571
CHAPTER FIFTEEN
Cardiovascular System
Cerebrum
(frontal
section)
Medulla
(transverse
section)
Spinal cord
(transverse sections)
(b)
Hypothalamus
Cardiac
center
Parasympathetic
vagus nerve
Sympathetic
nerve
Sympathetic trunk
SA node
AV node
Sensory
fibers
Carotid
sinus
Carotid
baroreceptors
Common
carotid
artery
Aorta
Aortic
baroreceptors
Effector
(muscle or gland)
Central
Nervous
System
Sensory or
afferent neuron
Motor or
efferent neuron
(a)
Receptor
tors
(pressoreceptors), can detect changes in blood pressure
(f
g. 15.24). Rising pressure stretches the receptors, and they
signal the cardioinhibitor center in the medulla. In response,
the medulla sends parasympathetic motor impulses to the
heart via the vagus nerve, decreasing the heart rate. This
action helps lower blood pressure toward normal.
Another regulatory refl
ex uses stretch receptors in the
venae cavae near the entrances to the right atrium. IF venous
blood pressure abnormally increases in these vessels, the
FIGURE 15.24
±Baroreceptor±ref
ex. (
a
)
Schematic oF a general ref
ex arc. (
b
) Autonomic
nerve impulses alter the activities oF the SA and
AV nodes.
balance between inhibitory eFFects oF the parasympathetic
f bers and excitatory eFFects oF the sympathetic f
bers. In this
region oF the brain, masses oF neurons Function as
cardio-
inhibitor
and
cardioaccelerator refl
ex centers.
These centers
receive sensory impulses From throughout the cardiovascu-
lar system and relay motor impulses to the heart in response.
±or example, receptors sensitive to stretch are located in
certain regions oF the aorta (aortic arch) and in the carotid
arteries (carotid sinuses). These receptors, called
barorecep-
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