CHAPTER 6 • NEUROHUMORAL CONTROL OF THE HEART AND CIRCULATION
131
Glossopharyngeal Nerve
(Cranial Nerve IX)
■ FIGURE 6.6 Location and innervation of arterial
baroreceptors. Carotid sinus receptors are located
on the internal carotid artery just above the junc-
tion w ith the external carotid artery. These recep-
tors are innervated by the sinus nerve of Hering,
which joins the glossopharyngeal nerve (cranial
nerve IX) before traveling up to the medulla. A ffer-
ent nerves from the aortic arch receptors join the
vagus nerve (cranial nerve X), which then travel to
the medulla. R, right; L, left.
rate of individual receptors and nerves. Each
individual receptor has its own threshold and
sensitivity to changes in pressure; therefore,
additional receptors are recruited as pressure
increases. Overall, the receptors of the carotid
sinus respond to pressures ranging from about
60 to 180 mm Hg. Therefore, if arterial blood
pressure decreases from normal, it lowers
the firing rate of the carotid sinus barorecep-
tors; conversely, increased arterial pressure
increases receptor firing.
Baroreceptors are sensitive to the rate of pres-
sure change and to a steady or mean pressure.
At a given mean arterial pressure, decreasing
the arterial pulse pressure decreases firing rate.
This is important during conditions such as
hemorrhagic shock in which pulse pressure (as
well as mean pressure) decreases because of the
decline in stroke volume caused by decreased
ventricular preload and increased heart rate.
Therefore, reduced pulse pressure reinforces
the baroreceptor reflex when mean arterial
pressure falls. The curve representing the fre-
quency of baroreceptor firing in Figure 6.7 is
the integrated receptor firing at a given pulse
pressure. At reduced pulse pressures, the curve
shifts to the right, thereby decreasing the firing
at any given mean arterial pressure.
Maximal
carotid
sinus
sensitivity
(the
point of greatest slope of the response curve
in Fig. 6.7) occurs near the “set point” of nor-
mal mean arterial pressures (~95 mm Hg in
adults). Therefore, small deviations from this
set point elicit large changes in barorecep-
tor firing frequency. This set point, and the
entire receptor response curve, is not fixed.
Chronic shifts in this curve can occur during
hypertension, heart failure, and other disease
100
Integrated
Receptor
50
Firing Rate
(% max)
0
0
100
200
Mean Arterial Pressure
(mmHg)
Carotid Sinus
Maximal
Sensitivity
Normal
Pulse
Pressure
Reduce
//.
Pulse
Pressure
f
*
Arterial Pressure Pulse
Mean
-\A A
Receptor
Firing
I I
I HIM I
■HH-H
■ FIGURE 6.7 Effects of arterial pressure on integrated carotid sinus firing rate. Left panel: The thresh-
old for receptor activation occurs at mean arterial pressures of about 60 mm Hg; maximal firing occurs
at about 180 mm Hg. Maximal receptor sensitivity occurs at normal mean arterial pressures. The receptor
firing-response curve shifts to the right w ith decreased pulse pressures; therefore, a decrease in pulse
pressure at a given mean pressure decreases firing. Right panel: Single receptor firing in response to pulsa-
tile pressure. Receptors fire more rapidly when arterial pressure is rapidly increasing during cardiac systole.
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