Autonomic nervous system

Autonomic nervous system or voluntary nervous system . It has the important function of controlling all the acts that the organism carries out of its own free will, such as those of the heart , those of the intestine and those of other internal organs. It has two different elements: the central nervous system and the peripheral nervous system.. It is related to the regulation of internal functions, and regarding its ontogeny, it was classified as visceral due to the innervation of structures and organs that derive from the endoderm and mesoderm, such as the viscera, glands, and cardiovascular system, which are controlled or regulated by the SNA. It is closely associated with the nervous system of relationship life, which innervates structures and organs derived from the ectoderm and mesoderm, such as the skin and the locomotor system, which is why it is called somatic. The Autonomous Nervous System is also represented by nerves, ganglia, and plexuses that supply the heart., the blood vessels, glands, other visceral organs and smooth muscles. It has a wide distribution throughout the body and controls the so-called automatic or vegetative functions, which are not easily modifiable at will, which does not mean that there is no subordination to higher structures.


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  • 1 Central nervous system
  • 2 Microscopic structure of the vegetative or autonomous nodes and their comparison with other nodes
  • 3 Morphofunctional characteristics of the sympathetic division
    • 1 Characteristics of the synapses. Neurotransmitters. Postsynaptic receptors. Enzymes
  • 4 Morphofunctional characteristics of the parasympathetic division
    • 1 Structure
    • 2 General function
    • 3 Postsynaptic receptors
    • 4 Enzyme
  • 5 Functions
    • 1 Sympathetic Nervous System
    • 2 Parasympathetic System
    • 3 Spinal cord
  • 6 Source

Central Nervous System

In the central nervous system can distinguish two nerve centers: the brain and spinal cord . The first one is located inside the skull and consists of millions of neurons grouped together forming a unit, within which the various nerve functions take place. The most relevant neuronal groupings are the medulla oblongata , the cerebellum, and the brain .

Microscopic structure of the vegetative or autonomous nodes and their comparison with other nodes

The autonomic or vegetative nodes are of variable size, there are very small ones, such as the intramural of the parasympathetic, located on the wall of the viscera, which may have few neurons. These nodes do not have a well-defined capsule. In larger autonomic nodes, such as sympathetic chains, there is a well-defined connective tissue capsule and a larger number of neurons. The neurons of the vegetative ganglia are multipolar, smaller than those of the craniospinal ganglia, the nerve fibers are unmyelinated (when they come out of the ganglion they form the gray communicating branches) and are distributed irregularly intermingled with the neuronal somata. The neuronal bodies are crashed, the nucleus is eccentric, and presents the typical characteristics of those of neurons: it is voluminous, spherical, vesicular and with a prominent nucleolus. In the cytoplasm, the typical organs of neurons are present, with the difference that the lipofucsin granules are not as evident as in the neurons of the craniospinal ganglia.

