Disparity of motoneurone and muscle differentiation following spinal transection in the kitten.

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Abstract
The spinal cord of kittens, 3-5 days of age, was transected at the lower thoracic level. Isometric contractions of the medial gastrocnemius and soleus muscles as well as intracellular potentials of their motoneurons were recorded after varying postoperative periods of up to 110 days. Similar observations were made 52-59 days after cord transection in adult cats. In cord-transected kittens, contraction time of the gastrocnemius muscle showed normal development, whereas the soleus muscle failed to maintain slow contraction. In adult cats, cord transection increased the speed of contraction in the soleus muscle without significant changes in contraction times of the gastrocnemius muscle. Soleus motoneurons showed a normal postnatal increase in the duration of afterhyperpolarization (a.h.p.) up to a cerain stage (61-71 days in age) following cord transection. The subsequent increase in the duration of a.h.p. of soleus motoneurons observed in normal kittens was lacking in cord-transected kittens. Soleus motoneurons possibly show 2 stages of differentiation in terms of the duration of a.h.p. In adult cats, cord transection caused a decrease in the duration of a.h.p. of soleus motoneurons approximately to the value observed at the end of the 1st stage of differentiation in kittens. The duration of a.h.p. of gastrocnemius motoneurons remained virtually unchanged following cord transection in both kittens and adult cats. The positive correlation between the duration of a.h.p. of soleus motoneurons and contraction time of the innervated muscle fibers normally observed in kittens and adult cats was absent following cord transection. Alterations in contraction time of the muscle following cord transection are probably due to virtual elimination of motoneuron discharge and the duration of a.h.p. reflects the discharge pattern of motoneurons under normal conditions. Based on these assumptions, a possible process for normal postnatal differentiation of motoneuron and muscle is proposed.