A comparative study of physiological and structural changes at the myoneural junction in two species of frog after transection of the motor nerve.

Structural and functional behaviour of motor end-plates after transection of the motor nerve has been studied in two species of frog: Rana esculenta and Rana temporaria. The physiological results show that in both species there is a transient cessation of spontaneous activity followed by a resumption of miniature end-plate potentials (min. e.p.p.s.) after denervation. The characteristics of these potentials (frequency, distribution of amplitudes, time-course) are similar in the two species. However, some differences have been observed: Firstly, the period of silence lasts for 2--4 days in the case of Rana temporaria whereas it is prolonged to about 15 days in Rana esculenta. Secondly, the resumption of min. e.p.p.s. is gradual and after the 10th day of denervation remains constant in Rana temporaria. It is inconstant independent of the period of denervation in Rana esculenta. The morphological results show that the Schwann cell is constantly in contact with the post-synaptic membrane after about 6 days of denervation in both species. It is suggested that either the Schwann cell is capable of functioning for a limited period of time in Rana esculenta or is activated to produce min. e.p.p.s. only in certain cases.