Neuroanatomy of the mesothoracic ganglion of the cockroachPeriplaneta americana(L) I . The roots of the peripheral nerves

Abstract

Bodian silver-stained sections and Procion Yellow impregnation w ere used to examine the general neuroanatomy and details o f the nerve roots in the mesothoracic ganglion of adult males ofPeriplaneta americana(L .). General structure of the ganglion is summarized and the basic plan of its tracheation outlined. Fresh details are given of the seven paired longitudinal fibre tracts and ten transverse commissures, and three new oblique tracts of characteristic shape are described : the ring tract, G -tract and I-tract. Regions of the ventral association centre are briefly described. Groups of neuron cell bodies, which lie peripherally in the ganglion, are designated according to position. The fibre bundles that form the roots of each of the six paired peripheral nerves (nerves 2 -7 ) and the single median nerve (nerve 8) are numbered from dorsal to ventral and their courses and fibre com position described. In all, 30 roots are characterized on each side o0f the ganglion, containing over 150 efferent, presumed motor, fibres and over 2000 afferent, presumed sensory, fibres. Function and fibre diameter are not consistently related and almost all sizes of both motor and sensory fibres occur. Nerves 2 and 6 divide into distinct dorsal and ventral roots; the roots of nerves 4 and 8 are dorsal and of nerve 7 solely ventral; roots of nerves 3 and 5 form fairly continuous series from dorsal to ventral. The more dorsal nerve roots tend to be motor in function and the ventral roots sensory. Nearly all motor cell bodies are located ventrally or ventrolaterally and their processes run m ore or less dorsally to give off dendritic branches into dorsal or lateral neuropile before sending axons peripherally in the nerve trunks. Cell bodies of nerves 3 -6 , except for one in the midline, are ipsilateral, those of nerves 3 and 4 lying almost wholly anteriorly and of nerves 5 and 6 both anteriorly and posteriorly. Cell bodies of nerve 2 are contralateral and anterior except for one in the dorsal midline. Nerve 8 receives axons from posterior cell bodies of both sides. An apparently common inhibitorym otoneuron branches to nerves 3 -6 . The topology of some other motoneurons, chiefly of nerves 4 and 5, is outlined and the probable correspondence of some of them with previously identified metathoracic neurons is discussed. Sensory roots, predominantly of very small fibres, of nerves 2, 3 and 5 -7 enter the ventral association centre. Mainly coarser sensory fibres branch into mid-level neuropile or below , though some of those of nerves 2, 3 and 7 ascend more dorsally.