Our brain controls and allows our movements. Although this may seem like a very reductionist description, it is still real. Our nervous system, within which the brain is located, is responsible for sending signals to all the muscles in our body so that they move.
To be more exact, These signals are sent through motor neurons or motoneurons Thanks to which we can walk, breathe, sneeze or make our heart beat.
Motor neurons, also known as motoneurons, are a set of neurons in the central nervous system whose main mission is to send a series of nerve impulses to the muscles or glands. These neurons They are found in the brain of all vertebrate species In the human species, they are located especially 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 muscles in the body; unlike afferent or sensory neurons that carry out the opposite route, sending information from the muscles to the rest of the nervous system.
This transmission of nervous impulses is intended to exert control over the skeletal muscles and the smooth muscles that make up the organs and glands. That is, thanks to motor neurons we are able to perform any type of movement, just as our organs are capable of functioning correctly.
However, in order to carry out these functions, motor neurons need the information sent to them by sensory or efferent neurons. Since in order to perform muscle movements appropriate to the situation, our brain must receive information from outside. Hence the need for both types of neurons to work in harmony.
In this way, our nervous system integrates the information 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 transmitting information, some results obtained in recent studies point towards the idea that These nerve cells have much more complex operating dynamics being capable of producing behaviors or motor patterns on their own.
Motoneurons and motor units
Since each neuron aims to activate a specific muscle fiber to be able to carry out a certain movement, each of these connections are 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 known as red fibers, which carry out very slow contraction movements.
These types of fibers tolerate tiredness and fatigue very well, making them especially suitable for maintaining a muscle contraction or posture without becoming fatigued. For example, They help us stay on our feet without getting tired
2. Fast fatigue 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 exhaust their energies more quickly and therefore tire much sooner.
These motor units are extremely effective 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 are located halfway between the two previous groups. Although they perform their function on medium-sized muscles, your reaction time is slower than in FF units and have the ability to tolerate fatigue for longer.
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 muscles, so They have a transcendental role in locomotor skills
These skeletal muscles are made up of striated fibers, which make up most of the body mass and are distinguished from the rest by being muscles that we can move at will.
Furthermore, 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 connect.
Classification according to position
Classification according to fibers
2. Visceral motor neurons
Visceral motor neurons are responsible for innervating all those muscle fibers that we cannot move voluntarily; that is, the smooth muscles. These muscles control, for example, the movements of our heart, 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 muscles
This last group of neurons has the sole mission of activating the muscles present in the face and neck, known as gill muscles.
Associated pathologies
There are a series of diseases or pathologies of neurological origin that are distinguished by a gradual degeneration of motor neurons, presenting different symptoms depending on whether the affected neurons are superior or inferior
Those diseases in which degeneration of the upper motor neurons is experienced are characterized by general muscle weakness When the affected motor neurons are the lower ones, the person may suffer from muscle tension, rigidity and hyperactivity of reflexes that cause involuntary muscle contractions.
Some of the diseases related to the degeneration of motor neurons are:
