Tactile innervation densities across the whole body

Abstract

The skin is our largest sensory organ and innervated by afferent fibers carrying tactile information to the spinal cord and onto the brain. The density with which different classes of tactile afferents innervate the skin is not constant but varies considerably across different body regions. However, precise estimates of innervation density are only available for some body parts, such as the hands, and estimates of the total number of tactile afferent fibers are inconsistent and incomplete. Here we reconcile different estimates and provide plausible ranges and best estimates for the number of different tactile fiber types innervating different regions of the skin, using evidence from dorsal root fiber counts, microneurography, histology, and psychophysics. We estimate that the skin across the whole body of young adults is innervated by approximately 230,000 tactile afferent fibers (plausible range: 200,000-270,000), with a subsequent decrement of 5-8% every decade due to aging. 15% of fibers innervate the palmar skin of both hands and 19% the region surrounding the face and lips. Slowly and fast-adapting fibers are split roughly evenly, but this breakdown varies with skin region. Innervation density correlates well with psychophysical spatial acuity across different body regions, and additionally, on hairy skin, with hair follicle density. Innervation density is also weakly correlated with the size of the cortical somatotopic representation, but cannot fully account for the magnification of the hands and the face.

Personalized Biometrics of Physical Pain Agree with Psychophysics by Participants with Sensory over Responsivity

“…The peripheral nervous systems include an interconnected network of afferent nerve fibers carrying information from the skin to the spinal cord and onto the brain [1]. This flow of activity can be modeled as it updates the brain moment by moment, reflecting the trajectories of our bodies in motion [2,3] or of the fluctuations in bodily signals at rest [4][5][6][7], within a given environment where sensory input is processed and integrated with ongoing movements making up intended [8,9] or spontaneous [10] behavioral states.…”

“…This flow of activity can be modeled as it updates the brain moment by moment, reflecting the trajectories of our bodies in motion [2,3] or of the fluctuations in bodily signals at rest [4][5][6][7], within a given environment where sensory input is processed and integrated with ongoing movements making up intended [8,9] or spontaneous [10] behavioral states. The afferent fibers from the periphery carry information about touch, pressure and movements sensed by the mechanoreceptors [11], while thermoreceptors and nociceptors process information about temperature and pain, respectively [1,12]. Collectively, they give rise to the sense of touch, which is important to manipulate objects [13], to control our movements [14], to gain a sense of body ownership [15] and affection [16], and to develop and maintain our overall psychological and social wellbeing [17].…”

Neural response during temporal – and spatial luminance contrast processing and its manifestation in the blood-oxygen-level-dependent-signal in striate and extra-striate cortex

“…Analogous to the visual system, the somatosensory cortex (S1) receives input from fast-adapting and fast-conducting Pacini and Meissner corpuscle – and slow-adapting and slow-conducting Ruffini receptors and Merkel’s disc. The proportion of fast- and slow-adapting fibres entering S1 varies across the body, with the latter representing 61% of the input from the upper and 47% from the lower extremities [ 22 ] Noxious stimuli are transmitted via fast-conducting A-fibres and slow-conducting C-fibres. The fast-adapting A-fibres elicit the initial sharp, stabbing pain and the slow-adapting C-fibres the persisting dull, burning pain sensation that follows.…”

Feel-Good Requirements: Neurophysiological and Psychological Design Criteria of Affective Touch for (Assistive) Robots

“…This goes along with differences in the function of glabrous and hairy skin. While hairy skin is more relevant for affective touch, which is the topic of this review, glabrous skin is more involved in discriminative touch, although, both types of skin are able to receive mediated touch as for example recently discussed in Corniani and Saal (2020). In order to transfer physiological findings to technical systems such as robots, sensing and feeling of the environment by receptors in the skin has been studied and characterized primarily with respect to the manipulation of objects (Johansson and Flanagan, 2009;Dahiya et al, 2010), i.e., active touch (Gibson, 1962).…”