Chapter 18: Nervous Regulation of the Circulation, and Rapid Control of Arterial Pressure

Written By: Razaullah Khattak (DPT Medical Professional)

Student Source Reference: Guyton and Hall Textbook of Medical Physiology.

Chapter: 18

To Read Previous Chapter 17🔗:

https://mednotesbyrazaullah.blogspot.com/2026/08/chapter-17-local-and-humoral-control-of.html

Introduction to Nervous Control of the Circulation

​The nervous system exerts rapid, global control over the circulatory system, complementing the localized intrinsic metabolic mechanisms that regulate individual tissue perfusion. While local control alters blood flow within isolated capillary beds, nervous regulation coordinates body-wide responses. It redistributes cardiac output to essential organs, modulates heart rate and contractile force, and maintains systemic arterial blood pressure within narrow physiological limits. The autonomic nervous system acts as the primary pathway for these systemic adjustments, integrating peripheral sensory feedback with brainstem and higher brain centers.

​The sympathetic division represents the primary controller of vascular resistance and capacitance, innervating almost all blood vessel segments across the body. Concurrently, the parasympathetic division delivers powerful cardioinhibitory signals via the vagus nerves to regulate cardiac chronotropy. Together, these autonomic pathways participate in tonic tone maintenance, acute stress responses, and continuous negative feedback reflex loops. Understanding these neural circuits is essential for analyzing short-term hemodynamic adjustments during physical exertion, postural changes, and shock.

​Autonomic Nervous System Organization

• ​Sympathetic nerves primarily regulate overall circulatory function.
• ​Parasympathetic nerves selectively regulate cardiac chronotropic function.
• ​Sympathetic vasomotor fibers exit thoracic spinal nerves.
• ​First two lumbar nerves contain sympathetic vasomotor fibers.
• ​Sympathetic fibers enter bilateral paravertebral sympathetic chains.
• ​Visceral sympathetic nerves innervate internal organs and heart.
• ​Peripheral spinal nerves distribute sympathetic fibers outward.
• ​Vascular capillaries lack direct sympathetic nerve innervation.
• ​Precapillary sphincters receive sparse mesenteric sympathetic innervation.
• ​Small arteriolar innervation controls local vascular resistance.
• ​Venous sympathetic innervation modifies total capacitance volume.
• ​Direct cardiac sympathetic fibers increase heart rate.
• ​Sympathetic cardiac stimulation enhances myocardial contractile strength.
• ​Parasympathetic vagus fibers carry cardiac inhibitory signals.
• ​Parasympathetic stimulation markedly decreases total heart rate.
• ​Parasympathetic activation slightly decreases atrial muscle contractility.

​Sympathetic Vasoconstrictor System and Central Control

• ​Sympathetic nerves carry abundant vasoconstrictor nerve fibers.
• ​Vasoconstrictor fibers densely innervate kidneys, spleen, skin.
• ​Skeletal muscle and brain receive less vasoconstrictor innervation.
• ​The vasomotor center lies in bilateral medullary reticular substance.
• ​The lower third of the pons contains vasomotor neurons.
• ​Anterolateral upper medulla houses the vasoconstrictor area.
• ​Upper medullary neurons excite spinal preganglionic vasoconstrictor cells.
• ​Anterolateral lower medulla contains the vasodilator area.
• ​Vasodilator neurons project upward to inhibit vasoconstrictor neurons.
• ​Posterolateral medulla houses the sensory tractus solitarius.
• ​Tractus solitarius receives vagus and glossopharyngeal sensory inputs.
• ​Sensory area coordinates reflex circulatory feedback responses.
• ​Vasomotor center maintains continuous partial vascular constriction.
• ​Sympathetic vasoconstrictor tone fires half to two impulses per second.
• ​Continual baseline firing creates normal baseline vasomotor tone.
• ​Total spinal anesthesia abolishes sympathetic tone, dropping pressure.
• ​Norepinephrine injection restores arterial pressure following spinal anesthesia.
• ​Lateral vasomotor center transmits excitatory signals to heart.
• ​Medial vasomotor center stimulates dorsal vagal motor nuclei.
• ​Higher brainstem reticular areas excite or inhibit vasomotor output.
• ​Posterolateral hypothalamus exerts strong excitatory vasomotor effects.
• ​Anterior hypothalamus exerts mild excitatory or inhibitory effects.
• ​Motor cortex stimulation excites vasomotor center via hypothalamus.
• ​Basal brain structures modulate central autonomic vasomotor output.
• ​Norepinephrine acts directly on vascular alpha-adrenergic receptors.
• ​Sympathetic impulses trigger adrenal medullary hormone release.
• ​Circulating epinephrine activates vascular alpha and beta receptors.

