurons according to their position, while the second divides them according to the fibers to which they are connected.

Classification according to position

• Upper motor neuronThese neurons are located throughout the cerebral cortex and their nerve endings are arranged to form a pyramidal pathway connected to the spinal cord.
• Lower motor neuronIn this case, the neurons are arranged in circuits, located in the anterior horn of the spinal cord, which are involved in reflex movements and involuntary movements.

Classification according to fibers

• Alpha motor neuronsalpha motor neurons: these are the largest motor neurons and their main function is to activate the extrafusal fibers. That is to say, all those fibers that make up the skeletal musculature. Thanks to them we can generate the force necessary to contract and move our muscles.
• Beta motor neuronsThese neurons are connected both to fibers of the skeletal musculature and to fibers outside the muscle spindle (intrafusal) and are responsible for receiving sensory information.
• Gamma motor neuronsFinally, the gamma motor neurons are only responsible for innervating the intrafusal fibers; regulating the sensitivity to contraction and helping to maintain muscle tone.

2. Visceral motor neurons

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

In order to carry out their function, visceral motor neurons also synapse with neurons in the ganglia of the autonomic nervous system, sending signals to the relevant organ and innervating the visceral musculature..

3. Special visceral motor neurons

This last group of neurons has the sole mission of activating the musculature present in the face and neck, known as gill musculature.

Associated pathologies

There are a series of diseases or pathologies of neurological origin that are distinguished by a gradual degeneration of the motor neurons, presenting a different symptomatology depending on whether the affected neurons are upper or lower..

Those diseases in which there is degeneration of the upper motor neurons are characterized by a general muscular weakening. When the affected motor neurons are the lower motor neurons, the person may suffer from muscle tension, stiffness and hyperactivity of reflexes causing involuntary muscle contractions.

Some of the diseases related to motor neuron degeneration are:

• Progressive bulbar palsy.
• Pseudobulbar palsy.
• Amyotrophic lateral sclerosis (ALS). (ALS).
• Primary lateral sclerosis.
• Progressive muscular atrophy..
• Spinal muscular atrophy.
• Post-polio syndrome.