Transmitter secretion in neuro-muscular junctions of different length and RAMIFI-cation

Nerve terminals with different length of their branches and with different types of ramification have been studied in the experiments performed on frog cutaneous-pectoralis muscle using electrophysiological and morphological techniques. It has been found that the spatial distribution of the nerve impulse-evoked transmitter secretion along each terminal branch is complex: The secretion increases distalward along the proximal by nearly a third of the terminal length and decreases linearly along its more distal portions. An increase in terminal length is followed by distalward shift of the secretion peak, by more gradual decay in the secretion if plotted along distal portions of the terminal, and by a decreased contribution of the secretion from distal portions to total secretion from the terminal. The critical terminal length at which total secretion from the terminal reached its maximum has been estimated. The distalward decrease of a secretion along the secondary terminal branches is similar to that found in the distal portion of the main terminal branch next to the ramification knot. An increase in terminal length and in the number of branchings is followed by decreased conduction velocity and transmitter release synchronism. The mechanisms determining limitations of the terminal length and the appearance of secondary terminal branches are discussed.Neirofiziologiya/Neurophysiology, Vol. 25, No. 3, pp. 170–175, May–June, 1993.