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Autonomic Reflexes


Many visceral functions of the body are regulated by
autonomic reflexes. Throughout this text, the functions
of these reflexes are discussed in relation to individual
organ systems; to illustrate their importance, a few are
presented here briefly.
Cardiovascular Autonomic Reflexes. Several reflexes in the
cardiovascular system help to control especially the
arterial blood pressure and the heart rate. One of these
is the baroreceptor reflex, which is described in Chapter
18 along with other cardiovascular reflexes. Briefly,
stretch receptors called baroreceptors are located in the
walls of several major arteries, including especially the
internal carotid arteries and the arch of the aorta.When
these become stretched by high pressure, signals are
transmitted to the brain stem, where they inhibit the
sympathetic impulses to the heart and blood vessels and
excite the parasympathetics; this allows the arterial
pressure to fall back toward normal.
Gastrointestinal Autonomic Reflexes. The uppermost part of
the gastrointestinal tract and the rectum are controlled
principally by autonomic reflexes. For instance, the
smell of appetizing food or the presence of food in
the mouth initiates signals from the nose and mouth
to the vagal, glossopharyngeal, and salivatory nuclei
of the brain stem.These in turn transmit signals through
the parasympathetic nerves to the secretory glands of
the mouth and stomach, causing secretion of digestive
juices sometimes even before food enters the mouth.
When fecal matter fills the rectum at the other end
of the alimentary canal, sensory impulses initiated by
stretching the rectum are sent to the sacral portion of
the spinal cord, and a reflex signal is transmitted back
through the sacral parasympathetics to the distal parts
of the colon; these result in strong peristaltic contractions
that cause defecation.
Other Autonomic Reflexes. Emptying of the urinary bladder
is controlled in the same way as emptying the rectum;
stretching of the bladder sends impulses to the sacral
cord, and this in turn causes reflex contraction of the
bladder and relaxation of the urinary sphincters,
thereby promoting micturition.
Also important are the sexual reflexes, which are initiated
both by psychic stimuli from the brain and by
stimuli from the sexual organs. Impulses from these
sources converge on the sacral cord and, in the male,
result first in erection, mainly a parasympathetic function,
and then ejaculation, partially a sympathetic
function.
Other autonomic control functions include reflex
contributions to the regulation of pancreatic secretion,
gallbladder emptying, kidney excretion of urine, sweating,
blood glucose concentration, and many other visceral
functions, all of which are discussed in detail at
other points in this text.