Interneurons involved in the control of multiple motor centers in crayfish.

A number of studies have suggested that abdominal positioning interneurons (APIs) in the crayfish Procambarus clarkii can influence activity in multiple motor centers. Previous work on this population of neurons has demonstrated that they control the activity of tonic abdominal motor neurons (MNs) which generate postural movements of the abdomen and, to some extent, the activity of swimmeret MNs. This investigation demonstrates that many identified APIs also affect the activity of two populations of MNs which open and close the uropods and, in some cases, that of the swimmeret MNs as well. The majority (64%) of APIs examined in this study have an excitatory effect on both populations of uropod MNs. A smaller number (23%) increase the level of activity in one population of uropod MNs, and suppress, or have no effect on, the activity of the other population of uropod MNs. Approximately 25% of the APIs which were examined influence the output of swimmeret MNs, in addition to affecting the activity of uropod MNs. There are also indications that previous estimates of the number of APIs may have been too low. This is based on the observation that many APIs possess what appear to be similar morphologies but generate different patterns of motor output. Taken together, these findings support the idea that APIs influence the output of multiple motor centers which play a role in the control of general body posture and balance in crayfish.