The peripheral nervous system is made up of nerves and ganglia that connect the central nervous system to the rest of the body and control voluntary movements, digestion, or the fight-or-flight response.
In this article we will describe the peripheral nervous system and its two subdivisions : the autonomic or vegetative nervous system and the somatic.
What is the peripheral nervous system?
The nervous system of animals, including humans, is concerned with the transmission of electrochemical impulses that enable a large number of biological processes to function. It is divided into two sets of connections: the central nervous system, composed of the brain and 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 fibres 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 the internal organs, smooth muscles and physiological functions such as digestion, and the somatic system, composed mainly of the cranial and spinal nerves.
Unlike the central nervous system, the peripheral nervous system n is either protected by the skull, spine and blood-brain barrier . This makes it more vulnerable to different types of threats, such as traumatic injuries or exposure to toxins.
The vegetative or autonomic nervous system
The autonomic, vegetative or involuntary nervous system is composed of sensory and motor fibres which connect the central nervous system to the smooth and cardiac muscles , as well as to the exocrine glands, which are found throughout the body and fulfil idiosyncratic functions.
The 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 of the skin, in the blood vessels, in the walls of the digestive system and in the sphincters of the urinary and biliary vesicles.
Through the action of the autonomic nervous system the control of digestion, heart and respiratory rate , urination, sexual response and fight-flight reaction is produced. This process, also known as “acute stress response”, consists of a discharge of neurotransmitters with a protective function in the face of threats.
Also depending on the vegetative system are the autonomous or visceral reflexes , a series of automatic responses that appear as a consequence of certain types of stimulation. Among these are the ocular, cardiovascular, glandular, urogenital and gastrointestinal reflexes, mainly peristalsis.
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 the maintenance of homeostasis or balance of the internal environment of the body. However, there is a third branch that is often left out: the enteric nervous system, which is 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 the body to allow functions such as the release of catecholamines, bronchodilation or mydriasis (pupil dilation). The parasympathetic system controls the relaxation of the sphincters , digestion or myosis (pupil contraction).
These two branches of the autonomic nervous system always act together; however, different physiological stimuli and signals can cause them to become unbalanced so that the functions of one branch predominate over those of the other. For example, sexual arousal responses are associated with activation of the parasympathetic system.
For its part, the enteric nervous system deals with the innervation (both sensory and motor) of the digestive tract, the pancreas and the gall bladder, and therefore with the control of the smooth muscles, the blood vessels and 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 bundles of nerve fibers, i.e. nerve axons, and therefore specialize in the transmission of electrochemical impulses. Nerve ganglia are composed of the somas or cell bodies of the neurons of the peripheral nervous system; in them takes place the relay of signals between the different structures of the nervous system.
This subdivision of the peripheral nervous system is related to the voluntary control of skeletal muscle contraction , as well as to that of the reflex arcs, which allow the execution of automatic responses by the motor neurons themselves, before the central nervous system receives the corresponding sensory afferences.
Cranial and spinal nerves
The 43 pairs of nerves in the human body constitute the somatic nervous system. Of these, 12 are found in the brain stem and 31 in the spinal cord , both in its dorsal and ventral roots. The former are called “cranial nerves” and the latter “spinal nerves”.
The transmission of information between the brain and the peripheral nervous system occurs through the 12 cranial pairs: the olfactory (I), optic (II), oculomotor (III), pathetic or trochlear (IV), trigeminal (V), abducens (VI), facial (VII), vestibulocochlear or auditory (VIII), glossopharyngeal (IX), vagus or pneumogastric (X), accessory (XI) and hypoglossal (XII).
The spinal nerves connect the spinal cord to the rest of the body. While the nerves that send sensory information to the central nervous system are located in the dorsal or posterior root of the cord, the somas of motor or efferent neurons are located in their ventral horns .