Selective synaptic changes following spinal motoneuron axotomy.

Fibers descending in the lateral columns (LC) of the frog spinal cord form synapses more proximal to the soma than those derived from dorsal root (DR) fibers. Following ventral root transection (motoneuron axotomy), the risetime of the LC-EPSP increased while that of the DR-EPSP decreased such that the risetime of the LC-EPSP was now longer than that of the DR-EPSP. Focal-potential analysis showed that the location of LC synapses remained in the region of the motor nucleus while DR synapses were located closer to the motor nucleus after axotomy. Proximal synapses thus persist after axotomy, but they undergo physiological changes that alter the LC-EPSP time course. In addition membrane properties of axotomized motoneurons were investigated. Input resistance, rheobasic current, and threshold depolarization all increased. Resting membrane potentials and afterhyperpolarizations were unchanged. Because axotomy had opposite effects on LC- and DR-EPSPs, changes in membrane properties cannot account for changes in EPSPs. Other possible mechanisms are discussed.