Sympathetic Neurotransmitters & Receptors

Sympathetic preganglionic neurotransmitter

Acetylcholine (ACh).

Sympathetic preganglionic synapse

Preganglionic axons communicate with postganglionic neurons at excitatory synapses using acetylcholine (ACh).

Sympathetic postganglionic neurotransmitters

Norepinephrine, epinephrine, or acetylcholine (ACh).

Most common sympathetic postganglionic neurotransmitter

Norepinephrine; about 80% of postganglionic sympathetic neurons release it.

Adrenergic receptors

Receptors that bind norepinephrine or epinephrine.

Two major classes of adrenergic receptors

Alpha (α) receptors and beta (β) receptors.

Alpha receptor subtypes

Alpha-1 (α1) and alpha-2 (α2) receptors.

Beta receptor subtypes

Beta-1 (β1), beta-2 (β2), and beta-3 (β3) receptors.

Alpha-1 (α1) receptor general location

Located on plasma membranes of various smooth muscle cells.

α1 receptors in blood vessels

Found in smooth muscle of blood vessels supplying the skin, gastrointestinal organs, and kidneys.

α1 receptors in the skin

Found on arrector pili smooth muscle cells in the dermis.

α1 receptors in the eye

Found on smooth muscle cells around the pupil.

Other α1 receptor locations

Found in the uterus during pregnancy and certain organs of the genitourinary system.

Alpha-2 (α2) receptor locations

Found on certain sympathetic target cells in the pancreas and adipose tissue, but most are on preganglionic sympathetic neurons.

Effect of α2 receptors on preganglionic sympathetic neurons

Activation causes hyperpolarization, which slows or cancels the action potential.

Purpose of α2 receptor activity

Dampens or shuts off sympathetic activity to prevent an excessive sympathetic response.

Beta-1 (β1) receptor locations

Located on cardiac muscle cells and certain cells of the kidneys and adipose tissue.

Beta-2 (β2) receptor locations

Located on smooth muscle of bronchioles, skeletal muscle fibers, smooth muscle of the urinary bladder, blood vessels supplying skeletal muscle, liver and pancreas cells, and salivary glands.

Beta-3 (β3) receptor locations

Located on adipose tissue cells and smooth muscle cells in the walls of the digestive tract.

Cholinergic receptors

Receptors that bind acetylcholine (ACh).

Two cholinergic receptor types in the sympathetic system

Muscarinic receptors and nicotinic receptors.

Sympathetic muscarinic receptor location

Found on sweat glands in the skin.

Sympathetic nicotinic receptor locations

Found on membranes of all postganglionic neurons and cells of the adrenal medulla.

Adrenal medulla cells

Modified postganglionic neurons that contain nicotinic receptors.

Preganglionic ACh receptor

Preganglionic sympathetic neurons release ACh that binds nicotinic receptors on postganglionic neurons.

Sympathetic sweat gland neurotransmitter

Postganglionic sympathetic neurons supplying sweat glands release ACh.

Sympathetic sweat gland receptor

ACh binds to muscarinic receptors on sweat gland cells.

Norepinephrine receptor type

Norepinephrine generally acts on adrenergic alpha or beta receptors.

Epinephrine receptor type

Epinephrine acts on adrenergic alpha or beta receptors.

ACh receptor types

ACh binds to cholinergic receptors, which include nicotinic and muscarinic receptors.

α1 key association

Smooth muscle, including blood vessels and muscles around the pupil.

α2 key association

Helps dampen excessive sympathetic activity through inhibition of sympathetic neurons.

β1 key association

Cardiac muscle cells.

β2 key association

Bronchioles and smooth muscle of blood vessels supplying skeletal muscle, along with several other target tissues.

β3 key association

Adipose tissue and smooth muscle of the digestive tract.

Nicotinic receptor key association

All sympathetic postganglionic neurons and adrenal medulla cells.

Muscarinic receptor key sympathetic association

Sweat glands.

Sympathetic neurotransmitter-receptor summary

Preganglionic ACh → nicotinic receptor; most postganglionic neurons release norepinephrine → adrenergic receptors; sympathetic sweat glands use ACh → muscarinic receptors.