The acetylcholine sensitivity in the vicinity of the neuromuscular junction of the frog

Abstract

For distancesr larger than 15 μm the ACh-response can be described by the diffusion law: the sensitivity depends onr −3; time to peak,T, and latency,L, depend onr 2 (r=distance junction-ACh-pipette). The diffusion coefficient for ACh has been determined to be 11.9 (S.E.±0.4)·10−6 cm2 sec−1 at 23°C. For a depolarization of 2 mV, the range of junctional ACh-sensitivity was normally 150–500 mV/nC with extreme values of 100 and 700 mV/nC. The time to peak was shorter than 6 msec. The range of maximal extrajunctional sensitivity was 0.1–45 mV/nC or, related to the respective junction, 0.1–12%, with low values at winter- and low to high values at summertime. The extension of ACh-sensitivity beyond the endplate (1% of junctional value) was 0 to 400 μm, being high at summertime. In some fibres a low ACh-sensitivity could be demonstrated over the whole length of the muscle. The extrajunctional sensitivity was highest at the entrance of the nerve fibre with 2 to 4 fold decrease within the endplate region. Extrajunctional ACh-receptors can be conveniently investigated on the surface of the muscle fibre antipodal to the neuromuscular junction. No extrajunctional “hot spots” or areas of high sensitivity neighbouring those of low sensitivity could be demonstrated.