Receptors

Adrenergic

Cholinergic

Receptors on the surface of the cells that are activated when they bind to acetylcholine (ACh). ACh is a neurotransmitter for both sympathetic and parasympathetic preganglionic nerve fibers as well as most parasympathetic postganglionic nerve fibers and its effects are always excitatory.

Nicotinic

Muscinaric

Transmission of the neurotransmitter norepinephrine (NE) released by the sympathetic postganglionic nerve fibers. Has both inhibitory and excitatory effects.

Nicotine and ACh stimulate the nicotinic receptors. The receptors are ligand-gated channels located in the CNS, neuromuscular junctions, and autonomic ganglia. Binding the ACh receptor to the the channel causes it to open allowing Na+ to diffuse inward and K+ to diffuse outward. Since the Na+ gradient is larger than the K+ gradient, the net effect is depolarization and the nicotinic receptors are excitatory.

Muscinaric and ACh stimulate the ACh receptors in visceral organs. These G-protein-coupled receptors are stimulated by ACh that is released by postganglionic parasympathetic axons to produce parasympathetic nerve effects in the heart, smooth muscles, and glands. The G-proteins bind with the receptor proteins to open and close the channel and activate different membrane enzymes. Thus the effects are excitatory and inhibitory. There are 5 different types of muscinaric receptors.

Causes hyperpolariaztion in the heart. ACh binds with G-proteins to open K+ channels

Causes depolarization and the contraction of the smooth muscle and secretion of glands. ACh binds with G-proteins opening the Ca2+ channels and increasing the systolic Ca2+

Mediate excitatory effects

Decrease cAMP in the cell and produce inhibitory effects

Alpha- both subtypes have different functions.

Beta- All subtypes produce effects by stimulating the production of cAMP within the target cells.

Alpha 1

Alpha 2

Located in the eye, skin and visceral vessels, skeletal muscle vessels. stomach and intestine, and liver. When Ne binds to Alpha 1, a rise of cytoplasmic concentration of Ca2+ as a second-messenger occurs.The stimulation of these receptors causes contraction of smooth muscles

Located on presynaptic terminal buttons and in the brain and produce a decreased release of NE when activated by it in the synaptic cleft, producing a negative feedback over the amount of NE released.

Beta 1

Beta 2

Located in the heart, Force the contraction of cardiac muscles promoting and increase in heart rate.

Located in the skeletal muscle arterioles and bronchioles, and liver. It promotes the relaxation of the smooth muscles and promote vasodilation.