Synapses upon motoneurons of locusts during retrograde degeneration

Abstract

Four interneurons of the ventral cord, the descending movement detectors (DMD) have symmetrical synapses upon the fast extensor tibiae (FETi) motoneurons on each side of the metathoracic ganglion. Each impulse in a DMD interneuron generates an excitatory post-synaptic potential (e.p.s.p.) of constant and similar amplitude in both FETi motoneurons of a normal locust. The symmetry provides inherent controls which makes this a convenient system to study the effect on inputs to a motoneuron caused by peripheral section of its axon. On the operated side the retrograde changes in the FETi motoneuron include, first an increased amplitude of the e.p.s.ps, then a brief period when they are variable, followed by a progressive reduction over a period of days. Other inputs to the FETi motoneurons from head, abdomen and tympanum also decline, but not at equal rates. Changes in e.p.s.p. amplitude are the opposite to those expected from simultaneous changes in the time constant. The observed changes in the e.p.s.ps are attributed to instability and then progressive loss of synapses upon the FETi motoneuron. The results show that the integrity of the motoneuron is essential for maintenance of its synaptic inputs.