What controls the Rate and Contractility of the Heart?
Although the heart rate is set by SA node, the rate and contractility can be regulated by the Autonomic Nervous System (ANS), which includes the Sympathetic (SNS) and the Parasympathetic (PNS).
Our heart rhythms are controlled by a number of factors but most powerfully by the interplay between the SNS and PNS branches of the autonomic nervous system. The SNS accelerates the heart; whereas the PNS acts as a brake a decelerates the heart.
When the two systems are working in proper balance, the heart becomes exquisitely responsive to the body’s ever changing needs and yet maintains a strong underlying stability.
When the two systems are not working well together, the heart shows a poor response to changing body needs and less stability in heart rate at rest. Chronic stress leads to autonomic imbalance with increased SNS activity and reduced PNS influence (Note: Imagine a powerful car with a very sensitive and twitchy gas pedal and very poor brakes.) and shows up as increased resting heart rate and reduced heart rate variability.