Abstract
A unitary reflex spike was recorded from the central cut end of ventral root filaments in decerebrated cats.
The lateral and medial gastrocnemius nerves were stimulated by a single shock to each of them. If the interval between the two shocks was within some 30msec and both were kept subthreshold with respect to the reflex discharge, the second of them was found to elicit a reflex spike. The reflex spike was undoubtedly produced by temporal and spatial summation of afferent impulses ascending through the two branches of the tibial nerve. Relation between shock intervals and response times was obtained. The response times were delayed at short intervals, and it was possible to draw four straight lines through each set of points in the graph of this relationship. Analysis suggests that the second postsynaptic potential was summed during the rising phase of the first postsynaptic potential.
In order to restrict the experiment to temporal summation alone, a unitary reflex spike was evoked by double shocks to the same muscular afferents. Although the intensity of each shock was subthreshold for reflex discharge, a reflex spike occurred after the second shock. The relation between the response time and the shock interval was obtained. As in the previous case of both temporal and spatial summation, it was again possible to draw four straight lines in the graph of this relationship. The slope of one line at short intervals serves to explain the relation between the rising rate of the postsynaptic potential and the critical threshold level. From the results it follows that the summed postsynaptic potential, as the inter-shock interval is prolonged, takes progressively longer time to reach the critical level for motoneurone firing during the production of postsynaptic potential. Analysis suggests that this delay is due to rise of threshold owing to accommodation of the motoneurone. The slope constant of relation between inter-shock interval and response time in tonic motoneurones differs from that in phasic motoneurons in a manner which suggests that accommodation is greater in the latter.
Differences between responses in tonic and phasic motoneurones to quick muscle stretch are attributed in part to differences in synaptic transmission.
