102 Neuroanatomy: Draw It to Know It
The Cardiac Refl ex (Advanced)
Here, we will draw the cardiovascular refl ex, which
maintains blood pressure and cardiac output. It involves
a wide range of autonomic receptors, fi bers, and nuclei,
so we will limit our drawing to a few fundamental nuclei
and fi ber types. To begin, let’s draw some of the key
involved structures. First, draw the heart; aorta and
aortic arch; and the common carotid artery — include its
bifurcation into the internal carotid and external carotid
arteries (denote the internal carotid artery, for reference).
Next, draw the inferior ganglia of the glossopharyngeal
and vagus nerves. Th en, draw the medulla; then, the
spinal cord; and lastly, the adjacent thoracic paraverte-
bral ganglia.
Now, let’s draw the specifi c components of the cardio-
vascular refl ex. First, draw the carotid body at the bifur-
cation of the common carotid artery and then the aortic
bodies below the arch of the aorta. Next, draw the carotid
sinus in the proximal walls of the internal carotid artery
and then the aortic arch baroreceptors in the aortic arch.
Th e carotid body and aortic bodies are chemorecep-
tors that respond to arterial oxygen and carbon dioxide
levels and blood acidity. Th e carotid sinus and aortic
arch baroreceptors are baroreceptors, which respond to
stretch changes in the arterial vasculature due to changes
in blood pressure. Now, show that the glossopharyngeal
nerve carries aff erents from the carotid body and carotid
sinus and that the vagus nerve carries aff erents from the
aortic bodies and aortic arch baroreceptors.
Next, within the lateral dorsal medulla, label the soli-
tary tract nucleus, and within the central medulla, label
the nucleus ambiguus. Th en, draw a coronal view through
the ventrolateral medullary reticular formation, called
simply the ventrolateral medulla. Divide the ventrolat-
eral medulla into rostral and caudal segments for reasons
we will show soon. Now, indicate that the glossopharyn-
geal and vagus nerves project their central processes to
the solitary tract nucleus. Th en, show that the solitary
tract nucleus innervates the nucleus ambiguus, which
projects to the parasympathetic cardiac ganglion and
induces heart rate deceleration. Note that the dorsal
motor nucleus of the vagus may play a parallel but very
minor role to that of nucleus ambiguus in cardiac inner-
vation. Now, show that the rostral ventrolateral medulla
provides tonic sympathetic stimulation to the interme-
diolateral cell column of the spinal cord, which produces
heart rate acceleration. Th en, indicate that the solitary
tract nucleus excites the caudal ventrolateral medulla,
which inhibits the rostral segment of the ventrolateral
medulla, which provides further means for heart rate
deceleration. Note that we have left out some of the
nuclei and regions involved in this refl ex for simplicity;
they are the parabrachial pontine nucleus, sensorimotor
cortex, amygdala, and hypothalamus.
A simple way to test the cardiovascular response is by
varying your pulse. Take your pulse and get a good sense
of your heart rate. Th en, take a deep breath and hold it
for 5 or 6 seconds. Your heart rate should speed up
because when you inhale deeply, you open up lung tissue
and shunt blood into the lung capillaries, which reduces
your eff ective circulating blood volume (ie, your stroke
volume). Cardiac output is stroke volume multiplied by
heart rate; therefore, to compensate for a decreased
stroke volume, your heart rate increases (typically by 8
beats per minute).
An additional, slower response to a reduced stroke
volume is to increase the eff ective circulating blood
volume, itself. For instance, when we stand, blood pools
in our veins, so aft er we stand upright for a full minute,
T5 sympathetic splanchnic fi bers command our abdom-
inal vessels to shunt roughly 1.5 units of blood from
our abdomen into our peripheral vasculature. Because
there is a delay in the shunting of blood between systems,
when we check orthostatic blood pressure, we must
wait at least a few minutes in between measuring supine
and standing blood pressure (and possibly longer,
even).^4 – 9 , 15 – 17