The most commonneurotransmittersreleased by neurons of the autonomic nervous system areacetylcholineandnorepinephrine.
Cholinergic neuronsrefer to those neurons that release acetylcholine at the axon terminal. In the autonomic nervous system, all preganglionic fibers are cholinergic, and all postganglionicparasympatheticfibers are cholinergic.
Cholinergic receptorsThe acetylcholine released from the autonomic nervous system bind to two types of cholinergic receptors on target tissues:
Nicotinic receptors: In the autonomic nervous system, nicotinic receptors are located in the adrenal medulla, and in the autonomic ganglia at the postganglionic sympathetic and parasympathetic neurons. In the somatic nervous system, nicotinic receptors are found at the neuromuscular junction where they induce voluntary contraction of skeletal muscles when stimulated.
Muscarinic receptors: Widely distributed throughout the body and subdivided into 5 classes (M1-5). When stimulated by acetylcholine, the muscarinic receptors induce changes such as decreased heart rate, increased secretion in salivary glands and stomach, bronchoconstriction, increased gastrointestinal motility.
Adrenergic neuronsare those that release noradrenaline (norepinephrine) from the axon terminal. Most postganglionicsympatheticneurons are adrenergic. (The exception being sympathetic fibers supplying sweat glands, which are cholinergic.)
Adrenergic receptors去甲肾上腺素释放的(去甲肾上腺素)utonomic nervous system binds to two families of G-coupled protein receptors on target tissues:
Alpha receptors: Located in the smooth muscle cells of arteries, except those supplying skeletal muscles. When the alpha receptors are stimulated by norepinephrine, the smooth muscle cells contract and thus decrease the diameter of the artery. This increases blood flow to the heart and increases blood pressure. Some medications (Alpha-blockers) lowers blood pressure by binding to the alpha receptors to prevent epinephrine to bind to them.
Beta receptors:Beta 1receptors are located in the heart where binding of norepinephrine induces several changes such as increased heart rate (increased chronotropy), increased contraction strength (increased inotropy), increased conduction speed through the AV node (increased dromotropy), and increased relaxation of cardiomyocytes during diastole (increased lusitropy).Beta 2receptors are located in the smooth muscle surrounding the bronchioles and arteries supplying blood to skeletal muscles. When the beta 2 receptors are stimulated by norepinephrine, smooth muscle cell relax causing dilation of airways and arteries.