Peripheral Nervous System (autonomic And Somatic): Parts And Functions
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-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.
The nervous system of animals, including humans, is responsible for the transmission of electrochemical impulses that allow the functioning of a large number of biological processes. It is divided into two sets of connections: the central nervous system, made up 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 fibers that make up the peripheral nervous system connect the brain and spinal cord with the rest of the body making possible the exchange of electrochemical signals with the entire body.
In turn, the peripheral nervous system consists of two subdivisions: the autonomic nervous system, which controls internal organs, smooth muscles and physiological functions such as digestion, and the somatic nervous system, composed mainly of cranial and spinal nerves.
Unlike the central nervous system, the peripheral is not 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 autonomic, vegetative or involuntary nervous system is composed of sensory and motor fibers that They connect the central nervous system with the smooth and cardiac muscles as well as with exocrine glands, which are found throughout the body and fulfill idiosyncratic functions.
Smooth muscles are located in the eyes, where they are associated with the dilation and contraction of the pupil and the 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 gallbladder.
Through the action of the autonomic nervous system, control of digestion, heart rate and respiration, urination, sexual response and fight-flight reaction. This process, also known as the “acute stress response,” consists of a release 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 consequence of certain types of stimulation. Among these we find 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 maintaining homeostasis or balance of the body’s internal environment. However, there is a third branch that is frequently left aside: the enteric nervous system, responsible for the functioning of the intestinal tract
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 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 vessels and mucous membranes that are located in these regions.
The somatic nervous system
The somatic nervous system is made up 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, neuronal axons, which is why they specialize in the transmission of electrochemical impulses. The nervous ganglia are composed of the somas or cell 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 motor neurons themselves, before the central nervous system receives the corresponding sensory inputs.
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 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 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 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 spinal cord, the somas of motor or efferent neurons are located in their ventral horns
