Somatic nervous system: parts, functions and characteristics

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

It is the main system for managing voluntary movements and the nerve center of dozens of sensory nerves. and motors that enter and exit 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.

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

The nervious system

The somatic nervous system is part of a larger whole, the nervous system, guarantor of 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 to transmit, process the information and generate effective responses depending on what each situation requires.

From an anatomical point of view, 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 made up of sensory and motor fibers that connect the central nervous system (CNS) with visceral organs, smooth muscles, and glands secretory; and the somatic nervous system, which regulates the voluntary functions of the body 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 sensory receptors that we have distributed throughout our body (mainly in the head, skin and extremities) and that information is transmitted to the system central nervous (CNS), which is responsible for executing orders through motor neurons that conduct nerve impulses to muscles skeletal.

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

The sensory nerves are responsible for transmitting bodily sensations to the CNS and motor nerves. They are responsible for sending orders from the CNS to the body's organs, stimulating muscle contraction.

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

Composition of the SNS

The nerves of which the somatic nervous system is composed can be classified according to the place of where they enter and leave in: cranial nerves, those that emerge directly from the brain or at the level of the trunk cerebral; 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 aim to transport 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 of the brain that is responsible for processing and encoding said information to send it to higher structures of the brain.

• You may be interested: "Olfactory bulb: definition, parts and functions"

2. Optic nerve

Receive visual sensory information to transmit it to higher brain regions responsible for vision.

3. Internal ocular motor nerve

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

4. Trochlear nerve

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

5. Trigeminal nerve

It has a sensitive and a motor portion, and is responsible for receiving somatosensory information (tactile sensations, pain, etc.) of the receptors of the face and head, in addition to controlling the muscles of chewing.

6. External ocular motor nerve or abducens

Its function is control lateral rectus muscle movement, allowing abduction of the eye (turn away from the nose).

7. Facial nerve

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

8. Vestibulocochlear nerve

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

9. Glossopharyngeal nerve

This nerve emerges from the medulla oblongata and receives taste information from the posterior part. of the tongue, somatosensory information from the tonsils, pharynx, middle ear and tube auditory. It is also involved in swallowing.

10. Vagus nerve

It emerges from the medulla oblongata and innervates the pharynx, esophagus, larynx, trachea, bronchi, heart, stomach, pancreas, and liver. Receive sensitive information from all these glands and participate 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 union of a spinal root and a neurocranial one. It controls the muscles of the neck and head that are used for its movement.

12. Hypoglossal nerve

It is mainly responsible for manage tongue movements.

Spinal nerves

The somatic nervous system is made up 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 spinal cord and from it to the glands, and to the skeletal and smooth muscles. They innervate the entire body, except for the head and some parts of the neck.

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

Along its path, each spinal nerve emits four branches: the meningeal, which innervates the meninges of the spinal cord; the communicators, which connect with the sympathetic ganglia and are responsible for carrying information to the body related to stress and the classic fight or flight responses; the posterior ones, which innervate the deep muscles of the dorsum of the trunk and the skin; and the former, which innervate the muscles and skin of the rest of the trunk and limbs.

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

Features

The main functions of the somatic nervous system can be reduced to the following: transmit sensory information to the brain and connect the central nervous system with organs, muscles and skin; send and transmit commands to the muscles to produce voluntary movements; and activate involuntary body movements or reflexes.

The process is as follows: sensory or afferent neurons transmit 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 efferent neurons are in charge of receiving the signal to send it to muscles and organs.

The somatic nervous system, in addition to managing voluntary movements of the muscles, it also controls reflex acts in which there is no direct brain intervention. This occurs when a nerve pathway connects directly through the spinal cord. For example, the withdrawal reflex when we put our hand in a fire and burn ourselves or the knee reflex, when we are hit with a hammer at the level of the patellar tendon.

SNS diseases

Disorders that affect the somatic nervous system they can seriously incapacitate the person who suffers them. Some of the most common are listed below:

1. Disc herniation

Herniated disc occurs when one of the discs in the spine is damaged. The disc may slip out of place (herniate) or rupture from injury or strain. This creates excess pressure on the spinal nerves, causing pain, numbness, or weakness in the patient.

Hernias can occur at any level of the spinal cord, and symptoms will vary depending on where the injury or displacement of the disc occurs. When symptoms occur in the legs, the disorder is called sciatica.

2. Neuralgia

Neuralgia is pain affecting the nerves of the face, skull, or neck, due to irritation, compression or infection thereof. It is one of the most common neuropathies (diseases of the nervous system).

The most common symptoms are more or less intense pain in different parts of the body, similar to an electric shock. This pain comes and goes suddenly, usually due to innocuous stimuli such as washing the face or chewing, and usually lasts a few minutes.

3. Spinal stenosis

Spinal stenosis involves a narrowing and constriction of the spinal canal (which houses the spinal cord) due to arthritis that causes the bones of the vertebrae to overgrow and the ligaments to widen. When the growth is excessive, there can be pinching and compression on the nerves of the spine, causing pain and loss of sensation in the patient.

The most common causes of spinal stenosis are: aging, arthritis (bone and rheumatoid), inherited conditions (such as scoliosis or a narrow spinal canal) and tumors, injuries, or fractures vertebral.

Bibliographic references:

• Brodal, P. (2004). The central nervous system: structure and function. Oxford University Press.
• 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. Panamerican Medical Publishing House