Morphofunctional characteristics of the sympathetic division

Autonomic nervous system

The sympathetic division of the autonomic nervous system has a central and a peripheral potion. The central portion is made up of a series of nuclei located in the lateral horns of the spinal cord from TI to L II and the peripheral portion by the sympathetic trunks (formed by a chain of ganglia interconnected by fibers and located on both sides of the spine vertebral), in addition prevertebral nodes such as celiac, upper and lower mesenteric, plexuses and nerves. It constitutes most of the SNV, including in its peripheral portion the two sympathetic ganglionic trunks, its branches, plexuses and second and third order ganglia. It has a wider distribution than the parasympathetic since it innervates all the sweat glands of the skin, the piloerector muscles, the muscle walls of most blood vessels, the heart, lungs, and other viscera. Its distribution is universal. The sympathetic division has thoracolumbar outlets to ganglia along the spinal cord. The sympathetic preganglionic neurons have their body in the lateral (or intermediate lateral) horns of the spinal cord, from segment T1 to L23 (thoracic or dorsal1 medullary segments to lumbar23). The axons of these neurons leave the medulla at the ventral roots and then with the spinal nerve. They can synapse with sympathetic postganglionic neurons that are found in the sympathetic ganglion chain, on either side of the spinal cord, or in three prevertebral ganglia, or in differentiated neurons in endocrine cells of the adrenal medulla. The axons of the preganglionics leave the medulla at the specific level, but they can innervate contiguous ganglia, rostral and caudally, traveling through the nerve trunks that connect to the ganglia. Most of the preganglionic muscles are small, slow myelin fibers. Each preganglionic synapse with many postganglionic synapses in different nodes. The ratio is from 1 to 10, so there is a great divergence. Postganglionic are non-myelinic and leave the ganglion for a communicating non-myelinic branch. From the point of view of the macroscopic structure, the peripheral part of the sympathetic system is made up of a long double chain of ganglia located on each side of the spinal column joined by thin cords, the sympathetic trunks and a series of prevertebral ganglia such as celiac nodes, the superior mesenteric node and the inferior mesenteric node, among others of less importance. The cervical sympathetic chain is made up of 3 nodes: 1) superior cervical node, the largest at the level of the 2nd and 3rd cervical vertebrae; 2) the middle cervical ganglion, sometimes absent, at the level of the 6th cervical vertebra or in the vicinity of the inferior thyroid artery when it approaches the thyroid gland through its lower pole; 3) the lower cervical node, which is generally fused with the first thoracic node, receiving the name of cervicothoracic node, or stellar node. It is located in front of the head of the rib, in the vicinity of the origin of the vertebral artery. The sympathetic system has, in addition to the sympathetic chain, 2 pure sympathetic nerves, the major splanchnic nerve originating from branches coming from the thoracic ganglia from 6 to 9 and the minor splanchnic nerve originating from branches coming from the thoracic ganglia 10 and 11, both together or separately, cross the diaphragm (lumbar portion) and end in the corresponding celiac node. They generally supply the smooth muscles and blood vessels of the small and large intestine. The thoracolumbar sympathetic chain still has 4 defined lumbar nodes, a pelvic portion with sacral nodes, medial to the anterior sacral holes and at the end an odd coccygeal node in front and sometimes below the coccyx. they cross the diaphragm (lumbar portion) and end in the corresponding celiac ganglion. They generally supply the smooth muscles and blood vessels of the small and large intestine. The thoracolumbar sympathetic chain still has 4 defined lumbar nodes, a pelvic portion with sacral nodes, medial to the anterior sacral holes and at the end an odd coccygeal node in front and sometimes below the coccyx. they cross the diaphragm (lumbar portion) and end in the corresponding celiac ganglion. They generally supply the smooth muscles and blood vessels of the small and large intestine. The thoracolumbar sympathetic chain still has 4 defined lumbar nodes, a pelvic portion with sacral nodes, medial to the anterior sacral holes and at the end an odd coccygeal node in front and sometimes below the coccyx.

Synapse characteristics. Neurotransmitters. Postsynaptic receptors. Enzymes

Brain: Autonomous Nervous System

Synapses of the sympathetic system are more complex than those of the parasympathetic system, while offering more possibilities of specific pharmacological action on them. The sympathetic preganglionic neuron has the same characteristics as the parasympathetic: both release acetylcholine. Furthermore, nicotinic receptors and acetylcholinesterase itself also correspond to this acetylcholine. Therefore, it can be affirmed that the synapse in the ganglion of the sympathetic system is functionally the same as in the parasympathetic. Sympathetic postganglionic neurons generally release norepinephrine, except for a few that innervate the sweat glands and muscle blood vessels; Acetylcholine (sympathetic cholinergic) is released in these structures. The adrenal medulla, It can be considered as a differentiated sympathetic node in the endocrine gland, it releases the hormones adrenaline and, to a lesser extent, norepinephrine. Consequently, most sympathetic postganglionic neurons are called adrenergic (noradrenergic), because they release norepinephrine at their terminations. The adrenergic receptors are classified into two types: alpha, related to adrenaline, but mainly to norepinephrine and beta, related almost exclusively to adrenaline. In turn, each type can be subdivided into the corresponding subtypes. The enzyme in the synapse in the ganglion is acetylcholinesterase. In adrenergic synapses, there are two types of enzymes that degrade norepinephrine and epinephrine: monoamine oxidase (MAO), that it is located in the sympathetic nerve endings and destroys the norepinephrine that is recaptured, as well as in the effectors, where it destroys it and also the adrenaline that penetrates; and catecholortomethyltransferase (COMT), which is found in all tissues, but especially the liver. Stimulation of the sympathetic causes responses of pupillary dilation, increased heart rate and the force of contraction of the heart, decreases the secretion and contraction of glands and muscles of the digestive tract, dilates the bronchi and causes sweating, as well as metabolic responses that provide energy . Being sympathetic stimulation generally “en masse”, it involves the stimulation of both adrenal medullae, resulting in an increased release of the hormones adrenaline and norepinephrine, those that will reach the target tissues with some delay in relation to the effects of the released norepinephrine on the nerve endings, making the effects of sympathetic stimulation more energetic and lasting. Furthermore, these hormones can reach tissues that have adrenergic receptors, but not sympathetic innervation. In conclusion, adrenal hormones potentiate the effects of sympathetic stimulation making them more durable and energetic and allow it to reach places that would not otherwise receive this regulatory influence. On the other hand, its main hormone, adrenaline, has a predominant effect on beta receptors (which are the most abundant in the heart), determining that cardiac stimulation receives an important contribution from the adrenals. This has practical medical importance,

