MOTOR NEURONS: CHARACTERISTICS, TYPES AND FUNCTIONS - GEOGRAFÍA - 2024

The motor neurons or motor neurons are nerve cells that conduct nerve impulses to the outside of the central nervous system. Its main function is to control the effector organs, mainly the skeletal muscles and the smooth muscles of the glands and organs.

Motor neurons are efferent, that is, they transmit messages to other nerve cells (afferent neurons are the ones that receive information). They are located in the brain, mainly in Brodmann's area 4, and in the spinal cord.

The brain is the organ that moves muscles. This statement may seem very simple, but in reality, movement (or behavior) is a product of the nervous system. To emit the correct movements, the brain must know what is happening in the environment.

In this way, the body has specialized cells to detect environmental events. Our brains are flexible and adapt so that we can react differently based on circumstances and experiences in the past.

These capabilities are made possible through the billions of cells that are in our nervous system. One of these cells are sensory neurons that capture information from the environment. While the motor neurons are those that control the contraction of the muscles or the secretion of the glands, in response to certain stimuli.

Relationship between sensory, transmission and motor neurons. Source: By Ruth Lawson Otago Polytechnic / CC BY (https://creativecommons.org/licenses/by/3.0)

Motor neurons differ from sensory neurons in that the latter are afferent, that is, they transmit information from the sensory organs to the central nervous system.

The latest research has found that motor neurons are not only passive receptors of motor commands, they are more complex than we think. Rather, they seem to play a fundamental role in circuits by generating motor behavior by themselves.

Classification of motor neurons

Motor neurons can be classified according to the tissue they innervate; There are several types that are described below.

- Somatic motor neurons

The movement of the musculoskeletal system is possible thanks to the synchrony between the contraction and relaxation of certain muscles. These are called skeletal muscles and are made up of striated fibers.

Striated muscle is the one that forms the majority of body mass. It is characterized by being of conscious action, that is, it can be stretched and contracted voluntarily. These coordinated movements require the intervention of numerous nerve fibers. Thus certain very complex movements of the skeleton are achieved.

Each somatic motor neuron has its cell body in the central nervous system and its axons (nerve processes) reach the muscles. Some studies have shown that certain axons are one meter long.

Axons form motor nerves. Two examples are the median nerve and the ulnar nerve, which run from the cervical vertebrae to the finger muscles.

Somatic motor neurons make only one synapse outside the central nervous system. For this reason they are called monosynaptic. They precisely synapse with the muscle fibers, through a specialized structure called the neuromuscular junction (described later).

Depending on the position, these neurons are divided into:

- Upper motor neuron: it is located in the cerebral cortex. It has nerve endings that form the pyramidal pathway that connects to the spinal cord.

- Lower motor neuron: it is located in the anterior horn of the spinal cord. At this point, neurons are organized into circuits that participate in automatic, stereotyped, reflex, and involuntary movements. For example, the sneeze or the withdrawal reflex of a painful stimulus.

The motor neurons in these circuits are organized into nuclei, arranged in longitudinal columns that can occupy 1 to 4 spinal segments.

Distinction between the upper motor neuron and the lower motor neuron. Source; Rcchang16 / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)

Depending on the muscle fibers they innervate, somatic motor neurons can be classified into:

- Alpha motor neurons: they are large, and their conduction speed is 60-130 m / s. They innervate the muscle fibers of skeletal muscle (called extrafusal fibers) and are located in the ventral horn of the spinal cord. These fibers are the main element of force generation in the muscle.

These neurons are responsible for the voluntary contraction of skeletal muscle. In addition, they help muscle tone, necessary for balance and posture.

- Beta motor neurons: innervates both the extrafusal fibers and the intrafusal fibers. That is, inside and outside the muscle spindle. This is the sensory receptor of the muscle, and is responsible for transmitting information about the length of the extension.

- Gamma motor neurons: innervate intrafusal fibers. They are responsible for regulating the sensitivity to muscle contraction. They activate the sensory neurons of the muscle spindle and the tendon reflex, which acts as protection against excessive stretching. It also tries to maintain muscle tone.

- Visceral motor neurons

Some movements of the muscle fibers are not consciously controlled by the subject, as is the case with the movement of our heart or our stomach. The contraction and relaxation of these fibers is involuntary.

This is what happens in the so-called smooth muscles, which are present in many organs. Visceral motor neurons innervate this type of muscle. It includes the heart muscle, and that of the viscera and organs of the body, such as the intestine, urethra, etc.

These neurons are dysynaptic, which means that they make two synapses outside the central nervous system.

In addition to the synapse it performs with muscle fibers, it also performs another involving neurons from the ganglia of the autonomic nervous system. These send impulses to the target organ to innervate the visceral muscles.

- Special visceral motor neurons

They are also known as branchial motor neurons, as they directly innervate the branchial muscles. These neurons regulate gill movement in fish. Whereas, in vertebrates, they innervate the muscles related to the movement of the face and neck.

Motor unit concept

A motor unit is a functional unit composed of a motor neuron and the muscle fibers that it innervates. These units can be classified into:

Slow motor units (S-slow)

Also known as red fibers, they stimulate small muscle fibers that contract slowly. These muscle fibers are very resistant to fatigue and are helpful in maintaining muscle contraction. They serve to remain in an upright position (in bipidestation) without tiring.

