Autonomic Nervous System: Structure, Function, and Regulation (Chapter 14 Study Guide)

Autonomic Nervous System Overview

Definition and Branches

The autonomic nervous system (ANS) is a branch of the peripheral nervous system that regulates involuntary functions, such as those of smooth muscle, glands, and the heart. The ANS is divided into two main branches:

• Parasympathetic Nervous System (PNS): "Rest & digest" responses

• Sympathetic Nervous System (SNS): "Fight or flight" responses

Parasympathetic vs. Sympathetic Divisions

Functional Differences

• Parasympathetic: Promotes maintenance activities and conserves energy (e.g., digestion, salivation, urination)

• Sympathetic: Mobilizes body systems during activity (e.g., increases heart rate, dilates pupils, inhibits digestion)

Functional outcomes:

• Parasympathetic: Stimulates salivation, slows heart rate, increases digestive activity

• Sympathetic: Inhibits salivation, accelerates heart rate, decreases digestive activity

Example: During stress, the SNS increases heart rate and redirects blood flow to muscles, while the PNS is active during rest, promoting digestion and energy storage.

Neurotransmitters of the ANS

Types and Functions

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

• Norepinephrine (NE): Released by most sympathetic postganglionic neurons

Receptors:

• Adrenergic receptors: Bind norepinephrine and epinephrine (Alpha and Beta subtypes)

• Cholinergic receptors: Bind acetylcholine (Nicotinic and Muscarinic subtypes)

Example: The heart rate increases when NE binds to beta-adrenergic receptors in cardiac tissue.

Adrenergic Receptors

Alpha and Beta Subtypes

• Alpha receptors: Generally cause vasoconstriction and increased blood pressure

• Beta receptors: Generally cause vasodilation and increased heart rate

Both epinephrine and norepinephrine can bind to these receptors, but their effects vary depending on receptor subtype and tissue location.

Dynamic Antagonism and Cooperation

PNS and SNS Interactions

• Dynamic antagonism: The PNS and SNS often have opposing effects to maintain homeostasis (e.g., heart rate regulation)

• Cooperation: In some processes, both systems work together (e.g., sexual arousal and orgasm)

Example: The SNS increases heart rate during exercise, while the PNS slows it during rest.

Control Centers and Regulation

Role of the Hypothalamus

• The hypothalamus is the major control center for autonomic activity, integrating signals and coordinating responses.

Duration of Action

Sympathetic vs. Parasympathetic Effects

• Sympathetic effects: Generally longer-lasting due to slower breakdown of norepinephrine

• Parasympathetic effects: Shorter duration due to rapid breakdown of acetylcholine

Example: The "fight or flight" response persists after the initial stimulus due to prolonged sympathetic activity.

Summary Table: Parasympathetic vs. Sympathetic Effects

Organ/System	Parasympathetic Effect	Sympathetic Effect
Heart	Decreases heart rate	Increases heart rate
Digestive Tract	Stimulates digestion	Inhibits digestion
Pupils	Constricts pupils	Dilates pupils
Salivary Glands	Stimulates salivation	Inhibits salivation
Urinary Bladder	Promotes urination	Inhibits urination

Key Equations

• Neurotransmitter breakdown rate (generalized): $\text{Rate} = k[\text{Neurotransmitter}]$

Additional info:

• "Dynamic antagonism" refers to the balance between the PNS and SNS in regulating organ systems.

• "Dual innervation" means most organs receive input from both divisions, allowing fine control.