Chapter 14: The Autonomic Nervous System – Structure, Function, and Clinical Relevance

The Autonomic Nervous System (ANS)

Introduction and Importance

The autonomic nervous system (ANS) is a critical component of the nervous system that oversees involuntary physiological functions, including heart rate, blood pressure, and the regulation of digestive and urinary systems. Understanding the ANS is essential for predicting the effects and side effects of various drugs, especially in clinical settings.

• Involuntary control: The ANS operates without conscious input, maintaining homeostasis and responding to internal and external stimuli.

• Clinical relevance: Many medications target the ANS, affecting patient outcomes.

Structural Organization of the Nervous System

Placement of the ANS

The nervous system is divided into the central nervous system (CNS) and peripheral nervous system (PNS). The PNS includes sensory (afferent) and motor (efferent) divisions. The motor division is further subdivided into:

• Somatic nervous system (SNS): Voluntary control of skeletal muscles.

• Autonomic nervous system (ANS): Involuntary control of smooth muscle, cardiac muscle, and glands.

Comparison: ANS vs. Somatic Nervous System (SNS)

Key Differences

Feature	Somatic Nervous System (SNS)	Autonomic Nervous System (ANS)
Effectors	Skeletal muscles	Smooth muscle, cardiac muscle, glands
Neurotransmitter	Acetylcholine (ACh)	ACh, norepinephrine (NE), epinephrine (E)
Pathway	One neuron (CNS to effector)	Two neurons (preganglionic and postganglionic)
Response	Always excitatory (muscle contraction)	Excitatory or inhibitory (depends on receptor and organ)

Pathways of the SNS and ANS

SNS Pathways

• Efferent neuron: Cell body in CNS; single, thick, myelinated axon extends to skeletal muscle.

• Neurotransmitter: All somatic motor neurons release ACh; effect is always excitatory.

ANS Pathways

• Preganglionic neuron: Cell body in CNS; thin, lightly myelinated axon extends to autonomic ganglion.

• Postganglionic neuron: Cell body in ganglion; nonmyelinated axon extends to effector organ.

• Neurotransmitter release:

  • Preganglionic fibers release ACh.

  • Postganglionic fibers release NE (sympathetic) or ACh (parasympathetic).

• Effect: Can be excitatory or inhibitory, depending on receptor type.

Functional Overlap: Somatic and Autonomic Systems

Example: Swallowing

• Oral phase: Voluntary (SNS) – chewing and moving food to the back of the mouth.

• Pharyngeal phase: Involuntary (ANS) – food moves through the throat, airways close.

• Esophageal phase: Involuntary (ANS) – food moves to the stomach via peristalsis.

Divisions of the ANS

Parasympathetic Division

• Promotes maintenance functions and conserves energy ("rest and digest").

• Ganglia located near or within target organs.

• Long preganglionic and short postganglionic fibers.

• Localized and fast-acting effects.

Sympathetic Division

• Mobilizes the body during activity ("fight or flight").

• Ganglia located near the spinal cord.

• Short preganglionic and long postganglionic fibers.

• Widespread and long-lasting effects.

Dual Innervation and Homeostasis

• Most visceral organs receive input from both divisions, which usually have opposing effects.

• Dynamic antagonism between divisions maintains homeostasis (e.g., heart rate regulation).

Roles of the Parasympathetic and Sympathetic Divisions

Parasympathetic Division

• Keeps energy use low during maintenance activities.

• Directs digestion, urination, and defecation.

• "Rest-and-digest" system.

• Example: After a meal, blood pressure and heart rate are low, pupils constricted, and digestion is active.

Sympathetic Division

• Prepares the body for emergencies ("fight or flight").

• Increases heart rate, dilates pupils, and mobilizes energy stores.

• Shunts blood to skeletal muscles and heart, dilates bronchioles, and causes the liver to release glucose.

• Maintains homeostasis during physical activity and stress.

Key Anatomical Differences

• Fiber length: Parasympathetic – long preganglionic, short postganglionic; Sympathetic – short preganglionic, long postganglionic.

• Ganglia location: Parasympathetic – near or within target organs; Sympathetic – close to spinal cord.

Pathways and Effects

Parasympathetic Pathways

• Preganglionic neurons travel directly to target organs and synapse in ganglia close to or within those organs.

• Short postganglionic axons allow for precise, localized responses.

Sympathetic Pathways

• Short preganglionic and long postganglionic neurons.

• Divergence allows one preganglionic neuron to synapse with many postganglionic neurons, producing widespread effects.

• Some preganglionic fibers synapse directly in the adrenal medulla, causing release of epinephrine (80%) and norepinephrine (20%) into the bloodstream.

Neurotransmitters and Receptors

Major Neurotransmitters

• Acetylcholine (ACh): Released by all preganglionic neurons and parasympathetic postganglionic neurons.

• Norepinephrine (NE): Released by most sympathetic postganglionic neurons (except sweat glands, which use ACh).

Cholinergic Receptors

• Nicotinic receptors: Found on all postganglionic neurons, adrenal medulla cells, and skeletal muscle sarcolemma. Always excitatory (depolarization).

• Muscarinic receptors: Found on all effector cells stimulated by postganglionic cholinergic fibers. Can be excitatory or inhibitory depending on the target organ.

Adrenergic Receptors

• Alpha (α) receptors: Subtypes α1 and α2.

• Beta (β) receptors: Subtypes β1, β2, and β3.

• Effects depend on receptor subtype and target organ.

Receptor	Location	Main Effect
α1	Vascular smooth muscle, iris, urinary sphincters	Vasoconstriction, pupil dilation, sphincter contraction
α2	Presynaptic terminals, pancreas, blood vessels	Inhibits NE release, reduces sympathetic outflow
β1	Heart, kidneys	Increases heart rate and force, renin release
β2	Bronchioles, blood vessels of skeletal muscle	Bronchodilation, vasodilation
β3	Adipose tissue	Stimulates lipolysis

Summary Table: ANS vs. SNS

Feature	Somatic Nervous System	Autonomic Nervous System
Control	Voluntary	Involuntary
Effectors	Skeletal muscle	Smooth muscle, cardiac muscle, glands
Neurotransmitter	ACh	ACh, NE, E
Pathway	One neuron	Two neurons
Effect	Excitatory	Excitatory or inhibitory

Additional info: This summary integrates textbook slides and expands on the pharmacological and clinical relevance of the ANS, as well as the anatomical and functional distinctions between its divisions.