Delayed depolarization and the repetitive response to intracellular stimulation of mammalian motoneurones

The present work was undertaken in order to investigate quantitative and qualitative aspects of tonic firing of motoneurones by the intracellular technique, being in this respect a sequence to studies of maintained unit discharges in ventral roots pursued over some years (e.g. recently by Granit & Rutledge, 1960, and Granit & Renkin, 1961). On the basis of the experiments by Araki & Otani (1955), Coombs, Curtis & Eccles (1957a, b) and Fuortes, Frank & Becker (1957), the axon hillock (Aor IS-zone) is nowadays visualized as a specific firing zone of low electrical threshold. The latter makes it, as it were, so sensitive to depolarization that its spike activates the rest of the motoneurone antidromically. If this also be true for tonic discharges, one is curious to know why the soma-dendrite region (Bor SD-zone) is invaded at all to deliver an ionic display, or, at least, what this region thereby might contribute to the sum total of events, because-on the theory-synaptic knobs would be expected to occur preferentially on the axon hillock, where, in reality, they are known to be absent or scarce. Quite consistently Coombs et al. (1957b) have also concluded that remote synapses are functionally ineffective. It is not a particularly attractive notion that demonstrable structures because of geometrical reasons would be lacking functional significance and this a priori standpoint has recently been strengthened by the work of Hild & Tasaki (1962) on tissue cultures of cerebellar neurones. By stimulating a visible dendrite they found impulses started 100 ,u from the soma to be capable of travelling and eliciting a soma spike. Most of our experiments were performed on rats, though specific points were checked with cat motoneurones. The feasibility of using rats has recently been demonstrated by Bradley & Somjen (1961). They observed that after-hyperpolarization as a sequence to the spike was smaller there