Our brain controls and allows our movements. Although this may seem like a very reductionist description, it is nevertheless real. Our nervous system, which includes the brain, is in charge of sending signals to all the muscles in our body so that they can move.
To be more exact, these signals are sent by motor neurons or motor neurons . Thanks to them we can walk, breathe, sneeze or get our heart beating.
What are motor neurons?
Motor neurons, also known as motor neurons, are a group 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 are found in the brains 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 in charge of sending information from these regions to the rest of the muscles of the body; unlike afferent or sensory neurons that make the opposite journey, sending information from the muscles to the rest of the nervous system.
This transmission of nerve impulses is intended to exert control over the skeletal and smooth muscles that make up the organs and glands. In other words, thanks to the motor neurons we are able to perform any type of movement, just as our organs are able to function properly.
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 be able to perform the muscular movements appropriate to the situation , 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 information from both types of neurons and allows us to move and react according to the demands and circumstances of our external context.
Despite the fact that motor neurons have traditionally been considered passive channels of information transmission, some results obtained in recent studies point to the idea that these nerve cells possess much more complex dynamics of functioning , being capable of producing motor behaviour or patterns by themselves.
Motor neurons and motor units
Since each neuron aims to activate a specific muscle fibre in order to carry out a certain movement, each of these connections is called a motor unit. These functional units can be divided into several types:
1. Slow motor units (S or slow)
In this type of motor unit, the neurons stimulate a small muscle fiber, also called a red fiber, which performs very slow contraction movements.
This type of fibre tolerates fatigue and tiredness very well and is therefore particularly suitable for maintaining muscle contraction or posture without fatigue. For example, help us to stand without getting tired .
2. Fast fatiguing (FF or fast fatiguing) motor units
In this second case the fibres involved are the white fibres, which are responsible for innervating larger muscle groups. Compared to slow motor units, fast fatigue motor units have very short reaction times but exhaust their energies more quickly and therefore tire much earlier.
These motor units are extremely efficient for movements that require rapid bursts of energy, such as jumping or running .
3. Fatigue Resistant Fast Motor Units
Finally, this last type of motor unit is halfway between the two previous groups. Although they function on medium sized muscles, their reaction time is slower than in FF units and they 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 fibre 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
These types of motor neurons act on the skeletal muscles, and therefore play a transcendental role in locomotor skills .
These skeletal muscles are made up of striated fibres, which make up most of the body mass and are distinguished from the rest by the fact that 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 the 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 neuron : These neurons are located along the entire length of the cerebral cortex and their nerve endings are arranged so that they form a pyramidal pathway connected to the spinal cord.
- Lower motor neuron : in this case the neurons are arranged to form circuits, located on the anterior pole of the spinal cord, which intervene in reflex movements and involuntary movements.
- Alpha motor neurons : are the largest motor neurons and their main function is to activate the extrafusal fibres. In other words, all those fibres that make up the skeletal muscles. Thanks to them we can generate the necessary force to contract and move our muscles.
- Beta-motor neurons : these neurons connect to both fibers of the skeletal musculature and fibers outside the muscle spindle (intrafusals) and are responsible for receiving sensory information.
- Gamma motor neurons : finally, the gamma motor neurons are only responsible for innervating the intrafusal fibres; regulating sensitivity to contraction and helping to maintain muscle tone.
2. Visceral motor neurons
The visceral motor neurons are in charge of innervating all those muscle fibers that we cannot move voluntarily; that is, the smooth muscles. This musculature controls, for example, the movements of our heart, viscera and intestines, etc.
In order to carry out their function, the visceral motor neurons also perform synapses with the ganglion neurons of the autonomic nervous system, sending the 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 the brachial musculature.
There are a series of diseases or pathologies of neurological origin that are distinguished by presenting a gradual degeneration of motor neurons, presenting a different symptomatology according to whether the affected neurons are superior or inferior .
Those diseases in which a degeneration of the upper motor neurons is experienced are characterized by a general muscular weakening . When the motor neurons affected are the lower ones, the person may suffer from muscular tension, rigidity and an overactivity of the reflexes that causes involuntary muscular contractions.
Some of the diseases related to the degeneration of motor neurons are
- Progressive bulbar palsy.
- Pseudobulbar palsy.
- Amyotrophic lateral sclerosis (ALS).
- Primary Lateral Sclerosis.
- Progressive muscular atrophy .
- Spinal muscular atrophy.
- Post-polio syndrome.