Sympathetic signaling activates the sympathetic nervous system through the interactions between neurotransmitters and their receptors on target cells.
The preganglionic fibers of the sympathetic division release the neurotransmitter acetylcholine, which binds to nicotinic receptors on postganglionic neurons, activating them.
All activated sympathetic postganglionic fibers, with the exception of those that innervate sweat glands, release norepinephrine.
The released norepinephrine binds to specific adrenergic receptors present on effector cells.
There are two kinds of adrenergic receptors: alpha and beta.
Alpha receptors can further be categorized into alpha-1 and alpha-2 receptors.
Alpha-1 receptors, found in smooth muscles, generally exert an excitatory effect on the host cell, while alpha-2 receptors, located on preganglionic fibers, exert an inhibitory effect.
Beta receptors are divided into beta-1, beta-2, and beta-3 receptors.
Beta-1 receptors, found in cardiac muscles, pituitary gland, and adipose cells, increase heart rate and boost metabolism.
Beta-2 receptors, located in smooth muscles lining the airways, relax respiratory muscles, widening airways for easier breathing.
Beta-3 receptors, found in the brown adipose tissues, cause lipolysis.