Structure and function of the giant fibre system in the cockroach, Periplaneta americana

Abstract

All cockroach giant axons surgically separated from the terminal ganglion collapsed in 10 to 30 days after cord transection, indicating that giant fibre somata are located in the last abdominal ganglion. With nerve cord hemisection near the terminal ganglion, giant fibres remained intact in the anterior contralateral connectives, while ipsilateral giant fibres collapsed. In transverse sections of hemisectioned nerve cord, it was evident that the ventral group of giant fibres extend from the terminal to the head ganglia, while the dorsal giants could not be followed beyond the mesothoracic ganglion. Electrophysiological experiments also indicated the presence of long fibres terminating in the mesothoracic ganglion. These fibres were activated by cercal stimulation and had a conduction velocity of 4 to 6 m/sec. Giant axons were traced to clusters of cell bodies in the last abdominal ganglion, indicating a syncytial origin. These axons do not branch in the abdominal ganglia, but branches of giant axons have been identified in the thoracic ganglia. As demonstrated in electrophysiological spike collision experiments, the ventral group of giant axons is activated by cercal stimulation and can conduct descending spikes, although they may not normally do so. Giant axon injury responses and cercal nerve degenerative changes were used to identify axon profiles in thin sections of the terminal ganglion neuropile examined by electron microscopy. Cercal nerve endings contained membrane-bound vesicles 500 Å in dia., similar in appearance to vesicles observed at vertebrate cholinergic junctions. These vesicles were sometimes heavily accumulated at electron-dense areas of cercal and giant fibre membrane, with the membranes separated by about a 200 Å gap. Observations support the view that the cercalgiant fibre synapse is mediated by a chemical transmitter liberated from vesicles within the axoplasm of the presynaptic fibre.