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 discharge
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 potentials
in early diastole (Figure 33–5). At lower mean pressures,
this phasic change in firing is even more dramatic with
activity only occurring during systole. At these lower pressures,
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 systemic
blood pressure and heart rate. At perfusion pressures
above 150 mm Hg there is no further increase in response, presumably
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 baroreceptors
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
Phasic aortic
pressure
Mean arterial pressures (mm Hg)
50
75
100
125
200
0
0.5
1.0
Time (s)
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.)
1.5
2.0
% fall in systemic blood pressure
80
70
60
50
40
30
20
10
0
50 100 150 200
Pressure in carotid sinus (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: Response
in a hypertensive monkey, demonstrating baroreceptor resetting
(arrow).
rate. Any drop in systemic arterial pressure decreases the
inhibitory discharge in the buffer nerves, and there is a compensatory
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 lowers
the elevated systemic pressure, and decreasing the perfusion
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 humans
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 increasing
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 important
in short-term control of arterial pressure. Activation of
the reflex allows for rapid adjustments in blood pressure in