The Motor neurons Or motoneurons are the nerve cells that drive the 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 musculature of glands and organs.
They are efferent, that is to say they transmit messages to other nerve cells (the afferent neurons are those that receive information).
These neurons are located in the brain , Mainly in area 4 of Brodmann , and in the spinal cord .
The brain is the organ that moves the muscles. This statement may seem very simple, but, in fact, movement (or behavior) is a product of the nervous system . In order to emit the correct movements, the brain must know what is happening in the environment.
In this way, the organism has specialized cells to detect environmental events. Our brains are flexible and adaptable so that we can react differently depending on the circumstances and what we experienced in the past.
These capabilities are possible through the billions of cells that are in our nervous system. One of these cells are sensory neurons that pick up information from the environment. While motor neurons control contraction of muscles or secretion of glands, in response to certain stimuli.
Motor neurons differ from sensory neurons in which the latter are afferent, that is, they transmit information from the sensory organs to the central nervous system.
Recent research has found that motor neurons are not only passive recipients of motor orders, but are more complex than we think. Rather they seem to play a fundamental role in circuits generating motor behavior by themselves.
Classification of motor neurons
Motor neurons can be classified according to the innervating tissue, so there are several types that are described below.
Somatic motor neurons
The movement of the locomotor apparatus is possible thanks to the synchrony between the contraction and the relaxation of certain muscles. These are called skeletal muscles and are composed of striated fibers.
Striated muscle is what forms the majority of body mass. It is characterized by being conscious action, that is, it can be stretched and contracted voluntarily.
These coordinated movements require the involvement of numerous nerve fibers. Thus certain very complex skeletal movements are achieved.
Each somatic motor neuron has its cellular body in the central nervous system and its axons (nerve extensions) reach the muscles. Some studies have indicated that certain axons have a length of one meter.
Axons form motor nerves. Two examples are the median nerve and the ulnar nerve, which extend from the cervical vertebrae to the muscles of the finger of the hand.
Somatic motor neurons perform only one synapse out of the central nervous system. For this reason they are called monosynaptic. They precisely perform the synapse with the muscle fibers, through a specialized structure called neuromuscular junction (described later).
Depending on the position, these neurons are divided into:
- Upper motor neuron: Is located in the cerebral cortex . It has nerve endings that form the pyramidal path that connects to the spinal cord.
- Lower motor neuron: Is located in the anterior horn of the spinal cord. At this point, neurons are organized in circuits that participate in automatic movements, stereotypes, reflexes and involuntary movements. For example, sneezing or reflex withdrawal from a painful stimulus.
The motor neurons of these circuits are organized into nuclei, arranged in longitudinal columns that can occupy 1 to 4 spinal segments.
Depending on the muscle fibers they innervate, somatic motor neurons can be classified into:
- Alpha motor neurons: Have a large size, and their driving 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 the generation of force in the muscle.
These neurons are responsible for the voluntary contraction of skeletal muscle. In addition, they help the muscle tone, necessary to maintain the balance and the posture.
- Beta motor neurons: It innervates both extrafusal fibers and 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: They innervate the intrafusal fibers. They are responsible for regulating the sensitivity to muscle contraction. They activate the sensory neurons of the muscle spindle and the osteotendinous reflex, which acts as protection against excessive stretching. It also seeks to maintain muscle tone.
Visceral motor neurons
Some movements of muscle fibers are not consciously controlled by the subject, as is 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 musculature, which is present in many organs. Visceral motor neurons innervate this type of muscle. It includes the heart muscle, and the organs and viscera of the body, such as the intestine, urethra, etc.
These neurons are dysynaptic, meaning they perform two synapses outside the central nervous system.
In addition to the synapse performed with muscle fibers, it also performs another involving nerve neurons 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 gill motor neurons, since they directly innervate the gill muscles. These neurons regulate the movement of gills in fish. While in vertebrates, they innervate the muscles related to movement of the face and neck.
