Peripheral nervous system

The peripheral nervous system is, along with the central nervous system (made up of the brain and spinal cord), one of the two divisions of the nervous system. The peripheral nervous system is made up of the nerves and nerve ganglia that extend outside the central nervous system. Its main function is to connect the central nervous system to the limbs and organs. It lacks a protective bony lining, which differentiates it from the central nervous system which is surrounded by the skull and vertebral column.

In the human species, it is made up of 12 pairs of cranial nerves and 31 pairs of spinal nerves, thus giving a total of 43 pairs of nerves. Each of the nerves follows a defined path and innervates a specific sector of the body. The division between the central nervous system and the peripheral nervous system is anatomical or structural. From a functional point of view, the peripheral nervous system can be divided into the somatic nervous system that is controlled voluntarily and the autonomic nervous system that functions involuntarily or automatically.

Structure

Image of the cervical spine. In yellow you can see the spinal nerves that arise on both sides of the vertebrae. In red the vertebral arteries.

The sciatic nerve is a motor nerve that originates in the sacral plexus.

• Spinal nerves. They are a total of 31 pairs of nerves each with two parts or roots that join one another: one sensitiva and another motor. The sensitive part is the one that moves the information from the receptors to the spinal cord, while the motor part is the one that carries the impulses from the spinal cord to the corresponding effects. They are distributed as follows:
  • 8 pairs of cervical nerves
  • 12 pairs of dorsal or chest nerves
  • 5 pairs of lumbar rachid nerves
  • 5 pairs of sacred rachid nerves
  • 1 pair of coccygeal rachid nerves

Each spinal or spinal nerve attaches at two different points to the spinal cord; It has a posterior root and an anterior root. The posterior and anterior roots unite to form the spinal nerve. Since the posterior root contains sensory fibers and the anterior root contains motor fibers, the result is a mixed nerve. Its route is very short, since it immediately divides into two branches, a smaller dorsal one and a larger ventral or anterior one. In the thorax, the ventral rami branch off from small communicating branches that carry fibers belonging to the autonomic nervous system.

Scheme in which a spinal nerve is represented with its two roots, sensitive and motor, leaving the spinal cord.

• Cranial nerves, also called cranial pairs, are 12 nerves that send sensory information from the neck and head to the central nervous system or transfer motor orders to control the skeletal musculature of the neck and head.
  • I. Olfactory nerve. It is a sensory nerve only, leads the nervous impulses generated by the odoriferous substances from the nose to the brain.
  • II Optical nerve. Exclusively sensory, convey visual information from the eye to the brain.
  • III Oculomotor nerve. It has motor fibers that control the eye and parasympathetic movement that modify the diameter of the pupil.
  • IV Troclear nerve. Its function is motor over one of the muscles whose contraction moves the eyeball.
  • V Trigem nerve. It is a mixed nerve that consists of a sensitive portion and another motor.
  • VI Nervio abducens or External Eye Motor. It intervenes in eye mobility, it is only motor.
  • VII Facial nerve. It is a mixed nerve with sensitive and motor fibers.
  • VIII Nerve vestibulococlear. It conveys to the brain the auditory and sensory information from the inner ear.
  • IX Gloosopharyngeal nerve. It's a mixed nerve. The sensitive portion carries signals from the tongue and the pharynx.
  • X Nervio vago. It is sensitive and motor, it also provides parasympathetic fibers that act on different organs, including the stomach and the heart.
  • XI Spinal nerve. It is an engine nerve that activates among others the sternocleidomastoid muscle, causing the spin of the head.
  • XII Hypogloss nerve. It's a nerve engine for the musculature of the tongue.

In yellow the olfactory nerve (I pair cranial)

In yellow color the trigem nerve and its branches (V to cranial) are represented

In yellow colour the facial nerve and its branches can be seen (VII par cranial)

Plexuses

The brachial plexus originates from the previous branch of the rachid nerves C5-C8 and T1.

After exiting the spinal cord, spinal nerves form networks made up of the union of adjacent nerves. These networks are called plexuses. There are four main ones:

• Cervical plexus. There are two, one right and one left. They are formed by the previous branches of the first four cervical nerves and some branches from the fifth.
• Brachial plexus. It originates from the rachid nerves C5-C8 and T1. It gives rise to several nerves that carry orders to the muscles of the shoulder and the upper limbs, including the cubital nerve that makes possible the contraction of the muscles that move the hand.
• Plexo lumbar. It is formed by the previous branches of the rachid nerves L1-L4. It gives rise to nerves that carry motor orders to the muscles of the anterolateral region of the abdomen and lower limbs.
• Plexo sacro. It is formed by the previous roots of the spinal nerves L4-l5 and S1-S4. It gives rise to nerves that carry motor orders to the lower members. From the sacred plexus comes the cytic nerve that is the longest of the human organism.

Function

The function of the peripheral nervous system is twofold. On the one hand, it transports the stimuli that the organism receives, both external and internal, to the central nervous system to be processed. On the other hand, it transfers the orders generated by the central nervous system to the organs and muscles of the whole body so that they carry out the different functions. If the peripheral nervous system did not exist, the muscles could not be governed by the brain, so voluntary movement would be impossible, since the cerebral orders would not reach their destination.

Dermatomes

In the right half of the body is represented the map of dermatomas. Previous Vision.

A dermatome is a patch of skin innervated by the sensory portion of a spinal nerve. Each dermatome corresponding to a medullary segment and a spinal nerve. They are distributed horizontally downward along the trunk, in the extremities the distribution is longitudinal. The map showing the distribution of dermatomes is important in medicine, as it allows determining the location of a lesion.

Nerves and nerve ganglia

Nerves are bundles of nerve fibers that carry information and are part of the peripheral nervous system. Afferent nerves carry sensory information to the central nervous system, while efferent nerves carry information from the central nervous system to effector organs. The nerves that carry pain signals are afferent, while the motor nerves that terminate in the different muscles and make muscle contraction and movement possible are efferent nerves. There are mixed nerves that carry both types of information.

Nerve ganglia are formed by accumulations of neuronal bodies that are interspersed in the course of some nerves and are part of the peripheral nervous system, they should not be confused with lymph nodes that are part of the lymphatic system.

Scheme in which the section is represented a peripheral nerve. The nerve fibers that carry the information are visible.

Pathology

Peripheral Neuropathy

Carpo Tunnel Syndrome is caused by mid nerve entrapment at wrist level.

It is caused by damage to the nerve fibers that make up one of the nerves. The causes can be multiple. They are generically divided into mononeuropathies when they only affect the functioning of one nerve and polyneuropathies when it affects several. As an example of mononeuropathy, we can cite carpal tunnel syndrome, which is produced by compression of the median nerve as it passes through the wrist. One of the most frequent polyneuropathies is toxic by alcohol consumption (alcoholic polyneuropathy).

The symptoms of neuropathies depend on the type of nerve affected and its location. When motor nerves are affected, muscle weakness, uncontrolled muscle contractions (fasciculations), and muscle atrophy occur. When the sensory nerves are affected, the sense of touch can be impaired, especially in the feet and hands. The alteration in the function of the fibers that transmit the sensations of pain and temperature causes insensitivity, which is why the affected people sometimes suffer serious injuries to their feet and burns without perceiving any pain.