Guyton and Hall Physiology Chapter 18 - Nervous Regulation of the Circulation

Vasomotor Center

• Located in the reticular substance of the medulla and lower portion of the pons
• Responsible for controlling vasoconstriction and vasodilation, and regulating heart activity
• Contains three areas: vasoconstrictor, vasodilator, and sensory.

Sympathetic Vasomotor Tone

• Continuous signals from the vasoconstrictor area of the vasomotor center to sympathetic vasoconstrictor nerve fibers lead to partial constriction of blood vessels
• This helps maintain a baseline level of blood pressure
• Spinal anesthesia blocks sympathetic nerve impulses, demonstrating the effect of vasoconstrictor tone on arterial pressure.

Control of the Vasomotor Center

• Higher brain centers (hypothalamus, cerebral cortex) can excite or inhibit the vasomotor center, influencing blood pressure and heart rate
• The hypothalamus has a significant role, with the posterolateral portions causing excitation and the anterior portion causing either excitation or inhibition.
• Stimulation of the motor cortex excites the vasomotor center, increasing heart rate and arterial pressure.

Functions of the Vasomotor Center

• Regulation of vascular tone through vasoconstriction and vasodilation
• Regulation of heart rate and strength of contractions through sympathetic and parasympathetic nerve fibers
• Control of blood pressure under various conditions, including exercise and stress
• Coordination of cardiovascular responses through its interplay with higher brain centers.

Baroreceptor Arterial Pressure Control

• Baroreceptors, located in large arteries, detect changes in blood pressure
• Increased blood pressure stretches baroreceptors, triggering signals to the CNS
• The CNS then sends feedback signals to the vasomotor center, adjusting heart rate and blood vessel constriction to maintain normal blood pressure.
• This is a negative feedback mechanism, meaning it reduces the stimulus that initiated it.

Sympathetic Nerve Control During Exercise

• During exercise, the vasomotor center increases sympathetic activity, leading to increased vasoconstriction, heart rate, and contractility
• This helps deliver more oxygen and nutrients to working muscles.

Anatomy of the Vasomotor Center

• The vasomotor center is located in the reticular substance of the medulla and lower third of the pons.
• It transmits parasympathetic impulses through the vagus nerves to the heart.
• It transmits sympathetic impulses through the spinal cord and peripheral sympathetic nerves to arteries, arterioles, and veins throughout the body.
• Sympathetic nerves carry vasoconstrictor nerve fibers (more numerous) and a few vasodilator fibers.
• The sympathetic vasoconstrictor effect is strongest in the kidneys, intestines, spleen, and skin.

Important Areas of the Vasomotor Center

• Vasoconstrictor area: located bilaterally in the anterolateral portions of the upper medulla.
  • Neurons from this area distribute fibers throughout the body.
• Vasodilator area: located mainly in the lower medulla.
  • Neurons from this area project to sympathetic vasoconstrictor neurons and inhibit vasoconstriction.

Neural Control of Blood Pressure

• The vasomotor center controls blood pressure by adjusting the degree of sympathetic vasoconstriction.
• Increased sympathetic discharge (e.g., during excitement, exercise) constricts blood vessels, increasing blood pressure.
• Decreased sympathetic discharge (e.g., during sleep) relaxes blood vessels, decreasing blood pressure.

Baroreceptors and Blood Pressure Regulation

• Baroreceptors: sensory receptors in the aortic arch and carotid sinuses that detect changes in blood pressure.
• Baroreceptor reflex: Responds to changes in blood pressure to regulate it.
• Mechanism of Baroreceptor Reflex:
  • An increase in blood pressure stretches the baroreceptors.
  • Impulses from the baroreceptors travel to the vasomotor center.
  • Signals from the vasomotor center inhibit sympathetic vasoconstrictor activity and increase vagal activity to the heart.
  • This results in vasodilation, decreased heart rate, and decreased blood pressure.
• Baroreceptor system as a pressure buffer:
  • Prevents rapid changes in blood pressure.
  • Reduces minute-to-minute fluctuations in blood pressure.

Denervated Baroreceptor System

• After denervation, baroreceptors are less effective at regulating blood pressure.
• This results in higher fluctuations in blood pressure.

Chemoreceptors and Blood Pressure Regulation

• Chemoreceptors: sensory receptors in the aortic arch and carotid bodies that detect changes in blood oxygen, carbon dioxide, and pH levels.
• Chemoreceptor reflex: Responds to changes in blood gas levels to regulate blood pressure.
• Mechanism of Chemoreceptor Reflex:
  • Decrease in blood oxygen, increase in carbon dioxide, or decrease in pH stimulates chemoreceptors.
  • Signals from the chemoreceptors travel to the vasomotor center.
  • The vasomotor center increases sympathetic vasoconstrictor activity, increasing blood pressure.

CNS Ischemic Response

• Mechanism:
  • Increase in intracranial pressure compresses cerebral vessels.
  • This triggers a CNS ischemic pressure response.
  • The vasomotor center increases sympathetic vasoconstrictor activity, increasing blood pressure.
  • This alleviates cerebral ischemia.
• The CNS ischemic pressure response is a powerful mechanism to increase blood pressure in extreme cases of cerebral ischemia.

