Page 91 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

3 Autonomic Nervous System (ANS)

Organization of the Autonomic Simple reflexes can take place within an
Nervous System organ (e.g., in the gut, ! p. 244), but complex
reflexes are controlled by superordinate auto-
In the somatic nervous system, nerve fibers ex- nomic centers in the CNS, primarily in the spi-
tend to and from the skeletal muscles, skin and nal cord (! A). These centers are controlled by
sense organs. They usually emit impulses in re- the hypothalamus, which incorporates the ANS
sponse to stimuli from the outside environ- in the execution of its programs (! p. 330). The
ment, as in the withdrawal reflex (! p. 320). cerebral cortex is an even higher-ranking cen-
Much somatic nervous activity occurs con- ter that integrates the ANS with other systems.
sciously and under voluntary control. In con- The peripheral ANS consists of a sympathetic
trast, the autonomic nervous system (ANS) is division and a parasympathetic division (! A)
mainly concerned with regulation of circula- which, for the most part, are separate entities
tion and internal organs. It responds to chang- (! also p. 80ff.). The autonomic centers of the
ing outside conditions by triggering ortho- sympathetic division lie in the thoracic and
static responses, work start reactions, etc. to lumbar levels of the spinal cord, and those of
regulate the body’s internal environment the parasympathetic division lie in the brain
(! p. 2). As the name implies, most activities of stem (eyes, glands, and organs innervated by
the ANS are not subject to voluntary control. the vagus nerve) and sacral part of the spinal
For the most part, the autonomic and so- cord (bladder, lower parts of the large in-
matic nervous systems are anatomically and testine, and genital organs). (! A). Pregan-
functionally separate in the periphery (! A), glionic fibers of both divisions of the ANS ex-
but closely connected in the central nervous tend from their centers to the ganglia, where
system, CNS (! p. 266). The peripheral ANS is they terminate at the postganglionic neurons.
efferent, but most of the nerves containing ANS Preganglionic sympathetic neurons arising
fibers hold also afferent neurons. These are from the spinal cord terminate either in the
called visceral afferents because their signals paravertebral ganglionic chain, in the cervical
originate from visceral organs, such as the or abdominal ganglia or in so-called terminal
esophagus, gastrointestinal (GI) tract, liver, ganglia. Transmission of stimuli from pregan-
lungs, heart, arteries, and urinary bladder. glionic to postganglionic neurons is choliner-
Some are also named after the nerve they ac- gic, that is, mediated by release of the neu-
company (e.g., vagal afferents). rotransmitter acetylcholine (! p. 82). Stimula-
Autonomic nervous activity is usually regu- tion of all effector organs except sweat glands
lated by the reflex arc, which has an afferent by the postganglionic sympathetic fibers is
limb (visceral and/or somatic afferents) and an adrenergic, i.e., mediated by the release of
efferent limb (autonomic and/or somatic effer- norepinephrine (! A and p. 84ff.).
ents). The afferent fibers convey stimuli from Parasympathetic ganglia are situated near
the skin (e.g. nociceptive stimuli; ! p. 316) and or within the effector organ. Synaptic trans-
nocisensors, mechanosensors and chemosen- missions in the parasympathetic ganglia and
sors in organs such as the lungs, gastrointesti- at the effector organ are cholinergic (! A).
nal tract, bladder, vascular system and geni- Most organs are innervated by sympathetic
tals. The ANS provides the autonomic efferent and parasympathetic nerve fibers. Nonethe-
fibers that convey the reflex response to less, the organ’s response to the two systems
such afferent information, thereby inducing can be either antagonistic (e.g., in the heart) or
smooth muscle contraction (! p. 70) in organs complementary (e.g., in the sex organs).
such as the eye, lung, digestive tract and blad- The adrenal medulla is a ganglion and hor-
der, and influencing the function of the heart mone gland combined. Preganglionic sympa-
(! p. 194) and glands. Examples of somatic thetic fibers in the adrenal medulla release
nervous system involvement are afferent acetylcholine, leading to the secretion of epi-
stimuli from the skin and sense organs (e.g., nephrine (and some norepinephrine) into the
78
light stimuli) and efferent impulses to the bloodstream (! p. 86).
skeletal muscles (e.g., coughing and vomiting).
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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