The Autonomic Nervous System (ANS)

Introduction

• Functions outside of conscious awareness

• Makes routine adjustments in body systems

• Regulates:

  • Body temperature

  • Cardiovascular, respiratory, digestive, excretory, and reproductive functions

Comparison with Somatic Nervous System

• Autonomic Nervous System (ANS)

  • Has afferent and efferent neurons

  • Efferent axons innervate visceral organs

  • Preganglionic axons synapse on postganglionic neurons at a ganglion

  • A ganglion is a cluster of neurons outside the CNS

• Somatic Nervous System

  • Has afferent and efferent neurons

  • Efferent axons innervate skeletal muscles

  • Somatic motor neurons do not involve ganglia

Sympathetic and Parasympathetic Divisions

• The ANS consists of two major subdivisions:

  • Sympathetic Division (Thoracolumbar)

    • Active during stress, exertion, or emergencies (“fight-or-flight”)

  • Parasympathetic Division (Craniosacral)

    • Active during resting conditions (“rest-and-digest”)

Sympathetic Division Details

• Preganglionic fibers from thoracic and upper lumbar spinal segments synapse in ganglia near the spinal cord

• Increases metabolism, alertness, and prepares the body for emergencies

• Neurotransmitter release mechanism:

  • Preganglionic fibers release acetylcholine (ACh)

  • Postganglionic fibers release norepinephrine (NE)

Key Response Mechanisms

• Fight or flight responses that include:

  • Increased alertness

  • Enhanced cardiovascular and respiratory activity

  • Mobilization of energy reserves

Parasympathetic Division Details

• Preganglionic fibers originate from brainstem and sacral spinal cord

• Synapses occur near or within target organs

• Promotes energy conservation and ongoing processes, such as digestion

• Neurotransmitter release mechanism:

  • All fibers release ACh, effects can be excitatory or inhibitory

Major Functions of Parasympathetic Stimulation

• Effects:

  • Relaxation

  • Food processing and nutrient absorption

  • Pupil constriction

  • Increased secretion of digestive enzymes

  • Contraction of the urinary bladder

Summary Comparison of Sympathetic and Parasympathetic

• Location of CNS Neurons:

  • Sympathetic: Lateral horns of T1-L2

  • Parasympathetic: Brainstem and S2-S4

• Ganglia Location:

  • Sympathetic: Chain and collateral ganglia

  • Parasympathetic: Near target organs

• Preganglionic Fibers:

  • Sympathetic: Short and myelinated

  • Parasympathetic: Long and myelinated

• Postganglionic Fibers:

  • Sympathetic: Long and unmyelinated

  • Parasympathetic: Short and unmyelinated

• Neurotransmitter Released:

  • Sympathetic: Norepinephrine

  • Parasympathetic: Always ACh

• Degree of Divergence:

  • Sympathetic: Approx. 1:32

  • Parasympathetic: Approx. 1:6

Visceral Reflexes

• Provides autonomic motor responses to modify or facilitate higher centers.

• Consists of a receptor, sensory nerve, processing center in CNS, and two visceral motor neurons (preganglionic and postganglionic).

• Reflexes can be:

  • Long Reflexes: Involve the CNS for processing

  • Short Reflexes: Bypass CNS, with sensory impulses directly to ganglia

Conclusion

• Most vital organs innervated by both systems; known as dual innervation.

• The sympathetic and parasympathetic divisions often have opposing effects on target organs.