Within the hierarchy of sensory interneurones in the crayfish, individual cells exist which code specifically for directional components of waterborne stimuli. The caudal photoreceptor interneurone (CPR) exhibits this mechanoreceptive response pattern, in addition to its primary light response. However, despite some variability the CPR is bidirectional, responding to water currents in both rostral and caudal directions.
The peripheral mechanoreceptive fields providing input to the CPR have been examined, using selective root ablation procedures. Receptors from the exopodite and from the distal portion of the telson are the primary sources of input, this being carried by the 2nd and 5th roots respectively of the caudal ganglion. Input from the ipsilateral roots is normally excitatory; input arising contralaterally has an inhibitory influence.
Populations of mechanoreceptors presynaptic to the CPR appear to be segregated in terms of the directional information encoded by the CPR. Specifically, ablation of the 2nd ipsilateral root decreases caudal sensitivity predominantly, whereas ablation of the 5th ipsilateral root primarily decreases rostral sensitivity.
Therefore, only primary sensory cells which originate from the exopodite, and respond to caudal deflexions of the hair shaft, effectively excite the CPR. Similarly, only the sensory cells originating from the telson which are responsive to anterior deflexions appear to effectively excite the CPR.