ACCOMMODATION AND LOCAL RESPONSE IN MOTONEURONS OF TOAD'S SPINAL CORD

Abstract

1. Electrical activities of single motoneurons in excised toad's spinal cord were explored with intracellular microelectrodes.
2. On the oscilloscopic tracings, the initiation of spike potentials was classified into two types: an LR-type and an abrupt-type. It was shown that in the former spike potentials were initiated from the soma introduced by local response, while in the latter they were initiated from the initial segment of motor axon.
3. Single motoneurons were stimulated by exponentially rising currents applied directly with intracellular electrodes and the threshold depolarization was investigated as a function of the latent time. The threshold depolarization remained unchanged while the latent time was less than about 10 msec. Beyond this limit, it rose gradually with increase in the latent time, showing accommodation of the motoneuron to sustained depolarization. Accommodation was more prominent in the initial segment of motor axon than in so ma.
4. Hill's constant of accommodation λ was found ranging from 25.5 to 70 msec. in the initial segment.
5. The local response was often observed in soma separated from the spike, with stimulating currents rising slow enough.
6. Wedenski inhibition was proven to occur in the synaptic transmission to spinal motoneurons.
7. Accommodation and local response in motoneurons were explained referring to the ionic hypothesis of excitation.