Headrest with safety actuating system

The head support (1) comprises a mounting pin comprising two vertical rods (6,6') on a seat back. A support (4) is fixed on the pin and a support cushion (5) can move relative to this support by means of an adjustment means between a position in which the cushion is adjacent to the support and a position where it is at a distance from the support. The head support comprises a mechanical system (18) activating the cushion towards its second position which is activated by a collision detection device.