Ions and mucoid substances in sensory organs--microanalytical data from insect sensilla

In insect sensory organs, the receptor endings are surrounded by an extracellular subcuticular matrix, the sensillum lymph. This matrix provides the correct aqueous and ionic milieu around the sensory processes, plays a role in the generation of the receptor current, and is also important for stimulus transport and/or stimulus inactivation in chemoreceptors. Histochemical evidence indicates the presence of proteoglycans (probably hyaluronate and/or chondroitin sulfate) in the sensillum lymph. X-ray micro-analytical studies demonstrate a very low sodium content, whereas potassium is present in high concentrations, comparable to those in intracellular compartments. The total positive charges of cationic elements are not balanced by the negative charges of chlorine, but by polyanions of the mucoid substances present in sensillum lymph. These fixed negative charges probably play a key-role in the establishment of the non-neural transepithelial voltage of insect sensilla, which in turn is a major source of the receptor current. Moreover, the mucoid substances of the sensillum lymph are probably involved in water conservation, since sensilla are prone to water loss, because the overlying cuticle must be permeable to the chemical stimuli. Functional analogies are pointed out between insect sensillum lymph and vertebrate olfactory mucus and, in particular, the endolymph of the vertebrate inner ear.