The Autonomic Nervous System: Structure, Function, and Organization

Chapter 14: The Autonomic Nervous System

Overview of the Autonomic Nervous System (ANS)

The autonomic nervous system (ANS) is a division of the peripheral nervous system that regulates involuntary physiological processes, including heart rate, blood pressure, respiration, digestion, and sexual arousal. The ANS operates largely below the level of consciousness to maintain homeostasis.

• Involuntary Functions: The ANS controls smooth muscle, cardiac muscle, and glands.

• Examples: Regulation of heart rate, pupil size, gastrointestinal motility, and glandular secretion.

• Principal Divisions: The ANS is divided into the sympathetic and parasympathetic divisions.

General Effects: The sympathetic division prepares the body for "fight or flight" responses, while the parasympathetic division supports "rest and digest" activities.

Anatomy of the ANS: Sympathetic and Parasympathetic Divisions

The ANS consists of two main divisions, each with distinct anatomical and functional characteristics.

• Sympathetic Division: Originates from the thoracolumbar region (T1-L2) of the spinal cord. Preganglionic neurons are short; postganglionic neurons are long.

• Parasympathetic Division: Originates from the craniosacral regions (brainstem and sacral spinal cord, S2-S4). Preganglionic neurons are long; postganglionic neurons are short.

• General Map: Sympathetic ganglia are located near the spinal cord (sympathetic chain or paravertebral ganglia), while parasympathetic ganglia are located near or within target organs.

Neurotransmitters: Both divisions use acetylcholine (ACh) at the preganglionic synapse. The sympathetic division primarily uses norepinephrine (NE) at the postganglionic synapse, while the parasympathetic division uses ACh.

Sympathetic Division: Organization and Pathways

The sympathetic division is responsible for mobilizing the body's resources during stress or activity. Its fibers originate in the thoracolumbar spinal cord and synapse in sympathetic ganglia.

• Preganglionic Neurons: Cell bodies are located in the lateral horns of the spinal cord (T1-L2).

• Sympathetic Chain (Paravertebral Ganglia): A series of ganglia adjacent to the vertebral column.

• Collateral (Prevertebral) Ganglia: Located anterior to the vertebral column; supply abdominal and pelvic organs.

• Pathways: Preganglionic fibers may synapse at the same level, ascend or descend the chain, or pass through to collateral ganglia.

Rami Communicantes: White rami carry preganglionic fibers to the sympathetic chain; gray rami carry postganglionic fibers to spinal nerves.

Parasympathetic Division: Organization and Pathways

The parasympathetic division conserves energy and promotes maintenance activities. Its fibers originate in the brainstem (cranial nerves III, VII, IX, X) and sacral spinal cord (S2-S4).

• Cranial Outflow: Preganglionic fibers travel with cranial nerves to innervate head, thoracic, and abdominal organs.

• Sacral Outflow: Preganglionic fibers exit via pelvic splanchnic nerves to supply pelvic organs.

• Ganglia: Terminal ganglia are located near or within the walls of target organs.

Functions: Stimulates digestion, urination, and defecation; slows heart rate; constricts pupils.

Comparison of Sympathetic and Parasympathetic Divisions

Feature	Sympathetic Division	Parasympathetic Division
Origin	Thoracolumbar (T1-L2)	Craniosacral (Brainstem, S2-S4)
Ganglia Location	Near spinal cord (paravertebral/collateral)	Near or within target organs
Preganglionic Fiber Length	Short	Long
Postganglionic Fiber Length	Long	Short
Main Neurotransmitter (Postganglionic)	Norepinephrine (NE)	Acetylcholine (ACh)
General Function	"Fight or flight"	"Rest and digest"

Specific Pathways and Effects

• Sympathetic Pathways: Include innervation to the head, thorax, abdomen, and pelvis. Examples: dilation of pupils, increased heart rate, bronchodilation, inhibition of digestion.

• Parasympathetic Pathways: Include innervation to the eye (constriction of pupils), stimulation of salivary glands, decreased heart rate, and increased gastrointestinal activity.

Dual Innervation: Most organs receive input from both divisions, which often have opposing effects (e.g., heart rate, pupil size).

Enteric Nervous System

The enteric nervous system is a specialized network of neurons within the walls of the gastrointestinal tract. It can function independently but is modulated by the ANS.

• Functions: Regulates digestive tract motility, secretion, and blood flow.

• Components: Myenteric plexus (motility) and submucosal plexus (secretion).

Summary Table: ANS Divisions and Effects

Organ/System	Sympathetic Effect	Parasympathetic Effect
Heart	Increases rate and force	Decreases rate
Pupils	Dilates (mydriasis)	Constricts (miosis)
Digestive Tract	Decreases motility and secretion	Increases motility and secretion
Bronchi	Dilates	Constricts
Bladder	Relaxes wall, contracts sphincter	Contracts wall, relaxes sphincter

Receptors and Neurotransmitters

• Cholinergic Receptors: Bind acetylcholine; include nicotinic (all ANS ganglia) and muscarinic (parasympathetic target organs).

• Adrenergic Receptors: Bind norepinephrine; include alpha and beta subtypes (sympathetic target organs).

Example: Beta-adrenergic receptors in the heart increase heart rate and contractility when stimulated by norepinephrine.

Summary

• The ANS is essential for regulating involuntary body functions and maintaining homeostasis.

• It consists of sympathetic and parasympathetic divisions with distinct anatomical and functional properties.

• Most organs receive dual innervation, allowing fine-tuned physiological control.