What is the result of norepinephrine and epinephrine binding to receptors on cardiac muscle fibers?

The binding of norepinephrine and epinephrine to receptors on cardiac muscle fibers leads to an increase in heart rate and strengthens contractility. This is primarily due to the action of these catecholamines on beta-adrenergic receptors located in the heart. When norepinephrine and epinephrine bind to these receptors, they initiate a cascade of intracellular events that enhance the heart's electrical activity and its ability to contract.

Increased heart rate occurs because the binding of these hormones accelerates the pacemaker activity of the sinoatrial (SA) node, which is responsible for initiating the heartbeat. Additionally, the increased strength of contractility means that each contraction of the cardiac muscle is more forceful, resulting in greater stroke volume and, consequently, increased cardiac output.

This physiological response prepares the body for 'fight or flight' situations, thus increasing blood flow to vital organs and muscles. Therefore, the correct answer highlights the stimulating effects of norepinephrine and epinephrine on the heart, which is well-documented in physiology.