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

Autonomic Nervous System (ANS) Overview

Definition and Branches

The autonomic nervous system (ANS) is a branch of the peripheral nervous system that regulates involuntary functions, such as smooth muscle activity, glandular secretion, and heart rate. 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 Nervous System

Functional Differences

• Parasympathetic (PNS): Promotes maintenance activities and conserves body energy (e.g., digestion, defecation, diuresis).

• Sympathetic (SNS): Mobilizes the body during activity (e.g., exercise, excitement, emergency).

Functional outcomes:

• PNS: Slows heart rate, increases digestive activity.

• SNS: Increases heart rate, inhibits digestion, dilates pupils.

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

Dynamic Antagonism

The PNS and SNS often have opposing effects on the same organs, maintaining homeostasis through dynamic antagonism.

• Example: SNS increases heart rate; PNS decreases it.

Parasympathetic Nervous System (PNS)

Functions and Effects

• Stimulates salivation, lacrimation, urination, digestion, and defecation.

• Decreases heart rate and blood pressure.

Example: The vagus nerve supplies PNS innervation to vital organs such as the heart and digestive tract.

Sympathetic Nervous System (SNS)

Functions and Effects

• Increases heart rate and blood pressure.

• Inhibits digestive and urinary functions.

• Stimulates release of glucose, dilates bronchioles.

Example: SNS activation during exercise increases blood flow to skeletal muscles and dilates airways.

Neurotransmitters in 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.

Example: ACh is excitatory at neuromuscular junctions, while NE can be excitatory or inhibitory depending on the receptor type.

Receptors

• Adrenergic receptors: Bind epinephrine and norepinephrine; divided into Alpha and Beta types.

• Cholinergic receptors: Bind acetylcholine; divided into nicotinic and muscarinic types.

Receptor Type	Main Ligand	Location	Effect
Alpha-adrenergic	Epinephrine/NE	Blood vessels, smooth muscle	Vasoconstriction, increased BP
Beta-adrenergic	Epinephrine/NE	Heart, lungs, skeletal muscle	Increased heart rate, bronchodilation
Nicotinic cholinergic	ACh	All autonomic ganglia	Excitatory
Muscarinic cholinergic	ACh	Parasympathetic target organs	Excitatory or inhibitory

Organs with Predominant Parasympathetic or Sympathetic Tone

• Parasympathetic tone: Heart, digestive tract, urinary tract

• Sympathetic tone: Blood vessels, sweat glands

Example: The PNS maintains resting heart rate, while the SNS controls blood vessel diameter.

Cooperation Between PNS and SNS

• Both systems may cooperate during sexual arousal and orgasm.

• PNS initiates erection; SNS triggers ejaculation.

Regulation of Body Temperature

• SNS controls thermoregulatory responses, such as sweating and vasodilation.

Duration of Action

• Parasympathetic effects are generally short-lived due to rapid breakdown of ACh.

• Sympathetic effects are longer-lasting due to slower breakdown of NE and epinephrine.

Central Control of Autonomic Activity

• The hypothalamus is the major control center for autonomic functions.

Summary Table: Parasympathetic vs. Sympathetic Effects

Organ/System	Parasympathetic Effect	Sympathetic Effect
Heart	Decreases rate	Increases rate
Digestive tract	Increases motility	Decreases motility
Pupils	Constricts	Dilates
Bronchioles	Constricts	Dilates
Blood vessels	Little effect	Constriction (increased BP)

Key Equations

• Blood Pressure Regulation (SNS): $\text{Blood Pressure} = \text{Cardiac Output} \times \text{Peripheral Resistance}$

Additional info: The study guide references dynamic antagonism, neurotransmitter types, and receptor subtypes, which are essential for understanding the physiological regulation of organ systems by the ANS.