Question

In an action potential mechanims, why is there a delayed voltage-gated response as potassium gates open?

Answer 1

• At steady state, when there is no external influence or stimulus, the cell remains in a steady state.
• This is achieved by ion leakage and pumping. The Sodium (Na⁺) voltage gated channels are closed. Potassium (K⁺) leak channels are open.
• During resting state there is no action potential and is described as resting potential, of normal value between -30mV to -70mV.
• When the cell receives a stimulus or electrical signal, first the Sodium (Na⁺) voltage gated channels are opened (when threshold stimulus is exceeded).
• Since concentration of Na⁺ is higher outside the cell than inside the cell (by factor of 10), Na⁺ ions move rapidly inside the cell.
• Overshoot: The sodium voltage dominates the potassium leak current, to cause positive membrane potential.
• Depolarization phase: Due to positive charge of Na⁺ ions, potential attains a positive value from initial value of -70mV. Thus, the membrane potential value moves towards zero, and the cell becomes depolarized.
• As cell becomes less negative, more Na⁺ channels open, increasing more influx of Na⁺ ions.
• Repolarization phase: With continuation of influx of Na⁺ ions, the potential reaches a peak value of +30 mV. At this point other gated channels, specifically Potassium (K⁺) gated channels open. Due to concentration gradient, K⁺ ions start leaving the cell. Thus, positive potential value again shifts back towards resting potential, and the cell is repolarized.
• Hyperpolarization phase: Potassium channels remain opens, Sodium channels close and reset.
• The voltage gated Potassium channels have a common structures plan with only single gate, which is sensitive to membrane potential of about -50 mV. Also, there are no or selective voltage sensors on Potassium channels, compared to Sodium channels.
• Thus, the time that is required to reach the potential of -50mV, during depolarization phase, causes delay in the opening of potassium gated channels. Same delay is observed during closing of potassium channels.