Brain Research, 149 (1978) 503-507 503 © Elsevier/North-Holland Biomedical Press Phasic gain control of the transmission in cutaneous reflex pathways to motoneurones during 'fictive' locomotion O. ANDERSSON, H. FORSSBERG, S. GRILLNER and M. LINDQUIST Department of Physiology 111, Karolinska Institutet, Lidingiiviigen 1, S-114 33 Stockholm (Sweden) (Accepted January 12th, 1978) A tactile stimulus applied to the dorsum of the foot during locomotion enhances flexion during the swing phase, but extension during the support phasea, 4. This phase- dependent reflex reversal can be regarded as a 'stumbling corrective response' built into the spinal circuitry of the tactile placing reactions 5. The present study investigates whether the fact that the pathways to flexors and extensors, respectively, are effective only in one phase of the stepcycle is due to central mechanisms (motoneuronal or pre- motoneuronal) or to interaction from other reflex pathways activated in the stepcycle. Intracellular recordings have been made from different species of a-motoneurones during 'fictive locomotion' (curarized preparation), during which the amplitude of the postsynaptic potentials (PSPs) elicited by electrical stimulation of the dorsum of the paw have been compared in the 'flexor' and the 'extensor' periods. A systematic differ- ence in amplitude of the PSPs in the different phases would imply that the two pathways are phasically modulated at a level before the motoneurone. Adult cats were acutely spinalized at the lower thoracic level, subjected to a lower lumbar laminectomy, decorticated, and with their hindlimb nerves and nerve filaments dissected and mounted for stimulation 1. The cats were subsequently paralyzed (Tubo- curarin) and injected i.v. with nialamide and DOPA, which activated the spinal loco- motor generator 6. Alternating 'locomotor activity' could then be recorded in muscle nerve filaments. The dorsum of the paw was weakly stimulated electrically (1-5 mA, 0.5-5 msec) and the responses in antidromically identified motoneurones were recorded intracellularly. One chronic spinal cat (spinal transection at 6 days) was subjected to the same procedure at 9 months. The intracellular data were stored on Disc in a HP21 MX computer. The individual responses were related to the 'locomotor cycle' and were categorized in different groups. The amplitudes of the responses in the groups were then statistically compared as well as presented in averaged form. Fig. 1 shows the alternating activity in a flexor motoneurone as well as a flexor filament. A stimulus was initiated at l, 2, 3 and 4, and the responses are shown at ex- panded time scale below. The response in the early flexion is large, which contrasts with the responses in the other parts of the stepcycle. In Fig. 2A the responses from the different groups are averaged. In the early flexion they are significantly larger (P < 0.01,