THE MULTISEGMENTAL MOTOR SUPPLY TO TRANSVERSE MUSCLES DIFFERS IN A CRICKET AND A BUSHCRICKET

Abstract

Most abdominal sternites of the cricket Gryllus bimaculatus and the bushcricket Decticus albifrons are bridged by a transverse muscle (TM) which supports expiratory movements. In the cricket, ventilatory contractions are controlled both within each segment, by a bilateral pair of excitatory motoneurones in the abdominal ganglion supplying the left and right halves of the TM independently, and intersegmentally, by peripheral collaterals of homologous motoneurones from adjacent segments. The axons of these motoneurones run in the ipsilateral paramedian nerve. This unique divergence of excitatory motoneurones to different muscles also results in massive convergence of excitatory inputs from different ganglia, especially on the TMs of the middle abdominal segments. TM contraction rates are increased by this intersegmentally divergent and convergent motor supply, especially in the middle abdominal segments. In bushcrickets, each transverse muscle in segments 3–7 is innervated bilaterally by four pairs of neurones: (i) two pairs of contralateral excitatory motoneurones with axons that diverge, supplying two adacent muscles; (ii) one pair of contralateral excitatory neurones found in the second anterior ganglion and (iii) a pair of median inhibitory neurones in the segmental ganglion. Transverse muscles 2 and 8 receive reduced innervation. The excitatory motoneurones generate slow excitatory postsynaptic potentials (EPSPs), which must sum to cause muscle contractions. During ventilation, contralateral paired transverse motoneurones fire at similar frequencies, thus sychronizing the contractions of the left and right halves of the muscle so that the whole muscle acts as a single unit.