The adequate stimulus is the specific form of energy (light, mechanical, chemical) to which a receptor is uniquely adapted and most sensitive, having the lowest threshold for activation. It is not the only energy that can activate it (e.g., a hard punch to the eye causes seeing "stars").
The adequate stimulus is the specific form of energy (light, mechanical, chemical) to which a receptor is uniquely adapted and most sensitive, having the lowest threshold for activation. It is not the only energy that can activate it (e.g., a hard punch to the eye causes seeing "stars").