Heart Rate ..Part 1 ..sympathetic and parasympathetic Balance

Despite the  widespread measurement of heart rate. The  physiological implications of high and low heart rates are complex and not always fully appreciated.

So in this post  we will discuss the basic Physiology Of HR and in the next posts we will go into the clinical aspects of HR.

First :Parasympathetic input slows the discharge rate and Beta input increases it

The intrinsic heart rate without any autonomic input was studied  by giving the muscarinic receptor antagonist atropine to block parasympathetic and the B1 adrenergic receptor antagonist propranolol to block sympathetic activity

Healthy adults aged 16–70 yrs, the intrinsic heart rate was 106 beats/min

There was an age-related decline in the intrinsic rate of 0.057 beats/min per year so that in a 60 yr old, intrinsic heart rate was only approximately 90 beats/min

Because the normal resting heart rate is approximately 70 beats/min, parasympathetic input must dominate in the resting state. I say dominate because there also is resting sympathetic activation.

The selective advantage of having both active sympathetic and parasympathetic activity is that it allows rapid
changes in heart rate at the onset of exercise or any other sudden stress by allowing rapid withdrawal of parasympathetic tone and a simultaneous increase in sympathetic activation.Perhaps not as advantageous, this also can result in large rapid changes in heart rate under pathologic conditions.