Final Answer:
No, a closed but not inactivated Na+ channel cannot be opened because the channels behind the open channels are temporarily inactivated.
Step-by-step explanation:
The reason a closed but not inactivated Na+ channel cannot be opened is due to the phenomenon of temporary inactivation of the channels behind the open channels. When a Na+ channel opens, it allows the influx of Na+ ions into the cell. However, this influx also triggers a process called inactivation, where the channels behind the open ones become temporarily inactivated. This means that even if a closed channel is not inactivated, it cannot be opened because the channels behind it are temporarily unable to open, preventing the flow of ions.
This phenomenon can be explained using the concept of refractory periods. After a Na+ channel opens and then closes, it enters a refractory period during which it cannot be opened again until it has fully recovered from inactivation. This refractory period ensures that the flow of ions is tightly regulated and prevents excessive depolarization of the cell membrane.
In summary, due to the temporary inactivation of the channels behind the open ones, a closed but not inactivated Na+ channel cannot be opened, as it is influenced by the refractory period and the regulation of ion flow within the cell.