Nor epinephrine is the neurotransmitters at the sympathetic post ganglionic nerve terminal that innervate visceral effectors the stimulation of adrenoceptor on these tissues always do not produce identical response. Therefore, based on the nature and physiological response obtained, adrenoceptors are classified as alpha and beta-receptors. Generally alpha-receptors are excitatory (vasoconstriction) and beta-receptors are inhibitory (vasodilatation) in nature with the exception of heart where beta-receptors are stimulatory while in visceral smooth muscles where alpha and beta-receptors both are inhibitory in nature. The distribution of these receptors in different organs is also different; some organs show the presence of one-type receptors and some are showing both type.
Fig 2.10 Signaling in sympathetic system
Fig 2.11 Secretion of Adrenal Medulla
Fig 2.12 Mechanism of Neuro transmission
Characteristic of important subtypes of alpha (α) receptor
There are two types of α receptor α 1 and α 2 . α 1 is located on post junction of effecter organ and α 2 is located in prejunctional nerve ending, also postjunctional in brain, pancreatic beta cells, platelets and extra junctional in certain blood vessels. α 1 is linked with IP 3 /DAG and phospholipase A 2 which will result in increase concentration of cytosolic calcium which is responsible for contraction of smooth muscles in blood vessels (vasoconstriction) and arrhythmia (contraction of cardiac muscles) but one point should be noted that it is responsible for gut smooth muscle relaxation because there two sets of muscles are present; circular and radial if you contract one other will relax contraction of circular muscle results in gut relaxation. IP 3 /DAG also result in activation of protein kinases, which signal nucleus for formation of specific proteins which are responsible for gland secretion. α 2 is also linked with IP 3 /DAG which will result in increase concentration of cytosolic calcium but is also linked with cAMP which is down regulated mood because of opening of k+ channel and closing of Ca2+ channel or blocking of calcium channel. As α2 is located in prejunctional site of synapse it will result in re uptake of neurotransmitters i.e. decreased depolarization or reduced activity sit’s a good target in the treatment of CNS disorders, as it reduces the central sympathetic outflow as its linked with IP 3 /DAG it results in vasoconstriction as of α 1 .
8.2Characteristic of important subtypes of beta (β) receptor
β 1 , β 2 and β 3 are three sub types of β receptors and all are c AMP linked but differ in their location and accordingly in their activity with an exception that β 2 is linked with down regulated c AMP sit will responsible for decreased activity. β 1 is present in bronchi, blood vessels, uterus, urinary tract and eye. β 2 is located in heart and juxta glomerular cells in kidney. β 3 is present on adipose tissue and its much detail is not available. As β 1 will result in activation of protein kinases which are responsible for contraction of smooth muscles and secretion from glands and depolarization in case nerve conduction while opposite for β 2 .