Second-order spinal cord pathway contributes to cortical responses after long recoveries from dorsal column injury in squirrel monkeys

Abstract

Months after the occurrence of spinal cord dorsal column lesions (DCLs) at the cervical level, neural responses in the hand representation of somatosensory area 3b hand cortex recover, along with hand use. To examine whether the second-order spinal cord pathway contributes to this functional recovery, we injected cholera toxin subunit B (CTB) into the hand representation in the cuneate nucleus (Cu) to label the spinal cord neurons, and related results to cortical reactivation in four squirrel monkeys () at least 7 months after DCL. In two monkeys with complete DCLs, few CTB-labeled neurons were present below the lesion, and few neurons in the affected hand region in area 3b responded to touch on the hand. In two other cases with large but incomplete DCLs, CTB-labeled neurons were abundant below the lesion, and the area 3b hand cortex responded well to tactile stimulation in a roughly somatotopic organization. The proportions of labeled neurons in the spinal cord hand region reflected the extent of cortical reactivation to the hand. Comparing monkeys with short and long recovery times suggests that the numbers of labeled neurons below the lesion increase with time following incomplete DCLs (<95%) but decrease with time after nearly complete DCLs (≥95%). Taken together, these results suggest that the second-order spinal cord pathway facilitates cortical reactivation, likely through the potentiation of persisting tactile inputs from the hand to the Cu over months of postlesion recovery.

Malleability of the cortical hand map following a finger nerve block

“…Yet, it is likely that some D2-related residual input, though reduced, still reached S1, as clearly shown in our active condition. This is consistent with deafferentation models for studying deprivation -even when considering extreme cases of spinal cord injuries and whole-hand amputation, some rudimentary peripheral signals are likely to persist from the injured nerves (Liao et al 2018). Indeed, in our computational model we assumed the nerve block doesn't provide complete abolishment of all D2 inputs.…”

The Evolution of the Pulvinar Complex in Primates and Its Role in the Dorsal and Ventral Streams of Cortical Processing

“…Some reactivation of MT may occur after V1 lesions in adult monkeys and humans, possibly by new or indirect inputs from the pulvinar, and possibly the extrastriate projections from the koniocellular system in the LGN are also involved [140]. In addition, compensations for sensory loss can be greatly enhanced by only a few surviving inputs, as shown by the reactivation of somatosensory cortex and the recovery of tactile sensation after nearly complete sensory loss due to spinal cord injury [141].…”

Cortical Reorganization of Sensorimotor Systems and the Role of Intracortical Circuits After Spinal Cord Injury

“…Midcervical dorsal column lesions result in the interruption of ascending afferents and an initial loss of responsiveness of area 3b hand representations, followed by an eventual shift to responsiveness to tactile face stimulation [108]. Incomplete dorsal column injury results in an eventual recovery of hand representations at several months after injury [109]. This recovery is attributed to a few spared axons and a second-order spinal cord pathway arising from lamina IV below the level of the lesion that projects to the cuneate nucleus through the lateral funiculus [109,110].…”

“…Incomplete dorsal column injury results in an eventual recovery of hand representations at several months after injury [109]. This recovery is attributed to a few spared axons and a second-order spinal cord pathway arising from lamina IV below the level of the lesion that projects to the cuneate nucleus through the lateral funiculus [109,110].…”