Fast Fatiguing Motor Units (FF)

Known as white fibers, they stimulate larger muscle groups, but they tire quickly. Their motor neurons are large, and they have high conduction and excitation speeds.

These motor units are useful for activities that require bursts of energy such as jumping or running.

Fatigue resistant fast motor units

They stimulate muscles with a moderate size, but they do not react as fast as the previous ones. They are somewhere in the middle between S and FF motor units. They are characterized by having the necessary aerobic capacity to resist fatigue for several minutes.

Motor neuron related diseases

Very high magnification micrograph of the medulla oblongata showing the fourth ventricle, the nuclei of the hypoglossal nerve, and the vagus nerve. Source: Nephron

Motor neuron diseases are a group of neurological disorders characterized by progressive degeneration of motor neurons. These diseases can be classified according to whether the upper motor neurons or the lower motor neurons are affected.

When there is an interruption in the signal sent by the lower motor neurons, the main consequence is that the muscles do not work properly. The result of these disorders can be general wasting, pathological thinning (emaciation), as well as fasciculations (uncontrollable tics).

When upper motor neurons are affected, muscle stiffness and hyperresponsiveness of tendon reflexes occur. This refers to stronger-than-normal involuntary muscle contractions, which can present as jerks in the knees or ankle.

Motor neuron diseases can be inherited or acquired. They generally occur in adults and children. They are more common in men than women. In adults, symptoms appear after 40 years of age.

The causes of acquired motor neuron diseases are generally unknown. However, some cases are related to exposure to radiotherapy or toxins. It is currently being investigated whether this type of disease is related to the body's autoimmune response to viruses such as HIV.

Here are some of the most common motor neuron diseases:

Amitrophic lateral sclerosis (ALS)

It affects the classic motor neurons, and is also known as Lou Gehrin's disease. It is a degenerative disease that mainly damages the motor neurons of the cortex, the trochoencephalon and the spinal cord.

Patients affected by ALS develop muscle atrophy, which fatally leads to severe paralysis, although there are no mental or sensory alterations. This disease has become famous for affecting the well-known scientist Stephen Hawking.

People with this disease have weakness and wasting of the bulbar muscles (those that control speech and swallowing). Symptoms first appear in the limbs and swallowing muscles. Exaggerated reflexes, cramps, fasciculations, and speech problems are also observed.

Progressive bulbar palsy

It is characterized by the weakness of the muscles that innervate the motor neurons of the lower part of the brain stem. These muscles are the lower jaw, face, tongue, and pharynx.

As a consequence of this, the patient has difficulty swallowing, chewing and speaking. There is a great risk of suffocation and aspiration pneumonia (inhaling food or liquids into the airways).

In addition, affected patients present with laughter or crying attacks, which are known as emotional lability.

Pseudobulbar palsy

It shares many characteristics with the previous disorder. In it there is a progressive degeneration of the upper motor neurons, causing weakness of the facial muscles.

This causes problems with speaking, chewing, and swallowing. In addition, a deep voice and immobility of the tongue may develop.

Primary lateral sclerosis

There is an involvement of the upper motor neurons. Its cause is unknown and it occurs more in men than in women. It begins after the age of 50, approximately.

There is a gradual degeneration of the nerve cells that control voluntary movement. These cells are located in the cerebral cortex, which is where higher mental functions are carried out.

This disease is characterized by stiffness in the muscles of the legs, trunk, arms, and hands.

Patients have problems with balance, weakness, sluggishness, and spasticity in the legs. The facial muscles can be affected producing dysarthria (difficulty articulating sounds and words).

Progressive muscular atrophy

In this disease there is a slow and progressive degeneration of the lower motor neurons. It mainly affects the hands and then spreads to the lower parts of the body. Its symptoms are cramps, tics, and pathological weight loss for no apparent reason.

Spinal muscular atrophy

It is an inherited disorder that affects the lower motor neurons. There is a progressive degeneration of the cells of the anterior horn of the spinal cord. The legs and hands are most severely affected. It can vary according to age, inheritance patterns, and severity of symptoms.

Postpolio syndrome

It is a disorder characterized by progressive weakness. It causes pain and fatigue in the muscles, and it occurs years after suffering from acute paralytic polio.

References

1. Carlson, NR (2006). Physiology of behavior 8th Ed. Madrid: Pearson.
2. Motor neuron diseases. (sf). Retrieved on February 28, 2017, from the National Institute of Neurological Disorders and Stroke: espanol.ninds.nih.gov.
3. Neuron motor. (sf). Retrieved on February 28, 2017, from Wikipedia: en.wikipedia.org.
4. Neurology, G. d. (July 7, 2004). Motor neuron diseases. Obtained from Sen: sen.es.
5. Newman, T. (January 14, 2016). A new role for motor neurons. Retrieved from Medical News Today: medicalnewstoday.com.
6. Takei, H. (April 28, 2014). Pathology of Motor Neuron Disorders. Obtained from Medscape: emedicine.medscape.com.
7. Tortora, GJ, & Derrickson, B. (2013). Principles of Anatomy and Physiology (13th ed.). Mexico DF; Madrid etc.: Editorial Médica Panamericana.
8. What role do motor neurons play in basic bodily functions? (February 24, 2013). Retrieved from Thingswedontknow: blog.thingswedontknow.com.