The importance of the motor nerve for the development of chemosensitivity at the neuromuscular junction

Effects of nerve activity on the postsynaptic membrane of skeletal muscle

The effect of nerve activity on the sensitivity of the neuromuscular junction to acetylcholine was studied. After 1 h of nerve stimulation the flexor hallucis longus muscles of cats become more sensitive to the blocking effect of suxamethonium than before stimulation. Similar results were found using rat soleus muscles. The increased sensitivity to suxamethonium was still observed in muscles from rats which had been curarized during the period of stimulation. Intracellular recordings from end plates of rat soleus muscles revealed an initial transient increase in miniature end-plate potential (MEPP) frequency after stimulation and an increase in the amplitude of MEPPs which persisted for as long as 4 h. Moreover the depolarising responses to acetylcholine and suxamethonium added to the bath were always greater in stimulated muscles. It was therefore concluded that activity of the motor nerve increases the sensitivity of the end plates to transmitter.

Effects of denervation and reinnervation on the responses of kitten muscles to acetylcholine and suxamethonium

1. The development of the high sensitivity of the adult neuromuscular junction to the blocking and depolarizing action of suxamethonium was studied in kittens of different ages.2. It was found that the sensitivity to the depolarizing effects of suxamethonium increases simultaneously with that to the blocking effects of the drug during the 3rd-6th week of life. The increase of sensitivity to the blocking effects of suxamethonium during development can therefore be accounted for by the depolarizing effect of the drug.3. When the nerve is crushed during the first week of the animal's life and then left to regenerate, the development of the high sensitivity of the neuromuscular junction to suxamethonium and acetylcholine is impaired. Thus during development the nerve is essential for the induction of the high sensitivity of the neuromuscular junction to acetylcholine as well as to other depolarizing drugs.4. The blocking and depolarizing effects of suxamethonium on reinnervated muscles of cats at different ages were compared. The depolarizing action of suxamethonium on reinnervated end-plates is always smaller, but its blocking effect is not. In young animals the blocking effect of suxamethonium on reinnervated muscles is smaller, whereas reinnervated muscles of adult animals are extremely sensitive to the blocking effects of the drug.5. 24-36 hr after denervation cat muscles were more sensitive to the blocking effects of suxamethonium but the depolarizing effect of the drug on the end-plate region was smaller. It thus appears that the nerve not only induces but also maintains the high sensitivity of the end-plate region to depolarizing drugs.