Variation in quantal secretion at different release sites along developing and mature motor terminal branches

The secretion of a quantum by groups of release sites (me) along the length of nerve terminal branches has been studied with extracellular electrodes at mature and juvenile toad neuromuscular junctions. The position of the recording sites with respect to the terminal branches was determined following zinc-iodide staining of terminals. me declined with distance from the last myelin segment along individual terminal branches: in a [Ca]o of 0.35 mM, me declined by 80% over a distance of 60 micron along mature terminal branches. Simultaneous recording of secretion from adjacent groups of release sites with two extracellular electrodes confirmed that secretion of a quantum at release sites declines with the distance of the sites from the last myelin segment. The secretion of a quantum by groups of release sites along juvenile terminal branches also declines with distance from the last myelin segment: me is always largest for the first release sites to be laid down. The facilitated secretion of quanta at sites with low me is larger than those with high me; as a consequence, all sites secrete similar numbers of quanta at the end of a short high-frequency train.