Organization of the giant axons of the cockroach Periplaneta americana.

Several authors have reported the presence of ascending giant fibres (AGF) in the nerve cord of the American cockroach, Periplaneta americana (Pumphrey & Rawdon-Smith, 1937; Roeder, 1948; Hess, 1958). Roeder (1948) presented a scheme showing a synaptic link (in the 6th abdominal ganglion) between the cereal sensory neurones and the AGF, the latter then ascending as continuous fibres to the metathoracic ganglion (T3). Part of the ascending pathways continue further and terminate at the mesothoracic ganglion (T2), while only a small fraction of the fibres reach the prothoracic (Tj) and the suboesophageal ganglion (So). Thus, in Roeder's scheme the AGF synapse at each thoracic ganglion. Since stimulation of a single cereal nerve evoked only 6-8 spikes in the ipsilateral and 2-3 in the contralateral connectives in the abdomen, Roeder (1948) claimed that the number of spikes elicited by a cereal stimulus is smaller than the number of giant axons (usually 8—10) in the abdominal cord. On this basis he proposed that some of these axons may not be activated by cereal stimulation and might conduct descending information. Hess (1958) found that sectioning of the nerve cord between the 5th and 6th abdominal ganglia resulted in degeneration of all the dorsal and most of the ventral giant axons in the abdomen, anteriorly to the point of severance, but two GFs of the ventral group remained intact after the operation. He therefore suggested that these two axons might serve as a descending or even as a common descendingascending route. This hypothesis has not been supported so far by experimental evidence. Hess found no sign of GF degeneration in the thorax and thus confirmed the scheme of Roeder (1948). Pipa, Cook & Richards (1959), however, traced the ventral giant group throughout the thorax, but did not exclude the possibility of a polysnaptic chain (Bullock & Horridge, 1965). In the present study an effort was made to demonstrate the existence of giant descending pathways. On the basis of the observations to be reported, the possibility of 'bi-directional conduction' will be considered.