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W. Hoobut and A. Niechaj

1. The dorsal root potentials (DRPs) produced on both sides of the spinal cord by prolonged repetitive stimulation of the alpha cutaneous afferents have been studied in spinal cats lightly anaesthetized by barbiturate.

2. Ipsilateral depolarization produced by long-lasting stimulation increases rapidly to maximum and then, in spite of a slight decline, it persists throughout the period of stimulation. Contralateral depolarization after its increase to maximum rapidly subsides to zero during continued stimulation.

3. Depression of the contralateral DRP during long-lasting stimulation depends on its frequency. It occurs at higher frequencies of tetanization (in excess of 20 c/s) while at low frequencies (3-15 c/s) contralateral DRP is maintained till the end of stimulation, displaying the pattern similar to depolarization of the ipsilateral dorsal root.

4. The effect of temporal facilitation on the DRPs is much greater and of longer duration on the contralateral than on ipsilateral side of the spinal cord.

5. After long-lasting conditioning stimulation there is prolonged depression of the testing DRPs produced by a single volley in the same nerve, Since duration of this depression is prolonged beyond the known duration of presynaptic inhibitory action, it is suggested to be due to homosynaptic depression in interneuronal pathways producing primary afferent depolarization.

6. More severe and prolonged depression of the contralateral DRPs evoked by preceding volleys indicates enhancement of homosynaptic depression on the other side of the spinal cord. It may be assumed that this increase depends on greater complexity of synaptic organization of the pathways producing contralateral depolarization of the primary afferent fibres.