Correlation between nerve terminal size and transmitter release at the neuromuscular junction of the frog

Abstract

SUMMARY1. End-plate potentials were recorded intracellularly at the frog neuromuscular junction bathed in a solution containing a low concentration of calcium and a high concentration of magnesium.2. The muscle was subsequently subjected to 'cholinesterase staining', and the area of the individual end-plates, studied with intracellular electrodes, was measured.3. A positive correlation was found between the end-plate area and the diameter of muscle fibres.4. The mean quantum content (m) showed a positive correlation with the size of end-plates.5. The frequency of spontaneous miniature end-plate potentials was positively correlated with m as well as with end-plate area.6. It is concluded that the amount of transmitter released following nerve stimulation is related to the size of nerve endings.

The relationship between motor endplate size and muscle fiber diameter in different muscle groups of the rat.

“…This result confirmed previous reports (HARRIS, 1954;KUNO et al, 1971), based on histochemical methods. This Vol.…”

“…Assuming that the efficacy of neuromuscular transmission is independent of fiber diameter, endplate size should be proportional to d3'2, theoretically. But the results from the experiments of KUNO et al (1971) and ours indicate that there is a linear correlation between them. This means that a largediameter fiber has lower efficacy of transmission than a small-diameter one.…”

“…As the input resistance of a muscle membrane is proportional to d'2 (d: fiber diameter) according to KATZ and THESLEFF (1957), large-diameter fibers would require large amounts of transmitter release following nerve stimulation to generate an epp large enough for maintenance of the efficacy of neuromuscular transmission, in comparison with small-diameter fibers. This situation was verified by KUNO et al (1971) revealing that there are positive correlations in the mean quantum content, endplate size and fiber diameter in frog muscles. Assuming that the efficacy of neuromuscular transmission is independent of fiber diameter, endplate size should be proportional to d3'2, theoretically.…”

“…It has been reported that there is a linear correlation between muscle fiber diameter and endplate size (HARRIS, 1954;KUNO et al, 1971). This is of physiological significance, since a large-diameter fiber would require a large area of the endplate membrane to generate an endplate potential (epp) large enough to maintain the efficacy of neuromuscular transmission, so that the negative correlation between input resistance of membrane and fiber diameter can be overcome (KATz and THESLEFF, 1957).…”

“…Photomicrographs of stained endplates were enlarged on photographic print paper, cut-out along the contour of the endplate and weighed. A microscopic calibration scale was similarly photographed and enlarged to the same magnification on photographic print paper and a rectangular cut-out was quantified in terms of area by weighing (KuNo et al, 1971). Because the majority of endplates revealed by AChE staining or autoradiography appeared "on edge" rather than "en face," we measured the longest diameter of the endplate instead of the area, as the index of endplate size.…”