The Histology of the Nervous System of an Insect Rhodnius prolixus(Hemiptera): II The Central Ganglia

Abstract

ABSTRACT The central nervous system of Rhodnius has been studied in sections stained with osmium / ethyl gallate. The single type of glial cell responsible for perineurium and axon sheaths in the peripheral nerves becomes differentiated in the thoracic ganglia into four types. Type i cells form the layer of perineurium cells filled with filamentous mitochondria. These cells which are rich in succinoxidase control the passage of all substances to and from the ganglion. Type ii cells produce the thick myelin-type sheaths for the lateral group of motor axons (see below). Type iii cells, the giant glial cells in the ganglionic layer, have a very extensive cytoplasm which sends deep tongue-like invaginations into the large ganglion cells. This cytoplasm must control the transmission of nutrients and excretory substances to and from the neurones. It contains some non-specific esterase. Type iv cells form an investment of the neuropile. They also have extensive cytoplasm which surrounds the axons and dendrites and provides intensely staining inter-neuronal material in the synaptic regions. This material, the ‘darkly staining neuropile’, is the chief site of acetylcholine esterase. The nerve-tracts are well displayed by the osmium / ethyl gallate method. The axons appear as unstained cords containing mitochondria, against the dark neuropile. The large motor axons of the legs separate into two groups. The medial group run directly into the neuropile and have thin sheaths. The lateral group remain outside the neuropile; they become narrow and enclosed in a thick myelin-type sheath; but as they turn into the neuropile the sheath becomes thin and the axons enlarge again.