Fundamental properties of rapidly and slowly adapting cells in the trigeminal ganglion of the awake freely moving rat
Authors:   , Leiser Steven C. , Moxon Karen Anne 
Published: July 2024

In order to perform tactile sensation, a system must be able to identify where in time and space an object was contacted and discriminate the tactile features of the object. Studies in analogous systems to the trigeminal somatosensory system of the rat have suggested that rapidly adapting (RA) and slowly adapting (SA) cells encode different aspects of tactile stimuli, yet the role of these cells in the rodent trigeminal vibrissal system is less clear. The goal of this thesis was to elucidate the basic properties of RA and SA cells and address how trigeminal ganglion (Vg) cells in general and RA and SA cells in particular, behave during natural whisking. Rats use their whiskers, which are laid out in rows and columns on either side of the face, to locate and discriminate tactile features of their environment in a manner similar to how primates use their fingertips, but the distribution of RA and SA cells across the whiskers is unknown. Thus the first aim of this thesis was to ascertain the ratio of RA and SA cells across the whiskers by recording 350 trigeminal ganglion neurons from 35 Long-Evans rats to assess the receptive field (RF) frequency of vibrissae-responsive cells and the relationship between RF frequency and the cells' response type (RA or SA). The results of this aim demonstrate that cells with RFs representing different mystacial whiskers have similar ratios of RA and SA cells, which contradicts the idea that rostral whiskers are devoted specifically to object localization and caudal whiskers to object discrimination, but rather suggests that each whisker has a similar ability to encode for multiple features of a tactile stimulus, inherent to RA and SA cells. The second aim of this thesis was to evaluate whether RA and SA cells maintain distinct firing patterns in the awake rat and was addressed by (1) developing a novel technique to chronically record from the trigeminal ganglion of awake freely-moving rats, (2) evaluating the responses of Vg cells to three distinct natural whisking behaviors-rest, whisking in air, and contact with an object-and (3) contrasting the responses of cells identified as RA and SA to test whether they exhibit distinguishable response properties during these natural whisking behaviors in the awake rat. This is the first study to correlate primary afferent signals to natural whisking behaviors in the awake rat. By chronically recording up to four single vibrissa-responsive neurons simultaneously in 14 Long-Evans rats for up to 118 days, our results show that all cells recorded had (1) no activity when the whiskers were not in motion, (2) significant activity when the rat was whisking in air-well correlated with whisk frequency-and (3) a great increase of activity when the whiskers contacted an object. In addition, SA cells were more active than RA cells during natural whisking, but RA cells increased their activity more than SA cells during object contact. Finally, both SA and RA cells can be time-locked to a particular phase of the whisk. The facts that both RA and SA cells fire continuously during contact and that both cells respond to a particular phase of the whisk discounts the implication that the function of RA and SA cells are separate and that it is the SA cells alone that encode object form and texture and the RA cells alone that encode object location.