Autonomic Nervous System: Neurotransmitters and Receptors

Overview

• The autonomic nervous system (ANS) is divided into sympathetic and parasympathetic divisions.
• Neurons in the ANS are classified based on the neurotransmitter they release:
  • Cholinergic neurons release acetylcholine (ACh).
  • Adrenergic neurons release norepinephrine (noradrenaline).
• Receptors for these neurotransmitters are integral proteins located on the plasma membrane of the postsynaptic neuron or effector cell.

Cholinergic Neurons

• Release neurotransmitter acetylcholine (ACh).
• Include:
  • All sympathetic and parasympathetic preganglionic neurons.
  • Sympathetic postganglionic neurons innervating sweat glands.
  • All parasympathetic postganglionic neurons.
• ACh is stored in synaptic vesicles and released by exocytosis. It diffuses across synaptic cleft and binds to cholinergic receptors.

Types of Cholinergic Receptors:

1. Nicotinic Receptors

   • Found in:
     • Plasma membrane of dendrites and cell bodies of sympathetic and parasympathetic postganglionic neurons.
     • Chromaffin cells in the adrenal medulla.
     • Motor end plate at the neuromuscular junction.
   • Activation causes depolarization and excitation of postsynaptic cell.

2. Muscarinic Receptors

   • Found in plasma membranes of all autonomic effectors (cardiac muscle, smooth muscle, glands) innervated by parasympathetic postganglionic axons.
   • Also present in sweat glands innervated by sympathetic postganglionic neurons.
   • Activation can cause depolarization or hyperpolarization, depending on the effector cell.
   • Example: ACh binding inhibits smooth muscle sphincters in the gastrointestinal tract, allowing food to move; it excites circular muscles in the iris, causing pupil constriction.

Adrenergic Neurons

• Release norepinephrine (NE).
• Most postganglionic neurons in the sympathetic division are adrenergic.
• NE is stored in synaptic vesicles and released by exocytosis, binding to adrenergic receptors.

Adrenergic Receptors:

• Bind both norepinephrine and epinephrine.
• NE can be released as a neurotransmitter or hormone.
• Types:
  • Alpha Receptors (subtypes 1 & 2)
  • Beta Receptors (subtypes 1, 2, & 3)

Effects of Receptor Activation:

• Alpha 1 and Beta 1 receptors: Cause depolarization or excitation.
• Alpha 2 and Beta 2 receptors: Cause hyperpolarization or inhibition.
• Beta 3 receptors: Found in brown adipose tissue; activation causes thermogenesis (heat production).

Summary of Neurotransmitter Functions

• Cholinergic Neurons: Release acetylcholine; bind to nicotinic and muscarinic receptors.
• Adrenergic Neurons: Release norepinephrine; bind to alpha and beta adrenergic receptors.
• Preganglionic Neurons: Always cholinergic, releasing acetylcholine.
• Sympathetic Postganglionic Neurons: Typically adrenergic, releasing norepinephrine, except those innervating sweat glands which are cholinergic.
• Parasympathetic Postganglionic Neurons: Always cholinergic.

Receptor Binding Effects:

• Nicotinic Receptors: Depolarization (excitation).
• Muscarinic Receptors: Depolarization or hyperpolarization.
• Adrenergic Receptors:
  • Alpha & Beta 1: Depolarization.
  • Alpha & Beta 2: Hyperpolarization.