MAPPING EXTRACELLULAR EXCITABILITY IN AN INSECT MECHANORECEPTOR NEURON

The cockroach tactile spine contains a single bipolar mechanosensory n euron. Extracellular stimulation of the neuron is possible by cutting the spine and lowering a microelectrode into the lumen, where the neur on is located, but neither the microelectrode nor the neuron can be vi sualized during stimulation. The threshold for electrical stimulation of the neuron was measured as a function of spatial position in the lu men. The spine was then fixed and serially sectioned for computer-aide d reconstruction. Alignment of threshold measurements with reconstruct ions produced maps of excitability around the neuron. The lowest thres hold was always close to the sensory dendrite or the adjacent soma. Th ese results are discussed in terms of models of action potential initi ation in this class of sensory neurons.