The Autonomic Nervous System: Structure, Function, and Clinical Relevance

The Autonomic Nervous System (ANS)

Introduction to the ANS

The autonomic nervous system (ANS) is a division of the peripheral nervous system that controls involuntary bodily functions. It consists of motor neurons that innervate smooth muscle, cardiac muscle, and glands, making adjustments to support body activities and maintain homeostasis. The ANS operates largely via subconscious control and is also known as the involuntary nervous system or general visceral motor system.

• Innervates: Smooth muscle, cardiac muscle, glands

• Functions: Adjusts heart rate, blood pressure, digestive processes, etc.

• Control: Subconscious

Structural Organization of the Nervous System

Place of the ANS

The nervous system is divided into the central nervous system (CNS) and peripheral nervous system (PNS). The PNS is further divided into sensory (afferent) and motor (efferent) divisions. The motor division includes the somatic nervous system (voluntary control of skeletal muscles) and the autonomic nervous system (involuntary control of visceral organs).

• ANS Subdivisions: Sympathetic and Parasympathetic divisions

ANS Versus Somatic Nervous System

Comparison of Effectors and Pathways

Both systems have motor fibers but differ in their effectors and neural pathways.

• Somatic Nervous System: Innervates skeletal muscles

• ANS: Innervates cardiac muscle, smooth muscle, and glands

• Somatic Pathway: Single, thick myelinated axon from CNS to muscle

• ANS Pathway: Two-neuron chain:

  1. Preganglionic neuron: Cell body in CNS, lightly myelinated axon to ganglion

  2. Postganglionic neuron: Cell body in autonomic ganglion, nonmyelinated axon to effector organ

Neurotransmitter Effects

Somatic vs. Autonomic Neurotransmitters

• Somatic: All motor neurons release acetylcholine (ACh); effect is always stimulatory

• ANS: Preganglionic fibers release ACh; postganglionic fibers release norepinephrine (NE) or ACh at effectors

• Effect can be stimulatory or inhibitory, depending on receptor type

Overlap of Somatic and Autonomic Function

Integration and Coordination

Higher brain centers regulate both systems. Many spinal and cranial nerves contain both somatic and autonomic fibers. Adaptations often involve both skeletal muscles and visceral organs.

• Example: During exercise, ANS nerves increase heart rate and open airways to supply active muscles with oxygen and glucose.

Comparison of Motor Neurons

Somatic vs. Autonomic Motor Neurons

The following table summarizes the differences between motor neurons in the somatic and autonomic nervous systems:

System	Neural Pathway	Neurotransmitter	Effector	Effect
Somatic	Single neuron from CNS to muscle	ACh	Skeletal muscle	Stimulatory
Autonomic (Sympathetic)	Two-neuron chain (preganglionic and postganglionic)	ACh (preganglionic), NE (postganglionic)	Cardiac/smooth muscle, glands	Stimulatory or inhibitory
Autonomic (Parasympathetic)	Two-neuron chain	ACh (both)	Cardiac/smooth muscle, glands	Stimulatory or inhibitory

Autonomic System Structure and Function

Overview

The ANS consists of complex neural pathways that connect the CNS to various organs, regulating involuntary functions such as heart rate, digestion, and respiratory rate.

Divisions of the Autonomic Nervous System

Parasympathetic and Sympathetic Divisions

• Parasympathetic division: Promotes maintenance functions and conserves energy ("rest-and-digest")

• Sympathetic division: Mobilizes body during activity ("fight-or-flight")

• Dual innervation: Most organs receive input from both divisions, which often have opposing effects to maintain homeostasis

Role of the Parasympathetic Division

Functions and Effects

The parasympathetic division keeps body energy use low while maintaining vital functions.

• Directs digestion, diuresis, and defecation

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

• Gastrointestinal tract activity is high

• Pupils constricted, lenses accommodated for close vision

• Example: Relaxing after a meal

Role of the Sympathetic Division

Functions and Effects

The sympathetic division prepares the body for stressful or energetic activity.

• Activates during exercise, excitement, emergency, embarrassment

• Increases heart rate, dilates bronchioles, causes liver to release glucose

• Shunts blood to skeletal muscles and heart

• Example: "Fight-or-flight" response

Key Anatomical Differences

Parasympathetic vs. Sympathetic Divisions

• Sites of origin: Parasympathetic fibers are craniosacral; sympathetic fibers are thoracolumbar

• Relative lengths of fibers: Parasympathetic has long preganglionic and short postganglionic fibers; sympathetic has short preganglionic and long postganglionic fibers

• Location of ganglia: Parasympathetic ganglia are near or within effector organs; sympathetic ganglia are close to the spinal cord

Differences Between the Parasympathetic and Sympathetic Divisions

Summary Table

Characteristic	Parasympathetic	Sympathetic
Origin	Craniosacral (brainstem, sacral spinal cord)	Thoracolumbar (thoracic, lumbar spinal cord)
Location of ganglia	Near/in effector organs	Close to spinal cord
Fiber lengths	Long preganglionic, short postganglionic	Short preganglionic, long postganglionic
Degree of branching	Minimal	Extensive
Functional role	Maintenance, energy conservation	Mobilization, stress response

Parasympathetic Division

Craniosacral Division

• Long preganglionic fibers extend from CNS almost to target organs

• Synapse with postganglionic neurons in terminal ganglia close to or within target organs

