38
Dynamics
of
the
Vascular
System
Fig.
2.4.2:
Discharge frequencies of carotid sinus stretch receptors
in
the control of
blood
pressure, via sympathetic and vagal stimulations. From Rushmore
(1
972).
A
perturbation that gives rise
to
an increase in blood pressure
stretches the wall
of
the carotid sinus. This in turn causes an increase in
discharge frequency from the baroreceptors (Fig.
2.4.2).
When the
pressure is low, the pressor-receptor impulse frequency decreases, the
vagus nerve impulses diminish and sympathetic cardiac nerve impulses
increase to accelerate the heart rate. This is accompanied by increased
activation of sympathetic vasoconstrictor fibers. The result
is
an increase
in peripheral resistance, hence an increase in blood pressure towards
normal level.
With an increase in blood pressure the carotid sinus
impulse frequency decreases, reducing the sympathetic discharge and
increasing the vagal discharge. The accompanied slowing in heart rate
and peripheral vasodilation restores blood pressure to normal level.
A
sigmoidal relationship between blood pressure and baroreceptor
impulse frequency has been found and the system is most sensitive over
the physiological range of blood pressures.
It
has been shown that the
baroreceptor discharge frequency is higher when pressure is pulsatile
