Evidence for the maintenance of motoneurone properties by muscle activity

1. Electrophysiological properties of soleus motoneurones in adult cats were examined with intracellular electrodes following alterations of activity of the soleus muscle induced by transection of the thoracic spinal cord or by conduction block of the muscle nerve with tetrodotoxin (TTX) cuffs. Attempts were also made to maintain muscle activity by daily stimulation of the maintain muscle activity by daily stimulation of the peripheral nerve. 2. Within 8 days after transection of the thoracic cord, soleus motoneurones showed a significant decrease in the duration of afterhyperpolarization following action potentials. This change in motoneurone properties induced by cord transection was prevented by daily stimulation of the sciatic nerve. 3. Soleus motoneurones showed a significant decrease in the duration of after-hyperpolarization within 8 days after conduction block of the soleus nerve with TTX. This change in montoneurone properties was prevented by daily stimulation of the nerve peripheral to the TTX cuff but not central to the cuff. 4. The soleus muscle showed a significant decrease in weight relative to body weight within 8 days after transection of the thoracic cord. This decrease in muscle weight following cord transection was prevented by daily stimulation of the sciatic nerve. 5. No fibrillation was detected in the soleus muscle 8 days after conduction block of the soleus nerve with TTX. The maximum twitch tension of the soleus muscle evoked by nerve stimulation showed no significant difference between the two sides treated and untreated with TTX. Fast axoplasmic transport measured with cholinesterase as a marker was not affected by TTX. Thus, there was no sign of functional although morphological abnormalities were found in some nerve fibres. 6. It is concluded that motoneurone properties in an adult depend partly upon some factors associated with activity of the innervated muscles and that such trophic signals are retrogradely carried by the motor axons.

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

1. 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 motoneurones were recorded after varying post-operative periods of up to 110 days. Similar observations were made 52--59 days after cord transection in adult cats. 2. 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. 3. Soleus motoneurones showed a normal post-natal increase in the duration of afterhyperpolarization (a.h.p.) up to a certain stage (61--71 days in age) following cord transection. However, the subsequent increase in the duration of a.h.p. of soleus motoneurones observed in normal kittens was lacking in cord-transected kittens. It is suggested that soleus motoneurones show two stages of differentiation in terms of the duration of a.h.p. 4. In adult cats, cord transection caused a decrease in the duration of a.h.p. of soleus motoneurones approximately to the value observed at the end of the first stage of differentiation in kittens. 5. The duration of a.h.p. of gastrocnemius motoneurones remained virtually unchanged follwoing cord transection in both kittens and adult cats. 6. The positive correlation between the duration of a.h.p. of soleus motoneurones and contraction time of the innervated muscle fibres normally observed in kittens and adult cats was absent following cord transection. 7. It was assumed that alteration s in contraction time of the muscle following cord transection are due to virtual elimination of motoneurone discharge and that the duration of a.h.p. reflects the discharge pattern of motoneurones under normal conditions. Based on these assumptions, a possible process for normal post-natal differentiation of motoneurone and muscle is proposed.