Analysis of the peripheral organization and the specificity properties of primary gustatory afferents

Abstract

The first-order nervous elements serving the sense of taste have been analysed with respect to peripheral organization and specificity. The basic specificity assumption made is the existence of sensory units consisting of a single nerve fibre which transmits sensory messages from one or more receptor sites. Each receptor site responds to one and only to one primary taste quality. Although only four gustatory primaries, i.e. salty, sour, sweet and bitter are commonly recognized, our analysis is not limited to any definite number of fundamental taste stimuli. The analytical procedure has been carried out for two hypothetical events: (i) that the receptor elements are distributed among single primary afferents according to a Poisson distribution, or (ii) that each primary nerve fibre carries an equal number of receptor sites. It has been found that if the receptors in the sensory units follow the Poisson distribution the correlation of the “across-fibre” response to two monogustatory stimuli equals zero. Deviations from the Poisson distribution changes this zero correlation. When the total response from a sensory unit is considered it is possible to show that an almost perfect summation of the receptor inputs impinging on a single afferent nerve fibre takes place when only a small fraction of its transmission capacity is engaged. If the engaged fraction approaches the total carrying capacity the summation process becomes increasingly defective. Thus, loss of information is prevented and specificity is accentuated only when a small part of the theoretical transmission capacity of the sensory unit is utilized at any one time. Some of the basic parameters used in our analysis have been estimated from the data published by Ogawa, Sato and Yamashita.