Chapter 14: The Autonomic Nervous System – Structured Study Notes

Chapter 14: The Autonomic Nervous System

Why This Matters

The autonomic nervous system (ANS) is essential for understanding how drugs affect body functions, especially in clinical settings. Knowledge of the ANS helps anticipate both therapeutic effects and side effects of medications.

• Clinical Relevance: Many drugs target ANS pathways, influencing heart rate, blood pressure, and organ function.

• Patient Care: Anticipating ANS effects is crucial for safe and effective treatment.

Autonomic Nervous System Overview

The autonomic nervous system (ANS) consists of motor neurons that regulate involuntary functions by innervating smooth muscle, cardiac muscle, and glands.

• Functions: Maintains homeostasis by adjusting heart rate, blood pressure, and digestive processes.

• Control: Operates via subconscious (involuntary) control.

• Alternate Names: Involuntary nervous system, visceral motor system.

Structural Organization of the Nervous System

The ANS is a subdivision of the peripheral nervous system (PNS), specifically within the motor (efferent) division.

• Motor Division: Includes the somatic nervous system (controls skeletal muscles) and the autonomic nervous system (controls visceral effectors).

• ANS Divisions: Sympathetic and parasympathetic.

14.1 ANS vs. Somatic Nervous System

Both systems have motor fibers but differ in their effectors, neural pathways, and neurotransmitter actions.

• Effectors: Somatic system targets skeletal muscles; ANS targets cardiac muscle, smooth muscle, and glands.

• Neural Pathways: Somatic uses a single neuron; ANS uses a two-neuron chain (preganglionic and postganglionic).

• Neurotransmitter Actions: Somatic always excitatory; ANS can be excitatory or inhibitory.

Efferent Pathways and Ganglia

The pathways differ in the number and type of neurons involved.

• Somatic Nervous System (SNS): Single, thick, myelinated axon from CNS to skeletal muscle.

• Autonomic Nervous System (ANS): Two-neuron chain:

  1. Preganglionic neuron: Cell body in CNS, thin, lightly myelinated axon to ganglion.

  2. Postganglionic neuron: Cell body in autonomic ganglion, nonmyelinated axon to effector.

Neurotransmitter Effects on Effectors

• Somatic Nervous System: Releases acetylcholine (ACh); always excitatory.

• ANS:

  • Preganglionic fibers release ACh.

  • Postganglionic fibers release:

    • Norepinephrine (NE) at most sympathetic fibers.

    • ACh at most parasympathetic fibers.

  • Effect can be excitatory or inhibitory, depending on receptor type.

Overlap of Somatic and Autonomic Function

Both systems are regulated by higher brain centers and often work together to coordinate body responses.

• Spinal and cranial nerves may contain both somatic and autonomic fibers.

• Adaptations often involve both skeletal muscles and visceral organs (e.g., increased heart rate during exercise).

Comparison of Motor Neurons in the Somatic and Autonomic Nervous Systems

Motor neurons differ in their structure, neurotransmitter release, and effect on target tissues.

• Somatic: One neuron, ACh, excitatory, targets skeletal muscle.

• Autonomic: Two neurons, ACh/NE, excitatory or inhibitory, targets cardiac/smooth muscle and glands.

14.2 Divisions of the Autonomic Nervous System

The ANS has two main divisions with opposing functions.

• Parasympathetic Division: Promotes maintenance, conserves energy.

• Sympathetic Division: Mobilizes body during activity.

• Dual Innervation: Most organs receive input from both divisions, maintaining homeostasis through dynamic antagonism.

Role of the Parasympathetic Division

The parasympathetic division is responsible for "rest-and-digest" activities.

• Keeps energy use low during maintenance activities (digestion, diuresis, defecation).

• Example: Relaxing after a meal—low heart rate, high gastrointestinal activity, constricted pupils.

Role of the Sympathetic Division

The sympathetic division prepares the body for "fight-or-flight" responses.

