The 7 types of nerves: classification and characteristics

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

In this article we will see what nerves are, what role they play in our nervous system and what types of nerves are there, among other issues.

• Related article: "Parts of the Nervous System: functions and anatomical structures"

What are nerves?

The nerves are structures made up of bundles of neuronal fibers (nerve extensions and axons), located outside the central nervous system, which are responsible for conducting impulses nerve centers and communicate the nerve centers of the brain and spinal cord with the rest of the organs of the body, and at the same time contrary.

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

As we will see later, some nerves have their origin in the spinal cord, while others are born in the brain. There are different types of nerves, and they can be sensitive, motor or mixed, and this will depend on the function that each of them fulfills within our nervous system.

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

The human nervous system

The human nervous system functions as a large system responsible for managing and coordinating bodily activities and functions. through its wiring network, which communicates all the 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 exit or enter the CNS. The SNP is in charge of sending the information and, after being evaluated, the brain sends the appropriate responses to necessary body parts, such as muscles or other types of of organs.

The main function of the PNS is thus connect the CNS to the organs, extremities, and skin. Its nerves extend from the CNS to the outermost areas of our body. And it is the SNP 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 it in different ways, and with different functions. Next, we will classify these nerves according to the following criteria:

1. According to the direction in which the nerve impulse is transmitted

Nerves can be classified in 3 ways, depending on the direction in which they transmit the nerve impulse.

1.1. motor nerves

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

1.2. sensory nerves

The sensory nerves are in charge of conducting the nerve impulse towards 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.

2. According to the origin from where the nerves come out

Nerves can also be classified based on where they start from in our body. In this case, there are two types of nerves:

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 brainstem. Some are sensitive, 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 in the thorax and abdomen.

2.2. spinal nerves

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

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

• You may be interested in: "Spinal cord: anatomy, parts and functions"

3. According to its role in coordinating 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; that is to say, whether they innervate the autonomic nervous system or the somatic or voluntary nervous system.

3.1. Somatic nervous system nerves

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

3.2. Autonomic nervous system nerves

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

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

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

On the other hand, we have the nerves of the sympathetic nervous system. This system dominates in moments of stress, and their nerves share the rest of the spinal nerves. The nerve fibers that this system houses 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 coverings

Spontaneous repair of peripheral nerves is possible thanks to a type of cells called Schwann cells, whose function is act as an insulating layer, wrapping 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 in carrying out a highly regulated process of differentiation and dedifferentiation, a unique characteristic of this type of cells and which is lacking in the rest of the cells of the system highly strung. This advantage means that they have great plasticity and allows them to go from a state in which they produce myelin to another, less differentiated one, in which they contribute to the repair of damaged nerves.

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

Researchers have discovered that the chemical messenger called cyclic AMP plays an important role in this process. This substance sends a protein to the nucleus of the Schwann cells that, once the nerve is repaired, starts myelination again. This is done by inactivating a specific gene (the c-Jun gene).

In short, this process is what makes possible the spontaneous repair of a nerve and even, in In some cases, the reimplantation of amputated limbs or members of the body, such as a finger. In this case, the Schwann cells would help to recover the ability to move and touch itself. Although, unfortunately, in certain types of nerves, regeneration is not complete and sequelae remain for life.

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). Clinical neuroanatomy. Padua: Piccin.