Autonomic Nervous System and Circulation Regulation

• The sympathetic nervous system plays a major role in circulatory regulation, increasing heart rate and contractility.
• Sympathetic fibers directly connect to the heart, enhancing its pumping action.
• Parasympathetic fibers influence cardiovascular function, mainly by controlling heart rate via the vagus nerve.
• Parasympathetic stimulation decreases heart rate and contractility.

Baroreceptors and Blood Pressure Regulation

• Baroreceptors, located in the carotid sinuses and aortic arch, sense changes in arterial pressure.
• Baroreceptor activation triggers a reflex that adjusts heart rate, vessel constriction, and blood pressure.
• The baroreceptor reflex helps maintain stable arterial pressure during postural changes, preventing dizziness upon standing.

Chemoreceptors and Arterial Pressure Regulation

• Chemoreceptors, located in the carotid bodies and aortic bodies, sense changes in oxygen, carbon dioxide, and hydrogen ion levels.
• Chemoreceptor activation triggers a reflex that modifies blood pressure, contributing to respiratory control.

Atrial Reflexes and Volume Regulation

• Atrial stretch receptors respond to increased atrial pressure, which often signifies increased blood volume.
• Atrial reflex activation reduces renal sympathetic nerve activity, promoting diuresis and blood volume regulation.

Nervous System Control Mechanisms and Oscillations

• Oscillations in arterial pressure, known as vasomotor waves, can be caused by fluctuations in baroreceptor and chemoreceptor reflexes.
• Delays in feedback responses can lead to oscillations in control systems, as seen in the example of an airplane's automatic pilot.

Sympathetic Vasoconstriction

• The sympathetic nervous system utilizes norepinephrine as the primary vasoconstrictor neurotransmitter.
• Norepinephrine directly acts on alpha-adrenergic receptors in vascular smooth muscle, leading to vasoconstriction.
• Adrenal medullae play a vital role in the sympathetic vasoconstrictor system.
• Sympathetic impulses stimulate the adrenal medullae, causing the release of epinephrine and norepinephrine into the bloodstream.
• These hormones act on all blood vessels, generally causing vasoconstriction.
• Notably, epinephrine can also cause vasodilation in some tissues due to its stimulation of beta-adrenergic receptors.
• A notable exception to vasoconstriction is the potential for sympathetic vasodilation in skeletal muscles during initial stages of exercise. This may be mediated by epinephrine or nitric oxide released from the vascular endothelium.
• Emotional fainting (vasovagal syncope) involves activation of the muscle vasodilator system and vagal heart inhibition, resulting in a rapid drop in arterial pressure.
• The drop in pressure reduces blood flow to the brain, leading to loss of consciousness, beginning with disturbing thoughts.

Role of the Nervous System in Rapid Arterial Pressure Control

• The nervous system is crucial for rapid increases in arterial pressure.
• The sympathetic nervous system coordinates vasoconstriction and cardioacceleration.
• This response involves simultaneous inhibition of parasympathetic vagal signals to the heart.
• Three major changes occur simultaneously:
  • Systemic arterioles constrict, increasing total peripheral resistance and raising arterial pressure.
  • Veins (and other large vessels) are strongly constricted, further contributing to the pressure rise.
• The rapid response of nervous control makes it a vital mechanism for immediate pressure regulation.
• Within seconds of stimulation, arterial pressure can rise significantly.
• Conversely, inhibiting nervous stimulation can quickly decrease arterial pressure by up to half within seconds.

Pressure Buffer Function of the Baroreceptor Control System

• The baroreceptor system opposes fluctuations in arterial pressure, acting as a pressure buffer.
• The nerves connecting the baroreceptors to the central nervous system are referred to as “buffer nerves.”
• Denervation of baroreceptors results in significantly increased variability in arterial pressure, demonstrating the importance of this system.
• The system greatly reduces minute-to-minute variations in pressure.

Baroreceptors and Long-Term Regulation

• The significance of baroreceptors in long-term regulation of arterial pressure remains controversial.
• Baroreceptors have been suggested to “reset” after 1 to 2 days to the prevailing pressure level, potentially reducing their long-term effectiveness.
• Despite resetting, studies suggest a persistent contribution of baroreceptors to long-term pressure control.

Chemoreceptor Reflex

• The chemoreceptor reflex can also oscillate, often alongside the baroreceptor reflex.
• The chemoreceptor reflex primarily influences vasomotor waves at low arterial pressures.
• It is important at pressure ranges of 40 to 80 mm Hg, while baroreceptor control weakens.

CNS Ischemic Pressure Control Mechanism

• Oscillation of the CNS ischemic pressure control mechanism occurs during cerebral ischemia.
• Cerebrospinal fluid pressure elevation causes compression of cerebral vessels, initiating a sympathetic surge to restore blood flow.
• When pressure rises, the sympathetic system becomes inactive.

This quiz covers the structure and function of the vasomotor center, including its role in controlling vasoconstriction and vasodilation. It explores how sympathetic vasomotor tone affects blood pressure and the influence of higher brain centers on the vasomotor center's activity. Test your knowledge on these physiological processes.