Sympathetic vs. Parasympathetic Divisions of the Autonomic Nervous System

Overview

• Both divisions contain motor pathways with two neurons: preganglionic and postganglionic.
• Preganglionic Neuron:
  • Cell body in the brain or spinal cord.
  • Axon extends to an autonomic ganglion (myelinated).
• Postganglionic Neuron:
  • Lies outside the central nervous system in the peripheral nervous system.
  • Axon extends to effectors (unmyelinated).

Sympathetic Division

• Preganglionic Neurons:
  • Cell bodies located in the lateral horns of the gray matter in the thoracic and lumbar segments of the spinal cord.
• Ganglia Locations:
  • Sympathetic Trunk Ganglia (Vertebral Chain Ganglia):
    • Located on either side of the vertebral column.
    • Innervate organs above the diaphragm (head, neck, shoulders, heart).
  • Prevertebral Ganglia:
    • Located anterior to the vertebral column.
    • Innervate organs below the diaphragm.
• Neuron Lengths:
  • Short preganglionic neurons.
  • Long postganglionic neurons.
• Synapse Pathways:
  • Can synapse in the first ganglion it reaches.
  • Can ascend or descend to other ganglia before synapsing.
  • Can pass through to a prevertebral ganglion or to adrenal medulla's chromaffin cells.
• Divergence:
  • One preganglionic neuron synapses with many postganglionic neurons (~20+).
  • Causes widespread body responses (e.g., nervousness causing increased heart rate, sweating).

Parasympathetic Division

• Preganglionic Neurons:
  • Cell bodies located either in brainstem nuclei or the sacral spinal cord.
• Ganglia Locations:
  • Terminal Ganglia:
    • Located close to or within the walls of visceral organs.
• Neuron Lengths:
  • Long preganglionic axons.
  • Short postganglionic axons.
• Synapse Characteristics:
  • Preganglionic neurons synapse with a few postganglionic neurons (up to 4-5), leading to localized responses.

Summary Table

Sympathetic Division:

• Preganglionic cell bodies in thoracic and lumbar spinal cords.
• Ganglia close to spinal cord (sympathetic trunk or prevertebral).
• Short preganglionic fibers, long postganglionic fibers.

Parasympathetic Division:

• Fibers originate in brain (cranial nerves) and sacral spinal cord.
• Ganglia close to or within the effector organs.
• Long preganglionic fibers, short postganglionic fibers.

Functional Differences:

• Sympathetic: Widespread effects due to divergence.
• Parasympathetic: Localized effects due to fewer synapses with postganglionic neurons.