Final answer:
The strength of cardiac contraction is regulated by the autonomic nervous system and hormones, with pacemaker cells, preload, contractility, and afterload all influencing heart function.
Step-by-step explanation:
Under normal conditions, the strength of cardiac contraction is regulated primarily by the autonomic nervous system (ANS) and various hormones. The contraction of the heart is controlled by pacemaker cells which respond to signals from the ANS to regulate heart rate. Signaling molecules like adrenaline can increase heart rate and contractility, while other factors such as preload (ventricular stretch before contraction), contractility (force of contraction), and afterload (resistance to eject blood) also play a crucial role in heart function.