• Short postganglionic fibers synapse with effectors

Visceral Sensory Neurons

Function and Pathways

• Send information about chemical changes, stretch, temperature, and irritation of viscera

• Receptors are free nerve endings scattered throughout viscera

• Cell bodies in dorsal root ganglia and sensory ganglia of cranial nerves

• Axons travel with autonomic motor fibers

• Can play a role in referred pain

Visceral Reflexes

Components and Examples

Visceral reflex arcs have the same basic components as somatic reflex arcs but differ in key ways:

1. Visceral reflex arc has two consecutive neurons in the motor pathway

2. Afferent fibers are visceral sensory neurons

3. Effectors are smooth muscle, cardiac muscle, and glands

• Examples: Sneezing, coughing, swallowing, vomiting, pupil dilation, contraction of smooth muscles

• Involves the enteric nervous system in the gastrointestinal tract

Neurotransmitters

Types and Effects

• Acetylcholine (ACh): Released by cholinergic fibers (all ANS preganglionic axons, all parasympathetic postganglionic axons)

• Norepinephrine (NE): Released by adrenergic fibers (almost all sympathetic postganglionic axons except sweat glands)

• Effect depends on receptor type: cholinergic or adrenergic

Cholinergic Receptors

Nicotinic and Muscarinic

• Nicotinic receptors: Found on all postganglionic neurons, adrenal medulla cells, and skeletal muscle cells at neuromuscular junction; effect is always stimulatory

• Muscarinic receptors: Found on all effector cells stimulated by postganglionic cholinergic fibers; effect can be inhibitory or excitatory depending on receptor subtype and target organ

• Example: ACh binding to cardiac muscle slows heart rate; binding to intestinal smooth muscle increases motility

Adrenergic Receptors

Alpha and Beta Receptors

• Alpha (α) receptors: Subclasses α1, α2

• Beta (β) receptors: Subclasses β1, β2, β3

• Effects depend on which subclass predominates on the target organ

• Example: NE binding to cardiac muscle β1 receptors increases heart rate; binding to β2 receptors causes bronchial relaxation

Cholinergic and Adrenergic Receptors Table

Summary Table

Neurotransmitter	Receptor Type	Main Locations	Effect on Binding
Acetylcholine (ACh)	Nicotinic	All postganglionic neurons, adrenal medulla, skeletal muscle	Excitation
Acetylcholine (ACh)	Muscarinic	All parasympathetic target organs, some sympathetic targets	Excitation (most), inhibition (cardiac muscle)
Norepinephrine (NE)/Epinephrine	Alpha (α)	Most sympathetic target organs	Excitation (α1), inhibition (α2)
Norepinephrine (NE)/Epinephrine	Beta (β)	Heart, lungs, blood vessels	Excitation (β1), inhibition (β2), variable (β3)

Selected Drug Classes That Influence the ANS

Drug Effects Table

Drug Class	Receptor Target	Effects	Example	Clinical Application
Cholinergic agents	Muscarinic ACh receptors	Stimulate parasympathetic effects	Pilocarpine	Treats glaucoma
Anticholinergic agents	Muscarinic ACh receptors	Inhibit parasympathetic effects	Atropine	Preoperative medication
Sympathomimetic agents	Adrenergic receptors	Stimulate sympathetic effects	Albuterol	Treats asthma
Sympatholytic agents	Adrenergic receptors	Inhibit sympathetic effects	Propranolol	Treats hypertension

Sympathetic and Parasympathetic Tone

Vasomotor and Organ Tone

• Sympathetic tone: Maintains partial constriction of blood vessels, controls blood pressure

• Parasympathetic tone: Dominates heart and smooth muscle of digestive and urinary tracts, slows heart rate, dictates normal activity

• Sympathetic division can override these effects during stress

Unique Roles of the Sympathetic Division

Specialized Functions

• Thermoregulatory responses to heat (sweat glands, blood vessel dilation/constriction)

• Release of renin from kidneys

• Metabolic effects: increases metabolic rate, raises blood glucose, mobilizes fats

Localized Versus Diffuse Effects

Parasympathetic vs. Sympathetic Control

• Parasympathetic division: short-lived, localized effects (ACh quickly destroyed)

• Sympathetic division: longer-lasting, bodywide effects (NE inactivated slowly; adrenal medulla hormones prolong effects)

Effects of Parasympathetic and Sympathetic Divisions on Various Organs

Organ System Effects Table

Target Organ/System	Parasympathetic Effects	Sympathetic Effects
Eye (iris)	Stimulates sphincter pupillae (constricts pupils)	Stimulates dilator pupillae (dilates pupils)
Heart	Decreases heart rate	Increases heart rate and force
Digestive tract	Increases motility and secretion	Decreases motility and secretion
Bronchioles	Constricts airways	Dilates airways
Liver	No effect	Stimulates glucose release

Control of ANS Function

Central Regulation

• Cortical controls: Communication with limbic system; some voluntary control possible

• Hypothalamic controls: Main integrative center; anterior regions direct parasympathetic, posterior direct sympathetic functions

• Brain stem: Reticular formation exerts direct influence

• Spinal cord: Location of several visceral reflexes

Discussion Points

Integration and Receptor Comparison

• Example of integration: Somatic system moves body away from danger, ANS increases heart rate and prepares body for action

• Muscarinic vs. Nicotinic receptors: Nicotinic are always stimulatory; muscarinic can be excitatory or inhibitory

• Neurotransmitter effects: A single neurotransmitter can have different effects depending on the receptor and organ system