[0:07]So now let me discuss the action of the histamine on the heart. Now, if you take this particular histamine, the action on the heart is via both the H1 receptors and as well as the H2 receptors. Right, via the H1 receptors and as well as H2 receptors. Now, for suppose, the H1 receptors, if they are being stimulated, they have what is called as negative dromotropic effect. Right, when H1 receptors are stimulated, they have negative dromotropic effect. Right, why is this negative dromotropic effect? This negative dromotropic effect, it is due to decrease in the AV conduction.
[1:17]That is decrease in the atrioventricular conduction. So that is the reason why there is negative dromotropic effect. Now, when the histamine, it is acting on the H2 receptors. Right, that will increase the force of contraction. Right, that will increase the force of contraction, that is, it increases the ionotropy. Okay, so that is what is the action of the histamine on the heart. When it is stimulating the H1 receptors, that will result in negative dromotropy. When it is stimulating the H2 receptors, that is increasing the force of contraction. Now, if you take the action of the histamine on the visceral smooth muscles. Right, so remember, this particular histamine, it acts on visceral smooth muscles, right? The histamine, it acts on visceral smooth muscles via or through the H1 receptors. And when this particular H1 receptors are stimulated, which are present on the smooth muscle, this will result in broncho constriction. Because where do we have this visceral smooth muscle is within the bronchus, that is in the respiratory tree, and as well as the smooth muscle is present within the gastrointestinal tract. So, when the H1 receptors are stimulated, which are present on the visceral smooth muscle, that will result in, right, that will result in broncho constriction and the second thing is that will cause abdominal cramps. Right, that will cause abdominal cramps. So, this is the action of the histamine on H1 receptors which are present on the visceral smooth muscle. All right. Next. Now, this particular histamine, by acting on the H2 receptors. Remember, H2 receptors, they are also present on the gastric parietal cells.
[3:55]Right, which are present on the gastric parietal cells. This will result in increase in the gastric secretion. Okay, so histamine increases the gastric secretion by acting on the H2 receptors which are present on the parietal cells. Next, the other important thing is,
[5:01]you take this histamine. Histamine, it is also synthesized within the brain. Right, histamine is also synthesized within the brain. Now, the histamine, whichever is being synthesized within the brain, it stimulates the reticular activating system. All right. So, the histamine, which is being synthesized within the brain, will stimulate the reticular activating system. So, when the histamine which is being synthesized in the brain, when it is stimulating the reticular activating system, that will maintain the wakefulness of the individual. All right. So, this will maintain the wakefulness. Right? And where are through via via which receptors, the reticular activating system gets activated is via the H1 receptors. So remember this point, the histamine, which is synthesized in the brain, acting through H1 receptors will stimulate the reticular activating system, and will result in the wakefulness or activeness of the individual.
[7:54]Now, after having discussed about the action of the histamine on H1 and H2. Now let me discuss the action of the histamine on H3 receptors.
