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1. Rat soleus and extensor digitorum longus (EDL) muscles were examined following complete blockade of sciatic nerve impulses with anaesthetics or diphtheria toxin for periods up to 14 days. 2. Muscles showed atrophy equivalent to that seen after similar periods of denervation. 3. Nerve blockade appeared to have little or no effect on neuromuscular transmission when tested by stimulation beyond the block. Normal spontaneous miniature end‐plate potentials were present. 4. Nerve impulse blockade caused the entire muscle membrane to become sensitive to iontophoretically applied acetylcholine. 5. The increase in sensitivity in soleus could be prevented by chronic nerve stimulation distal to the region of block. 6. Tenotomy, of 5‐12 days duration, which produced atrophy, had no effect on the sensitivity of soleus to acetylcholine. 7. Chronic direct stimulation of denervated soleus or EDL muscles could prevent the usual denervation supersensitivity, or cause it to decline towards normal once it had appeared. However, the sensitivity of the end‐plate region remained normal.

SUMMARY1. Post-tetanic potentiation (PTP) was studied at the neuromuscular junction of the frog. The magnitude and time course of PTP was dependent on the number of stimuli in the tetanus, rather than on the frequency or duration of the tetanus. At low temperature the maximum amplitude of PTP was unchanged, but the time course was prolonged.2. The magnitude and time course of PTP was accounted for quantitatively by estimated changes in the fraction of transmitter released per stimulus. 4. It is concluded that PTP is associated with an intracellular accumulation of Ca during the tetanus.

Signs of Schwann cell encroachment into the synaptic cleft were readily found and large areas of post-junctional membrane apposed only by Schwann cell were evident. 4. It is suggested that the mechanics of the process of synapse elimination in neonates is similar to that occurring during degeneration in the denervated adult. Transmission failure occurs abruptly at the supernumerary endings and they are disposed of by the Schwann cell.

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