Chapter 14: The Autonomic Division – Study Notes

Chapter 14: The Autonomic Division

Introduction

The autonomic nervous system (ANS) is a critical component of the peripheral nervous system that regulates involuntary physiological processes, including heart rate, blood pressure, respiration, digestion, and sexual arousal. This chapter explores the structure, function, and mechanisms of the ANS, with a focus on its divisions, neurotransmitters, and effects on target organs.

The Nervous System: Overview and Organization

Main Divisions

• Central Nervous System (CNS): Consists of the brain and spinal cord.

• Peripheral Nervous System (PNS): Includes all neural tissue outside the CNS and is divided into sensory (afferent) and motor (efferent) divisions.

Motor Division Subdivisions

• Somatic Nervous System (SNS): Controls voluntary movements of skeletal muscles.

• Autonomic Nervous System (ANS): Controls involuntary actions of cardiac muscle, smooth muscle, and glands.

Comparison of Autonomic and Somatic Nervous Systems

Key Differences

Feature	Autonomic Nervous System	Somatic Nervous System
Site of Origin	Sympathetic: thoracic/lumbar spinal cord; Parasympathetic: brain and sacral spinal cord	Ventral horn of spinal cord (most); some cranial nerves in brain
Number of Neurons from CNS to Effector	Two-neuron chain (preganglionic and postganglionic)	Single neuron
Organs Innervated	Cardiac muscle, smooth muscle, glands	Skeletal muscle
Neurotransmitter at Effector	ACh (parasympathetic); NE (sympathetic)	ACh only
Effect on Effector Organs	Excitatory or inhibitory (depending on receptor type)	Always excitatory (stimulates contraction)
Type of Control	Involuntary	Voluntary

Dual Innervation

Definition and Significance

• Dual innervation: Most visceral organs receive input from both sympathetic and parasympathetic divisions, allowing precise regulation of organ function.

• Sympathetic dominance: "Fight-or-flight" responses (e.g., exercise, excitement, emergency) increase heart rate, dilate pupils, and inhibit digestion.

• Parasympathetic dominance: "Rest-and-digest" responses (e.g., after eating, relaxation) decrease heart rate, constrict pupils, and stimulate digestion.

Comparison of Nervous and Endocrine Systems

Key Differences

Property	Nervous System	Endocrine System
Anatomic Arrangement	"Wired" system; neurons connect directly to target cells	"Wireless" system; hormones travel via blood to target cells
Type of Chemical Messenger	Neurotransmitters (released into synaptic cleft)	Hormones (released into blood)
Distance of Action	Short (across synapse)	Long (via bloodstream)
Speed of Response	Rapid (milliseconds)	Slower (seconds to days)
Duration of Action	Short-lived	Long-lasting
Major Functions	Coordinates rapid, precise responses	Regulates long-term processes (growth, metabolism)

The Adrenal Medulla and Sympathetic Nervous System

Role and Function

• The adrenal medulla is a modified sympathetic ganglion and part of the endocrine system.

• Upon stimulation, it secretes catecholamines (mainly epinephrine and norepinephrine) into the bloodstream, amplifying sympathetic effects throughout the body.

Autonomic Pathways: The Two-Neuron Chain

Structure

• All autonomic pathways consist of a preganglionic neuron (cell body in CNS) and a postganglionic neuron (cell body in autonomic ganglion).

• The preganglionic neuron releases acetylcholine (ACh) at the ganglion.

• The postganglionic neuron releases either ACh (parasympathetic) or norepinephrine (NE) (sympathetic) at the effector organ.

Sympathetic vs. Parasympathetic Divisions

Comparison Table

Feature	Sympathetic System	Parasympathetic System
Origin of Preganglionic Fiber	Thoracic and lumbar regions of spinal cord	Brain and sacral region of spinal cord
Origin of Postganglionic Fiber	Sympathetic chain near spinal cord or collateral ganglia	Terminal ganglia in or near effector organ
Fiber Length	Short preganglionic, long postganglionic	Long preganglionic, short postganglionic
Neurotransmitter Released	PREG: ACh; POST: NE (except sweat glands: ACh)	PREG: ACh; POST: ACh
Types of Receptors	Nicotinic (ganglia), adrenergic (effectors: α, β)	Nicotinic (ganglia), muscarinic (effectors)
Dominance	"Fight or flight"	"Rest and digest"

Receptor Types in the Autonomic System

Cholinergic Receptors

• Nicotinic receptors: Found on all postganglionic neurons (sympathetic and parasympathetic), adrenal medulla cells, and skeletal muscle cells at neuromuscular junctions. 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 and receptor subtype.

• Example: ACh binding to muscarinic receptors in cardiac muscle slows heart rate; in intestinal smooth muscle, it increases motility.

Adrenergic Receptors

• Alpha (α) receptors: Generally excitatory (e.g., vasoconstriction).

• Beta (β) receptors: β1 is usually excitatory (increases heart rate), β2 is usually inhibitory (bronchodilation).

Effect of Sympathetic and Parasympathetic Systems on Organs

Organ	Effect of Sympathetic Stimulation	Effect of Parasympathetic Stimulation
Heart	Increases heart rate and force of contraction (β1)	Decreases heart rate (vagus nerve)
Blood Vessels	Constriction (α); dilation in skeletal muscle (β2)	Dilates vessels supplying penis and clitoris only
Lungs	Dilates bronchioles (β2); decreases mucus secretion	Constricts bronchioles; stimulates mucus secretion
Digestive Tract	Decreases motility and secretion	Increases motility and secretion
Pupils	Dilates (mydriasis)	Constricts (miosis)

Summary

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

• It consists of sympathetic and parasympathetic divisions, which often have opposing effects on target organs.

• Dual innervation allows fine-tuned control of organ systems.

• Neurotransmitters and receptor types determine the specific effects on each organ.

Additional info: Some diagrams and tables were inferred and expanded for clarity and completeness based on standard Anatomy & Physiology content.