Soleus Motor Units in Chronic Spinal Transected Cats: Physiological and Morphological Alterations

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1. Intracellular recording and stimulation were used to study electrical properties of soleus motoneurons and isometric contractile properties of their muscle units. In addition, some muscle units were depleted of glycogen for purposes of studying their morphology. These data were compared for two experimental groups of barbiturate-anesthetized adult cats: those with spinal cords intact (normal) and others with spinal cords transected (ST) 4 mo before study.

2. The greatest change in soleus motoneurons after ST was a decrease in afterhyperpolarization (AHP) duration. Axonal conduction velocity (CV) increased slightly, yet consistently. These data confirm previous findings.

3. The voltage threshold of motoneurons, estimated from the product of rheobase current and input resistance, increased from the normal value of 7.5 mV to 10.1. This resulted from a significant increase in rheobase current after ST, because input resistance was not altered significantly. These data suggest that motoneuron excitability decreased after ST and therefore cannot account for hyperreflexia in chronic ST cats.

4. Measurement of soleus muscle unit isometric contractile properties generally confirmed earlier reports of decreases in twitch time course [both contraction time (CT) and half-relaxation time] and in twitch (Ptw) and maximum tetanic (Pmax) tensions after chronic ST. The ratio Ptw/Pmax increased as a result of a greater decline in tetanic than in twitch tension. No significant change was observed in the fatigue index.

5. Direct measures of muscle unit morphology showed that the specific tension for three ST units fell below the range of four normal units. This was expected given that the innervation ratio was not altered and that the decrease in muscle fiber area after ST was proportionately smaller than the decrease in Pmax.

6. Two lines of evidence showed that motoneuron and muscle unit properties were coordinated before and after ST. First, the normal correlation between AHP duration and CT was nearly as strong after as before ST, and the slope of this relation was not significantly different for normal and ST groups. Second, multivariate statistics (canonical analysis) showed that a combination of all motoneuron electrical properties listed above was strongly correlated with a combination of muscle unit twitch time, Pmax, and Ptw/Pmax for normal and ST groups. In contrast with the conclusions of Gallego et al. (26), we suggest from these data that SOL motor-unit properties are coordinated after ST.

7. Unlike most motor units in normal soleus, those in ST cats could be classified into two groups based on 1) position in the population distribution of percentage of maximal tension at a stimulation rate of 10 pulses per second (pps), 2) presence or absence of sag, and 3) statistical analysis that clustered units using a combination of CT, Ptw/Pmax, and CV. The ability to classify motor units consistently into the same groups by more than one set of criteria was taken to mean that there are two qualitatively unique populations of motor units in the soleus of ST cats.

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