OBJECTIVES FOR THE REFLEX CONTROL OF THE CARDIOPULMONARY SYSTEM

Blair H. Turner, Ph.D.

What kind of tissue gives rise to the heart's rhythmic activity?

Where is it located? What cranial nerves are involved?

What kind of tissue gives rise to the rhythmicity of breathing?

Where is it located?

By what cranial nerves is pulmonary rhythmicity brought about?

What is a baroreceptor? Where is it located?

What activates cardiovascular baroreceptors?

In what nerves are the sensory afferents of this reflex located?

In what nerves are the motor efferents of this reflex located?

What are the central connections between these afferent and efferent nerves?

By what means does this information modify the heart 's activity?

What is a chemoreceptor? Where is it located?

What activates cardiovascular chemoreceptors?

Where is chemoreceptor information sent?

In what nerves are the sensory afferents of this reflex located?

In what nerves are the motor efferents of this reflex located?

What are the central connections between these afferent and efferent nerves?

By what means does this information modify cardiopulmonary

function?

© Blair H. Turner, Ph.D. 1997

THE REFLEX CONTROL OF THE CARDIOPULMONARY SYSTEM

Blair H. Turner, Ph.D.

The metabolic situation of the body changes from minute to minute. In changing from a relaxed condition to one of exercise, for example, the metabolic rate of the body increases as the muscles work, consume oxygen and give off carbon dioxide. Consequently, more oxygen is needed by the body, and the carbon dioxide levels of the blood need to be reduced. It is a major function of the organs of the thoracic cavity to maintain appropriate blood levels of these gases by making adjustments that bring more oxygen into the system and expel more carbon dioxide. The cardiovascular system provides the channels and motive force by which blood-borne gases are exchanged with the tissues it perfuses, and with the external environment (at its interface with the lungs). The pulmonary system provides the mechanism (breathing) by which these gases are exchanged with the environment. These two systems together form a larger anatomical and functional unit, the cardiopulmonary system. This unit is based on the separation of the chambers of the heart, the right side bringing deoxygenated blood in from the body and pumping it toward the lungs; the left side bringing oxygenated blood in from the lungs and pumping to the tissues of the body. What makes the cardiopulmonary system responsive to metabolic changes is its short-term (second-to-second) control by the central and peripheral nervous systems, and its long-term control by the endocrine system.

The purpose of this lecture/laboratory sequence is to introduce you to the structures and mechanisms by which short-term control in the cardiopulmonary system is effected.

THE REFLEX CONTROL OF BLOOD PRESSURE

An Autonomic Reflex

Baroreceptors of the Carotid Sinus and Aortic Arch

A. Verbal Definition.

1. Afferent (sensory) pathway and brainstem termination

CN IX, via cardiac depressor nerves

Cell body in petrosal (inferior) ganglion

Terminates in caudal nucleus of the solitary tract

Function: stretch receptors sense amount of maintained tension in vessel wall, (i.e., amount of filling and back pressure mean arterial pressure) and discharges streams of impulses to the brainstem at a pressure-dependent rate.

2. Interneuronal and motor connections in the brainstem. Nucleus of solitary tract projects to nucleus ambiguus, ending on preganglionic parasympathetic cardioinhibitory neurons there. These latter exit the brainstem in CN X, ending on postganglionic parasympathetic neurons on the surface of the heart. Function: decreases heart rate,contractility and A-V conduction velocity, thus lowering blood pressure.

3. Interneuronal and motor connections to the spinal cord. Nucleus of the solitary tract, via relays, also sends inhibitory axons to the preganglionic sympathetic neurons of the intermediolateral horn, spinal levels T1-T4. These latter exit the spinal cord in the ventral roots and terminate on postganglionic sympathetic cell bodies, located in the superior and middle sympathetic ganglia. These then project to arteries. Function: Lowering arterial tone, leading to peripheral pooling of the blood. This, again, lowers blood pressure.

4. Further effect; Once blood pressure is lowered, the stretch eceptors decrease firing, decreasing activation of the parasympathetic cardioinhibitory neurons and disinhibiting sympathetic activation to the heart and arteries.

 

B. GRAPHIC DEFINITION

C. Table Definition.

The following illustrations at first appear daunting. However, keep in mind that they are only a pictorial way of representing the information you have charted in the tables for both somatic and autonomic reflexes. That is, they show pathways of simple reflexes consisting of afferents (GVAs), efferents (GVEs and GSEs) and one or two interneurons. Note that the baroreceptor reflex is purely autonomic, having only GVAs and GVEs. The chemoreceptor reflex is mixed, having GVAs and GSEs.

THE REFLEX CONTROL OF O2 LEVELS OF THE BLOOD

A Mixed Autonomic and Somatic Reflex

Chemoreceptors of the Carotid and Aortic Bodies

A. Verbal Definition.

1. Afferent (sensory) pathway and brainstem termination

CNs IX (via the carotid sinus nerve) and X

Cell body in petrosal (inferior) ganglion

Terminates in caudal nucleus of the solitary tract, and respiratory centers of the medulla.

2. Function: lowered blood levels of O2, raised levels of CO2, and raised pH all activate the chemoreceptors.

3. Interneuronal connections to the spinal cord

Nucleus of solitary tract projects, via cardiorespiratory neurons in the nucleus ambiguus, to motor neurons of the phrenic nucleus of the spinal cord, C3-C5, causing contraction of the diaphragm; and to the ventral motor neurons of T1-T12, causing contraction of the intercostal muscles. Function: increases rate and depth of the respiratory cycle (inspiration and expiration), causing lowering of CO2 levels and raising of O2 levels in the blood.

4. Final effect: Respiratory function is matched to the current metabolic situation of the organism.

5. Further effect: Once O2 levels in the blood rise, the chemoreceptors decrease firing. This reverses the functional sequence, resulting in a decrease in the rate and amplitude of breathing.

6. Note. CO2 and pH levels are also monitored by chemosensitive cells in the medulla.

B. Graphic Definition.

C. Table Definition.

The forebrain regulates both sympathetic and parasympathetic nervous system. Cells in both the amygdala and hypothalamus project (green pathway) to the intermediolateral cell column of T1-L2 (sympathetics) and to the craniosacral divisions of the parasympathetic nervous system. This setup allows the amygdala and hypothalamus to shift dominance between the two divisions of the autonomic nervous system. This shift is in turn controlled by both internal (hypothalamus) and external environments (amygdala). The amygdala receives information from the external world via inputs from the uncus (odor) and superior (hearing) and middle and inferior (vision) temporal gyri. These, in turn, ultimately receive input from the sense receptors. For example, the autonomic responses of fear of something smelled, heard or seen are controlled over this pathway.

Copyright©1996-99 B. Turner. All rights reserved.