The somatic nervous system is part of the peripheral nervous system and is responsible for transmitting sensory information and sending motor control information to the skeletal muscles.

It is the main management system of voluntary movements and the nerve centre of dozens of sensory and motor nerves that enter and leave the central nervous system, in connection with the skin, organs and muscles of the body.

In this article we explain what the somatic nervous system is, what its functions are, its composition and the main diseases that affect it.

The nervous system

The somatic nervous system is part of a greater whole, the nervous system, which guarantees the control and management of the vast majority of the vital functions of our body , capturing the stimuli of the environment and those of the organism itself in order to transmit, process the information and generate effective responses according to what each situation requires.

Anatomically, the nervous system can be divided into two parts: the central nervous system (CNS), which includes the brain and spinal cord; and the peripheral nervous system, which comprises the set of nerves and ganglia that connect the CNS with the rest of our body.

The peripheral nervous system can be divided, from a functional point of view, into two parts: the autonomic nervous system, which is composed of sensory and motor fibres that connect the central nervous system (CNS) with the visceral organs, smooth muscles and secretory glands; and the somatic nervous system, which regulates the voluntary functions of the organism and of which we will give more details below.

The somatic nervous system (SNS)

The somatic nervous system is responsible for capturing sensory information from the environment, using the sensory receptors that we have distributed throughout our body (mainly in the head, skin and extremities) and this information is transmitted to the central nervous system (CNS), which is responsible for executing the orders through the motor neurons that conduct nerve impulses to the skeletal muscles.

This system is associated with the voluntary control of body movements , as well as the processing of sensory information coming from the senses (sight, hearing and touch). The somatic nervous system is composed of afferent or sensory nerves and motor or efferent nerves.

The sensory nerves are responsible for transmitting the body’s sensations to the CNS and the motor nerves are responsible for sending the commands of the CNS to the body’s organs, stimulating muscle contraction.

The 43 segments of nerves of which our organism is composed are in the somatic nervous system. Each segment is made up of a sensory nerve and a motor nerve . Of the total, 31 emerge from the spinal cord (spinal nerves), while the remaining 12 emerge from the skull (cranial nerves).

Composition of the SNS

The nerves of which the somatic nervous system is composed can be classified according to where they enter and leave in: cranial nerves, those that emerge directly from the brain or at the level of the brain stem; and spinal nerves, those that emerge from the spinal cord.

Cranial nerves

There are 12 pairs of cranial nerves in the somatic nervous system , which emerge from the brain and are intended to carry sensory information, control certain muscles and regulate some glands and internal organs.

These are the twelve pairs of cranial nerves:

1. Olfactory nerve

It is responsible for receiving olfactory sensory information to transmit it to the olfactory bulb , a structure in the brain that processes and encodes this information to send it to higher structures in the brain.

2. Optic nerve

Receives the visual sensory information to transmit it to higher brain regions in charge of vision.

3. Internal ocular motor nerve

It controls eye movements and regulates processes such as pupil dilation and contraction.

4. Trochlear nerve

It innervates the upper oblique muscle of the eye and its main function is to control eye movements (up and down, and also out).

5. Trigeminal nerve

It has a sensory and a motor portion, and is responsible for receiving somatosensory information (tactile, pain sensations, etc.) from receptors in the face and head, as well as controlling the muscles of mastication.

6. External ocular motor nerve or abducens

Its function is to control the movement of the lateral rectus muscle, allowing the abduction of the eye (turning in the opposite direction to the nose).

7. Facial nerve

It contains both sensory and motor fibers, is responsible for receiving information from the tongue receptors (taste) and somatosensory information from the ears, and manages the movements of the muscles of the neck and face involved in facial expressions.

8. Vestibulocochlear nerve

It is a sensory afferent nerve and is responsible for balance and hearing function .

9. Glossopharyngeal nerve

This nerve emerges from the brain’s spinal cord and receives gustatory information from the back of the tongue, somatosensory information from the tonsils, pharynx, middle ear, and auditory tube. It is also involved in swallowing.

