Cholinergic Receptors

Cholinergic receptors are crucial for the parasympathetic nervous system as they respond to acetylcholine. There are two main categories:

Nicotinic Receptors

• Types:
  • Nicotinic Neuronal (Nn):
    • Found on neurons in autonomic ganglia and central nervous system.
    • Function:
      • Respond to acetylcholine and nicotine.
      • Involved in generating an excitatory post-synaptic potential (EPSP) by opening ligand-gated ion channels allowing sodium ions to flow in.
  • Nicotinic Muscular (Nm):
    • Located at neuromuscular junctions on skeletal muscles.
    • Function:
      • Binding of acetylcholine causes sodium to enter the cell, creating a motor endplate potential leading to muscle contraction.
• Mechanism:
  • Ligand-gated ion channels that open with acetylcholine binding, causing depolarization and action potentials.

Muscarinic Receptors

• Types:
  • M1, M3, M5 (GQ protein-coupled):
    • Stimulate target organs by increasing intracellular calcium levels via phospholipase C pathway.
  • M2, M4 (G inhibitory):
    • Inhibit target organs by reducing cyclic AMP levels and opening potassium channels.
• Locations & Functions:
  • M1:
    • Found in the central nervous system (memory, cognition) and gastric glands (stimulates HCl production).
  • M2:
    • Heart (reduces heart rate) and presynaptic nerve terminals (inhibits further acetylcholine release).
  • M3:
    • Found in exocrine glands (stimulates secretions), smooth muscles (causes contraction), and involved in processes like bronchoconstriction and GI tract mobility.
  • M4, M5:
    • Mainly found in the central nervous system and involved in cognitive functions, arousal, and analgesia.

Mechanisms of Action

• Nicotinic Receptors:
  • Ligand-gated ion channels facilitate sodium influx leading to depolarization and action potentials.
• Muscarinic Receptors:
  • G-protein coupled receptors alter cell function through second messengers like calcium and cyclic AMP.

Applications and Significance

• Central Nervous System:
  • Cholinergic receptors are essential for memory, arousal, and analgesia.
• Autonomic Functions:
  • Involved in heart rate modulation, glandular secretion, and smooth muscle contraction.
• Muscle Contraction:
  • Nicotinic receptors at neuromuscular junctions are crucial for skeletal muscle contractions.

Understanding these receptors is critical for exploring their roles in physiology and potential therapeutic targets.