The autonomic nervous system (physiology)

The autonomic nervous system (ANS) (also known as the autonomic nervous system, ANS) is part of the peripheral nervous system together with the somatic efferent (motor) system (carrying impulses from the CNS to the will-controlled striated muscles) and sensory (afferent) fibers (conducting sensation and pain). VNS ensures the transfer of impulses between the central nervous system (CNS) and effector tissues independent of volitional control (smooth muscle, myocardium, exocrine glands...).

The vegetative nervous system is divided into motor (efferent) and sensory (afferent) parts. Although afferent fibers play an important role in the regulation of homeostasis (e.g. cholinergic fibers from baroreceptors in the glomus caroticum ), they are pharmacologically affected by a minimum of drugs. Attention is therefore focused on the study of the efferent part. It anatomically consists of 2 systems - sympathetic and parasympathetic. A somewhat separate unit is the enteric nervous system (ENS), which, because of its relative independence from other parts of the VNS, is often singled out as an independent third part of the VNS with a specific function in the control of function gastrointestinal tract.

ANS anatomy
The efferent (motor) pathways of the ANS consist of 2 neurons, which are interpolated in the vegetative ganglia – preganglionic and postganglionic neurons are distinguished according to their position. There are significant differences in the arrangement of these pathways between the sympathetic and parasympathetic.

Sympathicus
The bodies of the sympathetic preganglionic neurons are located in the lateral corners of the gray matter of the spinal cord in the area of ​​the thoracic and lumbar segments (Th1-L3). The axons of these neurons leave the spinal cord in the corresponding segments, therefore the sympathetic system is referred to as the thoraco-lumbar system. The sympathetic autonomic ganglia are well organized and form two paravertebral chains along the spine. Excitations to the abdominal and pelvic organs are interpolated in ganglia located prevertebrally. It follows from the above that the axons of preganglionic neurons are much shorter than the axons of postganglionic neurons.

Parasympathicus
The preganglionic fibers of the parasympathetic leave the CNS via some cranial nerves(n. III, VII, IX and X) and the anterior horns of the spinal cord in the area of ​​the sacral segments (S2–S4) – the parasympathetic is therefore referred to as  the cranio-sacral system. Vegetative parasympathetic ganglia are located mainly in the wall of effector organs, preganglionic fibers are therefore much longer than axons of postganglionic neurons. With exceptions (ggl. cilliare, submandibulare and pelvic ganglia), parasympathetic ganglia are not precisely organized and the connections between pre- and postganglionic neurons are rather diffusely scattered in the wall of the effector organs.

ANS mediators
Noradrenaline is a mediator on most postganglionic sympathetic fibers.

Acetylcholine acts as a mediator on all preganglionic ANS fibers, all postganglionic parasympathetic fibers and some sympathetic postganglionic fibers (e.g. sweat glands). Acetylcholine is also used as a mediator in the adrenal medulla, where its cells developmentally correspond to the sympathetic ganglion and are therefore innervated by preganglionic sympathetic fibers. The cells themselves release adrenaline after stimulation.

According to mediators, ANS fibers are divided into adrenergic and cholinergic.

ANS function
Table 1 offers an overview of the basic functions that are influenced by the vegetative nervous system. Most of the tissues controlled by the ANS receive innervation from both the parasympathetic and sympathetic nerves, which usually act in opposition to each other. However, the influence of one system in a given tissue usually prevails – for example, blood vessels are primarily under the influence of the sympathetic, GIT and SA node under the influence of the parasympathetic.

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 * Autonomic nervous system (pharmacology) • Sympathomimetics • Sympatholytics • Parasympathomimetics • Parasympatholytics
 * Autonomic nervous system (anatomy)