Autonomic Nervous System Regulation of Heart and Circulation

Autonomic Nervous System: Overview

Introduction to the Autonomic Nervous System (ANS)

The autonomic nervous system (ANS) is a division of the peripheral nervous system that regulates involuntary physiological processes, including heart rate, blood pressure, respiration, digestion, and sexual arousal. The ANS is subdivided into the sympathetic and parasympathetic branches, which often have opposing effects to maintain homeostasis.

• Sympathetic branch: Prepares the body for 'fight-or-flight' responses, increasing heart rate and blood flow to muscles.

• Parasympathetic branch: Promotes 'rest-and-digest' activities, decreasing heart rate and facilitating digestion.

• Homeostasis: Dynamic balance between sympathetic and parasympathetic activity.

Example: During rest, parasympathetic activity dominates; during exercise or stress, sympathetic activity increases.

Autonomic Nervous System Pathways

Neural Pathways and Neurotransmitters

The ANS uses distinct neural pathways and neurotransmitters to communicate with target organs.

• Parasympathetic pathway: Originates in the CNS, uses acetylcholine (ACh) at both ganglionic and effector synapses.

• Sympathetic pathway: Originates in the CNS, uses acetylcholine at ganglia and norepinephrine (NE) at effector synapses.

• Adrenal sympathetic pathway: Involves the adrenal medulla releasing epinephrine (E) into the bloodstream.

Key receptors: Muscarinic (parasympathetic), adrenergic (sympathetic).

Example: Sympathetic stimulation of the heart uses NE on β1-adrenergic receptors.

Regulation of Heart Rate and Stroke Volume

Control of Heart Rate (Chronotropy)

Heart rate is primarily determined by the rate of depolarization in the sinoatrial (SA) node, which is modulated by autonomic input.

• Parasympathetic stimulation: Releases ACh, increases K+ efflux and decreases Ca2+ influx, hyperpolarizing SA node cells and slowing heart rate.

• Sympathetic stimulation: Releases NE, increases Na+ and Ca2+ influx, depolarizing SA node cells and increasing heart rate.

Equation:

$\text{Cardiac Output (CO)} = \text{Heart Rate (HR)} \times \text{Stroke Volume (SV)}$

Control of Stroke Volume (Inotropy)

Stroke volume is influenced by the force of contraction in ventricular myocardium, which is regulated by sympathetic activity and venous return.

• Sympathetic stimulation and epinephrine: Increase contractility (inotropy) via β1-adrenergic receptors.

• Venous return: Enhanced by venoconstriction (SNS), skeletal muscle pump, and respiratory pump.

• Frank-Starling mechanism: Increased end-diastolic volume (EDV) leads to stronger contractions.

Example: During exercise, both heart rate and stroke volume increase, raising cardiac output.

Regulation of Arteriolar Resistance and Blood Flow Distribution

Sympathetic Control of Vascular Tone

Arteriolar resistance is modulated by the frequency of sympathetic nerve firing and the release of norepinephrine.

• Tonic sympathetic activity: Maintains baseline vasoconstriction ('tone').

• Increased NE release: Causes vasoconstriction, raising resistance and redirecting blood flow.

• Decreased NE release: Causes vasodilation, lowering resistance and increasing blood flow to specific tissues.

Example: During exercise, vasodilation in skeletal muscle increases blood flow, while vasoconstriction in the GI tract reduces flow.

The Baroreceptor Reflex

Mechanism and Function

The baroreceptor reflex is a rapid negative feedback system that helps maintain stable arterial blood pressure.

• Baroreceptors: Stretch-sensitive receptors located in the carotid arteries and aorta.

• Function: Monitor blood pressure and send signals to the medulla oblongata.

• Response to low BP: Increased sympathetic, decreased parasympathetic activity; raises HR, SV, and vasoconstriction.

• Response to high BP: Increased parasympathetic, decreased sympathetic activity; lowers HR, SV, and vasodilation.

Equation:

$\text{Mean Arterial Pressure (MAP)} = \text{Cardiac Output (CO)} \times \text{Total Peripheral Resistance (TPR)}$

Cardiovascular Adjustments During Hypotension

Response to Postural Changes and Blood Loss

Sudden drops in blood pressure, such as from standing up quickly or hemorrhage, trigger compensatory mechanisms.

• Short-term (seconds to minutes): Baroreceptor reflex increases SNS outflow, raising CO and TPR to restore BP.

• Long-term (minutes to hours): Hormonal systems activate (e.g., angiotensin II, vasopressin) to increase fluid retention and vasoconstriction.

Example: Orthostatic hypotension is counteracted by increased heart rate and vasoconstriction.

Cardiovascular Adjustments During Exercise

Mechanisms Ensuring Oxygen Delivery to Muscles

During exercise, the cardiovascular system adapts to meet increased metabolic demands of skeletal muscle.

• Increased cardiac output: Both HR and SV rise due to increased SNS activity and venous return.

• Redistribution of blood flow: Vasodilation in active muscles, vasoconstriction in non-essential organs.

• Venous return: Enhanced by muscle and respiratory pumps.

• Mean arterial pressure: Rises slightly due to increased CO, but is moderated by vasodilation in muscle.

Equation:

$\text{CO} = \text{HR} \times \text{SV}$

Example: During running, sympathetic activity increases HR and contractility, while vasodilation in leg muscles ensures adequate oxygen delivery.

Summary Table: Autonomic Effects on Cardiovascular Function

Parameter	Sympathetic Effect	Parasympathetic Effect
Heart Rate	Increases (via NE, β1 receptors)	Decreases (via ACh, muscarinic receptors)
Stroke Volume	Increases (via contractility, venous return)	No direct effect
Arteriolar Resistance	Increases (vasoconstriction)	Decreases (vasodilation in some beds)
Blood Pressure	Increases	Decreases
Blood Flow Distribution	Redirects to muscles, heart	Promotes flow to GI tract, skin

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