Final answer:
Variations in the size of action potentials within the heart are primarily due to differences in membrane permeability to sodium ions, the density of potassium ion channels, the duration of the absolute refractory period, and the speed of depolarization.
Step-by-step explanation:
The variations in the size of action potentials within different regions of the heart can be attributed to various factors. A crucial aspect affecting action potential size is membrane permeability to sodium ions (Na+). During depolarization, the membrane becomes more permeable to Na+, which rapidly enters the cell. This sodium influx causes a rapid rise in membrane potential. Another key factor is the density of potassium ion channels (K+). During repolarization, K+ channels open, allowing potassium to exit the cell and lower the membrane potential. The speed of depolarization and the duration of the absolute refractory period also contribute to the dynamics of action potentials. In contractile cells, like those in the ventricles, a stable resting membrane potential is maintained at around -90 mV, and depolarization leads to an action potential peak of about +30 mV. The subsequent plateau phase and repolarization are regulated by the opening and closing of calcium (Ca2+) and potassium channels, shaping the action potential's duration and size.