ANS receptors:
Adrenergic receptors:
In the ANS, adrenergic neurons release NA which binds with adrenergic receptors and propogate the nerve impulses.
The two main types of adrenergic receptors are α-receptors & β-receptors. These receptors further subclassified as α- α1, α2 and β- β1, β2, β3.
α1 & β1 mostly produces excitation & α2 & β2 mostly produces inhibition.
Location:
α1: It is presence at the post junctional on effector organs like radial and sphincter muscles of iris (eye), heart, some BV (blood vessels), bronchial glands (lungs), liver, gut, skin, sex organs etc.
α2: It is presence on the prejunctional at the nerve ending. On the brain, pancreatic β cells, fat cells, gut muscles, veins etc.
β1: They are located at heart, salivary glands, juxtaglomerular apparatus of kidney, posterior pituitary.
β2: Lungs, BV, uterus, liver, eye, gut, urinary bladder, spleen, skeletal muscle, certain veins etc.
β3: Brown adipose tissue, where there function is to generate the heat by thermogenesis. Mostly presence in to the children.
Cholinergic receptors:
In ANS, cholinergic neurons release Ach a neurotransmitter. All sympathetic and parasympathetic neurons are cholinergic and also all parasympathetic postganglionic neurons are cholinergic.
Nicotinic receptors are presence on the dendrites or the cell bodies of postganglionic neurons of both sympathetic & parasympathetic neurons.
Muscarinic receptors are present on the all visceral organs. The muscarine, obtain from mushroom, mimics the action of Ach on these receptors.
There are two types of cholinergic receptors: muscarinic and nicotinic.
Nicotinic receptors
o The nicotine is never observed in to normal (nonsmoker) person still the receptors known as nicotinic, this is because this type of receptors are known as nicotinic, because these types of receptors stimulated by nicotine which mimics the action of Ach but having more affinity than Ach.
o They are ligand gated ion channel having pentameric structure. Activation of this causes opening of ion channel which causes influx of cation & leads to depolarization and generate action potential (AP).
o Depending on the location they are classified as NM & NN.
NM: They are presence on the neuromuscular junction mainly on the skeletal muscles. They cause depolarization at the muscle end plate which leads to contraction of muscle. They are pentameric having 2α, β, δ and γ or ε subunits and agonist by nicotine and PTMA and antagonist by tubocurarine.
NN: These are present on autonomic ganglia, adrenal mdulla and CNS. At autonomic ganglia it causes depolarization of postsynaptic neurons and propogate impulses through it. In the adrenal medulla releases adr & NA by same mechanism. And at the CNS causes excitation & inhibition depending up on the neuronal chemical. Nicotine and di methyl phenyl piprizinium are agonist and hexamethonium is antagonist to them.
Muscarinic receptors
o The substance known as muscarine from mushroom (amatina muscaria) is activating these type of receptors, so named as muscarinic receptors. They are G-protein coupled receptors (GPCRs). When Ach binds with them, they activated by Gi , containing 7-helical segments of amino acids where the amino end of chain is extracellular and carboxyl end of chain is intracellular & inhibit action of AC.
o By molecular cloning they are subdivided in to M1, M2, M3, M4, and M5.
- M1 : It is presence on the autonomic ganglia, on the gastric gland and at the certain part of the brain like hippocampus from limbic system and at the corpous straitum. It has role in gastric secretion. And histamine release. It acts through Gq protein and activates phospholipase C (PLc) which generate DAG & IP3 as 2 messenger. Some time they also activate PL-A2.
- M2: they are act through Gi protein hich inhibits all the functional activities. Located on the heart (SA node, AV node, atria, ventricle), on the cholinergic nerve ending and visceral smooth muscle. They inhibit AC resulting in hyperpolarisation of the neurons and decrease activity of SA node & conduction through AV node leads to bradycardia.
- M3: it is located on the visceral smooth muscle, iris, ciliary muscle and exocrine glands. They are also GPCRs acts by Gq protein. Their activity is dominated in smooth muscle the\an M2.
- M4: not abundant in body. They transmit neurotransmitter in certain areas of brain and acts through Gi protein.
- M5: it acts by Gq protein. Derifinacin is selective antagonist & related to dopamine release.
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