The functional properties of barrel cortex neurons projecting to the primary motor cortex

Abstract

Nearby neurons, sharing the same locations within the mouse whisker map, can have dramatically distinct response properties. To understand the significance of this diversity, we studied the relationship between the responses of individual neurons and their projection targets in the mouse barrel cortex. Neurons projecting to primary motor cortex (MI) or secondary somatosensory area (SII) were labeled with red fluorescent protein (RFP) using retrograde viral infection. We used in vivo two-photon Ca(2+) imaging to map the responses of RFP-positive and neighboring L2/3 neurons to whisker deflections. Neurons projecting to MI displayed larger receptive fields compared with other neurons, including those projecting to SII. Our findings support the view that intermingled neurons in primary sensory areas send specific stimulus features to different parts of the brain.

Figure 1.

Retrograde labeling with a virus expressing a red fluorescent protein. A, Schematic showing retrograde labeling of neurons in SI by injection of the retrograde virus HSV1 into MI. B, In vivo image of RFP+ neurons (maximum-intensity side projection of an image stack of RFP+ neurons; 512 × 128 × 96; section spacing, 8 μm). C, Distribution of labeled neurons in SI barrel cortex after bead (left) or virus (right) injection into MI. The white lines indicate the pia and the border between the cortex and the white matter. D, Normalized distribution of labeled neurons after bead (black, 1293 neurons) and HSV (white, 808 neurons) injection into MI.

Figure 2.

The receptive fields of barrel cortex neurons with projections to MI or SII. A, Left, Cells were loaded with Fluo-4 AM; right, one cell in the field of view (RFP+, cell 1) sends axons to MI. B, Examples of whisker stimulation-evoked fluorescence transients for three cells on two different trials. Columns correspond to the three cells identified in A. Red traces indicate trials with action potential-evoked fluorescence transients, blue traces indicate trials without fluorescence transients. Whisker stimuli are indicated at the bottom of C. C, Multiple fluorescence transients from cells 1–3 aligned on the whisker stimulus (bottom). D, Plots of amplitude (A_F) and difference (F_d) (see supplemental Methods, available at www.jneurosci.org as supplemental material) for all trials for the cells identified in A. The points were clustered into two groups (red, successes; blue, failures). The 95% confidence ellipses are overlaid on the graph. E, The response probability of these three cells to stimulation of D4 and D5. F, The distribution of the ratios of response probabilities to PW and SW stimulation following the virus injection into MI (left) and SII (right). The red bars indicate data from RFP+ cells and the white bars indicate the data from RFP− cells.