The effect of a high concentration of the antibiotic colistin sulfate on neuromuscular transmission was examined by recording intracellularly from rat diaphragm blocked with tubocurarine chloride. The presynaptic effect of colistin was different from that of neomycin sulfate, which reduces the probability of acetylcholine release: Colistin reduced that the quantum content of the initial and the last 20 end-plate potentials of a train of 40 stimuli without altering the probability of acetylcholine release. The quantum size was reduced, while the frequency of miniature end-plate potentials was unchanged.
Abstract B uchthal , F. and L. E ngbæek , Refractory Period and conduclion velocity of the striated muscle fibre . Acta physiol. scand. 1963. 59 . 199–220. —Refractory period and conduction velocity of transmembrane potentials were determined in single frog muscle fibres by intracellular stimulation and recording at 14, 20 and 25° C. At the end of the absolutely refractory period the latency of the potential evoked by the second stimulus was substantially increased mainly due to a reduction in conduction velocity, delayed firing causing at most 10 per cent of the total delay. The absolutely refractory period terminated at the onset of the negative after potential, recovery in excitability, amplitude and conduction velocity in the relatively refractory period occurred within a time interval in which the membrane potential did not change more than about 3 mV. At 2–3 times the absolutely refractory period excitability and conduction velocity had a supernormal phase. At the absolutely refractory period the level of depolarization required to initiate a propagated response had increased from 40 to more than 60 mV (recorded 0.25–0.6 mm from the stimulating electrode). A local response appeared at a time interval of 70–80 per cent of the absolutely refractory period of the propagated response.
Summary. The electrical potential gradients through the frog skin bathed in sulphate Ringer's were studied with microelectrodes inserted through the skin from its outer surface. The total skin potentials ranged between 73 and 145 mV, the inside positive with reference to ground. In most instances the total skin potential was reached in two, rarely three, distinct potential jumps. Comparison with measurements of the frog skin thickness indicated that the site of the first jump is in the epidermis while the second potential jump may correspond to the junction between epidermis and corium.
