Columnar organization of mechanoreceptive neurons in the cortical taste area in the rat

Mechanoreceptive neurons with or without taste responsiveness were recorded in the cortical taste area (CTA) of rats every 50 or 100 pm along an electrode track made as perpendicular to the surface as possible. Three groups of mechanoreceptive neurons were recognized based on the adequate stimulus, i.e., low-threshold mechanoreceptive (n = 16), nociceptive-specific (n = 48), and wide-dynamic range neurons (n = 392). Except for nine neurons, almost all had receptive fields (RFs) in the oral cavity (n = 447). They were categorized into three RF types: those with RFs only in the oral cavity (OC type; n = 23), those with RFs both in the oral cavity and on the lip (OL type; n = 44) and those with RFs in the oral cavity and on the external surface of the body (WB type; n = 380), e.g., tail. Neurons with inhibitory RFs were often located in the infragranular layers. Several neurons with the same receptive features were sequentially recorded along the track, suggesting the presence of columnar organization. The diameter of the possible functional column was the largest (mean 113.85 mum) for WB type neurons, but smaller in the other two types (mean 85 mum for the OC type and 62.5 mum for the OL type). Neurons were segregated according to the adequate stimulus within the column for WB type neurons. Taste-responsive mechanoreceptive neurons (n = 33) were recorded at 15 tracks, and two taste neurons were sequentially recorded in five cases, in three of which two successive neurons sharing the best stimulus were recorded. Taste neurons are possibly arranged in a column with a very small diameter within the large column of mechanoreceptive neurons.