Baroreceptor Reflex: Detailed Anatomy

Sections

Summary
Full Text

Summary

RECEPTORS

Chemoreceptors

• The carotid body and aortic bodies are chemoreceptors that respond to arterial oxygen and carbon dioxide levels and blood acidity.

Baroreceptors

• The carotid sinus and aortic arch baroreceptors are baroreceptors, which respond to arterial stretch changes due to blood pressure changes.

Afferent innervation of the carotid body and carotid sinus

• The glossopharyngeal nerve carries afferents from the carotid body and carotid sinus.

Afferent innervation of the aortic bodies and aortic arch baroreceptors

• The vagus nerve carries afferents from the aortic bodies and aortic arch baroreceptors.

CIRCUITRY

CNs 9 and 10

• The glossopharyngeal and vagus nerves project to the solitary tract nucleus.

Solitary tract nucleus

• The solitary tract nucleus innervates nucleus ambiguus, which projects to the parasympathetic cardiac ganglion and induces heart rate deceleration (note: the dorsal motor nucleus of the vagus may play a parallel but very minor role to that of nucleus ambiguus in cardiac innervation).

Rostral ventrolateral medulla

• The rostral ventrolateral medulla provides tonic sympathetic stimulation to the intermediolateral cell column of the spinal cord, which produces heart rate acceleration.

Caudal ventrolateral medulla

• 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.

EXCLUSIONS

• We leave out the parabrachial pontine nucleus, sensorimotor cortex, amygdala, and hypothalamus for simplicity.

Key Clinical Correlations:

• Global Cerebral Ischemia (Presyncope, Syncope)
• Arterial Dissection

Full Text

Overview

Physiology

Here, we will learn the anatomy of the baroreceptor reflex.

• Start a table.
• Denote that blood pressure is affected by
• Cardiac Output (stroke volume x heart rate)
• Total Peripheral Resistance (a measure of arteriolar constriction) – imagine that it's like a faucet on a hose.

Anatomy

Denote that it involves several key anatomic regions:

• Receptors:
  • Carotid artery and aortic arch.
• Autonomic afferents
• Relay/modulation centers:
  • Brain, brainstem, and spinal cord
• Autonomic efferents
• Tissues:
  • Heart, blood vessels

Key Structures

Structures

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.
• Then, draw a vertical snippet of the brainstem and spinal cord.
• Draw an expanded axial section of the medulla
• And the T1 – L2 level of the spinal cord.
  • Indicate the intermediolateral cell column.

Cardiovascular reflex

Now, let's draw the specific components of the cardiovascular reflex.

Carotid sinus

• First, draw the carotid sinus at the origin of the internal carotid artery.

Aortic arch

• Then, draw aortic arch baroreceptors in the aortic arch.
  • They are mechanoreceptors, which respond to stretch changes in the arterial vasculature due to changes in blood pressure.

Solitary tract nucleus

• Next, draw the solitary tract nucleus within the medial dorsal medulla.

Nucleus ambiguus

• Draw nucleus ambiguus ventral and lateral to it.

Ventrolateral medulla

• Then, draw a vertical view through the ventrolateral medulla (aka ventrolateral medullary reticular formation) in rostral and caudal segments: we'll see that the rostral segment produces sympathetic activity whereas the caudal aspect generates parasympathetic activity.

Glossopharyngeal nerve (CN 9)

• Now, show that the glossopharyngeal nerve (CN 9) projects from the carotid sinus to the solitary tract nucleus.

Vagus nerve (CN 10)

• And the vagus nerve (CN 10) projects from the aortic arch baroreceptors to the solitary tract nucleus.
• Show that these particular nerve branches attach to their respective cranial nerve inferior ganglia.

Cardiac ganglia

• Next, show representative cardiac ganglia within epicardial fat pads.

Parasympathetic Innervation

Solitary tract excitation of nucleus ambiguus

• Then, show that the solitary tract nucleus excites nucleus ambiguus.

Vagal cardiac nerve projections produce heart rate deceleration

• Indicate that it projects vagal cardiac nerve branches to the cardiac ganglia, which produce post-synaptic branches that induce heart rate deceleration.

Dorsal motor nucleus accessory circuitry

• Note that the dorsal motor nucleus of the vagus may play a parallel but much lesser role to that of nucleus ambiguus in cardiac innervation.

Sympathetic Innervation

Now, let's turn our attention to the sympathetic nervous system.

Paravertebral chain

• First draw the upper four segments of the paravertebral chain:

• Middle cervical
  • Inferior cervical
  • 1st thoracic
  • Indicate that the stellate ganglion comprises the inferior cervical and first thoracic ganglia.

Brainstem innervation of the spinal cord

• Now, show that the rostral ventrolateral medulla provides tonic sympathetic stimulation to the intermediolateral cell column of the spinal cord.

Paravertebral chain innervation of the heart

• Then, show that cardiac innervation comes from T1-T5 presynaptic branches which innervate the upper four segments of the paravertebral chain.
  • Show that they send cardiopulmonary splanchnic nerve branches to the cardiac ganglia via the cardiac plexus (which is a combination of parasympathetic and sympathetic nerve fibers).

Heart rate acceleration

• Then, show that post-synaptic sympathetic nerve fibers produce heart rate acceleration (which increases cardiac output).

Parasympathetic control of the sympathetic innervation

• As a parasympathetic control mechanism, show 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.

Additional Baroreflex actions

Vascular Constriction

We've completed the innervation of the heart, but we still need to address two other important baroreflex actions.

• Now create a nonsepcific box that encompasses both paravertebral and prevertebral ganglia.
• Then, draw a blood vessel in its resting state.
• Now, show T1 – L2 presynaptic sympathetic innervation to the paravertebral chain section.
• Then, show post-synaptic innervation to the blood vessel.
• Show that it produces blood vessel vasoconstriction (which increases total peripheral resistance).

Renin-angiotension-aldosterone system

• Lastly, draw the aorticorenal prevertebral ganglion.
• Then draw the kidneys.
• Now, show T9 - T11 presynaptic sympathetic innervation to the aorticorenal prevertebral ganglion.
• Then, show post-synaptic innervation to the kidneys.
• Indicate that the renin-angiotension-aldosterone system (of the kidney), adjusts blood pressure via change in blood volume (this is a slow mechanism).

Central nervous system modulators

• Draw a coronal section through the brain, so we can add some brain and brainstem modulators, after all, we can affect our heart rate through our own conscious fear and anxiety.
• Demarcate some major modulators:
  • Sensorimotor cortex
  • Insula
  • Hypothalamus
  • Amygdala
  • And in the brainstem, demarcate the parabrachial pontine nucleus.
• Show that these regions affect the autonomic balance of the baroreceptor reflex via innervation of the solitary tract nucleus.

Acute hypotension

• As a clinical, physiologic correlation, show that in acute hypotension (such as from blood loss or orthostasis):
  • a drop in blood pressure produces a drop in baroreceptor activity
  • which triggers a decrease in parasympathetic activity and an increase in sympathetic activity
  • which produces an increase in cardiac output and produces blood vessel constriction.
  • As mentioned at the beginning, blood pressure is a product of cardiac output and peripheral resistance, so an increase in cardiac output and arteriolar constriction results in a normalization of blood pressure.

Cardiovascular response & Orthostasis

Cardiovascular response

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.
• Then, 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 effective 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).

Orthostasis

• An additional, slower response to a reduced stroke volume is to increase the effective circulating blood volume, itself.
• For instance, when we stand, blood pools in our veins, so after we stand upright for a full minute, T5 sympathetic splanchnic fibers command our abdominal 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).