U01.05.014 Autonomic receptors

1. Autonomic Nervous System (ANS) Receptors

The autonomic nervous system (ANS) is divided into sympathetic and parasympathetic systems, each involving distinct receptors and pathways. Understanding these receptors and their locations is key for the USMLE Step 1 exam. Below is a detailed breakdown of the relevant receptors and pathways.

2. Key Components and Receptors

The receptors in the autonomic system are classified based on their location (e.g., smooth muscle, glands, cardiac muscle) and the neurotransmitters involved.

Somatic Nervous System

Voluntary Control:
- Neuromuscular junction
- Receptor: Nicotinic (ACh, NM receptor)
- Effect: Voluntary muscle contraction
- Neurotransmitter: Acetylcholine (ACh)

Sympathetic Nervous System

Receptors:
- Pre-ganglionic: Acetylcholine (ACh) acting on Nicotinic receptors (NN)
- Post-ganglionic:
  - Adrenal Medulla: Acetylcholine (ACh) acting on Nicotinic receptors (NN) → releases Catecholamines (Epinephrine, Norepinephrine)
  - Smooth Muscle, Gland Cells, Nerve Terminals, Cardiac Muscle, Sweat Glands, Renal Vasculature, Vessels: Norepinephrine (NE) acting on α and β adrenergic receptors
- Receptor Types:
  - Alpha (α) receptors: α1, α2
  - Beta (β) receptors: β1, β2
  - Dopamine (D) receptors: D1

Parasympathetic Nervous System

Receptors:
- Pre-ganglionic: Acetylcholine (ACh) acting on Nicotinic receptors (NN)
- Post-ganglionic:
  - Smooth Muscle, Gland Cells, Heart (Cardiac Muscle): Acetylcholine (ACh) acting on Muscarinic receptors (M)
  - Sweat Glands: Acetylcholine (ACh) acting on Muscarinic receptors (M)

3. Autonomic Pathways and Receptor Locations

Receptor	Location	Neurotransmitter	Receptor Type
Somatic System	Neuromuscular junction	Acetylcholine (ACh)	Nicotinic (NM)
Sympathetic	Cardiac Muscle, Smooth Muscle, Glands	Norepinephrine (NE)	α, β adrenergic
Parasympathetic	Heart, Smooth Muscle, Gland Cells	Acetylcholine (ACh)	Muscarinic (M)
Adrenal Medulla	Direct innervation by pre-ganglionic fibers	Acetylcholine (ACh)	Nicotinic (NN)
Sweat Glands	Sympathetic pathway but cholinergic innervation	Acetylcholine (ACh)	Muscarinic (M)

4. Important Points to Remember for USMLE Step 1

Pelvic Splanchnic Nerves and Cranial Nerves III, VII, IX, and X are part of the parasympathetic nervous system, mediating functions like rest and digest.
The Adrenal Medulla is directly innervated by preganglionic sympathetic fibers and releases catecholamines (epinephrine, norepinephrine) in response to sympathetic stimulation.
Sweat glands are an exception in the sympathetic pathway: they are innervated by cholinergic fibers despite being part of the sympathetic system. This leads to cholinergic (cold) sweat rather than adrenergic sweat.

5. Key Receptor Functions

Alpha-1 (α1) receptors: Mainly found in vascular smooth muscle, their activation leads to vasoconstriction.
Alpha-2 (α2) receptors: Located presynaptically, their activation inhibits neurotransmitter release.
Beta-1 (β1) receptors: Found in cardiac muscle, they increase heart rate and contractility.
Beta-2 (β2) receptors: Found in smooth muscle (e.g., bronchi), their activation causes vasodilation and bronchodilation.
Muscarinic receptors (M): Found in cardiac muscle, smooth muscle, and glands, they mediate parasympathetic effects like slowing of heart rate and contraction of smooth muscle.

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