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.