pre-med student with a question about antihistamines
First off, I'm just a pre-med student with a question that I thought you guys could easily answer: I know that antihistamines like diphenhydramine are CNS depressants. This would mean it decreases heart rate. BUT, I've taken it before and had an increase in heart rate. I did some reading and found out that antihistamines have anti-muscarinic effects, which I presume will only affect the parasympathetic nervous system. Since HR is controlled by the vagus nerve, a decrease in parasymp. activity should increase HR. I've tried researching it, but I'm having trouble. SO my question is, why is diphenhydramine supposed to decrease heart rate but can sometimes increase it? My second question is: what part of the CNS is specifically depressed and what is the general mechanism