The summation of stimuli

Abstract

The phrase, “summation of stimuli,” has been employed at times to signify only the phenomenon in which a stimulus of a fixed intensity, which at first is too weak to stimulate living substance, will upon repetition be followed by a response. It is more rational to include within the concept all cases of summation, whether the stimulus is at first below the stimulation threshold or above it. Summation is usually ascribed to an increase in the irritability of the protoplasm, but the conditions responsible for such increase have not been known. Two years ago, the senior author explained the increase in irritability found in the treppe of muscle, by the augmenting action of fatigue substances, notably carbon dioxide and lactic acid. This chemical theory of the treppe is here applied to the explanation of summation in general. The validity of this explanation has been confirmed by a large variety of experiments performed on the muscles of medusæ and crustaceans. It has long been known that summation with subminimal stimuli is very readily obtained in these forms. The authors have confirmed this. They have also studied the action on the muscles of carbon dioxide and lactic acid in small quantities.