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 consists of motor neurons that innervate smooth muscle, cardiac muscle, and glands, operating largely below the level of conscious awareness to maintain homeostasis.

• Innervation: Targets smooth muscle, cardiac muscle, and glands.

• Function: Adjusts body activities for optimal support.

• Control: Operates via subconscious mechanisms.

• Effectors: Primarily viscera (internal organs).

Nervous System Organization

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

• CNS: Central Nervous System

• PNS: Peripheral Nervous System

• Motor Division: Includes the ANS and somatic nervous system

• ANS Divisions: Sympathetic and Parasympathetic

Nervous System Flowchart

CNS
PNS
Motor Division	Sensory Division
Autonomic Nervous System	Somatic Nervous System
Sympathetic Division	Parasympathetic Division

Divisions of the ANS

Sympathetic and Parasympathetic Divisions

The ANS is divided into two main branches: the sympathetic and parasympathetic divisions. These divisions often have opposing effects on target organs, allowing precise control of visceral functions.

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

• Parasympathetic Division: Maintains body functions and conserves energy ("rest and digest").

• Dual Innervation: Most organs receive input from both divisions, resulting in dynamic antagonism.

Role of the Parasympathetic Division

Functions and Effects

The parasympathetic division is primarily concerned with conserving energy and promoting maintenance activities.

• "D" Activities: Digestion, Defecation, Diuresis

• Physiological Effects:

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

  • Gastrointestinal tract activity is high

  • Skin is warm, pupils are constricted

• Example: A person relaxing after a meal demonstrates parasympathetic activity.

Role of the Sympathetic Division

Functions and Effects

The sympathetic division prepares the body for stressful or emergency situations, often referred to as the "fight-or-flight" response.

• Origin: Thoracic and upper lumbar regions of the spinal cord (sympathetic chain ganglia)

• "E" Activities: Exercise, Excitement, Emergency, Embarrassment

• Physiological Effects:

  • Blood flow to organs is reduced, flow to muscles is increased

  • Heart rate increases, breathing becomes rapid and deep

  • Skin becomes cold and sweaty, pupils dilate

• Example: A person threatened or under stress exhibits sympathetic activation.

Neurotransmitters and Receptors

Major Neurotransmitters

The ANS uses two primary neurotransmitters: acetylcholine (ACh) and norepinephrine (NE).

• ACh: Released by all preganglionic fibers and by parasympathetic postganglionic fibers (cholinergic fibers).

• NE: Released by sympathetic postganglionic fibers (adrenergic fibers).

• Effect: Neurotransmitter effects depend on receptor type and can be stimulatory or inhibitory.

Receptor Types

Division	Neurotransmitter	Receptor Types	Effect
Parasympathetic	ACh	Nicotinic (excitatory), Muscarinic (excitatory or inhibitory)	Varies by organ
Sympathetic	NE (and epinephrine from adrenal medulla)	Alpha (generally excitatory), Beta (generally inhibitory; exception: heart)	Varies by organ

Additional info: NE binding to β-receptors at the heart is excitatory, increasing heart rate and contractility.

Nerve Roots and Ganglions

Structure and Pathways

ANS pathways involve a two-neuron chain: preganglionic and postganglionic neurons. Ganglia are clusters of neuronal cell bodies outside the CNS.

• Sympathetic Pathway: Preganglionic fibers pass through the sympathetic trunk to collateral ganglia (e.g., celiac ganglion).

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

Sympathetic Tone

Cardiovascular Regulation

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

• Vasomotor Tone: Partial constriction of blood vessels via α-receptors.

• Effects: Blood pressure rises as needed; blood vessels in skeletal muscle dilate during activity.

• Clinical Application: α-blocker drugs can treat hypertension by interfering with vasomotor fibers.

• Orthostatic Hypotension: Low blood pressure due to slow sympathetic response to positional changes.

Parasympathetic Tone

Regulation of Resting Functions

The parasympathetic division sets the resting activity levels of the heart, digestive, and urinary systems.

• Effects: Slows heart rate; promotes normal digestive and urinary function.

• Override: Sympathetic division can override parasympathetic effects during stress.

• Drugs: Agents that block parasympathetic responses increase heart rate and inhibit digestive/urinary functions.

Unique Roles of the Sympathetic Division

Functions Not Shared with Parasympathetic Division

The sympathetic division regulates several functions that are not influenced by the parasympathetic division.

• Adrenal Medulla Activity

• Sweat Glands

• Arrector Pili Muscles

• Kidneys

• Most Blood Vessels

• Thermoregulatory Responses

• Renin Release

• Metabolic Effects

Localized Versus Diffuse Effects

Duration and Spread of Effects

The two divisions of the ANS differ in the duration and localization of their effects.

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

• Sympathetic: Long-lasting, diffuse effects because NE is inactivated slowly and epinephrine remains in the blood until destroyed by the liver.

Levels of ANS Control

Central Regulation

ANS activity is regulated at multiple levels within the central nervous system.

• Main Integration Center: Hypothalamus

• Influences: Subconscious cerebral input via limbic system, cerebral cortex, reticular formation, and spinal cord.

Hypothalamic Control

• Regulates: Heart activity, blood pressure, body temperature, water balance, endocrine activity

• Controls: Emotional states (rage, pleasure), biological drives (hunger, thirst, sex), and fear responses

Referred Pain

Mechanism and Clinical Significance

Referred pain occurs when visceral pain is perceived as originating from somatic regions due to shared neural pathways.

• Mechanism: Visceral pain afferents travel along the same pathways as somatic pain fibers.

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

Developmental Aspects of the ANS

Changes Across the Lifespan

ANS function changes with age and injury.

• Youth: Impairments usually due to injury.

• Old Age: Efficiency decreases, leading to constipation, dry eyes, and orthostatic hypotension.