Autonomic Nervous System: Structure, Function, and Control

Autonomic Nervous System (ANS)

Overview of the ANS

The autonomic nervous system (ANS) is a division of the peripheral nervous system that regulates involuntary physiological processes. It primarily controls smooth muscle, cardiac muscle, and glands, operating largely below the level of conscious awareness to maintain homeostasis.

• Innervates smooth and cardiac muscle, and glands.

• Adjusts body functions to ensure optimal support for body activities.

• Operates via subconscious (involuntary) control.

• Effectors are mainly the viscera (internal organs).

Nervous System Organization

The nervous system is organized into central and peripheral components, with the ANS being a major part of the motor division of the peripheral nervous system.

• CNS (Central Nervous System): Brain and spinal cord.

• PNS (Peripheral Nervous System): Sensory (afferent) and motor (efferent) divisions.

• Motor Division: Subdivided into the somatic nervous system (voluntary control of skeletal muscles) and the autonomic nervous system (involuntary control of smooth/cardiac muscle and glands).

• ANS Divisions: Sympathetic and parasympathetic.

Divisions of the ANS

Sympathetic and Parasympathetic Divisions

The ANS is divided into two main branches that often have opposing effects on target organs, allowing precise regulation of visceral functions.

• Sympathetic Division: Mobilizes the body during extreme situations ("fight or flight").

• Parasympathetic Division: Performs maintenance activities and conserves body energy ("rest and digest").

• Most visceral organs receive dual innervation from both divisions, resulting in dynamic antagonism for fine control.

Role of the Parasympathetic Division

Functions and Effects

The parasympathetic division is primarily concerned with conserving energy and promoting housekeeping functions during rest.

• Involves the "D activities": digestion, defecation, diuresis (urination).

• Illustrated by a relaxed state after a meal:

  • Blood pressure, heart rate, and respiratory rates are low.

  • Gastrointestinal tract activity is high.

  • The skin is warm and the pupils are constricted.

Role of the Sympathetic Division

Functions and Effects

The sympathetic division prepares the body for situations requiring alertness or strength, such as emergencies or vigorous physical activity.

• Known as the "fight-or-flight" system.

• Originates from thoracic and upper lumbar regions of the spinal cord (sympathetic chain ganglia).

• Involves the "E activities": exercise, excitement, emergency, embarrassment.

• During exercise or threat:

  • Blood flow to organs is reduced; flow to skeletal muscles is increased.

  • Heart rate increases; breathing becomes rapid and deep.

  • The skin becomes cold and sweaty; pupils dilate.

Neurotransmitters and Receptors

Major Neurotransmitters

The ANS uses two primary neurotransmitters to communicate with target tissues:

• Acetylcholine (ACh): Released by all preganglionic fibers (both sympathetic and parasympathetic) and by parasympathetic postganglionic fibers.

• Norepinephrine (NE): Released by most sympathetic postganglionic fibers.

Neurotransmitter effects depend on the type of receptor present on the target organ.

Types of Fibers

• Cholinergic fibers: Release ACh (all ANS preganglionic axons, all parasympathetic postganglionic axons).

• Adrenergic fibers: Release NE (most sympathetic postganglionic axons).

Receptor Types

• Parasympathetic (ACh):

  • Nicotinic receptors: Always excitatory when ACh binds.

  • Muscarinic receptors: Can be excitatory or inhibitory depending on the target tissue.

• Sympathetic (NE):

  • Alpha (α) receptors: Generally excitatory.

  • Beta (β) receptors: Generally inhibitory, except NE binding to β-receptors in the heart is excitatory.

Nerve Roots and Ganglions

Structure and Pathways

Autonomic nerve fibers form complex networks involving ganglia outside the central nervous system.

• Sympathetic pathways: Preganglionic fibers may synapse in sympathetic chain ganglia or pass through to collateral ganglia (e.g., celiac ganglion).

• Effectors: Smooth muscle and glands of abdominal organs (e.g., intestine).

Sympathetic and Parasympathetic Tone

Sympathetic Tone (Vasomotor Tone)

The sympathetic division maintains a baseline level of activity in blood vessels, known as sympathetic tone, which is crucial for blood pressure regulation.

• Constriction of blood vessels via α-receptors increases blood pressure as needed.

• Vasodilation in skeletal muscle during increased activity.

• α-blocker drugs can reduce blood pressure by interfering with vasomotor fibers.

• Orthostatic hypotension: A drop in blood pressure upon standing, often due to slow sympathetic response.

Parasympathetic Tone

• Slows the heart rate.

• Regulates normal activity of digestive and urinary systems.

• Can be overridden by sympathetic division during stress.

• Drugs blocking parasympathetic responses can increase heart rate and inhibit digestive/urinary functions.

Unique Roles of the Sympathetic Division

Functions Not Shared by Parasympathetic Division

• Regulation of adrenal medulla, sweat glands, arrector pili muscles, kidneys, and most blood vessels.

• Thermoregulatory responses to heat.

• Release of renin from the kidneys.

• Metabolic effects (e.g., increased metabolic rate, blood glucose).

Localized Versus Diffuse Effects

Comparison of Parasympathetic and Sympathetic Effects

• Parasympathetic: Short-lived, localized effects due to rapid breakdown of ACh.

• Sympathetic: Long-lasting, diffuse effects because NE is inactivated more slowly and adrenal medulla releases epinephrine and NE into the bloodstream.

Levels of ANS Control

Central Regulation

The hypothalamus is the main integration center for ANS activity, but other brain regions also contribute.

• Hypothalamus: Controls heart activity, blood pressure, body temperature, water balance, endocrine activity, emotional states, and biological drives.

• Limbic system: Provides subconscious input to the hypothalamus.

• Cerebral cortex, reticular formation, spinal cord: Also influence ANS functions.

Referred Pain

Definition and Mechanism

Referred pain is the phenomenon where pain from internal organs is perceived as originating from the skin or other superficial sites. This occurs because visceral and somatic sensory fibers converge on the same pathways in the spinal cord.

• Example: Pain from a heart attack may be felt in the left arm.

Developmental Aspects of the ANS

Changes Across the Lifespan

• In youth, ANS impairments are usually due to injury.

• With aging, ANS efficiency decreases, leading to issues such as constipation, dry eyes, and orthostatic hypotension.

Summary Table: Comparison of Sympathetic and Parasympathetic Divisions

Feature	Sympathetic Division	Parasympathetic Division
Origin	Thoracic and lumbar spinal cord	Brainstem and sacral spinal cord
Main Function	"Fight or flight" (emergency, stress)	"Rest and digest" (maintenance, energy conservation)
Neurotransmitter (postganglionic)	Norepinephrine (NE)	Acetylcholine (ACh)
Receptors	Alpha (α), Beta (β) adrenergic	Nicotinic, Muscarinic cholinergic
Effect on Heart Rate	Increases	Decreases
Effect on GI Tract	Decreases activity	Increases activity
Pupil Response	Dilates		Constricts