Involuntary stretching during yawning in patients with pyramidal tract lesions: further evidence for the existence of an independent emotional motor system.

A variety of associated movements have been described in patients with pyramidal tract lesions. We report three patients in whom involuntary stretching of an otherwise plegic arm could be observed during yawning. These patients had radiologically verified lesions at different levels of the pyramidal tract. As yawning and stretching are an automatic behavioural pattern in animals, it is likely that stretches during yawning in man are also an automatic motor pattern, usually inhibited in the presence of an intact corticospinal tract. The physiological function of yawning is unclear at present. Yawning might be the somatomotor manifestation of a particular emotional state characterized by boredom and fatigue. Our observation that movements of an otherwise plegic arm occur in patients with pyramidal tract lesions supports therefore the concept of an emotional motor system which has an independent input to motoneurones in the brain stem and the spinal cord.