The probability of quantal secretion at release sites of different length in toad (Bufo marinus) muscle

1. The evoked quantal secretion recorded with an extracellular microelectrode (me) at selected sites along motor terminal branches visualized with the fluorescent dye 3-3 diethyloxardicarbocyanine iodide (DiOC2(5)) was compared with the maximum length of the synaptic contact at these release sites reconstructed from serial sections examined with the electron microscope. In addition the relationship between the binomial probability of secretion at release sites (pe) and the length of the synaptic contact was determined in an extracellular calcium. [Ca2+]o, of 0.35 mM. 2. Three of the six terminal branches studied in this way showed a decline in synaptic contact length of release sites from near the point of nerve entry (proximal) to the end of the branch (distal). The remaining three branches showed an increase in synaptic contact length near their middle, and in each case this was associated with a Schwann cell nucleus: contact length then declined to the end of the branches. 3. Both me and pe increased linearly with an increase in the maximum length of the synaptic contact over a range from 0.4 to 4.0 microns. This occurred independently of how the synaptic contact length varied along the length of terminal branches. The value of pe increased by about 0.05 for each 1 micron increase in synaptic contact length in a [Ca2+]o of 0.35 mM. 4. The decrease in synaptic contact length along the proximal parts of terminal branches, in which this occurs, is mostly due to a decrease in the length of close opposition (less than 0.2 micron) between the nerve terminal membrane and the postsynaptic membrane: the decrease in more distal parts of branches is due to the progressive encroachment of Schwann cell processes between the presynaptic and postsynaptic membranes as well as a decrease in synaptic contact length.