DUAL PATHWAYS FOR TACTILE SENSORY INFORMATION TO THORACIC INTERNEURONS IN THE COCKROACH

The escape system of the American cockroach is both fast and direction al. In response to wind stimulation both of these characteristics are largely due to the properties of the ventral giant interneurons (vGIs) , which conduct sensory information from the cerci on the rear of the animal to type A thoracic interneurons (TI(A)s) in the thoracic gangli a. The cockroach also escapes from tactile stimuli, and although vGIs are not involved in tactile-mediated escapes, the same thoracic intern eurons process tactile sensory information. The response of TI(A)s to tactile information is typically biphasic. A rapid initial depolarizat ion is followed by a longer latency depolarization that encodes most i f not all of the directional information in the tactile stimulus. We r eport here that the biphasic response of TI(A)s to tactile stimulation is caused by two separate conducting pathways from the point of stimu lation to the thoracic ganglia. Phase I is generated by mechanical con duction along the animal's body cuticle or other physical structures. It cannot be eliminated by complete lesion of the nerve cord, and it i s not evoked in response to electrical stimulation of abdominal nerves that contain the axons of sensory receptors in abdominal segments. Ho wever, it can be eliminated by lesioning the abdominal nerve cord and nerve 7 of the metathoracic ganglion together, suggesting that the rel evant sensory structures send axons in nerve 7 and abdominal nerves of anterior abdominal ganglia. Phase 2 of the TIA tactile response is ge nerated by a typical neural pathway that includes mechanoreceptors in each abdominal segment, which project to interneurons with axons in ei ther abdominal connective. Those interneurons with inputs from recepto rs that are ipsilateral to their axon have a greater influence on TI(A )s than those that receive inputs from the contralateral side. The pha se 1 response has an important role in reducing initiation time for th e escape response. Animals in which the phase 2 pathway has been elimi nated by lesion of the abdominal nerve cord are still capable of gener ating a partial startle response with a typically short latency even w hen stimulated posterior to the lesion. (C) 1995 John Wiley & Sons, In c.