Evidence that a long latency stretch reflex in humans is transcortical.

1. The hypothesis that the long latency reflex response to muscle stretch in humans uses a transcortical pathway was tested by looking for convergence onto cortical neurones in eleven normal subjects. 2. Postsynaptic events in single flexor pollicis longus (FPL) motoneurones were derived from changes in the firing probability of individual FPL motor units. 3. Extension of the terminal phalynx of the thumb resulted in both short latency and long latency facilitations of individual FPL motoneurones. These were not reproduced by electrical stimulation of afferents in the terminal phalynx. Magnetic stimulation over the contralateral motor cortex produced strong, short latency facilitation of FPL motoneurones. 4. When the facilitation produced by stimulation over the cortex was superimposed on the long latency facilitation produced by extension of the thumb, the facilitation produced by both stimuli was greater than the sum of the individual facilitations produced by either stimulus given alone. This was not the case when the superimposition occurred on the short latency response to stretch. 5. We conclude that afferent systems excited by the stretch of FPL converge onto cortical neurones which are known to facilitate motoneurones. Thus the cortex is likely to contribute to the long latency stretch reflex in humans.