Cutaneous reflex responses and their central nervous pathways studied in man.

1. Cutaneous reflex responses have been recorded in human first dorsal interosseous and extensor digitorum brevis muscles following electrical stimulation of the digital nerves of the index finger and second toe respectively.2. Recordings have been made in normal subjects and in patients with central nervous lesions.3. Cutaneous reflex responses in first dorsal interosseous were triphasic, consisting of initial short latency excitation, followed by inhibition, followed by prominent long latency excitation. Cutaneous reflex responses in extensor digitorum brevis were biphasic, consisting of short and long latency periods of excitation.4. Estimated central delay for the initial excitatory components of the cutaneous reflex in first dorsal interosseous and extensor digitorum brevis muscles ranged from 2.4 to 6.2 ms (mean 4.6 ms) and 0.6 to 4.1 ms (mean 2.3 ms) respectively.5. Differences in latency between short and long latency excitatory components of the cutaneous reflexes recorded in first dorsal interosseous and extensor digitorum brevis muscles ranged from 16 to 18 ms (mean 17.3 ms) and 27 to 32 ms (mean 29.3 ms) respectively.6. Differences in time delay between short and long latency excitation in first dorsal interosseous and extensor digitorum brevis muscles when compared in individual subjects ranged from 9 to 14 ms (mean 12 ms). These values lay within 0-7 ms (mean 4 ms) of estimates in each subject of conduction time along central pathways between T12 and C7 spinal segments.7. Differences in latency between short and long latency excitatory components of the cutaneous reflex recorded in first dorsal interosseous were 3.5-8.5 ms longer than the estimated minimum time for impulse conduction along a pathway travelling through the dorsal columns to cerebral cortex and returning by way of the corticospinal tract.8. The long latency excitatory component of the cutaneous reflex in first dorsal interosseous muscle is reduced and often delayed in patients with dorsal column lesions.9. The long latency excitatory and short latency inhibitory components of the cutaneous reflex in first dorsal interosseous muscle are absent in patients with damage to motor cortex.10. The long latency excitatory component of the cutaneous reflex in first dorsal interosseous muscle is reduced in amplitude and often delayed in patients with motoneurone disease causing damage to the corticospinal tract. The timing of short latency excitatory and inhibitory components is unchanged.11. It is concluded that the short latency excitatory and inhibitory components of the cutaneous reflex response of first dorsal interosseous muscle have a spinal pathway and that the interneurones mediating the inhibitory component are under descending extrapyramidal control from systems whose inputs are deranged by damage to motor cortex.12. It is concluded that the long latency excitatory component of the cutaneous reflex response of first dorsal interosseous muscle is of supraspinal origin requiring transmission of afferent impulses through the dorsal columns, a relay in the sensori-motor cortex and then descending transmission to the lower motoneurone pool by way of the corticospinal tract.