Autonomic Nervous System: Sympathetic vs Parasympathetic
Concept Name
Autonomic Nervous System
Genetic Loci
CHRNA3 (15q24) encodes nicotinic receptor subunit – polymorphisms associated with smoking risk. ADRA2A (10q25.2) encodes α2‑adrenergic receptor.
Intracellular Cascade
Preganglionic neurons release ACh → nicotinic receptors → postganglionic neuron. Sympathetic: NE release → α/β‑adrenergic receptors. Parasympathetic: ACh release → muscarinic receptors. Adrenal medulla: ACh → nicotinic receptors → epinephrine release.
Required Cofactors
No specific cofactors; dopamine β‑hydroxylase requires vitamin C (ascorbic acid) and copper for NE synthesis.
Histology Stains
Tyrosine hydroxylase (TH) immunohistochemistry marks sympathetic nerves. Choline acetyltransferase (ChAT) marks parasympathetic neurons.
EM Findings
Sympathetic varicosities contain dense‑cored vesicles (containing norepinephrine). Parasympathetic varicosities contain small clear vesicles (ACh).
Knockout Phenotype
Knockout of TH (tyrosine hydroxylase) in mice causes mid‑gestation embryonic lethality due to cardiovascular failure. DBH knockout results in elevated dopamine and absence of NE/epinephrine.
Specific Toxins
6‑Hydroxydopamine (6‑OHDA) selectively destroys catecholaminergic neurons. Botulinum toxin blocks ACh release. Reserpine depletes catecholamine stores by inhibiting VMAT.