[Laughter and smiling. The gesture between social philosophy and psychobiology].

The human individual experiences his gestuality unaware, making it even more revelatory of his current state of mind. Certain actions represent innate behaviour, while others are discovered or assimilated during learning. The actions that signify the sending of any kind of message to another person, become "gestures". Laughter is an example of expressive behaviour and, as such, plays an important part in social interaction, insofar as it supplies signals that influence the response of other individuals. After a review of the philosophical and sociological interpretations inherent in laughing and smiling, the correlations between weeping, smiling and laughing are examined specifically from as ethological standpoint. The neurophysiological aspects whereby the crying, smiling and laughing mechanisms depend on a multifactorial control system, that implies the activation of limbic and frontal circuits, a motor cortex response and an activation of the autonomic response of the vegetative nervous system, correlated to the cortico-hypothalamic ways, are then examined. The pathological observations carried out in Man are identified with the study of psychomotor epilepsy in the context of "gelastic crises", cataleptic-gelolegic crises "and the pseudo-bulbar syndrome" with crying and spastic laughing. Finally, the possible therapeutic implications correlated with the laughing mechanism are discussed. In fact, the facial contractions, associated with the emotional experience connected with them, would appear to function as a standard brain signal. Interesting are the findings of changes in neuromediator and endorphin modifications implied in the neurophysiological smiling and laughing mechanism. A further basic element is the involvement of the autonomous system which can be correlated with the concept of the trophotropic response according to Hesse, with an increase in parasympathetic tone and reduction in sympathetic tone.