• Activated by exercise, excitement, emergency, embarrassment.

• Effects: Increased heart rate, dilated pupils, shunted blood to muscles, dilated bronchioles, glucose release from liver.

Key Anatomical Differences Between ANS Divisions

Sympathetic and parasympathetic divisions differ in origin, fiber length, and ganglia location.

• Sites of Origin:

  • Parasympathetic: Craniosacral (brain stem and sacral spinal cord).

  • Sympathetic: Thoracolumbar (thoracic and lumbar spinal cord).

• Relative Lengths of Fibers:

  • Parasympathetic: Long preganglionic, short postganglionic.

  • Sympathetic: Short preganglionic, long postganglionic.

• Location of Ganglia:

  • Parasympathetic: In or near visceral effectors.

  • Sympathetic: Close to spinal cord.

Table: Anatomical and Physiological Differences Between Parasympathetic and Sympathetic Divisions

Characteristic	Parasympathetic	Sympathetic
Origin	Craniosacral (brain stem, sacral spinal cord)	Thoracolumbar (thoracic, lumbar spinal cord)
Location of ganglia	Near/in effector organs	Close to spinal cord
Fiber lengths	Long preganglionic, short postganglionic	Short preganglionic, long postganglionic
Rami communicantes	None	Present (white and gray)
Functional role	Maintenance, energy conservation	Mobilization, fight-or-flight
Neurotransmitters	ACh (preganglionic and postganglionic)	ACh (preganglionic), NE (postganglionic)

14.3 Parasympathetic Division

Also known as the craniosacral division, the parasympathetic system has long preganglionic axons and short postganglionic axons.

• Preganglionic fibers originate in brain stem and sacral spinal cord.

• Synapse in terminal ganglia near or within target organs.

Cranial Part of Parasympathetic Division

• Oculomotor nerves (III): Control eye muscles, pupil constriction, lens focusing.

• Facial nerves (VII): Stimulate nasal, lacrimal, and salivary glands.

• Glossopharyngeal nerves (IX): Activate parotid salivary glands.

• Vagus nerves (X): Account for ~90% of preganglionic parasympathetic fibers; serve thoracic and abdominal viscera.

Sacral Part of Parasympathetic Division

• Originates from S2–S4 spinal segments.

• Serves pelvic organs and distal large intestine.

• Axons travel via pelvic splanchnic nerves to synapse in pelvic and intramural ganglia.

14.4 Sympathetic Division

The sympathetic division is more complex, innervating more organs and some structures exclusively.

• Originates from thoracolumbar spinal cord (T1–L2).

• Preganglionic neurons located in lateral horns of spinal cord.

• Innervates sweat glands, arrector pili muscles, and all blood vessels.

Sympathetic Trunks

Sympathetic trunks (chains) run alongside the vertebral column and contain multiple ganglia.

• 23 ganglia per trunk: 3 cervical, 11 thoracic, 4 lumbar, 4 sacral, 1 coccygeal.

• Preganglionic fibers enter via white rami communicantes and synapse in trunk or collateral ganglia.

Table: Summary of Autonomic Ganglia

Name	Division	Location
Terminal ganglia	Parasympathetic	Within/near organ (intramural ganglia)
Sympathetic trunk ganglia	Sympathetic	Beside vertebral column
Collateral ganglia	Sympathetic	Anterior to vertebral column

Sympathetic Pathways with Synapses in Trunk Ganglia

• Postganglionic axons enter spinal nerves via gray rami communicantes.

• White rami: myelinated preganglionic axons; gray rami: nonmyelinated postganglionic axons.

• Only sympathetic neurons use rami communicantes.

Additional info:

• Further study should include neurotransmitter receptor types (nicotinic, muscarinic, adrenergic), visceral reflex arcs, and clinical disorders of the ANS.

• Equations are not directly relevant for this chapter, but understanding neurotransmitter mechanisms is essential for pharmacology.