Sensory activation and receptive field organization

32 33 Crayfish (Procambarus clarkii) have bilateral pairs of giant interneurons that control 34 rapid escape movements in response to predatory threats. The medial giant neurons 35 (MGs) can be made to fire an action potential by visual or tactile stimuli directed to the 36 front of the animal and this leads to an escape tail-flip that thrusts the animal directly 37 backwards. The lateral giant neurons (LGs) can be made to fire an action potential by 38 strong tactile stimuli directed to the rear of the animal and this produces flexions of the 39 abdomen that propel the crayfish upward and forward. These observations have led to the 40 notion that the receptive fields of the giant neurons are locally restricted and do not 41 overlap with each other. Using extraand intracellular electrophysiology in whole animal 42 preparations of juvenile crayfish, we found that the receptive fields of the LGs are far 43 more extensive than previously assumed. The LGs receive excitatory inputs from 44 descending interneurons originating in the brain; these interneurons can be activated by 45 stimulation of the antenna II nerve or the protocerebral tract. In our experiments, 46 descending inputs alone could not cause action potentials in the LGs, but when paired 47 with EPSPs elicited by stimulation of tail afferents, the inputs summed to yield firing. 48 Thus, the LG escape neurons integrate sensory information received through both rostral 49 and caudal receptive fields, and excitatory inputs that are activated rostrally can bring the 50 LGs’ membrane potential closer to threshold. This enhances the animal’s sensitivity to an 51 approaching predator, a finding that may generalize to other species with similarly 52 organized escape systems. 53 54