Receptor is a specific binding site of drug, which is situated on the surface or inside the effector cell. A large number of drugs act through these receptors. The action of drug is produce by activating or inhibiting the receptor activity.
In Autonomic Nervous System Receptor are divied into two types:
A. Cholinergic Recptors
B. Adrenargic Receptors
A. Cholinergic Receptors:
The receptors for acetylcholine are present at
Both sympathetic and parasympathetic ganglion (Nicotinic receptors)
Post ganglionic parasympathetic nerves endings (Muscarinic receptors.
The stimulant effect of acetylcholine on both sympathetic and parasympathetic ganglia is called ‘Nicotinic Action’. The nicotinic action of acetylcholine can be blocked by pentamethonium or hexamethonium.
The effect of acetylcholine at the post ganglionic parasympathetic nerve ending is called ‘Muscarinic action’. The muscarinic action of acetylcholine is:
Decrease in heart rate (Bradycardia)
Increase in glandular secretions like salivary and other gastro intestinal secretions.
Contraction of smooth muscle like intestine and bronchi.
All these muscarinic actions can be selectively blocked by atropine.
B. Adrenergic Receptors:
The receptors present at the postganglionic sympathetic nerve ending are called ‘Adrenergic receptors’.
The adrenergic receptors are classified as follows –
1. Alpha Receptors: The action on which is excitatory (except intestine).
A. Alpha receptors: Which are present in vascular smooth muscles, the effect occurs as constriction.
B. Alpha2 receptors: Which inhibit the release of rennin from the kidney, Alpha2 receptors are also present in human leucocytes and platelets.
Blood vessels of skin and mucus membrane – constriction
Spleen – contraction
Pupil – dilatation
Nictitating membrane (move the eyelids quickly) – contraction
Arrector pili muscle (of hair follicles) – contraction
2. Beta Receptor: The action of which is inhibitory (except heart)
A. Beta1 Receptor: Which are present in heart, the effect occurs as an increase in force and rate of contraction.
B. Beta2 Receptor: Which are present in bronchi, the effect occurs as relaxation.
The distribution of beta receptors and there effects are:
Blood vessels of skeletal muscles – Dilation
Uterus – Relaxation
Bronchi – Dilation
Intestine – Relaxation
Heart –Increase in force & rate (an exceptional effect)