Neuroanatomy of the mesothoracic ganglion of the cockroachperiplaneta americana(L.) II. Median neuron cell body groups

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Abstract
The groups of neuron cell bodies in the midline of the mesothoracic ganglion of adult malePeriplaneta americana(L.) were examined using Bodian silver-stained paraffin sections and toluidine blue-stained wholemounts to determine their general architecture, and axonal filling with Procion yellow or cobalt to study pathways of their neuron processes. Filling tracheae with trypan blue provided additional information on tracheation of the ganglion. Seven groups of somata were named according to position: the anterior m edian (AM), mid-dorsal median (MDM ), posterior dorsal median (PDM ), posterior median (PM ), anterior ventral median (AVM ), mid-ventral median (MVM ) and posterior ventral median (PVM ) groups. Each group is characterized by general appearance and position, numbers and sizes of constituent somata, and numbers and pathways of fibre bundles. Groups consist largely of interneurons, with a few efferent, presumed motor, neuron somata present in some. Different functional types of neurons (that is, motoneurons and interganglionic and intraganglionic interneurons) do not appear to be strictly segregated into different groups. The AM, PM , AVM and M VM groups contain apparently only interneurons, either interganglionic (AM), intraganglionic (PM, AVM ), or both together (M V M ). In the MDM group one motoneuron to each side supplies the dorsal root of peripheral nerve 2, four or five PDM cells (equivalent to dorsal unpaired median (DUM ) cells of other species) bifurcate to nerve 5 of both sides of the ganglion and some also branch to nerves 3, 4 and 6, and in the PVM group one cell to each side is the widespread common inhibitory motoneuron to nerves 3-6. Three other large, bifurcating neurons of the PDM group may be efferent but follow a somewhat different pathway and could not be traced into peripheral nerves. Numbers of somata in the seven groups total 200 or more, and intraganglionic interneurons seem greatly to outnumber interganglionic ones. The MDM , AVM , MVM and PVM groups show evidence of an apparently paired origin.