Autonomic Nervous System Overview

Autonomic Nervous System

• The autonomic nervous system (ANS) controls involuntary actions like cardiac muscle, smooth muscle, and glands
• It occupies both the peripheral nervous system (PNS) and central nervous system (CNS)
• The pathway from the CNS to an effector organ involves two neurons (except for the adrenal medulla)
• The first neuron, preganglionic fiber, is located in the CNS (e.g., cranial nerve nuclei or lateral horn of the spinal cord)
• The second neuron, postganglionic fiber, is within an autonomic ganglion and innervates the effector organ

ANS Divisions

• Anatomical divisions based on origin:
  • Cranial: arises from cranial nerve nuclei (III, VII, IX, X)
  • Thoracolumbar: arises from the lateral horn of the thoracic and upper lumbar spinal cord
  • Sacral: arises from the lateral horn of the sacral spinal cord
• Physiological divisions based on function:
  • Sympathetic system (thoracolumbar)
  • Parasympathetic system (craniosacral)

Autonomic Ganglia

• Definition: Collections of nerve cells outside the CNS where preganglionic and postganglionic fibers communicate
• Types:
  • Lateral (paravertebral): along the vertebral column
  • Collateral (prevertebral): in front of the aorta
  • Terminal: in the walls of organs

Properties of ANS

• One-way communication: Impulses travel in one direction within the ANS (preganglionic to postganglionic)
• One relay: Signals usually pass through one ganglion in the ANS
• One chemical transmitter (mostly acetylcholine (ACh) and norepinephrine (NE))
• Delay: due to chemical transmission at synapses

Autonomic Nervous System Actions

• Reciprocal action: When one system is stimulated, the other is inhibited within the same organ (e.g. heart)
• Antagonistic action: Opposite actions on different effector tissues within the same organ (e.g., iris)
• Complementary action: Actions working together; e.g., grief (sympathetic for energy, parasympathetic for crying)
• Same action: Some organs receive input from both systems leading to similar effects (e.g., salivary glands)
• Single innervation: Single system (sympathetic or parasympathetic) supplies certain organs, e.g., adrenal medulla, skin

Sympathetic System

• Originates from the thoracolumbar region of the spinal cord
• Preganglionic fibers are short, and postganglionic fibers are long
• Often described via "fight or flight" responses—increased heart rate, blood flow, respiration, etc.

Parasympathetic System

• Originates from Cranial nerves (III, VII, IX, X) and sacral spinal nerves
• Preganglionic fibers are long, and postganglionic fibers are short
• Often described via "rest and digest" responses—decreased heart rate, digestion, etc.

Neurotransmitters

• Acetylcholine (ACh): Released at all preganglionic neuron synapses, and by parasympathetic postganglionic neurons and in the adrenal medulla.
• Norepinephrine (NE): Released by sympathetic postganglionic neurons.
• Adrenaline/Epinephrine: released by Adrenal medulla