Congenital foot deformation alters the topographic organization in the primate somatosensory system

Abstract

Limbs may fail to grow properly during fetal development, but the extent to which such growth alters the nervous system has not been extensively explored. Here we describe the organization of the somatosensory system in a 6-year-old monkey (Macaca radiata) born with a deformed left foot in comparison to the results from a normal monkey (Macaca fascicularis). Toes 1, 3, and 5 were missing, but the proximal parts of toes 2 and 4 were present. We used anatomical tracers to characterize the patterns of peripheral input to the spinal cord and brainstem, as well as between thalamus and cortex. We also determined the somatotopic organization of primary somatosensory area 3b of both hemispheres using multiunit electrophysiological recording. Tracers were subcutaneously injected into matching locations of each foot to reveal their representations within the lumbar spinal cord, and the gracile nucleus (GrN) of the brainstem. Tracers injected into the representations of the toes and plantar pads of cortical area 3b labeled neurons in the ventroposterior lateral nucleus (VPL) of the thalamus. Contrary to the orderly arrangement of the foot representation throughout the lemniscal pathway in the normal monkey, the plantar representation of the deformed foot was significantly expanded and intruded into the expected representations of toes in the spinal cord, GrN, VPL, and area 3b. We also observed abnormal representation of the intact foot in the ipsilateral spinal cord and contralateral area 3b. Thus, congenital malformation influences the somatotopic representation of the deformed as well as the intact foot.

Intracortical connections are altered after long‐standing deprivation of dorsal column inputs in the hand region of area 3b in squirrel monkeys

“…Another series of brainstem and spinal cord sections were processed for CTB staining to reveal label from tracer injections in digits 1, 3, and 5 of both hands. We used cytochrome oxidase (CO; Wong‐Riley, ) and vesicular glutamate transporter 2 (VGLUT2) staining (Liao et al, ; Qi et al, ) in separate series to reveal the architecture in the thalamus, brainstem, and spinal cord.…”

“…We measured areas and optical intensities of CTB‐labeled regions in the brainstem cuneate nuclei on the two sides using ImageJ 64 software (National Institutes of Health, Bethesda, MD). The areas and optical intensities of CTB‐labeled regions on the two sides across the brainstem sections were statistically compared using the signed‐rank test (SigmaPlot 11.0, San Jose, CA; see Liao et al, ). Statistical significance was considered at P < 0.05.…”

Brain-Computer Interfaces for Communication: Preferences of Individuals With Locked-in Syndrome

“…Surprisingly, apart from the hand, toes and feet were the most chosen body parts for cBCI control, even though only 6 (out of 9 selecting toes/feet) NMD and 2 (out of 7 selecting toes/feet) SO participants could still move their toes and/or feet to a certain extent. Another explanation could be the fact that feet are large body parts of which dexterity is evolutionarily close to that of the hand [44][45][46] and, therefore, easy to imagine/attempt control. Nevertheless, a biased choice for these body parts cannot be completely ruled out because feet and toes were the first and the last examples of 4 body parts shown in the animation video ("foot, head, tongue, or toes").…”

Organized Toe Maps in Extreme Foot Users

“…We investigated whether extreme habitual foot behavior might associate with organized toe maps in SI, where they have not been found in typically developed humans (Akselrod et al, 2017), although they are identified in monkeys (Liao et al, 2016;Nelson et al, 1980). Using 7 tesla neuroimaging during passive toe touch, we studied the foot representation of two individuals with developmental upper-limb amelia.…”