We examined the effects of 6-wk chronic spinalization at the L(1)-L(2) level on composite monosynaptic Ia excitatory postsynaptic potentials (EPSPs) recorded in medial gastrocnemius (MG), lateral gastrocnemius (LG), soleus (SOL), and plantaris (PL) moto-neurons. Amplitudes, rise times, and half-widths of composite monosynaptic Ia EPSPs evoked by low-strength electrical stimulation of peripheral nerves were measured in barbiturate-anesthetized cats and compared between unlesioned and chronic spinal preparations. The mean amplitude of homonymous composite Ia EPSPs evoked by 1.2 times threshold(1.2T) stimulation and recorded in all four ankle extensor motoneuron pools increased 26% in chronic spinal animals compared with unlesioned controls. There was also an increased incidence of large-amplitude, short-rise time EPSPs. When the same data were separated according to individual motoneuron species, homonymous EPSP amplitudes in MG motoneurons were found to be unchanged. EPSPs recorded in LG motoneurons and evoked by stimulation of the combined LG and SOL nerve were increased by 46%. Mean EPSP amplitudes recorded in both SOL and PL motoneurons were larger after spinalization but statistical significance was only achieved when values from SOL and PL were combined to produce a larger sample size. In LG motoneurons from chronic spinal animals, all EPSPs evoked by 1.2T stimulation of the LGS nerve were greater than or equal to 0.5 mV in amplitude. In unlesioned preparations, one fourth of the LG cells had EPSPs that were less than or equal to 0.2 mV. The mean amplitude of heteronymous EPSPs evoked by 2T stimulation of LGS and MG nerves and recorded in MG and LG motoneurons, respectively, doubled in size after chronic spinalization. Because homonymous EPSP amplitudes were unchanged in MG motoneurons, synaptic mechanisms and not passive membrane properties are likely responsible for increased heteronymous EPSP amplitudes in MG. The mean 10-90% rise time of homonymous composite Ia EPSPs in pooled data from all motoneurons decreased 21% in 6-wk chronic spinal animals. Unlike EPSP amplitude, significant rise time decreases were found in all four motoneuron pools. Compared with the other motoneuron species, the mean homonymous rise time recorded in MG motoneurons was shortest and decreased the least in chronic spinal animals. Rise times of heteronymous Ia EPSPs in MG and LG motoneurons also decreased. The maximum rate of rise of homonymous EPSPs increased in all four motoneuron species. The mean half-widths of Ia composite EPSPs decreased in 6-wk spinalized preparations in all motoneuron species. For EPSPs evoked at 1.2T the overall mean decrease was 31%, with those recorded in LG motoneurons having the largest (46%) and those in MG the smallest (23%) mean change. Homonymous EPSP rise time and half-width were divided by membrane time constants measured in the same cells. Mean values of these normalized rise times and half-widths were identical in chronic spinal and unlesioned preparations. Despite an unchanged mean, the shortest normalized rise time and half-width values occurred in the chronic spinal preparation. The present results in anesthetized chronic spinal adult cats reveal an overall increase in homonymous and heteronymous Ia EPSP amplitude in ankle extensor motoneurons. Larger heteronymous and homonymous Ia EPSPs produced during movement-evoked stretch of ankle extensors should contribute to increased ankle extensor stretch reflexes observed in the chronic spinal cat.
