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 dilation, and digestive processes.

• 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

• Two-Neuron Pathway: The ANS uses a two-neuron chain to connect the central nervous system (CNS) to effector organs:

  • Preganglionic neuron: Cell body in the CNS; axon extends to an autonomic ganglion.

  • Postganglionic neuron: Cell body in the ganglion; axon projects to the target organ.

• Sympathetic Division: Preganglionic neurons originate in the thoracolumbar region (T1-L2) of the spinal cord.

• Parasympathetic Division: Preganglionic neurons originate in the brainstem and sacral spinal cord (craniosacral outflow).

General Map: Sympathetic fibers are generally shorter preganglionic and longer postganglionic; parasympathetic fibers are the opposite.

Sympathetic Division: Organization and Pathways

The sympathetic division is responsible for mobilizing the body's resources during stress or activity.

• Origin: Thoracolumbar region (T1-L2) of the spinal cord.

• Ganglia: Sympathetic chain (paravertebral) ganglia and prevertebral (collateral) ganglia.

• Pathways: Preganglionic fibers may synapse in the sympathetic chain, pass through to collateral ganglia, or innervate the adrenal medulla.

• Neurotransmitters: Acetylcholine (ACh) is released by preganglionic neurons; norepinephrine (NE) is typically released by postganglionic neurons.

Example: Sympathetic stimulation increases heart rate and dilates bronchioles.

Parasympathetic Division: Organization and Pathways

The parasympathetic division conserves energy and promotes maintenance activities.

• Origin: Brainstem (cranial nerves III, VII, IX, X) and sacral spinal cord (S2-S4).

• Ganglia: Terminal ganglia located near or within target organs.

• Neurotransmitters: Both pre- and postganglionic neurons release acetylcholine (ACh).

Example: Parasympathetic stimulation decreases heart rate and increases digestive activity.

Comparison of Sympathetic and Parasympathetic Divisions

Feature	Sympathetic	Parasympathetic
Origin	Thoracolumbar (T1-L2)	Craniosacral (Brainstem, S2-S4)
Ganglia Location	Near spinal cord	Near or in 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 heart, lungs, blood vessels, sweat glands, and adrenal medulla.

• Parasympathetic Pathways: Include innervation to the heart, digestive tract, and glands.

• Dual Innervation: Most organs receive input from both divisions, which often have opposing effects.

Example: The heart receives both sympathetic (increases rate) and parasympathetic (decreases rate) innervation.

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.

Summary Table: ANS Divisions and Effects

Organ/System	Sympathetic Effect	Parasympathetic Effect
Heart	Increases rate and force	Decreases rate
Bronchioles	Dilates	Constricts
Digestive Tract	Decreases motility	Increases motility
Pupils	Dilates (mydriasis)	Constricts (miosis)

Receptors and Neurotransmitters

• Cholinergic Receptors: Bind acetylcholine; include nicotinic and muscarinic receptors.

• Adrenergic Receptors: Bind norepinephrine and epinephrine; include alpha and beta receptors.

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

Summary

• The ANS is essential for regulating involuntary body functions.

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

• Most organs receive dual innervation, allowing fine control of physiological processes.