The morphology of the sensory neuron in the cockroach femoral tactile spine

1. The cockroach femoral tactile spine contains a single bipolar sensory neuron. The mechanosensitive dendrite in the wall of the spine leads through the spine lumen to a cell body, and then to an axon that proceeds proximally along the femur. The ultrastructure of the sensory ending has been examined before with electron microscopy. However, the morphology of the complete neuron and its relationship to the general spine structure have not been described before. 2. The tactile spine neuron has been extensively used in electrophysiological studies, including intracellular recordings. Action- potential amplitudes and thresholds were variable and inversely related in intracellular recordings, which could be caused by variability in the location of the action-potential initiation region, the position of the recording electrode, or the neuronal morphology. Attempts to observe the complete neuronal morphology by dye injection were hampered by the opaque and autofluorescent cuticle surrounding the neuron. 3. We examined 10 tactile spine neurons, and their surrounding structures, by taking serial 1-μm sections through the base of the spine, normal to its long axis. The sections were examined with light microscopy, digitized by tracing onto a graphics tablet, and then reassembled with the use of computer software. Reconstructions were made of the borders of the spine cuticle, neuron, neuronal nucleus, glial wrappings, and the main trachea in the spine lumen. 4. There was considerable variability in the size and shape of the neuronal cell body, although the sensory dendrite and axon had more consistent morphologies. The cell body was always extensively wrapped in glial cells, but this dense wrapping did not extend along the axon or dendrite. The size of the cell body and the nucleus increased approximately linearly with the external diameter of the tactile spine. 5. The variability in apparent electrical distance from a penetrating microelectrode to the site of action- potential initiation can probably be explained by the variable shape of the cell body. The location of the glial wrappings suggests that they assist passive conduction of the receptor current from the sensory dendrite through the cell body to the axon.