ACETYLCHOLINE AND NOREPINEPHRINE SENSITIVE RECEPTORS ON POSTSYNAPTIC MEMBRANE OF THE PREGANGLIONIC AND POSTGANGLIONIC JUNCTION AND TARGET ORGANS.

 

 

1. On all postsynaptic membranes of the preganglionic and postganglionic junction, cholinergic (acetylcholine sensitive) receptors are present since all preganglionic axons release acetylcholine to the postganglionic axons. There are two types of cholinergic receptors: nicotinic and muscurinic. The nicotinic cause fast action potential while the latter causes slow. The acetylcholine released binds predominantly with the nicotinic receptors on the postganglionic axons causing fast action potential also called fast EXCITORY POST SYNAPTIC POTENTIAL (EPSP). After the fast EPSP has been fired, the acetylcholine may bind with muscurinic receptors on the post synaptic membrane to cause slow EPSP or slow INHIBITORY POST SYNAPTIC POTENTIAL (IPSP).

During EPSP, when ACH (acetylcholine) binds with cholinergic receptors, the Na and Ca channels open causing their influx generating an action potential. During IPSP when ACH binds to muscurinic cholinergic receptors, k channels open causing their efflux resulting in hyperpolarization.

              

2.The target organs posses both adrenergic (noradrenaline sensitive receptors) for the sympathetic response and cholinergic MUSCURINIC receptors for the parasympathetic response. An exception is the receptors on the sweat glands and blood vessels of skeletal muscles which posses cholinergic MUSCURINIC receptors and respond to ACH for SYMPATHETIC response.

 

Note: the muscurinic receptors are found on postsynaptic membrane of the postganglionic axon and the target organs while nicotinic are only found on post synaptic membrane of the post ganglionic axons.

 

Atropine can competitively inhibit muscurinic receptor sensitivity to ACH. 

 

Similarly the axons that release ACH are called cholinergic fibers and those that release Noradrenaline are called adrenergic fibers.

 

There are also 4 types of adrenergic receptors a1 a2, b1 and b2 and are only present on target organs for sympathetic response. A1 is also present on the presynaptic membrane of the neuroeffector junction.

 

All respond to norepinephrine. A1 and a2 on target organs when stimulated cause excitory functions while b1 and b2 cause inhibitory functions. The a1 present on presynaptic membrane is stimulated when there is too much norepinephrine in the synaptic cleft. It inhibits the production of more norepinephrine into the synaptic cleft.

 

All beta receptors cause inhibitory effects (e.g. B2 receptors present on lungs cause bronchodilation) except for the b1 receptors present on myocardium that cause excitory effects.

Note: norepinephrine has a greater effect on alpha receptors than beta receptors.