Fig. 1.

Genetically identified spinal interneurons (INs) implicated in microcircuits integrating tactile information for sensorimotor control. A: diagram representing the typical spinal circuitry integrating tactile sensory information, where a low-threshold mechanoreceptor (LTMR) sensory neuron (SN) enters the spinal cord and bifurcates to project to spinal INs and neurons in supraspinal centers. Subpopulations of spinal INs (Spinal INs) receiving tactile inputs project to spinal targets, including motoneurons of the lateral motor column (LMC) and medial motor column (MMC) and/or to supraspinal centers. Roman numerals refer to the Rexed laminae. B–E: diagrams representing dI3 INs (B), retinoic acid receptor-related orphan receptor α (RORα) INs (C), dI4 INs (D), and dI1 INs (E). SNs (blue) connect to spinal IN populations. The established implication of some LTMR subtypes is indicated. IN connections to and from supraspinal centers are indicated. Ascending and descending projections are not represented at their exact location for simplicity purposes. B: dI3 INs project to motoneurons as well as to the lateral reticular nucleus (LRN). Note the weaker connection to MMC motoneurons. vGluT2, vesicular glutamate transporter 2. C: RORα INs project to both V0 cholinergic (V0c) INs (maroon) and motoneurons. RORα INs also receive inputs from the cortex via corticospinal tracts (CST) and the vestibular nucleus via the lateral vestibulospinal tract (LVST). D: dI4 INs are involved in a presynaptic axo-axonic inhibition of sensory projection onto spinal INs (pink). dI4 INs that mediate axo-axonic inhibition of proprioceptive afferents are not depicted. dI4 IN supraspinal connections have yet to be identified. GAD2, glutamate decarboxylase 2; GlyT2, glycine transporter 2; NPY, neuropeptide Y. E: two defined subpopulations of dI1 INs are represented by having either ipsilateral (ipsi; light purple) projections to the cerebellum via the ventral spinocerebellar tract (vSCT) or contralateral (cont; deep purple) projections to the dorsal SCT (dSCT). Subpopulation-specific genetic markers are listed above the IN symbols; a number of shared genetic identifiers are listed below the IN symbol. F: transcriptional code involved in the development of genetically identified spinal INs that integrate tactile information. See Table 1 for more description of the transcription factors listed. Note that the progenitor domain or domains from which RORα INs originate are not known [panel F adapted from Lu et al. (2015) under CC-BY].