​Sympathetic Vasodilator System and Emotional Fainting

• ​Skeletal muscle receives sympathetic vasodilator nerve fibers.
• ​Muscle vasodilator fibers release acetylcholine in some species.
• ​Epinephrine excites muscle beta-adrenergic receptors for dilation.
• ​Anterior hypothalamus controls central sympathetic vasodilator pathways.
• ​Sympathetic vasodilator system provides minimal resting blood flow control.
• ​Muscle vasodilators may cause anticipatory dilation before exercise.
• ​Intense emotional disturbance triggers severe vasovagal syncope.
• ​Vasovagal syncope activates muscle vasodilators and vagal cardioinhibition.
• ​Rapid arterial pressure drops reduce cerebral blood flow.
• ​Cerebral hypoperfusion during syncope causes sudden consciousness loss.
• ​Disturbing cortical thoughts initiate vasovagal brainstem pathways.

​Nervous Mechanisms of Rapid Pressure Control

• ​Sympathetic activation rapidly elevates systemic arterial pressure.
• ​Systemic arteriole constriction greatly increases peripheral resistance.
• ​Venous constriction displaces peripheral blood volume toward heart.
• ​Increased venous return stretches cardiac chambers for stronger pumping.
• ​Direct sympathetic stimulation increases heart rate up to threefold.
• ​Enhanced cardiac contractility doubles maximal cardiac output capability.
• ​Sympathetic activation doubles arterial pressure within 5 to 10 seconds.
• ​Sudden sympathetic inhibition halves arterial pressure within 40 seconds.
• ​Exercise activates brainstem reticular activating system instantaneously.
• ​Exercise-induced sympathetic surge elevates pressure 30 to 40 percent.
• ​Extreme fright triggers the acute alarm reaction pressure elevation.
• ​Alarm reactions provide immediate arterial pressure for muscle flight.

​Baroreceptor Reflex Mechanics and Arterial Pressure Maintenance

• ​Baroreceptors are stretch receptors in large systemic artery walls.
• ​Increased arterial pressure stretches arterial baroreceptor spray endings.
• ​Baroreceptor signals inhibit central sympathetic output to lower pressure.
• ​Carotid sinus baroreceptors lie above internal carotid bifurcations.
• ​Aortic baroreceptors locate within the aortic arch wall.
• ​Hering's nerves carry carotid baroreceptor signals to glossopharyngeal nerves.
• ​Glossopharyngeal nerves transmit carotid signals to tractus solitarius.
• ​Vagus nerves carry aortic arch baroreceptor signals to medulla.
• ​Carotid baroreceptors fire at pressures above 50 to 60 mm Hg.
• ​Carotid baroreceptor firing rates reach maximum at 180 mm Hg.
• ​Aortic baroreceptors operate at 30 mm Hg higher pressure thresholds.
• ​Baroreceptors exhibit highest sensitivity around normal 100 mm Hg pressure.
• ​Rapidly changing pressure evokes higher firing rates than steady pressure.

Disclaimer: This content is for educational purposes only and not a substitute for professional medical advice.