EFFECTS OF CHRONIC SPINALIZATION ON ANKLE EXTENSOR MOTONEURONS .1. COMPOSITE MONOSYNAPTIC IA EPSPS IN 4 MOTONEURON POOLS

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 lo w-strength electrical stimulation of peripheral nerves were measured i n barbiturate-anesthetized cats and compared between unlesioned and ch ronic 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 s pinal animals compared with unlesioned controls. There was also an inc reased incidence of large-amplitude, short-rise time EPSPs. When the s ame data were separated according to individual motoneuron species, ho monymous EPSP amplitudes in MG motoneurons were found to be unchanged. EPSPs recorded in LG motoneurons and evoked by stimulation of the com bined LG and SOL nerve were increased by 46%. Mean EPSP amplitudes rec orded in both SOL and PL motoneurons were larger after spinalization b ut statistical significance was only achieved when values from SOL and PL were combined to produce a larger sample size. In LG motoneurons f rom chronic spinal animals, all EPSPs evoked by 1.2T stimulation of th e LGS nerve were greater than or equal to 0.5 mV in amplitude. In unle sioned preparations, one fourth of the LG cells had EPSPs that were le ss than or equal to 0.2 mV. The mean amplitude of heteronymous EPSPs e voked by 2T stimulation of LGS and MG nerves and recorded in MG and LG motoneurons, respectively, doubled in size after chronic spinalizatio n. Because homonymous EPSP amplitudes were unchanged in MG motoneurons , synaptic mechanisms and not passive membrane properties are likely r esponsible 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 E PSP 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 decr eased 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 p reparations in all motoneuron species. For EPSPs evoked at 1.2T the ov erall mean decrease was 31%, with those recorded in LG motoneurons hav ing 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 unles ioned preparations. Despite an unchanged mean, the shortest normalized rise time and half-width values occurred in the chronic spinal prepar ation. The present results in anesthetized chronic spinal adult cats r eveal an overall increase in homonymous and heteronymous Ia EPSP ampli tude in ankle extensor motoneurons. Larger heteronymous and homonymous Ia EPSPs produced during movement-evoked stretch of ankle extensors s hould contribute to increased ankle extensor stretch reflexes observed in the chronic spinal cat.