The role of proprioceptive signals in the crayfish escape circuit

A single proprioceptor in the tailfan of the crayfish, Procambarus clarkii (Girard), innervated by only twelve sensory neurones encodes the position a nd the direction and velocity of movement of the exopodite relative to the endopodite. Most of the sensory neurones project to, and terminate in, the terminal abdominal ganglion where they form a map in which projection posit ion is based on the velocity threshold of the sensory neurone. The sensory signals from this small proprioceptor have significant effects on the neuro nal circuits mediating escape swimming and activate the lateral giant inter neurone directly through monosynaptic connections and indirectly via a disy naptic pathway involving a number of interposed intersegmental interneurone s. The lateral giant interneurones are activated through electrical synapse s whereas the ascending interneurones in the disynaptic pathway are excited through both electrical and chemical synapses. The proprioceptive signals are also responsible for evoking widespread presynaptic inhibition of exter oceptive afferents that reduces the efficacy of their outputs. This pathway therefore reduces afference caused by water movement as a result of an ani mals own escape movements. Movements of the chordotonal organ also lead to a delayed input to giant motor neurone that is timed to occur during flexio n movements of the abdomen. Thus not only do the proprioceptive signals act ivate the escape pathway leading to a tail-flip, but they also protect it f rom unwanted sensory input, and may also prevent depression of its neuromus cular synapses.