Interneurons between giant axons and motoneurons in crayfish escape circuitry. 
1.  Crayfish giant fibers are generally believed to generate tailflip movements by means of direct connections to two classes of phasic flexor muscle motoneurons, the motor giants (MoGs) and the nongiant fast flexor motoneurons (FFs).  It is shown here that the giants also stimulate a network of interneurons that make connections with the FFs.  2.  This network includes an intraganglionic neuron, the segmental giant (SG), in each abdominal hemisegment and a number of intersegmental neurons, two of which (I2 and I3) were studied in detail.  3.  The SGs are driven reliably by the giant fibers and they in turn drive the FFs of their hemisegment about as effectively as do the giant fibers themselves; it is possible that the giant fibers excite the FFs mainly by way of the SGs.  The SGs also have an efferent first root axon whose peripheral targets we have been unable to determine.  4.  I2 and I3 originate in the second and third abdominal ganglia, respectively, and descend to the last ganglion.  In their ganglia of origin they are reliably driven by the giant fibers and by the SGs.  In addition, I2 weakly excites I3 and both receive weak, apparently direct, excitatory input from FFs as well as less direct excitatory and inhibitory input from unidentified afferent sources.  Both weakly excite most FFs in ganglia behind the one in which they originate.  This excitation adds to that produced directly by giant fibers and SGs and, we believe, is sometimes decisive in causing FF firing.  Their firing also causes inhibition involved in suppressing effects of reafference, as do the giant fibers themselves.  5.  I3 strongly excites the motoneurons of certain tail fan muscles (the ventral and posterior telson flexors).  However, the contraction of these muscles would be maladaptive during some giant fiber-mediated tailflips.  Accordingly, when the giant fibers, which always recruit I3, fire, they cause an inhibition of the motoneurons that nullifies the excitatory input from I3.  At a formal level this means that the giants, viewed as command neurons, not only drive but also alter or modulate the subordinate motor pattern-generating network that they control.  6.  Tailflips that are less stereotyped than those mediated by giant fibers are known to occur without participation of the giants.  It is suggested that the presence of complex circuitry mediating between giant fibers and FFs may be related to the use of portions of this circuitry as well as the FFs themselves in production of nongiant tailflips.