Useful time is: the duration of the stimulus on the tissue; the duration of one excitation; the time from the onset of the stimulus to the appearance of a response; the minimum time during which the threshold stimulus causes the appearance of AP (action potential)

The correct answer is: the time from the onset of the stimulus to the appearance of a response.<br /><br />Useful time refers to the time interval between the onset of a stimulus and the appearance of a response. It is the time during which the stimulus is effective in producing a response. In the context of neural activity, useful time is the time from the onset of a stimulus to the generation of an action potential interval is important in understanding the relationship between the stimulus and the response, and it can be used to determine the effectiveness of a stimulus in eliciting a response.