Cutaneous afferent innervation of the human foot sole: what can we learn from single-unit recordings?

Abstract

Cutaneous afferents convey exteroceptive information about the interaction of the body with the environment and proprioceptive information about body position and orientation. Four classes of low-threshold mechanoreceptor afferents innervate the foot sole and transmit feedback that facilitates the conscious and reflexive control of standing balance. Experimental manipulation of cutaneous feedback has been shown to alter the control of gait and standing balance. This has led to a growing interest in the design of intervention strategies that enhance cutaneous feedback and improve postural control. The advent of single-unit microneurography has allowed the firing and receptive field characteristics of foot sole cutaneous afferents to be investigated. In this review, we consolidate the available cutaneous afferent microneurographic recordings from the foot sole and provide an analysis of the firing threshold, and receptive field distribution and density of these cutaneous afferents. This work enhances the understanding of the foot sole as a sensory structure and provides a foundation for the continued development of sensory augmentation insoles and other tactile enhancement interventions.

From barefoot hunter gathering to shod pavement pounding. Where to from here? A narrative review

“…42 Sensory feedback from the skin interacts with descending motor commands at the spinal cord which allows reflexive modulation of motor neuron excitability. 43 The density of mechanoreceptors on the plantar aspect of the foot appears to follow a proximal-distal and mediolateral gradient, that is, a higher density of mechanoreceptors towards the lateral edge of the foot, the metatarsals and digits. 43 An increase in the density of mechanoreceptors improves tactile sensitivity to small changes in stimulus amplitude and spatial awareness.…”

“…43 The density of mechanoreceptors on the plantar aspect of the foot appears to follow a proximal-distal and mediolateral gradient, that is, a higher density of mechanoreceptors towards the lateral edge of the foot, the metatarsals and digits. 43 An increase in the density of mechanoreceptors improves tactile sensitivity to small changes in stimulus amplitude and spatial awareness. Control of balance and gait is dependent on the control of the centre of mass over a base of support.…”

“…An increase in mechanoreceptors towards the lateral edge of the foot and the toes is thought to be important to subtly identify the outer limits of the base of the support. 43 This would be less important medially, first because the risk of falling or ankle sprain would be lower and second, because a well-developed arch is rarely in contact with the ground when barefoot. The lower density of mechanoreceptors and therefore, higher threshold required to stimulate the heel may be due to the heel's role in absorbing force during ground contact on walking and the fact that it cannot be independently manipulated.…”

“…The lower density of mechanoreceptors and therefore, higher threshold required to stimulate the heel may be due to the heel's role in absorbing force during ground contact on walking and the fact that it cannot be independently manipulated. 43 Interestingly, noxious plantar stimulation of the heel or plantar surface near the metatarsal-phalangeal joints result in plantar and digit flexion. 42 44 The result of this in a weightbearing position is the redistribution of load towards the lateral edge of the foot and distal digits as the arch rises.…”

Serial Histological Sectioning of the Intact Mouse Hind Paw for Quantification of the Anatomical Distribution of Meissner-like Corpuscles across the Skin

“…The skin forms a barrier with the surrounding environment, protecting the body from chemical, mechanical and thermal exposure and enabling the sensation of heat, cold, pain and touch [1][2][3]. Mammalian glabrous skin is specialized for dis-criminative touch and mobility, enabling controlled movements during gait and balance [2,4]. Critical to this function are afferent units, consisting of an afferent sensory neuron and a mechanoreceptor at its terminus in the dermis or epidermis.…”