Ganong's Review of Medical Physiology, 23rd Edition

560
SECTION VI
Cardiovascular Physiology

BARORECEPTOR NERVE ACTIVITY

Baroreceptors are more sensitive to pulsatile pressure than to
constant pressure. A decline in pulse pressure without any
change in mean pressure decreases the rate of baroreceptor dis-
charge and provokes a rise in systemic blood pressure and
tachycardia. At normal blood pressure levels (about 100 mm Hg
mean pressure), a burst of action potentials appears in a single
baroreceptor fiber during systole, but there are few action po-
tentials in early diastole (Figure 33–5). At lower mean pres-
sures, this phasic change in firing is even more dramatic with
activity only occurring during systole. At these lower pres-
sures, the overall firing rate is considerably reduced. The
threshold for eliciting activity in the carotid sinus nerve is
about 50 mm Hg; maximal activity occurs at about 200 mm Hg.
When one carotid sinus is isolated and perfused and the
other baroreceptors are denervated, there is no discharge in
the afferent fibers from the perfused sinus and no drop in the
animal’s arterial pressure or heart rate when the perfusion
pressure is below 30 mm Hg (Figure 33–6). At carotid sinus
perfusion pressures of 70–110 mm Hg, there is a near linear
relationship between perfusion pressure and the fall in sys-
temic blood pressure and heart rate. At perfusion pressures
above 150 mm Hg there is no further increase in response, pre-
sumably because the rate of baroreceptor discharge and the
degree of inhibition of sympathetic nerve activity are maximal.
From the foregoing discussion, it is apparent that the barore-
ceptors on the arterial side of the circulation, their afferent
connections to the medullary cardiovascular areas, and the
efferent pathways from these areas constitute a reflex feedback
mechanism that operates to stabilize blood pressure and heart

rate. Any drop in systemic arterial pressure decreases the

inhibitory discharge in the buffer nerves, and there is a com-

pensatory rise in blood pressure and cardiac output. Any rise

in pressure produces dilation of the arterioles and decreases

cardiac output until the blood pressure returns to its previous

normal level.

BARORECEPTOR RESETTING

In chronic hypertension, the baroreceptor reflex mechanism

is “reset” to maintain an elevated rather than a normal blood

pressure. In perfusion studies on hypertensive experimental

animals, raising the pressure in the isolated carotid sinus low-

ers the elevated systemic pressure, and decreasing the perfu-

sion pressure raises the elevated pressure (Figure 33–6). Little

is known about how and why this occurs, but resetting occurs

rapidly in experimental animals. It is also rapidly reversible,

both in experimental animals and in clinical situations.

ROLE OF BARORECEPTORS IN SHORT-

TERM CONTROL OF BLOOD PRESSURE

The changes in pulse rate and blood pressure that occur in hu-

mans on standing up or lying down are due for the most part

to baroreceptor reflexes. The function of the receptors can be

tested by monitoring changes in heart rate as a function of in-

creasing arterial pressure during infusion of the

α

-adrenergic

agonist phenylephrine. A normal response is shown in Figure

33–7; from a systolic pressure of about 120 to 150 mm Hg,

there is a linear relation between pressure and lowering of the

heart rate (greater RR interval). Baroreceptors are very impor-

tant in short-term control of arterial pressure. Activation of

the reflex allows for rapid adjustments in blood pressure in

FIGURE 33–5
Discharges (vertical lines) in a single afferent
nerve fiber from the carotid sinus at various levels of mean
arterial pressures, plotted against changes in aortic pressure with
time.
Baroreceptors are very sensitive to changes in pulse pressure as
shown by the record of phasic aortic pressure.
(Reproduced with
permission from Berne RM, Levy MN:
Cardiovascular Physiology,
3rd ed. Mosby, 1977.)

1.0 1.5 2.0

Time (s)

0 0.5

200

125

100

75

50

Phasic aortic

pressure

Mean arterial pressures (mm Hg)

FIGURE 33–6

Fall in systemic blood pressure produced by

raising the pressure in the isolated carotid sinus to various

values. Solid line:

Response in a normal monkey.

Dashed line:

Re-

sponse in a hypertensive monkey, demonstrating baroreceptor reset-

ting (arrow).

80

70

60

50

40

30

20

10

(^050100150200)
Pressure in carotid sinus (mm Hg)
% fall in systemic blood pressure