Ganong's Review of Medical Physiology, 23rd Edition

628 SECTION VII Respiratory Physiology
Heart
FIGURE 37–4 Location of carotid and aortic bodies. Carotid
bodies are positioned near a major arterial baroreceptor, the carotid sinus.
Two aortic bodies are shown near the aortic arch.
CAROTID & AORTIC BODIES
Carotid body
Carotid sinus
Common carotid
arteries
Aortic bodies
Aortic arch
There is a carotid body near the carotid bifurcation on each
side, and there are usually two or more aortic bodies near the
arch of the aorta (Figure 37–4). Each carotid and aortic body
(glomus) contains islands of two types of cells, type I and type
II cells, surrounded by fenestrated sinusoidal capillaries. The
type I or glomus cells are closely associated with cuplike endings
of the afferent nerves (Figure 37–5). The glomus cells resemble
adrenal chromaffin cells and have dense-core granules
containing catecholamines that are released upon exposure to
hypoxia and cyanide. The cells are excited by hypoxia, and the
principal transmitter appears to be dopamine, which excites
the nerve endings by way of D 2 receptors. The type II cells are
glia-like, and each surrounds four to six type I cells. Their
function is probably sustentacular.
Outside the capsule of each body, the nerve fibers acquire a
myelin sheath; however, they are only 2 to 5 μm in diameter and
conduct at the relatively low rate of 7 to 12 m/s. Afferents from
the carotid bodies ascend to the medulla via the carotid sinus
and glossopharyngeal nerves, and fibers from the aortic bodies
ascend in the vagi. Studies in which one carotid body has been
isolated and perfused while recordings are being taken from its
Type II cell
Type I
(glomus) cell
Glossopharyngeal
afferent axons
FIGURE 37–5 Organization of the carotid body. Type I (glomus)
cells contain catecholamines. When exposed to hypoxia, they release
their catecholamines, which stimulate the cuplike endings of the
carotid sinus nerve fibers in the glossopharyngeal nerve. The glia-like
type II cells surround the type I cells and probably have a sustentacular
function.
afferent nerve fibers show that there is a graded increase in
impulse traffic in these afferent fibers as the PO 2 of the perfusing
blood is lowered (Figure 37–6) or the PCO 2 is raised.
Type I glomus cells have O 2 -sensitive K + channels, whose
conductance is reduced in proportion to the degree of
hypoxia to which they are exposed. This reduces the K +
efflux, depolarizing the cell and causing Ca 2+ influx, primarily
via L-type Ca 2+ channels. The Ca 2+ influx triggers action
potentials and transmitter release, with consequent excitation
Impulses/s
8
6
4
2
0 100 200 400 600
Arterial PO2 (mm Hg)
FIGURE 37–6 Effect of PCO 2 on afferent nerve firing. The rate
of discharge of a single afferent fiber from the carotid body is plotted
at several PO 2 (circles) and fitted to a line. A sharp increase in firing rate
is observed as PO 2 falls below normal resting levels (ie, near 100 mm
Hg). (Courtesy of S Sampson.)