Organization of the Autonomic Nervous System

In the somatic nervous system, nerve fibers extend to and from the skeletal muscles, skin and sense organs. They usually emit impulses in response to stimuli from the outside environment, as in the withdrawal reflex (^ p. 320). Much somatic nervous activity occurs consciously and under voluntary control. In contrast, the autonomic nervous system (ANS) is mainly concerned with regulation of circulation and internal organs. It responds to changing outside conditions by triggering ortho-static responses, work start reactions, etc. to regulate the body's internal environment (^ p. 2). As the name implies, most activities of the ANS are not subject to voluntary control.

For the most part, the autonomic and somatic nervous systems are anatomically and functionally separate in the periphery (^ A), but closely connected in the central nervous system, CNS (^ p. 266). The peripheral ANS is efferent, but most of the nerves containing ANS fibers hold also afferent neurons. These are called visceral afferents because their signals originate from visceral organs, such as the esophagus, gastrointestinal (GI) tract, liver, lungs, heart, arteries, and urinary bladder. Some are also named after the nerve they accompany (e.g., vagal afferents).

Autonomic nervous activity is usually regulated by the reflex arc, which has an afferent limb (visceral and/or somatic afferents) and an efferent limb (autonomic and/or somatic effer-ents). The afferent fibers convey stimuli from the skin (e.g. nociceptive stimuli; ^ p.316) and nocisensors, mechanosensors and chemosen-sors in organs such as the lungs, gastrointestinal tract, bladder, vascular system and genitals. The ANS provides the autonomic efferent fibers that convey the reflex response to such afferent information, thereby inducing smooth muscle contraction (^ p. 70) in organs such as the eye, lung, digestive tract and bladder, and influencing the function of the heart (^ p. 194) and glands. Examples of somatic nervous system involvement are afferent 78 stimuli from the skin and sense organs (e.g., light stimuli) and efferent impulses to the skeletal muscles (e.g., coughing and vomiting). Despopoulos, Color Atlas of Physiology © All rights reserved. Usage subject to terms

Simple reflexes can take place within an organ (e.g., in the gut, ^ p. 244), but complex reflexes are controlled by superordinate auto-nomic centers in the CNS, primarily in the spinal cord (^ A). These centers are controlled by the hypothalamus, which incorporates the ANS in the execution of its programs (^ p. 330). The cerebral cortex is an even higher-ranking center that integrates the ANS with other systems.

The peripheral ANS consists of a sympathetic division and a parasympathetic division (^ A) which, for the most part, are separate entities (^ also p. 80ff.). The autonomic centers of the sympathetic division lie in the thoracic and lumbar levels of the spinal cord, and those of the parasympathetic division lie in the brain stem (eyes, glands, and organs innervated by the vagus nerve) and sacral part of the spinal cord (bladder, lower parts of the large intestine, and genital organs). (^ A). Preganglionic fibers of both divisions of the ANS extend from their centers to the ganglia, where they terminate at the postganglionic neurons.

Preganglionic sympathetic neurons arising from the spinal cord terminate either in the paravertebral ganglionic chain, in the cervical or abdominal ganglia or in so-called terminal ganglia. Transmission of stimuli from pregan-glionic to postganglionic neurons is choliner-gic, that is, mediated by release of the neu-rotransmitter acetylcholine (^ p. 82). Stimulation of all effector organs except sweat glands by the postganglionic sympathetic fibers is adrenergic, i.e., mediated by the release of norepinephrine (^ A and p. 84ff.).

Parasympathetic ganglia are situated near or within the effector organ. Synaptic transmissions in the parasympathetic ganglia and at the effector organ are cholinergic (^ A).

Most organs are innervated by sympathetic and parasympathetic nerve fibers. Nonetheless, the organ's response to the two systems can be either antagonistic (e.g., in the heart) or complementary (e.g., in the sex organs).

The adrenal medulla is a ganglion and hormone gland combined. Preganglionic sympathetic fibers in the adrenal medulla release acetylcholine, leading to the secretion of epi-nephrine (and some norepinephrine) into the bloodstream (^ p. 86).

I— A. Schematic view of autonomic nervous system (ANS)

Parasympathetic division

(Craniosacral centers) Transmitter substances: Preganglionic: Acetylcholine Postganglionic: Acetylcholine

Controlled by superordinate centers

Sympathetic division

(Thoracic and lumbar centers) Transmitter substances: Preganglionic: Acetylcholine Postganglionic: Norepinephrine (Exception: Sweat glands, some muscular blood vessels)

Parasympathetic division

(Craniosacral centers) Transmitter substances: Preganglionic: Acetylcholine Postganglionic: Acetylcholine

Reflex Arc Bladder

Genitals

Urinary bladder Adrenal medutla

Genitals

Cholinoceptors

Nicotinic receptors:

- All postganglionic, autonomic ganglia cells and dendrites

- Adrenal medulla Muscarinic receptors:

- All target organs innervated by postganglionic parasympathetic nerve fibers (and sweat glands innervated by sympathetic fibers)

Postganglionic: Cholinergic

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