The encoding of touch by somatotopically aligned dorsal column subdivisions

Abstract

The somatosensory system decodes a range of tactile stimuli to generate a coherent sense of touch. Discriminative touch of the body depends on signals conveyed from peripheral mechanoreceptors to the brain via the spinal cord dorsal column and its brainstem target the dorsal column nuclei (DCN). Models of somatosensation emphasize that fast-conducting low-threshold mechanoreceptors (LTMRs) innervating the skin drive the DCN. However, post-synaptic dorsal column neurons (PSDCs) within the spinal cord dorsal horn also collect mechanoreceptor signals and form a second major input to the DCN. The significance of PSDCs and their contributions to the coding of touch have remained unclear since their discovery. Here, we show that direct LTMR inputs to the DCN convey vibrotactile stimuli with high temporal precision, whereas PSDCs primarily encode touch onset and the intensity of sustained contact into the high force range. LTMR and PSDC signals topographically re-align in the DCN to preserve precise spatial detail. Different DCN neuron subtypes have specialized responses that are generated by unique combinations of LTMR and PSDC inputs. Thus, LTMR and PSDC subdivisions of the dorsal column encode different tactile features and differentially converge in the DCN to generate unique ascending sensory processing streams.

Paresthesia during spinal cord stimulation depends on synchrony of dorsal column axon activation

“…When many afferents are co-activated (by tactile stimulation or by SCS), huge amounts of feedforward, lateral, and feedback inhibition are engaged, dampening the impact of input from any one afferent. Neurons projecting from DC nuclei to thalamus experience strong surround inhibition activated by primary afferent input 74 . Surround Inhibition also dampens pain signals (see Introduction) as well as evoked potentials and reflexes (see Fig.…”

“…Asynchronous spikes are more readily blocked than synchronous spikes by feedforward inhibition, unless inhibition is particularly weak. Feedforward inhibition could occur at multiple points along the pathway conveying somatosensory information to the brain, including the first relay points in the spinal dorsal horn and dorsal column nuclei 74 . Absence of cortical activation during kf-SCS (see Fig 1D) suggests that kf-SCS-evoked signals are blocked subcortically, which would also explain the attenuation of SEPs and the H reflex (see Figs.…”

A multi-channel electrophysiology approach to non-invasively and precisely record human spinal cord activity

“…Even modern MSG-studies are currently limited to cervical or lumbar windows when investigating spatial properties of spinal somatosensory evoked fields (Akaza et al, 2021; Ushio et al, 2019), whereas our approach allows for a more holistic view. Obviously, the here presented non-invasive electrophysiological data do not allow for conclusions regarding the exact origin of these potentials within the spinal cord, but previous animal work suggests that these potentials are generated by interneurons in the deep dorsal horn (e.g., Beall et al, 1977; Willis et al, 1973; for review, see Shimoji, 1995), likely as part of the post-synaptic dorsal column pathway, which is a prominent source of tactile input to the dorsal column nuclei (Giesler et al, 1984; Turecek et al, 2022; for review, see Brown, 1981).…”