Essentials of Anatomy and Physiology

receptors are the vagus (10th cranial) nerves. If we
now put all of these facts together in a specific exam-
ple, you will see that the regulation of heart rate is a
reflex. Figure 12–7 depicts all of the structures just
mentioned.
A person who stands up suddenly from a lying posi-
tion may feel light-headed or dizzy for a few moments,
because blood pressure to the brain has decreased
abruptly. The drop in blood pressure is detected by
pressoreceptors in the carotid sinuses—notice that
they are “on the way” to the brain, a very strategic
location. The drop in blood pressure causes fewer
impulses to be generated by the pressoreceptors.
These impulses travel along the glossopharyngeal
nerves to the medulla, and the decrease in the fre-
quency of impulses stimulates the accelerator center.
The accelerator center generates impulses that are car-
ried by sympathetic nerves to the SA node, AV node,
and ventricular myocardium. As heart rate and force
increase, blood pressure to the brain is raised to nor-
mal, and the sensation of light-headedness passes.

When blood pressure to the brain is restored to nor-

mal, the heart receives more parasympathetic impulses

from the inhibitory center along the vagus nerves to

the SA node and AV node. These parasympathetic

impulses slow the heart rate to a normal resting pace.

The heart will also be the effector in a reflex stim-

ulated by a decrease in the oxygen content of the

blood. The aortic receptors are strategically located so

as to detect such an important change as soon as blood

leaves the heart. The reflex arc in this situation would

be (1) aortic chemoreceptors, (2) vagus nerves (sen-

sory), (3) accelerator center in the medulla, (4) sympa-

thetic nerves, and (5) the heart muscle, which will

increase its rate and force of contraction to circulate

more oxygen to correct the hypoxia.

Recall also from Chapter 10 that the hormone

epinephrine is secreted by the adrenal medulla in

stressful situations. One of the many functions of epi-

nephrine is to increase heart rate and force of contrac-

tion. This will help supply more blood to tissues in

need of more oxygen.

The Heart 285

Accelerator center

Inhibitory center

Medulla

Thoracic spinal cord

Sympathetic nerves

Right ventricle

Vagus (motor) nerves

Vagus (sensory)

nerve

Glossopharyngeal

nerves

Carotid sinus and

carotid body

Common

carotid

arteries

Aortic arch

Aortic sinus

and aortic body

SA node

Bundle of His

AV node

Figure 12–7. Nervous regulation of the heart. The brain and spinal cord are shown on

the left. The heart and major blood vessels are shown on the right.

QUESTION:Sympathetic impulses to the ventricles will have what effect?