Our findings show that Clarke’s column dSC neurons represent a focal target for the convergence of descending cortical and sensory afferent pathways, nucleating spinal microcircuits with the potential to predict and modulate proprioceptive feedback signals. Prior physiological studies by Hongo and coworkers have provided evidence for both rapid excitation and prolonged inhibition of dSC neurons by descending corticospinal projections18,19
. Our findings are in general agreement with these earlier in vivo studies, although it is worth emphasizing that Hongo’s analysis did not establish the direct nature of cortically-evoked excitation. Nor were Clarke’s column neurons distinguished from other classes of dSC neurons. In fact, the most complete physiological description of dSC neuronal activity and input characteristics was obtained at caudal lumbar segmental levels, a region devoid of Clarke’s column neurons18,19
. At these caudal levels, dSC neurons are known to receive prominent direct input from cutaneous sensory afferents6
, which contrasts with the nearly pure proprioceptive sensory origin of direct input to Clarke's column neurons. Thus, our findings provide clear evidence for cortical regulation of the activity and output of identified Clarke’s column dSC neurons. When taken together with Hongo’s findings, they suggest a common strategy for descending cortical control of distinct subclasses of dSC neurons.
Our findings also invoke the existence of distinct classes of Clarke’s column dSC neurons. Neurons positioned within the dorsomedial sector of the Clarke’s column annulus can be distinguished from their ventrolateral counterparts by synaptic bouton contacts from corticospinal axons. We presume that these neurons represent the set of physiologically-defined dSC neurons that exhibit excitatory responses to dorsal column stimulation. In addition, dSC neurons throughout the annulus receive dense input from GABAergic and glycinergic inhibitory interneurons, providing a plausible anatomical substrate for the prominent inhibitory responses elicited in most dSC neurons by dorsal column stimulation. Some dSC neurons nevertheless exhibited exclusively excitatory responses to dorsal column stimulation, raising the possibility that the inhibitory boutons that contact some, presumably dorsomedial, dSC neurons derive from interneurons that lack corticospinal input. The existence of three distinct synaptic arrangements on dSC neurons hints at the operation of multiple channels for proprioceptive processing within Clarke’s column (Supplementary Fig. 6a
Our anatomical analysis of excitatory synaptic terminals on Clarke’s column dSC neurons revealed a vast predominance of VG1- over VG2-labeled boutons, and the coexpression of Parv by essentially all VG1+ boutons. This synaptic phenotype implies that virtually all sensory inputs to this set of dSC neurons derive from proprioceptors -- an unanticipated finding when considered from the perspective of physiological studies arguing that dSC neurons serve as sensory monitors of limb position through the convergence of diverse cutaneous and proprioceptive sensory signals. One way of reconciling our findings with prior observations is to invoke the idea that the influence of cutaneous sensory input on the integrative sensory properties of Clarke’s column dSC neurons is achieved indirectly, via intermediary interneuronal pathways.
The functional circuitry of dSC neurons uncovered through our physiological analysis of dorsal column inputs provides evidence that Clarke’s column serves an integrative role well beyond that of a simple sensory relay nucleus. The existence of strong excitatory cortical inputs to a subset of dSC neurons provides a potential intraspinal pathway for the transfer of descending cortical commands onto a sensory relay system destined for the cerebellum (Supplementary Fig. 6a–i,ii
). These cortically-derived signals appear well-suited to deliver predictions of the sensory consequences of motor acts, anticipating peripherally-derived sensory feedback. The cortically-evoked inhibitory responses detected in dSC neurons typically persist for 100msec or more, a period which spans the temporal delay incurred through peripheral routing of proprioceptive sensory feedback33
. These findings argue in favor of an independent function for descending cortical commands in suppressing or modulating the impact of activation of dSC neurons by proprioceptive sensory feedback (Supplementary Fig. 6a–iii
). The origin of dorsal column inputs to Clarke’s column dSC neurons, from motor and/or somatosensory cortical areas, remains to be resolved.
The functions invoked for Clarke’s column dSC neurons in the regulation of proprioceptive sensory processing share certain features in common with the integrative properties of ventral spinocerebellar (vSC) tract pathways. Physiological studies have shown that vSC tract neurons receive inputs from sensory, local and descending axons that mirror those converging on spinal motor neurons34,35
. One major role of the vSC tract therefore appears to be to relay a corollary copy of inputs to spinal motor neurons directly to supraspinal processing centers36
. By analogy, our findings imply that dSC neurons might serve a similar corollary function, with the notable difference that information conveyed via the Clarke’s column pathway reports on anticipated proprioceptive input, rather than on imminent motor output. In turn, these considerations pose the downstream problem of how the cerebellum integrates inputs from multiple spinocerebellar signaling streams. In recent genetic tracing studies (AWH and TMJ, data not shown) we have found that the terminals of dSC and vSC axons converge on the same cerebellar folia, and are frequently be found in proximity to the same granule neuron, suggesting that cerebellar processing involves the convergence of spinal inputs onto a common granule cell target.
Intriguingly, the intraspinal circuitry for cortically-mediated inhibition of dSC signaling exhibits an organization that conforms to that of an insect corollary discharge circuit employed in the cancellation of self-generated auditory stimuli37
. Mammalian and arthropod circuits both rely on the activation of interneurons that exert pre-synaptic inhibition of sensory afferent input, as well as post-synaptic inhibition of a primary central relay neuron. The additional, predictive, role of dSC neurons uncovered in our studies may be more akin to the corollary activities observed the sensory processing centers of higher mammals12,38
. In the mouse, the molecular delineation of dSC neurons opens the way for future genetic manipulation of neuronal elements in this spinal corollary circuit.
Traditionally, corollary discharge circuits involved in motor planning have been assigned to pontocerebellar and intracortical pathways15,16
, raising the further question of the merits of constructing intraspinal circuits with similar design features (Supplementary Fig. 6b
). A spinally-focused corollary circuit will inevitably incur slightly greater temporal delays than its supraspinal counterparts. But, by way of compensation, it affords descending cortical systems direct access to a selective sensory channel, in principle permitting early and effective anticipation or cancellation of the proprioceptive consequences of movement.