Enhancement in Ia-motoneuron synaptic transmission caudal to chronic spinal cord transection

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Abstract
The spike-trigerred averaging technique was used to record Ia-evoked individual EPSP(excitatory post-synaptic potentials)s in triceps surae motoneurons in cats made spinal at L(lumbar)5 or T(thoracic)13 from 12 to 310 days before the experiment. These results were compared to those obtained using the same techniques in intact preparations. Following L5 transection, an increase in EPSP amplitude was observed for a period of 4 mo., after which it returned to normal. This increase was marked by a selective exaggeration of EPSP with short rise times. No change in the shape-index curve (half-width vs rise time) was observed. From this somatic EPSP are selectively enlarged and the distribution of Ia terminals on the motoneuron is similar to that in intact preparations. Following chronic T13 transection, EPSP were similar to those in intact preparations. Projection frequency increased to 100% following L5 transection and returned to approximately normal (80%) after several months. Increase in projection frequency was also seen following T13 transection in the same preparations for which the amplitudes were normal. Changes in EPSP amplitude and projection frequency are independent. The interpretation of these results is complicated by the finding that amplitude and projection frequency increase following acute transection at L5 or T13. Increases in EPSP amplitude in chronic spinal animal require denervation of motoneurons (which would be more extensive in triceps surae after L5 than T13 transection) and may be the result of collateral sprouting or denervation supersensitivity. Other mechanisms undoubtedly also contribute to these changes. Projection frequency increases are independent of the extent of motoneuron denervation since they are observed regardless of the level of transection.