Number and Laminar Distribution of Neurons in a Thalamocortical Projection Column of Rat Vibrissal Cortex

This is the second article in a series of three studies that investigate the anatomical determinants of thalamocortical (TC) input to excitatory neurons in a cortical column of rat primary somatosensory cortex (S1). Here, we report the number and distribution of NeuN-positive neurons within the C2, D2, and D3 TC projection columns in P27 rat somatosensory barrel cortex based on an exhaustive identification of 89 834 somata in a 1.15 mm(3) volume of cortex. A single column contained 19 109 +/- 444 neurons (17 560 +/- 399 when normalized to a standard-size projection column). Neuron density differences along the vertical column axis delineated cytoarchitectonic layers. The resulting neuron numbers per layer in the average column were 63 +/- 10 (L1), 2039 +/- 524 (L2), 3735 +/- 905 (L3), 4447 +/- 439 (L4), 1737 +/- 251 (L5A), 2235 +/- 99 (L5B), 3786 +/- 168 (L6A), and 1066 +/- 170 (L6B). These data were then used to derive the layer-specific action potential (AP) output of a projection column. The estimates confirmed previous reports suggesting that the ensembles of spiny L4 and thick-tufted pyramidal neurons emit the major fraction of APs of a column. The number of APs evoked in a column by a sensory stimulus (principal whisker deflection) was estimated as 4441 within 100 ms post-stimulus.