The Re-formation of the Neuromuscular Junction in Frog after Short and Long Durations of Denervation

Abstract

The mechanism of re-innervation depends on the period of denervation. After 2 1/2 months, when post-synaptic folds are intact but for the furrows, the reinner vation takes place at the former synaptic sites. The Schwann cell which is present in contact with the post-synaptic membrane till that time is withdrawn at the moment of reinnervation and the terminal bouton establishes contact with it. After 4 months of denervation, the post-synaptic sites are not always intact; at places there is sequestration and hence flattening of the folds. Thus, the reinnervation takes place, firstly, at old synaptic gutters where the synaptic sites are intact and secondly, by the formation of new neuromuscular junctions. The latter is marked by the specialisation of the parts of muscular membrane before the nerve is in contact with it. The process of reinnervation is gradual after short durations and is rapid after long durations. Similarly, the reappearance of the electric activity is gradual after short and abrupt after long durations. The miniature end-plate potentials (min. e.p.p.s.) in recently innervated muscle in both cases are characterised by their slow time course but normal in their frequency and distribution of the amplitudes. In case of some fibres, the min. e.p.p.s. had Gaussian as well as skewed amplitude distribution. This could be due to the unequal receptivity of the ACh or to the liberation of cholinesterases in variable amounts. The newly formed end-plates are capable of functionning before acquiring their normal morphological characters.