Mapping extracellular excitability in an insect mechanoreceptor neuron.

The cockroach tactile spine contains a single bipolar mechanosensory neuron. Extracellular stimulation of the neuron is possible by cutting the spine and lowering a microelectrode into the lumen, where the neuron is located, but neither the microelectrode nor the neuron can be visualized during stimulation. The threshold for electrical stimulation of the neuron was measured as a function of spatial position in the lumen. The spine was then fixed and serially sectioned for computer-aided reconstruction. Alignment of threshold measurements with reconstructions produced maps of excitability around the neuron. The lowest threshold was always close to the sensory dendrite or the adjacent soma. These results are discussed in terms of models of action potential initiation in this class of sensory neurons.