The 7 types of nerves: classification and characteristics.

A review of the main types of nerves in the human body, classified according to various criteria.

Our nervous system is responsible for coordinating the actions and activities we carry out throughout our lives, whether voluntary or involuntary, conscious or unconscious. And it does this mainly thanks to the nerves, which are responsible for conducting the nerve impulses that make it possible for everything to function properly.

In this article we will look at what nerves are, what role they play in our nervous system, and what types of nerves existamong other questions.

What are nerves?

The nerves are structures constituted by bundles of neuronal fibers. (nerve extensions and axons), located outside the central nervous system, which are responsible for conducting nerve impulses and communicating the nerve centers of the brain and spinal cord with the rest of the organs of the body, and vice versa.

These fiber bundles are enveloped by a thin membrane, the perineurium, which surrounds the bundle of nerve fibers; and in turn, the complete nerve formed by the union of several bundles is covered by another structure, called epineurium.

As we will see later, some nerves have their origin in the spinal cordwhile others originate in the encephalon. There are different types of nerves, being able to be sensitive, motor or mixed, and this will depend on the function that each one of them fulfills within our nervous system.

But before we go into this, we will briefly see how the human nervous system works and what are its characteristics.

The human nervous system

The human nervous system functions as a large system in charge of managing and coordinating bodily activities and functions through its network of wiring, which communicates all parts of our body..

The nervous system is divided into the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS is made up of the brain and spinal cord, the command control and nerve impulse transmission center, respectively.

The PNS is made up of various types of nerves that leave or enter the CNS. The PNS is responsible for sending information to it and, after being evaluated, the brain sends the appropriate responses to the necessary parts of the body, such as muscles or other organs.

The main function of the PNS is therefore to, to connect the CNS with the organs, limbs and skin.. Its nerves extend from the CNS to the outermost areas of our body. And it is the PNS that is responsible for helping us react to stimuli in our environment.

Types of nerves and classification

As we mentioned earlier, the nerves of the peripheral nervous system connect the central nervous system with the rest of the body. And they do so in different ways, and with different functions. In the following, we will classify these nerves according to the following criteria:

1. according to the direction in which the nerve impulse is transmitted 2. according to the direction in which the nerve impulse is transmitted.

Nerves can be classified in 3 ways, according to the direction in which the nerve impulse is transmitted.

1.1. Motor nerves

The motor nerves are responsible for all voluntary skeletal and somatic movement (such as moving a leg or an arm). (such as moving a leg or an arm), they conduct the nerve impulse to the muscles and glands.

Sensory nerves

Sensory nerves are responsible for conducting the nerve impulse to the central nervous system, that is, from the receptors to the coordination centers.

1.3. Mixed nerves

Mixed nerves conduct the nerve impulse in both directions and have both sensory and motor axons. have both sensory and motor axons.

2. According to the origin from which the nerves emanate

Nerves can also be classified on the basis of the place from which they depart in our body. In this case, two types of nerves are differentiated:

2.1. Cranial nerves.

There are 12 pairs of nerves (12 to the left and 12 to the right) that arise from the brain or at the level of the brain. arise from the brain or at the brainstem level.. Some are sensory, others motor and also mixed.

These nerves basically control the muscles of the head and neck, with the exception of one of them, the vagus nerve, which also acts on structures of the thorax and abdomen.

2.2. Spinal nerves

There are 31 to 33 pairs of nerves and they are all of the mixed type. They originate in the spinal cord and pass through the vertebral muscles and are distributed and are distributed in various areas of the body.

All of them have a dorsal or sensory root, made up of neuron bodies that receive information from the skin and organs; and a ventral or motor root, which transmits information to the skin and organs.

3. According to its function in the coordination of voluntary or involuntary acts.

Another of the criteria with which we can classify various types of nerves is their involvement in the coordination of voluntary or involuntary acts; i.e., whether they innervate the nervous system, whether they innervate the autonomic nervous system or the somatic or voluntary nervous system..

3.1. Nerves of the somatic nervous system

The somatic or voluntary nervous system is the one that totally or partially manages the actions and activities of our body, which can be conscious (such as picking up or manipulating an object) or unconscious (moving the left leg forward when walking, for example). Its nerves are made up entirely of myelinated fibers (an insulating layer that forms around the nerve to make transmission more efficient).

3.2. Nerves of the autonomic nervous system

The autonomic nervous system responds mainly to nerve impulses in the spinal cord, brain stem and hypothalamus. The nerves of this system are made up of efferent fibers that leave the central nervous system, except for those that innervate skeletal muscle.

Afferent nerves, which transmit information from the periphery to the central nervous system, are responsible for transmitting visceral sensation and regulating vasomotor and respiratory reflexes (heart rate or Blood Pressure control). (control of heart rate or blood pressure).

In the autonomic nervous system, two types of nerves can be distinguished. On the one hand, there are the nerves of the parasympathetic nervous system; this system predominates in moments of relaxation. predominates in moments of relaxationand is constituted by the vagus cranial nerve. It also shares the spinal nerves of the sacral region (lower part of the spinal column).

On the other hand, we have the nerves of the sympathetic nervous system. This system predominates in moments of tensionand its nerves share the rest of the spinal nerves. The nerve fibers housed in this system are partially separated from the rest of the spinal nerves and form two chains of ganglia, located on both sides of the spinal column.

Schwann cells: the protective sheaths

Spontaneous repair of peripheral nerves is made possible by a type of cell called Schwann cells, whose function is to act as an insulating layer, wrapping the nerve fibers with a substance called myelin. act as an insulating layer, enveloping the nerve fibers with a substance called myelin.. This fatty layer protects the nerves and improves the speed of transmission of nerve impulses.

In the peripheral nervous system, Schwann cells play a fundamental role by carrying out a highly regulated process of differentiation and dedifferentiation, a unique characteristic of this type of cell that is lacking in the rest of the cells of the nervous system. This advantage gives them great plasticity and allows them to move from a state in which they produce myelin to another, less differentiated state, in which they contribute to the repair of the damaged nerve.

When we suffer an injury to a peripheral nerve, these cells temporarily lose their ability to form myelin and regress to an earlier, highly undifferentiated stage. This occurs to help the nerve regenerate so that they can reach the target tissues.. Once the nerve is repaired, the cell regains its ability to produce myelin.

Researchers have discovered that in this process, the chemical messenger called cyclic AMP plays an important role. This substance sends a protein to the nucleus of the Schwann cells that, once the nerve has been repaired, sets myelination in motion again. This occurs through the inactivation of a specific gene (the c-Jun gene).

Ultimately, it is this process that makes possible the spontaneous repair of a nerve and even, in some cases, the reimplantation of amputated limbs or body parts, such as a finger. In this case, the Schwann cells would help to recover the ability to move and the touch itself. Unfortunately, however, in certain types of nerves, regeneration is not complete and lifelong sequelae remain.

Bibliographic references:

• Gomis-Coloma C, Velasco-Aviles S, Gomez-Sanchez JA, Casillas-Bajo A, Backs J, Cabedo H. (2018). Class IIa histone deacetylases link cAMP signaling to the myelin transcriptional program of Schwann cells. J Cell Biol. doi: 10.1083/jcb.201611150.
• Navarro X. (2002). Physiology of the autonomic nervous system. Rev Neurol ;35(6):553-62.
• Waxman, S. (2012). Neuroanatomia clinica. Padova: Piccin.