Morphofunctional characteristics of the parasympathetic division

Brain: Autonomous Nervous System

The parasympathetic division of the ANS is made up of a central and a peripheral portion. The central portion in turn is divided into a cranial and sacral portion. The cranial portion is made up of parasympathetic nuclei of the cranial nerves III, VII, IX and X, as well as nuclei located in the sacral spinal cord, in the lateral intermediate zones of the segments from S II to S IV. The peripheral portion is made up of the axons that start from the vegetative nuclei of those cranial nerves, the ganglia related to these nerves such as the ciliary, pterygopalatine, otic, and submandibular, as well as the pre and postganglionic fibers that reach the organs that supply these nerves and on the other hand at the sacral level is made up of the nerves, branches and plexuses that reach pelvic organs such as the rectum,


The parasympathetic preganglionic neurons have their body in the brainstem nuclei corresponding to the cranial pairs III, VII, IX and X. (Fig. 6) and in the lateral areas of the sacral medulla, segments S2 to S3 (sometimes it includes S1 and S4). The axons of these neurons leave the CNS and synapse with parasympathetic postganglionic neurons located in peripheral ganglia or plexuses that are in the vicinity or on the wall of the organs they supply. These organs are located mainly in the middle part of the body (head and trunk). Unlike SS, the axons of parasympathetic preganglionic neurons are long, while the axons of postganglionic neurons are short.

The parasympathetic cranial nuclei have already been studied when analyzing the cranial nerves, only the autonomous components in them should be remembered. The ganglia involved in the cranial nerves III, VII, IX and X.

The III NC (Common ocular motor), participates in the innervation of the intrinsic muscles of the eye, therefore participates in the processes of accommodation and pupillary control, as studied in the visual system. The Facial (VII). It supplies the submandibular and sublingual glands (both are salivary), lacrimal glands, and the nasopalatine mucosa.

The glossopharyngeal (IX). Innervates the parotid gland.

The Vagus (X), innervates the viscera of the neck, chest and abdomen.

The anterior branches of the second, third and often fourth sacral spinal nerves emit visceral branches (with the name of pelvic nerves, erector nerves) that go directly to the pelvic viscera joining the pelvic sympathetic branches and pelvic splanchnic nerves to form the Pelvic or hypogastric Plexus. The preganglionic fibers that come from sacral segments 2 to 4 synapse with tiny parasympathetic ganglia located on the walls of each pelvic viscera. The pelvic erector nerves supply the muscles of the bladder walls with motor fibers and its sphincter with inhibitory fibers, the erectile tissue of the penis and the clitoris with vasodilator and possibly inhibitory fibers.

General function

The regulatory function of SPS is generally associated with digestive and evacuation activities, the stimulation of which usually promotes energy conservation or synthesis reactions in the body. This is why it is sometimes claimed that the parasympathetic nervous system is an anabolic regulatory system. Its activity is generally of a discrete type, this means that it is capable of regulating one function with relative independence from the others. Eg activation of the urination reflex is carried out independently of the digestive reflexes regulated by the parasympathetic nervous system.

