Cardiac Prepotential

The prepotential is a gradually change in the electrical charge at the inner surface of the cell membrane. When the charge reaches a certain value (i.e., threshold potential) there is a rapid influx of calcium ions that leads to a heart beat. Section A of the action potential graph at the bottom of the model represents the prepotential.

Sodium-Potassium Pump

The ubiquitous membrane-bound sodium-potassium pump is represented as ATP within a circle. The block arrows indicate the removal of three sodium ions (Na⁺) from the cytoplasm in exchange for two potassium ions (K⁺) from the interstitial fluid. This type cell is unique in that, unlike most body cells, there are few if any potassium leakage channels. However, like most other cells, there is a small population of sodium leakage channels (cylinder).

The constant activity of this pump--assumming the ions it moves are available in the appropriate compartments:

establishes a sodium gradient that insures continued sodium influx (block arrow) back into the cell through passive leakage channels (cylinder), and
adds potassium (block arrow) to the cytoplasm that accumulates unless gated potassium channels are open.

Potassium channels are closed in this illustration; they are the paired ovals shown above the potassium ion. It is this accumulation of potassium ions that adds positive charge to the inner surface of the cell membrane and is reflected as section A of the action potential graph...the prepotential. Textbooks often describe this phenomenon as "due to the closure of potassium channels." This is indeed the case as will be shown later.

Last updated: 7/16/2005