Autonomous nervous system

This system daily emits diverse instructions that allow us to deal with the exterior world, but it also helps maintain the inner balance of the body.

The Human Body Chapter 5: Peripheral Nervous System Peripheral Nervous System Somatic nervous system Spinal nerves Cranial nerves Responses and reflexes Peripheral Nervous System Diseases What you must know Láminas bilingües para imprimir / Bilingual laminae to print

Imágenes The ANS includes sympathetic ganglions (in the picture) and parasympathetic ganglions in its organization. Picture of a nerve ganglion LÁMINAS Autonomous nervous system Ganglions: Connection structures

It is in charge of the regulation of the involuntary functions of the body, maintaining inner homeostasis (body equilibrium) and of the adaptation responses when faced with changes of the external or internal environment. From this point of view, the autonomous nervous system (ANS) is a clearly efferent system, which transmits impulses from the central nervous system (CNS) to the peripheral organs. Among the actions taken place we find: heart rate control, the contraction and dilation of the blood vessels, the contraction and relaxation of the smooth muscles of several organs, placement of the crystalline (in the eye), the regulation of pupil size, urinary and sweat emission, the secretion of exocrine and endocrine glands. The ANS exercises control over the functions of many organs and tissues in the body and along with the endocrine system allows fine internal adjustments to take place, necessary to maintain an optimum internal environment in the body. Just like the somatic nervous system, the ANS has afferent (sensory) neurons, connection and association ones, and efferent (motor) neurons. The afferent impulses originate in the receptors located, mainly, in the viscera and travel through afferent ways towards the CNS, where they are integrated by means of connection neurons at different levels, to then go out through efferent (motor) ways to the visceral effector organs. The autonomous system runs its effectors through nerves made up of two neurons, the preganglionic and postganglionic. The cellular bodies of the first are located at diverse sites of the grey matter of the spinal cord and in the motor nuclei of cranial nerves III (oculomotor n.), VII (facial n.), IX (glossopharyngeal n.) and X (vagus n.). The axons of these cellular bodies produce synapses over the cellular bodies of the postganglionic neurons, which get together to form the ganglions outside the central nervous system.

The two divisions

The ANS is made up of two divisions, the sympathetic and parasympathetic. Both are formed by preganglionic neurons that are in the CNS and by postganglionic ones located in the CNS. The number of synapses between preganglionic neurons and postganglionic ones from both divisions varies in relation to the number of neurons that participate. In the sympathetic, a preganglionic neuron establishes synapse with many postganglionic neurons, while in the parasympathetic, it makes a connection with a few postganglionic neurons. Due to the location of the preganglionic neurons of the sympathetic in the grey matter of the spinal cord, between the thoracic-lumbar segments (from T1 to L2), the sympathetic is also called the thoracolumbar system. On the other hand, the preganglionic neurons of the parasympathetic are in the encephalic trunk and in the lumbo-sacral spinal cord (between S2 and S4) and because of it this system is called craniosacral. In general, the sympathetic and parasympathetic integrate functionally in the regulation of diverse organs, carrying out opposite functions most of the time. Specifically, the sympathetic system prepares the body for emergency situations (speeding up functions) and the parasympathetic springs into action in relaxed situations (slowing down functions). Generally, the processes favored by sympathetic neurons tend to involve a loss of energy, while the parasympathetic stimuli reestablish and preserve the energy of the body.

Parasympathetic nervous system

Contrary to what goes on in the sympathetic, the parasympathetic nervous system is related to resting processes and its activation is aimed at saving energy. It gives digestion priority (acceleration of the digestive movements and enzyme secretion), in order to allow the body to count with the necessary energetic raw materials. Along these lines, parasympathetic activation produces, among other things, the decrease of the heart rate, increase in the secretion of digestive enzymes and insulin, among others. The parasympathetic nervous system also participates in the regulation of the sexual response and has to do with the stimulation of these organs (erection). In order for these and several other parasympathetic actions to take place, each preganglionic neuron forms a synapse with a few postganglionic neurons, which, combined with the proximity of the synapse to the innervated organ, makes the parasympathetic stimulation more localized than that of the sympathetic, which is more diffuse.

Sympathetic nervous system

The sympathetic system is the larger of the two divisions of the autonomous nervous system and is widely distributed all over the body because it innervates the heart and lungs, the muscles of the walls of many blood vessels, the erectores pilorum, the sweat glands and many viscera of the abdomen and pelvis. The sympathetic nervous system plays a fundamental role in the preservation of the body due to the fact that it prepares the body for an emergency, generating a fast and massive response to exterior stimuli that may threaten the integrity of the individual. This generalized reaction throughout the body is known as the alarm or stress (also known as fight or flight) reaction. In this sense, among its actions we find: acceleration of the heart and respiratory rates, the dilation of the respiratory canals, of the blood vessels of the skeletal muscles; the pilous erection and cutaneous sweating, among others. In order for all of these responses to take place, each sympathetic preganglionic neuron has synapse with many postganglionic neurons (from 20 to 30) which spread out through different organs. This is how the massive response of the sympathetic stimulation throughout the body is explained; a response that, at the same time, is augmented by the release of a neurotransmission substance, noradrenaline (or norepinephrine) on the part of the medulla of the suprarenal gland.

PALABRAS CLAVE

• "Autonomous nervous system"
• "Parasympathetic nervous system"
• "Sympathetic nervous system"
• "Enteric nervous system"
• Neurotransmitters

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