Dendritic activities of spinal motoneurones in pigs and rabbits enhanced through chronic stimulation of a dorsal root.

1. The L6 or L7 dorsal root in 7-10-day-old pigs was chronically stimulated for 3 or 4 h a day for 4 days/week. The stimuli were 50 or 500 Hz pulse trains delivered at 0.5/s. The induced movements were limited to the hip and hind leg on the stimulated side without any sign of pain. There was evidence that not only the monosynaptic, but also polysynaptic pathways to the motoneurones were chronically stimulated. After 2-6 days of chronic stimulation, all pigs were studied in acute experiments either under pentobarbitone (Nembutal) anaesthesia or in the decerebrated state. Normal (unstimulated) 15-28-day-old pigs were also studied in acute experiments under similar conditions, as the controls. 2. In thirty-three out of fifty-six motoneurones of normal pigs there were all-or-none potentials of 2-6 ms duration and 1-5 mV amplitude. These have been named D-spikes. They occurred in response to tibial nerve and/or dorsal root stimulation and/or spontaneously. They could be abolished by injection of a hyperpolarizing current. These results show that D-spikes are different from any of the well-established all-or-none events of the motoneurone, i.e. the spikes of the axon, initial segment, soma and unitary excitatory post-synaptic potentials. Several kinds of D-spikes could exist in the same motoneurone, as judged from their wave forms and thresholds. This argues for the dendrites as the sites of origin for D-spikes, because only the dendrites can have multiple sites for spike generation. D-spikes were shown to summate to form a larger depolarization. 3. In pigs chronically stimulated with 500 Hz pulses seventy-five motoneurones were studied in acute experiments. In sixty-one motoneurones D-spikes were observed. This indicated that D-spike occurrence was enhanced in the chronically stimulated motoneurones as compared with controls. 4. Characteristic of the chronically stimulated motoneurone was the presence of a depolarization of all-or-none nature which was more than 10 ms in duration and 2-12 mV in amplitude. The depolarization is referred to as the D-wave. The D-wave was similar in properties to the D-spike described above, except that it was of longer duration. Consequently, it is assumed that the D-wave is summed D-spikes. D-waves were observed in nineteen motoneurones.(ABSTRACT TRUNCATED AT 400 WORDS)