IPSILATERAL CORTICAL STIMULATION INHIBITED THE LONG-LATENCY RESPONSE TO STRETCH IN THE LONG FINGER FLEXORS IN HUMANS

1. Transcranial magnetic stimulation over the motor cortex of one hemi sphere is known to decrease the excitability of the motor cortex of th e other hemisphere. We investigated the effect of this interhemispheri c or transcallosal inhibition of the motor cortex on the reflex respon se to stretch in the long flexors of the fingers in human subjects. 2. Stretch of the long finger flexors, through extension of the proximal interphalangeal joints with a torque pulse, resulted in a reflex EMG response with short- and long-latency components. Magnetic stimulation was applied over the motor cortex ipsilateral to the muscles being st retched. When a magnetic shock but no stretch was given, a decrease in background EMG in the ipsilateral finger flexors occurred at a latenc y of 33 +/- 6 . 2 ms after the stimulus and with a duration of 25 +/- 8 . 5 ms. 3. If the magnetic shock and the stretch were given at appro priate interstimulus intervals, the long-latency stretch reflex (LLSR) showed inhibition in all subjects. LLSR was reduced to 49 . 2 +/- 19% (S.D.; n = 9) of the area of the control response. 4. The LLSR did no t act as a single event in response to the magnetic shock. That is, pa rt of the LLSR could be reduced in amplitude while the remainder was u naffected. The reduction in LLSR had an onset latency of 27 +/- 3 . 8 ms after the magnetic stimulus and a duration of 29-55 ms. Inhibition was only obvious when this interval after the magnetic stimulus coinci ded with the LLSR. 5. In most subjects the short-latency stretch refle x (SLSR) also showed some inhibition (83 . 4 + 11 . 2% of the control) . However, this mas less than the effect on the LLSR in all subjects. 6. The site of stimulation, over the ipsilateral motor cortex, was spe cific for inhibition of the LLSR. When the coil was moved anteriorly o r to the midline, inhibition was significantly decreased. 7. We sugges t that the inhibition of the LLSR of the long flexors of the fingers r esulted from a reduction in excitability of the motor cortex produced by an inhibitory transcallosal pathway and conclude that the LLSR in t his muscle has a transcortical component.