How Depolarization Triggers Action Potentials: Role of Voltage-Gated Sodium Channels in Electrically Excitable Cells

Which of the following situations occurs in electrically excitable cells?a. When Na+ ion channels open, K+ ion channels close.b. The sodium-potassium exchange pump moves sodium into the cell.c. Depolarization causes voltage-gated sodium ion channels to open.d. Ligand-gated sodium ion channels are opened by high extracellular calcium levels.e. Proteins tend to diffuse out of the cell.

Depolarization causes voltage-gated sodium ion channels to open.

The correct answer is c. Depolarization causes voltage-gated sodium ion channels to open.

Electrically excitable cells, such as neurons and muscle cells, are able to generate and conduct electrical impulses or action potentials. These cells have specialized structures called ion channels in their cell membranes, which regulate the flow of ions in and out of the cell.

In response to a stimulus, such as a neurotransmitter binding to a receptor or a mechanical stretch, the cell membrane depolarizes – that is, the membrane potential becomes less negative due to the movement of positive ions into the cell. This depolarization triggers the opening of voltage-gated ion channels, which in the case of electrically excitable cells are usually sodium channels.

When the voltage-gated sodium channels open, sodium ions rush into the cell, further depolarizing the membrane and causing more sodium channels to open. This rapid influx of positive ions generates an action potential, or a brief spike in electrical potential that propagates down the length of the cell.

The other options are not correct. A. When Na+ ion channels open, K+ ion channels close. This is not entirely accurate, as the opening of sodium channels does not directly affect potassium channels. However, during an action potential, potassium channels do open and contribute to the repolarization of the membrane. B. The sodium-potassium exchange pump moves sodium into the cell. This is not true – the sodium-potassium pump is responsible for moving sodium out of the cell and potassium into the cell. D. Ligand-gated sodium ion channels are opened by high extracellular calcium levels. This is also false – ligand-gated ion channels are opened by the binding of specific molecules or ligands to the channel protein, and calcium ions do not directly regulate sodium channels. E. Proteins tend to diffuse out of the cell. This statement is not relevant to electrically excitable cells or ion channels.

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