Ganong's Review of Medical Physiology, 23rd Edition

628
SECTION VII
Respiratory Physiology

CAROTID & AORTIC BODIES

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 end-
ings of the afferent nerves (Figure 37–5). The glomus cells re-
semble 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
μ
m 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

afferent nerve fibers show that there is a graded increase in

impulse traffic in these afferent fibers as the P

O 2

of the perfusing

blood is lowered (Figure 37–6) or the P

CO 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, primar-

ily via L-type Ca

2+

channels. The Ca

2+

influx triggers action

potentials and transmitter release, with consequent excitation

FIGURE 37–4
Location of carotid and aortic bodies.
Carotid
bodies are positioned near a major arterial baroreceptor, the carotid si-
nus. Two aortic bodies are shown near the aortic arch.

Carotid body

Carotid sinus

Common carotid

arteries

Aortic bodies

Aortic arch

Heart

FIGURE 37–5

Organization of the carotid body.

Type I (glo-

mus) cells contain catecholamines. When exposed to hypoxia, they re-

lease 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.

FIGURE 37–6

Effect of P

CO 2

on afferent nerve firing.

The rate

of discharge of a single afferent fiber from the carotid body is plotted

at several P

O 2

(circles) and fitted to a line. A sharp increase in firing rate

is observed as P

O 2

falls below normal resting levels (ie, near 100 mm

Hg).

(Courtesy of S Sampson.)

Type II cell

Type I

(glomus) cell

Glossopharyngeal

afferent axons

8

6

4

2

0 100 200 400 600

Arterial PO 2 (mm Hg)

Impulses/s