Autonomic Nervous System Details

Autonomic Nervous System Overview

Differences Between Sympathetic and Parasympathetic Systems

  • Origin

    • Sympathetic fibers originate from the thoracolumbar region (T1-L2).
    • Parasympathetic fibers originate from the craniosacral region (brainstem and sacral spinal cord).

  • Location of Ganglia

    • Parasympathetic: ganglia are located close to effector organs; referred to as terminal ganglia.
    • Short postganglionic neuron (compared to preganglionic).
    • Sympathetic: has two types of ganglia:
    • 1. Paravertebral Ganglia: located in a chain next to the spinal cord (sympathetic chain).
    • 2. Collateral Ganglia: situated close to effector organs (names include celiac, superior mesenteric, and inferior mesenteric ganglia).

  • Length of Fibers

    • Parasympathetic: long preganglionic neurons, short postganglionic neurons.
    • Sympathetic: short preganglionic neurons, long postganglionic neurons.

  • Ratio of Preganglionic to Postganglionic Neurons

    • Parasympathetic: a 1:1 ratio (one preganglionic neuron to one postganglionic neuron).
    • Sympathetic: varies (1:1, 1:2, etc.).

  • Physiological Role

    • Sympathetic: Generally responsible for the 'fight or flight' response and increases ATP consumption.
    • Parasympathetic: Active during 'rest and digest'; conserves energy.

Unique Organs and Systems

  • Fibers of sympathetic nerves only:
    • Sweat glands.
    • Erector pili (goosebumps).
    • Adrenal medulla.
    • Most blood vessels (exceptions include coronary blood vessels, which have dual innervation).

Neurotransmitters and Receptors

  • Preganglionic neurotransmitter in both systems: Acetylcholine.
  • Postganglionic neurotransmitter:
    • Parasympathetic: Acetylcholine.
    • Sympathetic: Norepinephrine (or epinephrine).
Receptor Types
  • Cholinergic Receptors: Receptors for acetylcholine.
  • Adrenergic Receptors: Receptors for norepinephrine/epinephrine (5 subtypes: alpha 1, alpha 2, beta 1, beta 2, beta 3/dopaminergic).

Pharmacology Terms

  • Sympathomimetic Drugs: Mimic sympathetic nervous activation (e.g., Ephedrine).
  • Parasympathomimetic Drugs: Mimic parasympathetic action (e.g., Pilocarpine).
  • Sympatholytic Drugs: Inhibit sympathetic activity.
  • Parasympatholytic Drugs: Inhibit parasympathetic activity (e.g., Atropine).

Understanding Sympathetic Effects on Organs

  • Vasodilation: Loss of sympathetic nerve supply results in blood vessel dilation.
  • Situational responses in drugs affecting heart rate:
    • Beta Blockers: Block sympathetic action, decreasing heart rate.
    • Agonists vs. Antagonists: Agile understanding of which drugs either stimulate or block the receptors corresponding to sympathetic or parasympathetic responses.