Question

Nerve impulses are initiated by nerve fibres only when the membrane shall become more permeable to

• A. Adrenaline
• B. Phosphorus
• C. Sodium ions
• D. Potassium ions

Answer 1

The correct option is C Sodium ions

Action potentials are generated by special types of voltage-gated ion channels in a cell's plasma membrane. When the channels open (in response to depolarization in transmembrane voltage), they allow an inward flow of sodium ions, which changes the electrochemical gradient, which in turn produces a further rise in the membrane potential. This then causes more channels to open, producing a greater electric current across the cell membrane. The process proceeds until all of the available ion channels are open, resulting in a large upswing in the membrane potential. The rapid influx of sodium ions causes the polarity of the plasma membrane to reverse, and the ion channels then rapidly inactivate. As the sodium channels close, sodium ions can no longer enter the neuron, and then they are actively transported back out of the plasma membrane. Potassium channels are then activated, and there is an outward current of potassium ions, returning the electrochemical gradient to the resting state.