Relationship between physiological response type (RA and SA) and vibrissal receptive field of neurons within the rat trigeminal ganglion.

Cells within the trigeminal ganglion (Vg) encode all the information necessary for the rat to differentiate tactile stimuli, yet it is the least-studied component in the rodent trigeminal somatosensory system. For example, extensive anatomical and electrophysiological investigations have shown clear somatotopic organization in the higher levels of this system, including VPM thalamus and SI cortex, yet whether this conserved schemata exists in the Vg is unknown. Moreover although there is recent interest in recording from vibrissae-responsive cells in the Vg, it is surprising to note that the locations of these cells have not even been clearly demarcated. To address this, we recorded extracellularly from 350 sensory-responsive Vg neurons in 35 Long-Evans rats. First, we determined three-dimensional locations of these cells and found a finer detail of somatotopy than previously reported. Cells innervating dorsal facial features, even within the whisker region, were more dorsal than midline and ventral features. We also show more cells with caudal than rostral whisker receptive fields (RF), similar to that found in VPM and SI. Next, for each vibrissal cell we determined its response type classified as either rapidly (RA) or slowly (SA) adapting. We examined the relationship between vibrissal RF and response type and demonstrate similar proportions of RA and SA cells responding to any whisker. These results suggest that if RA and SA cells encode distinct features of stimuli, as previously suggested, then at the basic physiological level each whisker has similar abilities to encode for such features.