STRUCTURE AND INNERVATION OF LONGITUDINAL AND TRANSVERSE ABDOMINAL MUSCLES OF THE CRICKET, GRYLLUS-BIMACULATUS

Detailed morphological and physiological studies on the insect abdomin al muscles, including their innervation and neuromuscular transmission , are essential for understanding their important role in respiratory movements. There are both longitudinal and transverse muscles in the v entral abdominal segments of the cricket, Gryllus bimaculatus. Muscle 202 was selected as an example of a longitudinal muscle. This muscle i s, on average, 1.4 mm long, paired on both sides of the abdomen, and c onsists of 127 fibers whose mean maximum diameter is 32 mu m; the aver age sarcomere length is 8.1 mu m. It is innervated by two ipsilateral motoneurons in the second abdominal ganglion, the axons of which run i n the ipsilateral first nerve root of the third abdominal ganglion. Tw o motor axons run in parallel from the two cell bodies and innervate i n close proximity. Accordingly, large and small excitatory junctional potentials (EJPs) are recorded from the same fiber with slightly diffe rent thresholds when the first nerve root of the third abdominal gangl ion is stimulated. Muscle 203, which is a transverse muscle that exten ds across the fifth abdominal sternum and is located over the fourth a bdominal ganglion and muscle 202 on both sides, is, on average, 2.9 mm long and consists of 86 fibers with a maximum diameter of 33 mu m. Th e average sarcomere length is 7.9 mu m. The right or left half of the muscle is innervated mainly by a contralateral motoneuron in the third abdominal ganglion through the ipsilateral first nerve root of the th ird abdominal ganglion. Nerve branches of the first nerve root also re ach muscles 188 and 218. Muscle 203 is additionally innervated by the first nerve roots of abdominal ganglia 1, 2, and 4. These innervations were ascertained both electrophysiologically and histologically. Indi vidual muscle fibers of muscle 203 produced small EJPs in response to stimulation of the first nerve roots of abdominal ganglia 2, 3, and 4 and large EJPs in response to stimulation of the root from the first a bdominal ganglion. The large and small EJPs in muscle 203 have propert ies similar to those in muscle 202. (C) 1995 Wiley-Liss, Inc.