Sensing taste and odours
Part 3: Nerve Cells and Accessory Cells
Sensory neurons are bipolar cells that innervate the cuticular sensilla apparatus. Distally the nerve cell extends out into a dendrite that may or may not branch. Proximally, the sensory neuron produces an axon that travels to the central nervous system. Contact chemoreceptors send projections to the ganglion of the segment on which the receptor occurs. Olfactory receptors send projections to the antennal lobes in the head.
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[Diagram: B.W. Cribb]
All sensilla that have sensory neurons associated with them have a similar set of accessory cells. The thecogen cell wraps around the nerve cell bodies. It produces, at its apex, the dendrite sheath. One or two trichogen cells wrap the thecogen cell. |
Outermost is a tormogen cell. This cell secretes the socket before pulling back to create a large lymph space. The cells bordering the lymph space have the cell membrane extended into microvillae.
In the silk moth, Antherea, the lymph that fills the sensillum shaft and surrounds the dendrite sheath produces molecules that bind with moth pheromone odours. The cells probably also produce an enzyme that breaks down odour molecules.
Taste receptors usually have two to four sensory neurons. In addition a mechanoreceptive neuron may be present, making five cells in total, as seen for example in the trichoid sensilla on the proboscis of blowflies.
The prominent taste hairs present on the labellum of Drosophila contain one cell that senses sucrose (and other sugars), a second cell that tastes salts, a third activated by pure water, and a fourth neuron that is the “bitter” neuron and responds to aversive compounds such as caffeine (see Dahanukar, A., Lei, Y.T., Kwon, J.Y. & Carlson, J.R. (2007) Two Gr genes underlie sugar reception in Drosophila. Neuron, 56, 503-516).
Sensilla responding to odour have a much wider range. Only two or three sensory neurons are associated with the pheromone receptive sensilla. Coeloconic sensilla often have three. But some have many more, for example, 22 sensory neurons are associated with some sensilla trichodea of the biting midge, F. (T.) townsvillensis.
Go on to part 4: Humidity receptors