Concept of motor unit
It is called a motor unit that functional unit composed of a motor neuron and the muscle fibers it innervates. These units can be classified in:
- Slow motion (S-slow): Also known as red fibers, stimulate small muscle fibers that contract slowly. These muscle fibers are very resistant to fatigue and are useful for maintaining muscle contraction. They are used to stay upright (in bipeds) without getting tired.
- Fast fatigue motor units (FF): Known as white fibers, stimulate larger muscle groups, but they tire quickly. Their motor neurons are large, and have high speeds of conduction and excitation.
These motor units are useful for activities that need energy bursts like jumping or running.
- Fast motor units resistant to fatigue: Stimulate muscles with a moderate size, but do not react as fast as the previous ones. They are in the middle between the motor units S and the FF. They are characterized by having the aerobic capacity necessary to withstand fatigue for several minutes.
Diseases
They are a set of neurological disorders characterized by progressive degeneration of motor neurons. These diseases can be classified according to whether upper motor neurons or 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 function properly. The result of these disorders may be a general weakening, pathological thinning ( Wasting ), As well as fasciculations (uncontrollable tics).
When upper motor neurons are affected, there is stiffness in the muscles and hyperreactivity of the tendon reflexes. This refers to involuntary muscle contractions that are more intense than normal, which can occur as shaking in the knees or ankle.
Diseases of the motor neuron can be inherited or acquired. Generally occur in adults and children. They are more common in men than in women. In adults, symptoms occur after the age of 40 years.
The causes of diseases of acquired motor neurons are generally unknown. However, some cases are related to exposure to radiation or to toxic. It is currently 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 diseases of motor neurons:
- Amyotrophic lateral sclerosis (ALS): In it classic motor neurons are affected, and is also known as Lou Gehrin's disease. It is a degenerative disease that mainly damages the motor neurons of the cortex, the Trokycephalic and the spinal cord .
Patients affected by ALS develop muscular atrophy, which inevitably 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 wear of the bulbar muscles (those who control speech and swallowing). Symptoms occur first in the extremities and muscles of swallowing. There are also exaggerated reflexes, cramps, fasciculations and speech problems.
- Progressive bulbar paralysis: Is characterized by the weakness of the muscles that innervate the motor neurons of the lower part of the cerebral stem. These muscles are the lower jaw, the face, the tongue and the pharynx.
As a result, the patient has difficulty swallowing, chewing and speaking. There is a high risk of asphyxia and aspiration pneumonia (inhalation of food or fluids into the respiratory tract).
In addition, the affected patients present attacks of laughter or crying, which are known as emotional lability.
- Pseudobulbar palsy: Shares many characteristics with the above disorder. It shows a progressive degeneration of the upper motor neurons, causing weakness of the facial muscles.
This causes trouble talking, chewing and swallowing. In addition, you may develop severe voice, and immobility of the tongue.
- Primary lateral sclerosis: There is an involvement of the upper motor neurons. Its cause is unknown and occurs more in men than in women. It starts after about 50 years of age, 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 the higher mental functions are performed.
This disease is characterized by stiffness in the muscles of the legs, trunk, arms and hands.
Patients have problems with balance, weakness, slowness, and Spasticity in the legs. The facial muscles can be affected, producing dysarthria (difficulty in 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 extends to lower parts of the body. Its symptoms are cramps, tics and pathological weight loss for no apparent reason.
- Spinal muscular atrophy: Is an inherited disorder that affects lower motor neurons. There is a progressive degeneration of the cells of the anterior horn of the spinal cord. It affects the legs and hands more severely. Variants may be presented according to age, inheritance patterns and severity of symptoms.
Postpolio syndrome: Is a disorder characterized by progressive weakness. It causes pain and fatigue in the muscles, and occurs years after suffering from acute paralytic poliomyelitis.
References
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- Newman, T. (January 14, 2016). A new role for motor neurons. Retrieved from Medical News Today: medicalnewstoday.com.
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