Differences in transmitter release and number of nerve terminals per motoneuron between two frog muscles

Abstract

Recent studies suggest that the number of nerve terminals maintained per motoneuron and perhaps other motoneuron properties influence transmitter release. We have examined, in detail, the structure and function of motoneurons innervating two different twitch muscles, the sartorius and the ext. long. dig. IV (e.l.d.) in the adult frog using histochemical and electrophysiological techniques. These studies were undertaken with a view towards learning whether differences in the number of nerve terminals per motoneuron were correlated with differences in transmitter release between motoneurons innervating these two muscles. Moreover, these studies provide detailed, quantitative data that are a necessary prerequisite for carrying out additional studies to examine the possible influence of muscle on transmitter release properties. In low Ca2+-high Mg2+ Ringer's solution, mean quantal content (m) and miniature end-plate potential (MEPP) frequency were larger at sartorius than at e.l.d. end-plates. The mean length of nerve terminals was also found to be greater in sartorius than in e.l.d. muscles. When release was normalized for the difference in terminal length, m remained larger in the sartorius, whereas MEPP frequency in the two muscles was similar. Cell bodies of individual sartorius motoneurons were greater in cross-sectional area and maintained approximately 5.6-fold greater aggregate length of nerve terminals per motoneuron than did e.l.d. motoneurons. This greater aggregate length of nerve terminal contact with their respective target muscles is primarily a result of a greater number of nerve terminals supported by sartorius than by e.l.d. motoneurons. Total spontaneous (MEPP frequency) and evoked (m) quantal release were 5.5 and 11.4 x greater, respectively, from individual sartorius motoneurons than from e.l.d. motoneurons. The results of this study are discussed in terms of the relationship between transmitter release and the number of nerve terminals maintained per motoneuron.