ISSN:
1432-1351
Source:
Springer Online Journal Archives 1860-2000
Topics:
Biology
,
Medicine
Notes:
Summary Neuromuscular transmission at temperatures between 8 °c and 24 °c was investigated in a leg muscle of the shore crab,Pachygrapsus crassipes, acclimated to 12 °c, to determine whether diversity of synaptic performance at different endings of the same motor axon could be maintained over a wide temperature range. Changes in amplitude of the excitatory junction potential (EJP), muscle membrane input resistance, and twin pulse facilitation over a range of interpulse intervals were measured. In addition, extracellular records of synaptic currents were made at individual synapses. In most fibers, EJP amplitude decreased with increasing temperature; this was correlated with lower input resistance of the responding muscle fiber. Time constant of decay of the EJP usually decreased with increased temperature, but in some fibers it was maximal at around 16 °c, probably because large EJP's activated additional membrane conductance channels at temperatures below 16 °c. The time constant of decay of the extracellular synaptic current decreased with temperature, but average amplitude was not strongly temperature-dependent. Thus, changes in amplitude of the EJP with temperature were strongly influenced by postsynaptic changes. Facilitation of all EJPs increased with temperature. The more significant changes were in the second component of short-term facilitation. Synapses producing large EJP's showed depression at short interpulse intervals at the lower temperatures. As temperature was increased, the depression was replaced by facilitation. Synapses producing small EJP's showed increase in total facilitation with increasing temperature. The time constants of decay of facilitation for these synapses were not strongly temperature sensitive. These results differ markedly from those at vertebrate neuromuscular synapses. A test of the hypothesis that changes in facilitation or early depression might be linked to changes in duration of the presynaptic action potential was made by broadening the terminal potential with 3-aminopyridine at a warm temperature. Net facilitation was reduced slightly at short interpulse intervals, but the second phase of facilitation was little affected. Thus, additional mechanisms besides changes in spike width are involved, and the two phases of short-term facilitation are governed by different processes. In functional terms, the neuromuscular system must operate at higher impulse frequencies as the temperature is increased to achieve a given level of muscle membrane depolarization. Differentiation of synaptic properties of different axonal terminals becomes less pronounced as the temperature is increased.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00612600
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