How fast are voltage gated Na channels in terms of opening?

Voltage-gated sodium channels are characterized by their rapid activation in response to changes in membrane potential. When a cell is depolarized, these channels open almost immediately, allowing sodium ions to flow into the cell, which contributes to the rapid upstroke of the action potential. This swift response is crucial for the rapid transmission of electrical signals along neurons and muscle fibers.

The rapid opening of these channels is essential for the phases of the action potential, where the influx of sodium leads to further depolarization of the membrane. The quick opening mechanism plays a significant role in the overall excitability of neurons and myocytes and supports the immediate response needed for physiological processes such as reflex actions and muscle contraction.

The other options do not accurately describe the behavior of voltage-gated sodium channels. Unlike "slow" or "moderate" descriptors, which would indicate a gradual response, voltage-gated sodium channels are specifically designed to react quickly to changes in voltage, making "fast" the most fitting description of their functional characteristics.