Figure 6. Specificity of Sensory-Interneuron Connectivity at Individual Joints.

(A) Assay of proprioceptive input to V1^R interneurons.

(B-C) CTB⁺; vGluT1⁺ proprioceptive input to V1^R interneurons. (C) Percent of V1^R interneurons with pool-specific input. p = 0.0007, one-way ANOVA; Bonferroni post-hoc test: p < 0.01, GL vs TA or IF, n = 3 animals. Innervation density (inputs/100 μm dendrite) for neurons receiving sensory input: GL, 5.1 ± 1.0; TA, 2.8 ± 1.9. All data are mean ± SEM. Scale bar = 2 μm.

(D) Assay of monosynaptic input from pool-specific Ia afferents onto V1^R interneurons.

(E) Left, V1^R interneurons (red, labeled in En1::Cre; Rosa.lsl.tdT mice) targeted for intracellular recording, filled with Alexa-488 hydrazide (green). Right, stimulation of L5 ventral root (vr-L5) at different intensities evoked graded short-latency synaptic potentials. Scale bar = 10 μm.

(F) V1^R interneurons (GL: 6/11 cells; TA: 1/11 cells; IF: 0/11 cells) receiving monosynaptically-evoked EPSPs from peripheral muscles.

(G) Monosynaptic EPSPs in a V1^R interneuron (black) or GL motor neuron (gray) following GL muscle stimulation.

(H) Short-latency EPSPs evoked in V1^R interneurons after GL stimulation (3 superimposed responses evoked at 0.1 Hz stimulation frequency, black). Stimulation from TA (gray) or IF (brown) muscle sensory fibers resulted in long (>20 msec), variable latencies, indicative of polysynaptic activation. Arrow, stimulation artifact.

(I) Left, latencies from synaptically evoked responses in V1^R interneurons after stimulation of GL, TA, and IF muscles in p4 to p5 mice. The latency from GL muscle stimulation was significantly shorter than from TA or IF (p < 0.05, one-way ANOVA). Right, coefficient of variation (CV_onset) of synaptic response latency. Red line = mean.

(J) Assay of proprioceptive sensory input onto V1^Sp8 interneurons.

(K-L) CTB⁺; vGluT1⁺ proprioceptive input to V1^Sp8 interneurons. (L) V1^Sp8 interneurons with pool-specific input. p = 0.004, one-way ANOVA; Bonferroni post hoc test: p < 0.01, GL vs TA; p < 0.001, TA vs IF; n.s., GL vs IF, n ≥ 3 animals. Average CTB⁺; vGluT1⁺ input density/100 μm of V1^Sp8 dendrite: GL: 0.92 ± 0.23; TA: 2.20 ± 0.14; IF: 0 ± 0; p < 0.0001, oneway ANOVA; Bonferroni post hoc test: p < 0.01, GL vs TA; p < 0.05, GL vs IF; p < 0.001, TA vs IF. Similar labeling efficiency was observed for GL, TA, and IF sensory afferents (Figure S7C). Scale bar = 2 μm. See also Figure S7.