Excitatory postsynaptic potentials (EPSPs) generated in soleus motoneurons by single homonymous Ia-fibers were measured using intracellular recording and the spike-triggered averaging technique. Two groups of barbiturate-anesthetized adult cats were studied: one with the spinal cord intact and the other with the spinal cord severed as thoracic segment 13 (T13) several hours prior to recording. In cord-transected cats, single homonymous Ia-fibers produced EPSPs in soleus motoneurons that were, on average, larger and faster rising relative to normal, as they are for those produced in medial gastrocnemius (MG) motoneurons. Specifically, mean EPSP amplitude and rise time were, respectively, 261 ± 22 μV and 0.65 ± 0.05 ms for the transected group vs. 160 ± 21 μV and 0.96 ± 0.08 ms for the intact group. The group means for each parameter were significantly different (P < 0.005). The group difference in EPSP amplitude was largely due to a decrease in number of small EPSPs in the transected group (11% < 100 μV compared with the normal 41%) and not due to the occurrence of unusually large ones. Ratios of the largest to smallest amplitude EPSPs produced in the same motoneuron were similarly distributed for intact and transected groups, implying that the effect of transection on EPSP size was uniform across different Ia-fiber synapses made with the same motoneuron. Mean EPSP amplitude for each transected cat (n = 5) was larger than normal, but in some cases the increase took > 10 h to express itself. The normal tendency for EPSP rise time to decline on average with amplitude was absent in the transected group, wherein rise time was reduced to similar average values in all amplitude categories. This suggests that the decrease in rise time occurred independently of the increase in amplitude. In contrast, EPSP half-width, which tended toward lower than normal values [5.63 ± 0.36 (SE) ms vs. 6.51 ± 0.44 ms; P > 0.10], decreased in proportion with rise time as evidenced by the preservation of the normal relation between those parameters in transected cats. Normalizing EPSPs by motoneuron time constant (τ) reduced the group differences in rise time and half-width, suggesting that a fall in τ contributes to the abbreviation of EPSP time course. The condition of the spinal cord best accounted for differences in synaptic strength between groups. Samples from the intact and transected groups were not significantly different in terms of motoneuron action potential amplitude or Ia-fiber mean firing rate. Group differences found for motoneuron and Ia-fiber conduction velocity and for the proximity of motoneuron recording site to Ia-fiber dorsal column entry zone could not account for the difference in EPSP size. Moreover, there was no indication that synaptic strength varied systematically with these factors after transection. This study confirms earlier but recently challenged findings that synaptic transmission at single Ia-fiber-motoneuron connections can be modified by transection of the spinal cord. The responses found here for EPSPs generated in soleus motoneurons were, however, distinct from those in MG and semitendinosus motoneurons in ways that give a more complete assessment of the effects of transection on transmission at Ia-fiber synapses.
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