Temporal changes in activity during destruction of the thoracic ventral eclosion muscle of the tsetse fly

The spontaneous intracellular activity of the thoracic ventral longitudinal eclosion muscle (VLEM) of Glossina is described for the period from eclosion up to a short time before the final breakdown of recorded fibres. The VLEM comprises a single motor unit with no inhibitory input. The firing frequency of the motor unit declines over 5 h after eclosion and leg release. Over a period of inactivity lasting between 19-24 h in the sample fibres, there is no loss of resting membrane potential and occasional miniature potentials. The inactivity is ended by the sudden onset of biphasic potentials very different in form to the motor potentials and having a greatly reduced amplitude. These potentials fired at 6 Hz. lasted 2-4 h and ended with a rise in frequency to 25 Hz. No further activity is recorded and the fibres are observed to lose their striations and rigor. Experiments to characterize the ionic basis of activity in the VLEM have been done on spontaneous and evoked activity. Like other insect muscles, the VLEM has a major Ca2+ potential but unlike insect skeletal muscles, it also appears to have a TTX-sensitive component. This Na+ component is revealed by pretreating the system in Na(+)-free-choline saline, or by treatment with TEA in Ca(2+)-free saline. Neither EGTA nor cobalt abolish this potential. Addition of EGTA does not inhibit nerve evoked activity suggesting that the VLEM neuromuscular junction is in some way protected. The similarity of this muscle to insect visceral muscles is discussed.