Parasympathetic Nervous System: Neurotransmitters and Receptors

Overview of the Parasympathetic Nervous System

  • Focus on receptors and neurotransmitters within the parasympathetic nervous system.

Parasympathetic Neurotransmitter

  • Acetylcholine (ACh): The sole neurotransmitter for the parasympathetic nervous system.
    • Identical neurotransmitter used to stimulate skeletal muscle.

Cholinergic Receptors

  • Cholinergic receptors: Receptors that respond to acetylcholine:
    • Named "cholinergic" because they are activated by choline.
    • Present in both parasympathetic and to some degree in sympathetic nervous systems.

Types of Cholinergic Receptors

  1. Nicotinic Receptors

    • Found at:
      • Neuromuscular junctions (connecting motor neurons to skeletal muscles).
      • Synapses between preganglionic and postganglionic neurons in the autonomic nervous system.
    • Mechanism of Action:
      • Preganglionic neurons release acetylcholine.
      • Binding to nicotinic receptors causes an excitatory response.
      • Mechanism involves direct ionotropic channels that allow ion flow (sodium ions) across the postsynaptic membrane.
    • Representation:
      • Preganglionic neurons synapse on postganglionic cells through cranial nerves III (oculomotor), VII (facial), IX (glossopharyngeal), and X (vagus) as well as the sacral nerves.
      • This leads to a signal that promotes the "rest and digest" activities of the parasympathetic system.

  2. Muscarinic Receptors

    • Found at:
      • Target organs of the parasympathetic nervous system.
      • Occasionally found on some sympathetic targets, such as eccrine sweat glands.
    • Mechanism of Action:
      • These receptors interact with ACh via a second messenger system, specifically G-protein coupled receptors (GPCR).
      • This system can lead to various cellular responses:
      • May open sodium channels, exciting the cell.
      • Conversely, may close other channels, inhibiting cell activity depending on the specific G protein activated.
      • Represents versatility in signal transduction, enabling different effects based on the type of target cell.
    • Exception Mentioned:
      • ACh generally has excitatory effects except in the heart, where it inhibits pacemaker cells.

Summary of Neurotransmission in the Parasympathetic Nervous System

  • All preganglionic autonomic neurons secrete acetylcholine to stimulate postsynaptic cells, regardless of whether they are part of the sympathetic or parasympathetic nervous systems.
  • All parasympathetic postganglionic neurons also release acetylcholine onto their specific targets, leading to excitatory transmission with noted exceptions (such as the heart).
  • Important to note a rare case where sympathetic postganglionic neurons also release acetylcholine.
  • The difference in receptor types explains the variation in responses at the target organs, with nicotinic being direct and muscarinic being mediated by second messengers.

Key Points to Remember

  • Memorize cranial nerves involved in parasympathetic preganglionic axons: III, VII, IX, and X.
  • Distinguish between nicotinic (excitatory, ionotropic) and muscarinic (varied effects via GPCR) receptors in terms of location and function.