Differences in Transmitter Release at Different Synapses of the Frog Sartorius Muscle

Abstract

Microelectrode recording from Mg 2+ -blocked end-plates was used to analyse changes in quantum content (m) , probability of release (p) and available transmitter store (n) in frog sartorius muscle fibres during repetitive nerve stimulation at 10 Hz. Measurements were made on the large fibres from the inner surface of the muscle, and on the small fibres on the outer surface (Zefirov, A.L., et al., Bull. exp. Biol. Med., 1978, in press). In large fibres repetitive nerve stimulation caused first an increase, and then a sharp fall in m. In small fibres, the initial increase was smaller and the subsequent decrease was very gradual. In small fibres the maximum rate of transmitter release was about half that of large fibres. Evaluation of binomial release parameters (Bennett, M.R. and Florin, T., J. Physiol. (Lond.), 238, 93–107, 1974) showed that n was smaller in small fibres (range 5–60) than in large fibres (range 30–160). During repetitive stimulation p increased steadily in large fibres, whereas it showed little change in small fibres. It is suggested that these differences are related to the different length of synaptic contact in large and small fibres (Kuno, M., et al., J. Physiol. (Lond.), 213, 545–556, 1971).