The 7 types of nerves: classification and characteristics

Our nervous system is responsible for coordinating the actions and activities that we carry out throughout our lives, whether they are voluntary or involuntary, conscious or unconscious. And it does this primarily through the nerves, which are responsible for driving the nerve impulses that keep everything running smoothly.

In this article, we will take a look at what nerves are, what role they play in our nervous system, and what types of nerves are there, Among other problems.

    What are nerves?

    nerves they are structures made up of bundles of neural fibers (Nerve extensions and axons), located outside the central nervous system, which are responsible for the conduct of nerve impulses and communication of the nerve centers of the brain and spinal cord with other organs of the body, and on the contrary.

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

    As we will see later, some nerves they originate in the spinal cord, While others are born in the brain. There are different types of nerves, they can be sensitive, motor or mixed, and it will depend on the function each of them performs within our nervous system.

    But before we dive into that, let’s briefly take a look at how the human nervous system works and its characteristics.

    The human nervous system

    The human nervous system functions as a large system responsible for the management and coordination of bodily activities and functions. thanks to its cabling network, which communicates all parts of our body.

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

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

    The main function of the SNP is therefore connect the CNS to organs, limbs and skin. Its nerves extend from the CNS to the outermost regions 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 noted above, nerves in the peripheral nervous system connect the central nervous system to 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. Depending on the direction in which the nerve impulse is transmitted

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

    1.1. motor nerves

    Motor nerves they are responsible for all voluntary skeletal and somatic movements (Like moving a leg or an arm), they drive nerve impulses to muscles and glands.

    1.2. sensory nerves

    Sensory nerves are responsible for driving nerve impulses to the central nervous system, i.e. from receptors to coordination centers.

    1.3. mixed nerves

    The mixed nerves conduct the nerve impulse in both directions and they have both sensitive and motor axons.

    2. Depending on where the nerves come from

    Nerves can also be classified according to their starting point in our body. In this case, two types of nerves are differentiated:

    2.1. cranial nerves

    There are 12 pairs of nerves (12 on the left and 12 on the right) which they come from the brain or at the level of the brainstem. Some are sensitive, other drivers and also mixed.

    These nerves mainly control the muscles of the head and neck, with the exception of one of them, the vagus nerve, which also acts on the 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 to be distributed over various areas of the body.

    They all 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.

      3. According to its function of coordinating voluntary or involuntary acts

      Another of the criteria making it possible to classify various types of nerves is their implication in the coordination of voluntary or involuntary acts; in other words that is to say, whether they are innervated in the autonomic nervous system or in the somatic or voluntary nervous system.

      3.1. Nerves of the somatic nervous system

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

      3.2. Nerves of the autonomic nervous system

      The autonomic nervous system, in turn, responds primarily to nerve impulses from the spinal cord, brainstem, and hypothalamus. The nerves in this system are made up of efferent fibers that exit the central nervous system, except for those that innervate 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 the one hand, there are the nerves of the parasympathetic nervous system; this system predominates in moments of relaxation, And consists of the cranial vagus nerve. It also shares the spinal nerves with the sacral region (lower part of the spine).

      On the other hand, we have the nerves of the sympathetic nervous system. this system predominates in times of tension, And their nerves share the rest of the spinal nerves. The nerve fibers that house this system are partially separated from the rest of the spinal nerves and form two chains of ganglia, located on either side of the spine.

      Schwann cells: protective coatings

      Spontaneous repair of peripheral nerves is made possible by a type of cell called Schwann, the function is acting as an insulating layer, enveloping nerve fibers in a substance called myelin. This layer of fat protects the nerves and improves the speed of transmission of nerve impulses.

      In the peripheral nervous system, Schwann cells play a key role in carrying out a highly regulated process of differentiation and dedifferentiation, unique to this type of cell and in which they lack the rest of the cells of the nervous system. This advantage makes them very plastic and allows them to go from one state in which they produce myelin to another, less differentiated, in which they help repair the damaged nerve.

      When we suffer injury to a peripheral nerve, these cells temporarily lose their ability to form myelin and return to a much undifferentiated earlier stage. This is done to help the nerve regenerate itself and allow it to reach the target tissues. Once the nerve is repaired, the cell regains its ability to produce myelin.

      The researchers found 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 cell which, once the nerve is repaired, starts myelination again. This occurs by the inactivation of a specific gene (the c-Jun gene).

      In short, this process is what makes possible the spontaneous repair of a nerve and even, in some cases, the reimplantation of limbs or amputated limbs, such as a finger. In this case, the Schwann cells would help regain the ability to move and feel the same. Although, unfortunately, in some types of nerves regeneration is not complete and there are still lifelong sequelae.

      Bibliographical references:

      • Gomis-Coloma C, Velasco-Aviles S, Gómez-Sanchez JA, Casillas-Bajo A, Backs J, Cabedo H. (2018). Class IIa histone deacetylases bind cAMP signaling to the myelin transcription 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.

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