Study Guide: The Autonomic Nervous System (Chapter 15, Principles of Anatomy & Physiology, 14th Edition)

Chapter 15: The Autonomic Nervous System

Overview

The autonomic nervous system (ANS) is a critical component of the nervous system responsible for regulating involuntary physiological functions, including heart rate, digestion, respiratory rate, and glandular activity. This chapter compares the ANS with the somatic nervous system and explores the anatomy, neurotransmitters, and physiological effects of the ANS.

Comparison of Somatic and Autonomic Nervous Systems

Somatic Nervous System

• Definition: The somatic nervous system controls voluntary movements and transmits sensory information from external stimuli.

• Sensory Neurons: Related to touch, pain, temperature, proprioception, sight, hearing, taste, smell, and equilibrium.

• Motor Neurons: Innervate skeletal muscles for voluntary movement.

• Motor Pathway: A single, myelinated motor neuron extends from the CNS to the skeletal muscle fiber.

• Neurotransmitter: Acetylcholine (ACh) at the neuromuscular junction.

Autonomic Nervous System

• Definition: The ANS regulates involuntary functions by receiving input from sensory receptors in organs, blood vessels, muscles, and the nervous system.

• Motor Pathway: Consists of two motor neurons in series: the preganglionic neuron (cell body in CNS) and the postganglionic neuron (cell body in autonomic ganglion).

• Effectors: Smooth muscle, cardiac muscle, and glands.

• Divisions: Sympathetic (fight-or-flight) and Parasympathetic (rest-and-digest).

Table: Comparison of Somatic and Autonomic Nervous Systems

Feature	Somatic Nervous System	Autonomic Nervous System
Sensory Input	From somatic senses and special senses	Mainly from interoceptors; some from somatic senses and special senses
Control of Motor Output	Voluntary (conscious control)	Involuntary (autonomic regulation)
Motor Neuron Pathway	One neuron from CNS to effector	Two neurons: preganglionic and postganglionic
Neurotransmitters	Acetylcholine only	Acetylcholine and norepinephrine
Effectors	Skeletal muscle	Cardiac muscle, smooth muscle, glands
Response	Contraction of skeletal muscle	Contraction/relaxation of smooth muscle, increased/decreased gland secretion, etc.

Anatomy of Autonomic Motor Pathways

General Organization

• Each division of the ANS has two motor neurons: preganglionic (cell body in brain or spinal cord) and postganglionic (cell body in autonomic ganglion).

Sympathetic Division

• Preganglionic neuron cell bodies are located in the lateral horns of the gray matter in the 12 thoracic and first 2-3 lumbar segments.

• Sympathetic ganglia are sites of synapses between preganglionic and postganglionic neurons.

• Two major types of sympathetic ganglia:

  • Sympathetic trunk ganglia: Lie in a vertical row on either side of the vertebral column.

  • Prevertebral ganglia: Lie anterior to the vertebral column, close to large abdominal arteries.

• Axons of sympathetic preganglionic neurons may connect with postganglionic neurons in four ways:

  1. Synapse in the first ganglion reached.

  2. Ascend/descend to a higher/lower ganglion before synapsing.

  3. Pass through trunk ganglion to prevertebral ganglion and synapse.

  4. Pass through both ganglia and extend to the adrenal medullae.

• Major autonomic plexuses in the abdomen and pelvis: celiac (solar) plexus, superior mesenteric plexus, inferior mesenteric plexus, renal plexus, hypogastric plexus.

Parasympathetic Division

• Preganglionic neuron cell bodies are in the nuclei of cranial nerves III, VII, IX, X (brain stem) and lateral gray matter of sacral segments 2-4.

• Cranial parasympathetic outflow: Extends from brain stem in 4 cranial nerves.

• Sacral parasympathetic outflow: Extends from 2nd-4th sacral spinal nerves.

ANS Neurotransmitters and Receptors

Types of Neurons

• Cholinergic neurons: Release acetylcholine (ACh).

• Adrenergic neurons: Release norepinephrine (noradrenaline).

Types of Receptors

• Cholinergic receptors: Nicotinic and muscarinic receptors.

• Adrenergic receptors: Alpha (α1, α2) and beta (β1, β2, β3) receptors.

Table: Location and Responses of Adrenergic and Cholinergic Receptors

Type of Receptor	Major Locations	Effects of Activation
Nicotinic (Cholinergic)	Postganglionic neurons, neuromuscular junctions	Excitation (impulse in postganglionic neuron or muscle fiber)
Muscarinic (Cholinergic)	Effectors innervated by parasympathetic postganglionic neurons	Excitation or inhibition (depending on effector)
α1 (Adrenergic)	Most smooth muscle, glands	Excitation (contraction, secretion)
α2 (Adrenergic)	Pancreas, platelets, smooth muscle	Inhibition (relaxation, decreased secretion)
β1 (Adrenergic)	Heart, kidney	Excitation (increased heart rate, renin release)
β2 (Adrenergic)	Bronchi, blood vessels, uterus	Inhibition (relaxation of smooth muscle)
β3 (Adrenergic)	Adipose tissue	Lipolysis

Physiology of the ANS

Antagonistic Effects

• The sympathetic and parasympathetic divisions generally have opposing (antagonistic) effects on target organs.

Sympathetic Activation: "Fight or Flight"

• Speeds up heart rate and force of contraction

• Relaxes airway muscles for increased oxygen intake

• Dilates pupils for improved vision

• Inhibits digestive system

• Enhances blood flow to skeletal muscles

Table: Effects of Sympathetic and Parasympathetic Stimulation

Visceral Effector	Sympathetic Stimulation	Parasympathetic Stimulation
Heart	Increases rate and force	Decreases rate
Bronchi	Bronchodilation	Bronchoconstriction
Pupils	Dilation	Constriction
Digestive Tract	Decreased motility	Increased motility
Blood Vessels (skeletal muscle)	Dilation	No effect

Integration and Control of Autonomic Functions

Autonomic Reflex Arcs

• Functions such as heart rate, blood pressure, and vessel diameter are regulated by autonomic reflexes.

• The reflex arc consists of:

  • Receptor

  • Sensory neuron

  • Integrating center

  • Motor neurons

  • Effector

The Nervous System and Homeostasis

The nervous system is essential for maintaining homeostasis by regulating and integrating the functions of all body systems. The ANS plays a key role in this process by adjusting physiological parameters in response to internal and external stimuli.

Key Terms and Definitions

• Autonomic Nervous System (ANS): Regulates involuntary physiological functions.

• Somatic Nervous System: Controls voluntary movements.

• Preganglionic Neuron: First neuron in the ANS motor pathway, cell body in CNS.

• Postganglionic Neuron: Second neuron, cell body in autonomic ganglion.

• Cholinergic Neuron: Releases acetylcholine.

• Adrenergic Neuron: Releases norepinephrine.

• Sympathetic Division: "Fight or flight" responses.

• Parasympathetic Division: "Rest and digest" responses.

Relevant Equations

• Nerve Impulse Transmission:

$\text{Nerve Impulse} = \text{Action Potential} = \text{Depolarization} + \text{Repolarization}$

• Neurotransmitter Release:

$\text{Neurotransmitter Release} \propto \text{Calcium Ion Influx}$

Additional info: Tables and diagrams have been summarized and expanded for clarity. All major anatomical and physiological features of the ANS are included for comprehensive exam preparation.