Introduction to the Autonomic Nervous System

Overview

• Autonomic Nervous System (ANS): Involuntary control over organs and glands. Consists of three branches:
  • Sympathetic Nervous System (SNS)
  • Parasympathetic Nervous System (PNS)
  • Enteric Nervous System (ENS)
• Peripheral Nervous System (PNS) sub-divides into:
  • Sensory System: Sensory input.
  • Motor System: Contains somatic (voluntary) and visceral (involuntary) components.
    • Somatic Motor: Voluntary control (e.g., skeletal muscles).
    • Visceral Motor (ANS): Involuntary control (e.g., heart, GI tract).

Sympathetic vs. Parasympathetic Nervous Systems

• Sympathetic Nervous System:

  • Purpose: Fight, flight, or fright; uses energy during stress.
  • Location: Thoracolumbar outflow (T1-L2 segment).
  • Preganglionic Neurons: Short, originate in T1-L2 region, synapse close to the spinal cord.
  • Postganglionic Neurons: Long, reach target organs.
  • Neurotransmitters:
    • Preganglionic: Acetylcholine (cholinergic fibers).
    • Postganglionic: Norepinephrine (adrenergic fibers).

• Parasympathetic Nervous System:

  • Purpose: Rest and digest; conserves energy.
  • Location: Craniosacral outflow (cranial nerves III, VII, IX, X and sacral S2-S4 segment).
  • Preganglionic Neurons: Long, synapse near or in target organs (intramural or terminal ganglia).
  • Postganglionic Neurons: Short.
  • Neurotransmitters:
    • Preganglionic: Acetylcholine (cholinergic fibers).
    • Postganglionic: Acetylcholine (cholinergic fibers).

Ganglia in the ANS

• Definition: Group of cell bodies in the PNS.
• Sympathetic Ganglia:
  • Chain (Paravertebral) Ganglia: Located close to the spinal cord, allow fibers to ascend or descend.
  • Collateral (Prevertebral) Ganglia: Located in front of the vertebral column (near the aorta).
• Parasympathetic Ganglia:
  • Terminal (Intramural) Ganglia: Located near or within target organs.

Detailed Anatomy and Features

• Sympathetic Pathways:

  • Descend/Ascend within Chain Ganglia: Access any level of the spinal cord.
  • Sympathetic Postganglionic Neurons: Different functions (pilo-motor, vasomotor, sudo-motor).
  • Splanchnic Nerves: Some fibers form unique nerves to specific organs (e.g., heart, lungs).
  • Exceptions: Sweat glands employ acetylcholine (not norepinephrine).

• Parasympathetic Pathways:

  • Cranial Nerves: III (oculomotor), VII (facial), IX (glossopharyngeal), X (vagus) - each nerve serves different organs.
  • Sacral Region: S2-S4 segment contribute to pelvic organs.

Neurotransmitter Differences

• Acetylcholine: Common in both sympathetic and parasympathetic ganglionic synapses.
• Norepinephrine: Predominantly in sympathetic postganglionic synapses (except sweat glands).

Key Takeaways

• Sympathetic: Thoracolumbar origin, short preganglionic/long postganglionic neurons, uses acetylcholine and norepinephrine.
• Parasympathetic: Craniosacral origin, long preganglionic/short postganglionic neurons, uses acetylcholine for both synapses.

Future Topics

• Further details on craniosacral outflow and ganglia.
• Muscarinic and nicotinic receptors.
• Detailed analysis of sympathetic pathways and adrenergic receptors.