EFFECTS OF CHRONIC SPINALIZATION ON ANKLE EXTENSOR MOTONEURONS .3. COMPOSITE IA EPSPS IN MOTONEURONS SEPARATED INTO MOTOR UNIT TYPES

In this paper we continue an examination of changes in composite Ia ex citatory postsynaptic potentials (EPSPs) in ankle extensor motoneurons after 6-wk (L(1)-L(2)) spinal cordotomy. The ratio of rheobase to inp ut resistance was used to divide motoneurons into three groups approxi mating fast-fatigable (FF), fast fatigue-resistant (FR), and slow(S) m otor units in barbiturate-anesthetized cats. Homonymous monosynaptic I a EPSPs evoked by low-strength 1.2 times threshold (T) electrical st imulation and heteronymous EPSPs evoked by 2T stimulation were compare d between groups of motoneurons in unlesioned and chronic spinal prepa rations. The distribution of motor unit types of triceps surae and pla ntaris (PL) motoneurons according to the present classification scheme agrees well with that obtained elsewhere using mechanical typing. Chr onic spinalization resulted in an increased proportion of type FF moto neurons in PL and type FR motoneurons in lateral gastrocnemius (LG) mo toneurons. There was a numeric but insignificant increase in the propo rtion of fast medial gastrocnemius motor units. Membrane time constant (tau(m)) and estimated total cell capacitance were significantly redu ced in FF and S motoneurons in chronic spinal preparations. FF motoneu rons from chronic spinal animals also had a reduced afterhyperpolariza tion duration. Mean values of membrane electrical properties in FR mot oneurons were unaltered after spinalization. Homonymous Ia EPSP change s after chronic spinalization occurred preferentially in type FR and S motor units. Amplitudes increased 69% in type FR and 38% type S motor units but were unchanged in type FF units. Furthermore, the amplitude s of heteronymous Ia EPSPs in type FF and S units in the chronic spina l preparation were almost double those in unlesioned preparations. Hom onymous EPSP 10-90% rise times decreased 25% in type FR motor units an d 15% in type S motor units and were unchanged in type FF motor units. Homonymous EPSP halfwidth decreased in all three motoneuron groups. N ormalization of EPSP rise time and half-width to tau(m) reduced the di fference bem tween EPSP shape indexes in unlesioned and chronic spinal preparations in type FF and S motoneurons but less so in type FR moto neurons. Normalized EPSP shape indexes in some type FR units were shor ter after chronic spinalization than any in unlesioned preparations. T he increased amplitude and decreased rise time of Ia EPSPs in type FR motoneurons after spinalization occurred without changes in the electr ical properties of type FR motor units. Evidence obtained using a comp artmental model of the motoneuron suggests that average location of sy naptic transmission in type FR and perhaps S motoneurons is nearer the soma in chronic spinal animals. The threshold voltage for eliciting a n action potential was determined from intracellular current injection . In FR motoneurons, homonymous 1.2T EPSP amplitude reached 13% of thr eshold voltage in unlesioned preparations and 21% in chronic spinal pr eparations. This suggests that FR motoneurons would be more readily re cruited by homonymous group Ia afferents in chronic spinal cats. Heter onymous EPSPs were also closer to threshold voltage levels in chronic spinal preparations. These results suggest that during simultaneous ac tivation of primary muscle spindle afferents from all four ankle exten sors muscles, the summation of heteronymous and homonymous group Ia EP SPs would result in increased recruitment and thus exaggerated reflexe s in all three motor unit types. The present findings indicate that me chanisms producing changes in Ia EPSPs after chronic spinal injuries a ct differentially on ankle extensor motoneurons according both to moto r unit type and to motoneuron species. The greatest increase in homony mous EPSP amplitude and decrease in EPSP rise time occurs in type FR a nd LG motoneurons. These changes in monosynaptic EPSPs could contribut e to increased reflex motoneuron recruitment in chronic spinal prepara tions.