Postsynaptic receptors

These receptors, although specific to acetylcholine (cholinergics), are different depending on their location. Those in the node are also stimulated by nicotine, which is why they are called nicotinic receptors, being almost the same as those found in the motor plate. Instead, the receptors in the effectors are stimulated by the muscarine toxin, which is why they are called muscarinic receptors. It should be noted that nicotine and muscarine are NOT natural substances in the body; but toxins that are used experimentally.


The enzyme, both in the lymph node synapse and in the effector, is acetylcholinesterase. Parasympathetic stimulation produces contraction of the pupillary sphincter, which produces miosis, and of the ciliary muscle, which produces accommodation of near vision. Decreases heart rate, increases lacrimal, salivary and digestive tract secretion, as well as digestive motility, produces broncho constriction.



The Autonomous Nervous System performs two very important functions that complement each other, one to accelerate and the other to slow down the internal activities of the body. This is very important because if it were not, the body could lose control. For example, when exercising, the heart accelerates to bring more oxygen and nutrients to the muscles , but the heart cannot keep up that rhythm all the time because it would “explode”, so the other function of this system is to slow it down when You stop exercising and allow yourself to regain your normal rhythm. The functioning of the whole organism depends on the nervous systemas a whole, none of the organs and “systems” that make it up work in isolation, if one fails, the others fail. Involuntary functions are coordinated by the Autonomous Nervous System, which in turn is divided into two systems:

  • The Sympathetic Nervous System
  • Parasympathetic System

That they carry out opposite actions of the same function.

Sympathetic Nervous System

Sympathetic Nervous System (SS) (or thoracolumbar) has a chain of ganglia (group of neurons) interconnected on either side of the spinal column, which sends nerve fibers to several larger nodes, such as the celiac ganglion. These, in turn, give rise to nerves that go to the internal organs. The ganglia of the sympathetic chains connect to the central nervous system through fine branches that connect each ganglion with the spinal cord.The Sympathetic Nervous System acts in cases of emergency and stress causing various reactions such as acceleration of the pulse and respiration, slows digestion , increases blood pressure and causes blood to reach the brain in greater quantity ,legs and arms , it also increases the blood sugar level. All this is done to prepare the person to make the most of their energy and to act in special situations.

Parasympathetic System

The Parasympathetic Nervous System (SPS) (or craniosacral). The fibers of the cranial parasympathetic, especially the nerves of the cranial nerves III, VII, IX and X, pass to the ganglia and plexuses located within various organs.The Parasympathetic System, on the other hand, stores and conserves energy and maintains the normal rhythm of the organs and glands of the body. After a scare, trauma, intense pain or any special situation of the body, the Parasympathetic ensures that everything returns to calm and normal. Of these two, obviously the Parasympathetic is the most important to survive, because if it did not normalize the functions, the body could not support them. The sympathetic nerves originate in different parts of the spinal cord, while the parasympathetic nerves originate in thenerve cranial and spinal. Most parasympathetic nerve fibers are found in a nerve called the “bum” that reaches all of the body’s internal organs. Among some of the functions performed by these two systems are:

  • The sympathetic opens the pupil of the eye, the parasympathetic closes it.
  • The sympathetic stimulates the production of saliva, the parasympathetic inhibits it.
  • The sympathetic makes the heart beat very fast and strong, the parasympathetic reduces the beating and its intensity.
  • The sympathetic relaxes the muscle so that you can urinate or defecate, the parasympathetic contracts them to close.

Spinal cord

The spinal cord is that part of the central nervous system that is located inside the spinal column . It is a kind of cord that extends from the medulla oblongata, where it joins the brain, to the beginning of the lumbar region . The medulla consists of a large number of elongated neurons. All these elements (brain and spinal cord) are surrounded by the meninges , which are protective membranes. The set of nerves that leave the brain and spinal cord and are distributed throughout the body constitute the peripheral nervous system. The nerves can be of two types, the sensory or sensory and the motor. The former are the conduction path for nerve impulses that transmit the preceding information from the sense organs to the central nervous system . Once there the information is conveniently interpreted, preparing a response that is driven by the motor nerves to the muscles or the organs in charge of executing it.


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