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 excitatory postsynaptic potentials (EPSPs) in ankle extensor motoneurons after 6-wk (L(1)-L(2)) spinal cordotomy. The ratio of rheobase to input resistance was used to divide motoneurons into three groups approximating fast-fatigable (FF), fast fatigue-resistant (FR), and slow(S) motor units in barbiturate-anesthetized cats. Homonymous monosynaptic Ia EPSPs evoked by low-strength [1.2 times threshold (T)] electrical stimulation and heteronymous EPSPs evoked by 2T stimulation were compared between groups of motoneurons in unlesioned and chronic spinal preparations. The distribution of motor unit types of triceps surae and plantaris (PL) motoneurons according to the present classification scheme agrees well with that obtained elsewhere using mechanical typing. Chronic spinalization resulted in an increased proportion of type FF motoneurons in PL and type FR motoneurons in lateral gastrocnemius (LG) motoneurons. There was a numeric but insignificant increase in the proportion of fast medial gastrocnemius motor units. Membrane time constant (tau(m)) and estimated total cell capacitance were significantly reduced in FF and S motoneurons in chronic spinal preparations. FF motoneurons from chronic spinal animals also had a reduced afterhyperpolarization duration. Mean values of membrane electrical properties in FR motoneurons were unaltered after spinalization. Homonymous Ia EPSP changes 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 amplitudes of heteronymous Ia EPSPs in type FF and S units in the chronic spinal preparation were almost double those in unlesioned preparations. Homonymous EPSP 10-90% rise times decreased 25% in type FR motor units and 15% in type S motor units and were unchanged in type FF motor units. Homonymous EPSP halfwidth decreased in all three motoneuron groups. Normalization of EPSP rise time and half-width to tau(m) reduced the difference bem tween EPSP shape indexes in unlesioned and chronic spinal preparations in type FF and S motoneurons but less so in type FR motoneurons. Normalized EPSP shape indexes in some type FR units were shorter after chronic spinalization than any in unlesioned preparations. The increased amplitude and decreased rise time of Ia EPSPs in type FR motoneurons after spinalization occurred without changes in the electrical properties of type FR motor units. Evidence obtained using a compartmental model of the motoneuron suggests that average location of synaptic transmission in type FR and perhaps S motoneurons is nearer the soma in chronic spinal animals. The threshold voltage for eliciting an action potential was determined from intracellular current injection. In FR motoneurons, homonymous 1.2T EPSP amplitude reached 13% of threshold voltage in unlesioned preparations and 21% in chronic spinal preparations. This suggests that FR motoneurons would be more readily recruited by homonymous group Ia afferents in chronic spinal cats. Heteronymous EPSPs were also closer to threshold voltage levels in chronic spinal preparations. These results suggest that during simultaneous activation of primary muscle spindle afferents from all four ankle extensors muscles, the summation of heteronymous and homonymous group Ia EPSPs would result in increased recruitment and thus exaggerated reflexes in all three motor unit types. The present findings indicate that mechanisms producing changes in Ia EPSPs after chronic spinal injuries act differentially on ankle extensor motoneurons according both to motor unit type and to motoneuron species. The greatest increase in homonymous EPSP amplitude and decrease in EPSP rise time occurs in type FR and LG motoneurons. These changes in monosynaptic EPSPs could contribute to increased reflex motoneuron recruitment in chronic spinal preparations.