Autonomic Nervous System: Structure, Function, and Clinical Relevance

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Chapter 14: The Autonomic Nervous System

Why This Matters

The autonomic nervous system (ANS) is essential for regulating involuntary physiological functions. Understanding the ANS is crucial for predicting drug effects and managing patient care, as many drugs target ANS receptors.

Key Point: Many drugs act by activating or inhibiting ANS neurotransmitter receptors on target organs.
Key Point: Knowledge of the ANS helps anticipate drug side effects and clinical outcomes.

Place of the ANS in the Structural Organization of the Nervous System

The ANS is a division of the peripheral nervous system (PNS) responsible for involuntary control of body functions.

Central Nervous System (CNS): Brain and spinal cord.
Peripheral Nervous System (PNS): Includes 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).
ANS Divisions: Sympathetic and Parasympathetic.

Comparison of Motor Neurons in the Somatic and Autonomic Nervous Systems

Somatic and autonomic motor neurons differ in structure and function.

Somatic Nervous System: Single, heavily myelinated axon from CNS to skeletal muscle; always stimulatory.
Autonomic Nervous System: Two-neuron chain: preganglionic (lightly myelinated) and postganglionic (unmyelinated) neurons; can be stimulatory or inhibitory.
Effectors: Somatic targets skeletal muscle; ANS targets smooth muscle, cardiac muscle, and glands.

Anatomical Arrangement of the Parasympathetic Nervous System

The parasympathetic division promotes rest and digest functions.

Ganglia: Located in or near target organs.
Neurons: Long preganglionic, short postganglionic fibers.
Vagus Nerve (X): Carries ~90% of preganglionic parasympathetic fibers.
Craniosacral Division: Preganglionic neurons in cranial (III, VII, IX, X) and sacral nerves.

Key Anatomical Differences Between ANS Divisions

The sympathetic division prepares the body for 'fight or flight' responses.

Ganglia: Mainly near the spinal column.
Neurons: Short preganglionic, long postganglionic fibers.
Thoracolumbar Division: Preganglionic neurons originate in the lateral horn of thoracolumbar spinal cord.

Sympathetic Trunk, or Chain, Ganglia

The sympathetic trunk consists of paired ganglia running alongside the spinal cord.

Preganglionic fibers: Pass through white rami communicantes to synapse in trunk or chain ganglia.
Postganglionic axons: Enter ventral rami via gray rami communicantes.
Collateral ganglia: Some fibers synapse outside the chain in collateral ganglia.

Three Pathways of Sympathetic Innervation

Sympathetic fibers can follow three main pathways to reach their effectors.

Synapse in trunk ganglion at the same level.
Synapse in trunk ganglion at a different level.
Pass through trunk to synapse in a collateral ganglion anterior to the vertebral column.

Sympathetic Innervation of the Adrenal Medulla

Some sympathetic fibers directly stimulate the adrenal medulla, leading to systemic effects.

Adrenal medulla: Releases epinephrine and norepinephrine into the bloodstream, amplifying sympathetic responses.

Sympathetic and Parasympathetic Tone

Both divisions maintain a baseline level of activity, known as 'tone,' in target organs.

Sympathetic tone (vasomotor tone): Maintains partial constriction of blood vessels, allowing rapid adjustment of blood pressure.
Parasympathetic tone: Dominates heart rate regulation at rest and controls smooth muscle of digestive and urinary tracts.
Example: During exercise, sympathetic tone increases, shunting blood to muscles and raising heart rate.

Visceral Reflexes

Visceral reflexes are automatic responses involving ANS effectors.

Components: Receptor, sensory neuron, integration center, motor neuron, and effector.
Examples: Reflexes that empty the rectum and bladder.

Cholinergic Receptors

Cholinergic receptors bind acetylcholine (ACh) and mediate parasympathetic effects.

Types: Nicotinic receptors and Muscarinic receptors.
Nicotinic receptors: Found on all postganglionic neurons, adrenal medulla cells, and skeletal muscle sarcolemma; always stimulatory.
Muscarinic receptors: Found on all effector cells stimulated by postganglionic cholinergic fibers; can be inhibitory or excitatory depending on the organ.
Example: ACh binding to cardiac muscle slows heart rate; binding to intestinal smooth muscle increases motility.

Adrenergic Receptors

Adrenergic receptors respond to norepinephrine (NE) and epinephrine, mediating sympathetic effects.

Alpha (α) receptors: Subtypes α1, α2.
Beta (β) receptors: Subtypes β1, β2, β3.

Table: Cholinergic and Adrenergic Receptors

Neurotransmitter	Receptor Type	Major Location(s)	Effect of Binding
Acetylcholine (ACh)	Nicotinic	All postganglionic neurons, adrenal medulla, skeletal muscle	Excitation
Acetylcholine (ACh)	Muscarinic	All parasympathetic target organs, some sympathetic targets	Excitation in most cases; inhibition of cardiac muscle
Norepinephrine (NE) and Epinephrine	Alpha (α1, α2)	Sympathetic target organs	Generally stimulatory
Norepinephrine (NE) and Epinephrine	Beta (β1, β2, β3)	Sympathetic target organs	β1: increases heart rate; β2: inhibitory (e.g., bronchodilation); β3: stimulates lipolysis

Effects of the Parasympathetic and Sympathetic Divisions on Various Organs

The two ANS divisions often have opposing effects on target organs.

Target Organ/System	Parasympathetic Effects	Sympathetic Effects
Eye (iris)	Constricts pupils	Dilates pupils
Salivary glands	Stimulates watery saliva	Stimulates thick, viscous saliva
Heart	Decreases heart rate	Increases heart rate and force
Lungs	Constricts bronchioles	Dilates bronchioles
Digestive tract	Stimulates motility and secretion	Inhibits motility and secretion
Bladder	Promotes voiding	Inhibits voiding
Genitals	Promotes erection	Promotes ejaculation

Levels of ANS Control

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

Hypothalamus: Main integration center for ANS functions.
Brainstem: Regulates heart rate, blood pressure, and digestive activities.
Spinal cord: Controls urination, defecation, and sexual function.
Cerebral cortex: Can influence ANS via limbic system connections.

Disorders of the ANS

Several clinical conditions are associated with ANS dysfunction.

Raynaud's disease: Exaggerated vasoconstriction in fingers and toes, causing pallor and cyanosis; treated with vasodilators.
Autonomic dysreflexia: Life-threatening, uncontrolled activation of autonomic neurons in quadriplegics and those with spinal cord injuries above T6; leads to dangerously high blood pressure.

Developmental Aspects of the Autonomic Nervous System

The ANS develops from neural tube and neural crest cells, with efficiency declining in old age.

Development: Preganglionic neurons from neural tube; postganglionic neurons, adrenal medulla, and ganglia from neural crest.
Nerve growth factor: Guides axons to target organs during development.
Aging effects: Constipation, dry eyes, orthostatic hypotension due to less responsive pressure receptors and cardiovascular centers.

Additional info: The ANS is essential for homeostasis, and its dysfunction can have widespread effects on health and disease management.