Opposing parallel connections through crayfish local nonspiking interneurons.

Unilateral local nonspiking interneurons in the terminal (sixth) abdominal ganglion of crayfish (Procambarus clarkii Girard) can be classified into two major groups of PL and AL types by their gross morphology and somatic position. These premotor interneurons are the neural components of uropod motor pattern formation. They receive sensory input from the exopodite of the contralateral side as well as that of the ipsilateral side. Small fluctuations in their membrane potentials cause sustained change in activity of the motoneurons innervating the uropod muscles. PL interneurons, which make noninverting connections to an identified closer, the reductor motoneuron No. 1, mainly receive excitatory input from the afferents of the contralateral exopodite, whereas inverting PL interneurons receive inhibitory input. AL interneurons receive distinctly different input from the afferents. Noninverting AL interneurons mainly receive inhibitory input, whereas inverting AL interneurons receive excitatory input. The rate of discharge of the reductor motoneurons is increased by sensory stimulation. The PL interneurons form either excitatory or disinhibitory pathways, which are relevant in function to the observed increase of the motoneuron. Conversely, the AL interneurons form either inhibitory or disfacilitatory pathways. Thus, the PL and the AL interneurons are fractionated in function and distinguishable in terms of their physiology by their input and output correlations. Functional meaning of the presence of these two types of unilateral local nonspiking interneurons of opposing connections in the uropod motor control system is discussed.