10. Vagus nerve

It emerges from the spinal cord and innervates the pharynx, esophagus, larynx, trachea, bronchi, heart, stomach, pancreas, and liver. It receives sensory information from all these glands and participates in cardiac and digestive processes , sending information to organs and muscles.

11. Spinal accessory nerve

It is a motor nerve that is formed by the junction of a spinal and a neurocranial root. It controls muscles in the neck and head that are used for movement.

12. Hypoglossal nerve

It is mainly in charge of managing language movements .

Spinal nerves

The somatic nervous system is composed of 31 pairs of cranial nerves. These nerves connect organs and muscles to the spinal cord ; they are responsible for transmitting sensory and visceral information to the cord and from the cord to the glands, and to the skeletal and smooth muscles. They innervate the whole body, except for the head and some parts of the neck.

Of the 31 existing pairs, 8 are cervical, 12 thoracic, 5 lumbar, 5 sacral and one coccygeal (located at pelvic floor level). All of them are mixed; that is, they have a sensitive part or root, where the spinal ganglion is located; and another motor part . These two roots join together and form the trunk of the spinal nerve, which emerges from the vertebral canal through the corresponding intervertebral foramen.

In its path, each spinal nerve emits four branches: the meningeal, which innervates the meninges of the spinal cord; the communicating, which connects to the sympathetic ganglia and is responsible for carrying information to the body related to stress and the classic responses of fight or flight; the posterior, which innervates the deep muscles of the back of the trunk and skin; and the anterior, which innervates the muscles and skin of the rest of the trunk and limbs.

Functions

The main functions of the somatic nervous system can be reduced to the following : transmitting sensory information to the brain and connecting the central nervous system to the organs, muscles and skin; sending and transmitting commands to the muscles to produce voluntary movements; and activating involuntary body movements or reflexes.

The process is as follows: the sensory or afferent neurons transmit the electrical impulses to the central nervous system and the brain; then, these stimuli are processed by the central nervous system; and finally, the motor or afferent neurons are in charge of receiving the signal to send it to muscles and organs.

The somatic nervous system, in addition to managing the voluntary movements of the muscles, also controls reflex acts in which there is no direct intervention by the brain .

4. Trochlear nerve

It innervates the upper oblique muscle of the eye and its main function is to control eye movements (up and down, and also out).

5. Trigeminal nerve

It has a sensory and a motor portion, and is responsible for receiving somatosensory information (tactile, pain sensations, etc.) from receptors in the face and head, as well as controlling the muscles of mastication.

6. External ocular motor nerve or abducens

Its function is to control the movement of the lateral rectus muscle, allowing the abduction of the eye (turning in the opposite direction to the nose).

7.
Facial nerve

It contains both sensory and motor fibers, is responsible for receiving information from the tongue receptors (taste) and somatosensory information from the ears, and manages the movements of the muscles of the neck and face involved in facial expressions.

8.
Vestibulocochlear nerve

It is a sensory afferent nerve and is responsible for balance and hearing function .

9. Glossopharyngeal nerve

This nerve emerges from the brain’s spinal cord and receives gustatory information from the back of the tongue, somatosensory information from the tonsils, pharynx, middle ear, and auditory tube. It is also involved in swallowing.

10. Vagus nerve

It emerges from the spinal cord and innervates the pharynx, esophagus, larynx, trachea, bronchi, heart, stomach, pancreas, and liver. It receives sensory information from all these glands and participates in cardiac and digestive processes , sending information to organs and muscles.

11. Spinal accessory nerve

It is a motor nerve that is formed by the junction of a spinal and a neurocranial root.

  • Martin, J. H. (2014). Neuroanatomy: Text and Atlas. AMGH Editor.
  • Moore, K.L and Agur, A.M.R. (2007). Fundamentals of Anatomy with clinical orientation. 2nd edition. Editorial Médica Panamericana