The Segmental Giant Neurone of the Signal Crayfish, Pacifastacus Leniusculus , and its Interactions With Abdominal Fast Flexor and Swimmeret Motor Neurones

Abstract

The anatomy and physiology of the segmental giant (SG) of the crayfish Pacifastacus leniusculus is described. The peripheral axon of the SG terminates in the 1st root near the base of the swimmeret, and has no direct target organ. There is a tuft of fine dendritic branches in the lateral neuropile where the SG axon has its smallest diameter. The central region of the SG has a conduction velocity of 1·3 ms−1, a length constant of 0·4–0·7 mm and an input resistance greater than 1·8MΩ. Dual microelectrode penetrations of pre- and postsynaptic neurones confirm that the SG receives input from the giant fibres through rectifying electrical synapses, and excites the fast flexor motor neurones of the abdomen, also through electrical synapses. The SG also excites a variety of swimmeret motor neurones, including power-and return-stroke classes. Attention is drawn to the similarity in anatomy between the SG and return-stroke swimmeret motor neurones, and a possible evolutionary explanation for the existence of the SG is proposed.