CB [054] Central Nervous control of the circulation 2024-2025

Central Nervous Control of the Circulation

Instructor

Dr. Sandra Younan, Professor of Physiology, Kasr Al Aini Faculty of Medicine, New Giza School of Medicine (2024-2025).

Aims

The primary aim is to thoroughly describe and elucidate the multifaceted functions of central nervous control of circulation, addressing both physiological states in health and pathophysiological conditions in disease.

Classification of Resting Values of Arterial Blood Pressure (BP)

Normal and High Resting BP:

• Normal BP:A young adult at rest typically presents a BP of 120/80 mmHg.

  • Notably, about 5% of individuals aged 60 years or older may exhibit systolic BP exceeding 160 mmHg, indicating a higher risk for cardiovascular events.

  • The average BP for individuals approximately 70 years old is around 170/90 mmHg.

High BP:

• Blood pressure is classified as high when readings are ≥140/90 mmHg. Consistent high BP can precipitate serious health issues such as cardiovascular and renal complications.

Blood Pressure Patterns

• Resting BP demonstrates diurnal variation influenced by multiple factors:

  • Graphical Analysis: Illustrates variations in systolic (SBP) and diastolic (DBP) BP throughout the 24-hour cycle.

  • A nocturnal dip is typically observed from 12 AM to 6 AM, which is essential for cardiovascular recovery during sleep.

Blood Pressure Influences

• Activity Dependency: Blood pressure levels vary across activities:

  • Elevated during engaging activities such as conversations, emotional arguments, or physical exertion.

  • Blood pressure levels fluctuate between states of sleep and wakefulness, demonstrating an inherent activity-state dependence.

White-Coat Syndrome

• Concept: Introduced to describe the phenomenon where BP readings increase during medical exams due to patient anxiety or stress.

  • For accurate assessment, BP should be recorded when patients are calm and relaxed to avoid misleading readings.

Isolated Clinic Hypertension

• This condition is identifiable only through ambulatory blood pressure monitoring (ABPM) or self-monitoring.

  • Current research suggests it may not be a benign condition, and further studies on its long-term outcomes are warranted.

  • Ongoing surveillance is advised every 1-2 years, as it does not typically respond well to standard antihypertensive therapies.

Interpreting ABP Profile

• Evaluation of ABP profiles must be performed alongside diary information related to activities and timing of medications to ascertain true blood pressure regulation

  • Normal ABP Values:

    • Daytime: < 135/85 mmHg.

    • Nighttime: < 120/75 mmHg.

    • 24-hour average: < 130/80 mmHg.

  • An ideal profile would maintain load increases of < 20% above normal values.

Effect of Pain on BP

• Painful stimuli lead to elevations in both BP and heart rate, mechanisms of which operate independently of brainstem regions.

  • Under certain conditions, normal physiological inputs can induce hypertensive crises.

CNS Areas Controlling Circulation

• Key Brain Regions:

  • Cerebrum and Diencephalon: Responsible for generating cardiovascular responses linked to behavioral changes and emotional states.

  • Brainstem and Spinal Cord: Critical for the fundamental reflexive control of the cardiovascular system.

Sympathetic and Parasympathetic Outflow

• The Sympathetic Nervous System (SNS) influences cardiac and vascular smooth muscle, promoting vasoconstriction and increased heart rate.

• The Parasympathetic Nervous System (PNS) primarily regulates heart function, modulating heart rate through activities such as vagal tone enhancement.

Cardiac Control Centers

• Cardiovascular control centers located within the medulla oblongata include:

  • Cardiostimulator Center: Enhances cardiovascular activity via sympathetic pathways, influencing heart rate and contractility.

  • Cardioinhibitory Center: Reduces cardiovascular activity through parasympathetic pathways, primarily lowering heart rate.

  • Both centers work to maintain a slight resting stimulation to the heart, contributing to overall autonomic tone and cardiovascular homeostasis.

Autonomic Nervous System Overview

• Sympathetic Nervous System: Involved in the fight or flight response, controlling physiological responses such as vasodilation or vasoconstriction, and the modulation of smooth muscle.

• Parasympathetic Nervous System: Responsible for restorative functions including lowering heart rate and regulating digestive processes.

Inputs and Feedback in Blood Pressure Regulation

• Regulation of blood pressure incorporates diverse inputs from various receptors, including:

  • Arterial and Atrial Baroreceptors: Monitor stretch variations due to blood pressure changes.

  • Mechanoreceptors: Respond to physical force in vessels.

  • Chemoreceptors: Detect changes in blood chemistry, adjusting cardiovascular function accordingly.

Negative Feedback Control Mechanism

• A vital methodology in sustaining stable blood pressure levels.

  • Breakdown of this mechanism can lead to significant instability in blood pressure regulation and cardiovascular health.

Baroreceptor Reflex and Control

• Baroreceptor Reflex Loop:

  • Encompasses interaction between the brainstem, sympathetic and parasympathetic pathways that facilitate efficient cardiovascular responses.

  • Activation occurs when BP surpasses established set points, leading to vasoconstriction or dilation in blood vessels, thereby restoring BP to baseline levels.

Locations of Receptors & Afferent Nerves

• Key Afferent Nerves:

  • Glossopharyngeal and Vagus Nerves: Critical for conveying baroreceptor signals to the CNS.

• Anatomical Sites:

  • Receptors are strategically located at the carotid arteries, aorta, and various structures within the heart.

Baroreceptor Function

• Baroreceptors operate as stretch receptors, which respond to fluctuations in blood pressure, ultimately providing necessary feedback to adapt heart rate and vascular tone as required.

Importance of the Baroreceptor Reflex

• The baroreceptor reflex is crucial for adaptive cardiovascular responses in situations such as

  • Standing from a sitting position (upright positioning),

  • During hemorrhage events,

  • In cases of cardiac failure, and

  • During exercise, supplying dynamic regulation of blood pressure across varying conditions.

Exercise Effects on BP

• During physical exercise, the following changes are typically observed:

  • Cardiac Output (CO) and Heart Rate (HR) both experience significant increases, enhancing blood flow to muscles.

  • Total Peripheral Resistance (TPR) often sees a reduction as blood vessels dilate to accommodate increased blood flow.

  • Systolic BP generally rises due to increased cardiac output, while diastolic BP may exhibit a decrease or remain steady depending on the level of exertion.

Testing Baroreceptors

• The Valsalva Maneuver serves as a method to evaluate baroreflex functionality by monitoring blood pressure responses during forced exhalation against a closed airway.

Atrial Baroreceptors Overview

• Atrial Baroreceptors: Located within the atria, these sensors respond to elevated filling pressures, facilitating an increased heart rate to optimize cardiac efficiency during adjustments of venous return.

  • The Atrial Reflex (Bainbridge Reflex) describes the heart's physiological response to changes in atrial dynamics, contributing significantly to cardiovascular regulation.

Ventricular Mechanoreceptors

• Ventricular Mechanoreceptors: These specialized receptors elicit vasodilation and bradycardia when stimulated, playing a pivotal role in blood pressure regulation and the body's adaptive responses.

Peripheral Chemoreceptors Function

• Stimulation of peripheral chemoreceptors typically raises BP and can induce changes in HR depending on lung ventilation status, adapting to varied conditions like airway obstruction and altitude changes.

References

• Green A., Schuyan Y. et al. Deep brain stimulation can regulate arterial blood pressure in humans. Neuroreport, somatosensory system.

Closing

• We thank you for your participation from the New Giza School of Medicine.