Question

The graph showing depolarization of every cardiac muscle cell in the body OTHER than SA Node muscle cells has a plateau. Draw that graph from your notes (draw with a pen, please) and then explain why the plateau exists and what effect(s) the plateau has on contraction of the heart musculature. You may hand draw any diagrams you use, but type your explanation. 10 points for a perfect answer.

Answer 1

The action potential in cardiac cell is quite different from what is seen in the SA node or a nerve cell. Phase 2 is the plateau phase of the cardiac action potential. Membrane permeability to calcium increases during this phase, maintaining depolarization and prolonging the action potential. Membrane permeability to calcium decreases towards the end of phase 2. The plateau is partially maintained by an inward sodium current. Sodium flows into the cell through the sodium–calcium exchanger. The exchanger transfers three sodium ions into the cell in exchange for one calcium ion flowing out, and so produces a net inward positive current.

Phases of Cardiac Action potential:

Phase 0 - Rapid Na influx through open fast Na+ channels
Phase 1 - Transient K + channels open and K+ efflux returns TMP to 0 mV
Phase 2- Influx of Ca+2 through L-type channels is electrically balanced by K+ efflux through delayed rectifier K+ channels
Phase 3- Ca+2 channels close but delayed rectifier K+ channels remain open and return TMP to - 90 mV
Phase 4- Na+, Ca+2, channels closed, Open K+ rectifier channels keep TMP stable at -90 mV