THE CONTRIBUTION OF POSTSYNAPTIC FOLDS TO THE SAFETY FACTOR FOR NEUROMUSCULAR-TRANSMISSION IN RAT FAST-TWITCH AND SLOW-TWITCH MUSCLES

1. At the rat neuromuscular junction, the postsynaptic folds and the v oltage-gated sodium channels (VGSCs) within them are thought to amplif y the effects of postsynaptic currents. In this study, the contributio n of this effect to the safety factor for neuromuscular transmission, the ratio of the normal quantal content to the number of quanta requir ed to reach threshold, has been estimated. 2. Normal quantal content w as determined in isolated nerve-muscle preparations of rat soleus and extensor digitorum longus (EDL) muscles in which muscle action potenti als were blocked by mu-conotoxin. The quantal content estimated from v oltage recordings was 61.8 and 79.4 in soleus and EDL, respectively, a nd from charge measurements derived from current recordings was 46.3 ( soleus) and 65.1 (EDL). 3. The threshold for action potential generati on in response to nerve stimulation was determined from endplate poten tials (EPPs) and endplate currents (EPCs) in preparations partially bl ocked with d-tubocurarine. The number of quanta required to reach thre shold was estimated from voltage recordings to be 19.7 (soleus) and 23 .2 (EDL) and from charge measurements derived from current recordings to be 13.3 (soleus) and 13.0 (EDL). 4. When intracellular electrodes w ere used to inject current into the muscle fibre, the total charge req uired to reach threshold was approximately twice that of the nerve-evo ked threshold (EPC). 5. The safety factor for nerve-evoked responses a t the junction was 3.5 (soleus) and 5.0 (EDL). In the extrajunctional region the safety factor estimated from injected currents was 1.7 (sol eus) and 2.5 (EDL). 6. It is concluded that the effect of the postsyna ptic folds and the VGSCs within them is to double the safety factor. A t normal frequencies of nerve impulse activity in vivo, this effect is likely to be crucial far ensuring effective neuromuscular transmissio n.