U01.05.011 Autonomic receptors

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The autonomic nervous system (ANS) regulates involuntary functions and consists of two divisions: parasympathetic and sympathetic. Understanding receptor types, neurotransmitters, and target organs is high-yield for licensing exams.

Overview of Autonomic Pathways

Feature	Parasympathetic	Sympathetic
Preganglionic neuron	Long	Short
Postganglionic neuron	Short	Long (except adrenal medulla)
Preganglionic neurotransmitter	Acetylcholine (ACh)	Acetylcholine (ACh)
Postganglionic neurotransmitter	ACh	Norepinephrine (NE), except: sweat glands → ACh; adrenal medulla → Epi/NE
Target organs	Smooth muscle, glands, cardiac muscle	Smooth muscle, glands, cardiac muscle, vessels, adrenal medulla, sweat glands
Receptors on target	Muscarinic (M1, M2, M3)	Adrenergic (α1, α2, β1, β2), Dopamine (D1), Muscarinic (M3 for sweat glands)

Receptor Types and Locations

Receptor	Neurotransmitter	Location / Function
Nicotinic N (Nn)	ACh	Autonomic ganglia (both parasympathetic and sympathetic)
Nicotinic M (Nm)	ACh	Neuromuscular junction (somatic)
Muscarinic (M1, M2, M3)	ACh	Smooth muscle, glands, cardiac muscle (parasympathetic)
α1	NE	Vascular smooth muscle → vasoconstriction
α2	NE	Presynaptic inhibition of NE release
β1	NE / Epi	Heart → ↑HR, ↑contractility; kidney → renin release
β2	NE / Epi	Smooth muscle relaxation (bronchi, vessels of skeletal muscle)
D1	Dopamine	Renal vascular smooth muscle → vasodilation

Special Notes

Pelvic splanchnic nerves and cranial nerves III, VII, IX, and X are parasympathetic.
The adrenal medulla is directly innervated by the preganglionic sympathetic fibers, releasing epinephrine and norepinephrine into the blood.
Sweat glands are sympathetic but cholinergic, causing “cold sweat”.
Somatic nervous system: voluntary motor neurons release ACh at nicotinic M receptors on skeletal muscle.

Summary Table: ANS Neurotransmitters and Receptors

Division	Preganglionic NT	Preganglionic receptor	Postganglionic NT	Postganglionic receptor	Target organ
Parasympathetic	ACh	Nn	ACh	M1, M2, M3	Smooth muscle, glands, heart
Sympathetic	ACh	Nn	NE (except: sweat glands → ACh)	α1, α2, β1, β2, M3	Heart, vessels, smooth muscle, adrenal medulla, sweat glands
Somatic	N/A	N/A	ACh	Nm	Skeletal muscle

Learning Objective (Step 1)

Understand the neurotransmitters, receptors, and target organs of the parasympathetic, sympathetic, and somatic nervous systems; recognize exceptions such as cholinergic sweat glands and the adrenal medulla, and be able to predict physiological responses to receptor stimulation.