Active Touch Laboratory, Department of Psychology, University of Sheffield, Sheffield, United Kingdom.
Sheffield Robotics, University of Sheffield, Sheffield, United Kingdom.
J Neurophysiol. 2020 Oct 1;124(4):1229-1240. doi: 10.1152/jn.00313.2020. Epub 2020 Sep 23.
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 ∼230,000 tactile afferent fibers (plausible range: 200,000-270,000), with a subsequent decrement of 5-8% every decade due to aging. Fifteen percent 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.
皮肤是我们最大的感觉器官,由传入纤维支配,这些纤维将触觉信息传递到脊髓和大脑。不同类别的触觉传入纤维支配皮肤的密度不是恒定的,而是在不同的身体区域有很大的变化。然而,只有一些身体部位(如手)有确切的传入密度估计值,而触觉传入纤维总数的估计值不一致且不完整。在这里,我们使用来自背根纤维计数、微神经生理学、组织学和心理物理学的证据,调和了不同的估计值,并为不同类型的触觉纤维支配皮肤不同区域的数量提供了合理的范围和最佳估计值。我们估计,年轻人全身的皮肤由大约 230,000 个触觉传入纤维支配(合理范围:200,000-270,000),由于衰老,每十年减少 5-8%。15%的纤维支配双手手掌皮肤,19%的纤维支配面部和嘴唇周围区域。慢适应和快适应纤维大致均匀分布,但这种分布随皮肤区域而变化。传入密度与不同身体区域的心理物理空间敏锐度密切相关,此外,在有毛发的皮肤上,与毛囊密度相关。传入密度与皮质躯体感觉代表区的大小也有微弱的相关性,但不能完全解释手和脸的放大。