The peripheral nervous system (autonomic and somatic)

The peripheral nervous system is made up of nerves and ganglia that connect the nervous system. central with the rest of the body and control voluntary movements, digestion or response of fight-flight.

In this article we will describe the peripheral nervous system and its two subdivisions: the autonomic or vegetative nervous system and the somatic one.

What is the peripheral nervous system?

The nervous system of animals, including humans, is concerned with the transmission of electrochemical impulses that allow a large number of biological processes to function. It is divided into two sets of connections: the central nervous system, made up of brain and the spinal cord, and the peripheral nervous system.

The term "peripheral" denotes the location of the components of this neural network in relation to the central nervous system. The neurons and fibers that make up the peripheral nervous system connect the brain and spinal cord to the rest of the body, making it possible to exchange electrochemical signals with the whole body.

In turn, the peripheral nervous system consists of two subdivisions: the autonomic nervous system, which controls internal organs, the smooth muscles and physiological functions such as digestion, and somatic, composed mainly of the cranial nerves and spinal.

Unlike the central nervous system, the peripheral not protected by the skull, spine, and blood-brain barrier. This makes you more vulnerable to different types of threats, such as traumatic injuries or exposure to toxins.

The autonomic or vegetative nervous system

The autonomic nervous system, vegetative or involuntary, is composed of sensory and motor fibers that connect the central nervous system with the smooth and cardiac muscles, as well as with the exocrine glands, which are found throughout the body and fulfill idiosyncratic functions.

Smooth muscles are located in the eyes, where they are associated with dilation and contraction of the pupil and accommodation of the lens, in the hair follicles in the skin, in blood vessels, in the walls of the digestive system and in the sphincters of the urinary gallbladder and bile.

Through the action of the autonomic nervous system, control of digestion, heart rate and breathing, urination, sexual response and fight-flight reaction. This process, also known as the “acute stress response”, consists of a discharge of neurotransmitters with a protective function against threats.

They also depend on the vegetative system autonomic or visceral reflexes, a series of automatic responses that appear as a result of certain types of stimulation. Among these we find the ocular, cardiovascular, glandular, urogenital and gastrointestinal reflexes, mainly peristalsis.

• You may be interested: "The 12 primitive reflexes of babies

The sympathetic, parasympathetic, and enteric branches

The subdivision of the autonomic nervous system into two branches is well known: the sympathetic and the parasympathetic, responsible for maintaining homeostasis or balance of the internal environment of the organism. However, there is a third branch that is frequently neglected: the enteric nervous system, responsible for the functioning of the intestinal tract.

The activation of the sympathetic nervous system is related to the fight-flight response: it increases energy consumption by of the body to allow functions such as catecholamine release, bronchodilation, or mydriasis (pupillary dilation). The parasympathetic system controls sphincter relaxation, digestion or miosis (pupillary contraction).

These two branches of the autonomic nervous system always act together; However, different stimuli and physiological signals can cause them to become unbalanced so that the functions of one of them predominate over those of the other. For example, sexual arousal responses are associated with the activation of the parasympathetic system.

For its part, the enteric nervous system is responsible for the innervation (both sensory and motor) of the digestive tract, the pancreas and the gallbladder, and therefore the control of smooth muscles, blood vesselsand of the mucous membranes that are located in these regions.

The somatic nervous system

The somatic nervous system is composed of nerves and ganglia with sensory and motor functions that allow the connection between the central nervous system and the rest of the body.

Nerves are sets of nerve fibers, that is, of neuronal axons, which is why they specialize in the transmission of electrochemical impulses. Nerve ganglia are composed of the cell bodies or bodies of the neurons of the peripheral nervous system; in them the relay of signals between the different structures of the nervous system takes place.

This subdivision of the peripheral nervous system is related to the voluntary control of skeletal muscle contraction, as well as with the reflex arcs, which allow the execution of automatic responses by the own motor neurons, before the central nervous system receives the sensory inputs corresponding.

The cranial and spinal nerves

The 43 pairs of nerves in the human body make up the somatic nervous system. Of these, 12 are found in the brainstem and 31 in the spinal cord, both in its dorsal and ventral roots. The former are called "cranial nerves" and the latter the "spinal or spinal nerves".

The transmission of information between the brain and the peripheral nervous system occurs through the 12 cranial nerves: the olfactory (I), the optic (II), the oculomotor (III), the pathetic or trochlear (IV), the trigeminal (V), the abducens (VI), the facial (VII), the vestibulocochlear or auditory (VIII), the glossopharyngeal (IX), the vagus or pneumogastric (X), the accessory (XI) and the hypoglossal (XII).

The spinal or spinal nerves connect the spinal cord to the rest of the body. While the nerves that send afferent sensory information to the central nervous system are found in the dorsal or posterior root of the medulla, the somas of motor or efferent neurons are located in their ventral horns.