Motor vehicle seat back comprising a structure, a headrest, a headrest, a support, a slide, a return element and a lock. The structure comprises at least one socket. The headrest comprises at least one stem mounted so as to slide in the socket between a high position and a low position. The stem has a curve. The support is mounted on the structure through an articulation element having at least a degree of freedom to rotate about an axis of orientation. The slide is connected to the stem and capable of sliding relative to the support in a direction of sliding. The return element is interposed between the slide and the support and biases the headrest towards its low position. The lock has an active state in which it is capable of holding the headrest in the high position and an inactive position in which it releases the headrest.