Kumar Siddarth, Liu Gang, Schloerb David W, Srinivasan Mandayam A
J Biomech Eng. 2015 Jun;137(6):061002. doi: 10.1115/1.4029985. Epub 2015 Mar 18.
When we touch an object, surface loads imposed on the skin are transmitted to thousands of specialized nerve endings (mechanoreceptors) embedded within the skin. These mechanoreceptors transduce the mechanical signals imposed on them into a neural code of the incident stimuli, enabling us to feel the object. To understand the mechanisms of tactile sensation, it is critical to understand the relationship between the applied surface loads, mechanical state at the mechanoreceptor locations, and transduced neural codes. In this paper, we characterize the bulk viscoelastic properties of the primate finger pad and show its relationship to the dynamic firing rate of SA-1 mechanoreceptors. Two three-dimensional (3D) finite element viscoelastic models, a homogeneous and a multilayer model, of the primate fingertip are developed and calibrated with data from a series of force responses to micro-indentation experiments on primate finger pads. We test these models for validation by simulating indentation with a line load and comparing surface deflection with data in the literature (Srinivasan, 1989, "Surface Deflection of Primate Fingertip Under Line Load," J. Biomech., 22(4), pp. 343-349). We show that a multilayer model with an elastic epidermis and viscoelastic core predicts both the spatial and temporal biomechanical response of the primate finger pad. Finally, to show the utility of the model, ramp and hold indentation with a flat plate is simulated. The multilayer model predicts the strain energy density at a mechanoreceptor location would decay at the same rate as the average dynamic firing rate of SA-1 mechanoreceptors in response to flat plate indentation (previously observed by Srinivasan and LaMotte, 1991 "Encoding of Shape in the Responses of Cutaneous Mechanoreceptors," Information Processing in the Somatosensory System (Wenner-Gren International Symposium Series), O. Franzen and J. Westman, eds., Macmillan Press, London, UK), suggesting that the rate of adaptation of SA-1 mechanoreceptors is governed by the viscoelastic nature of its surrounding tissue.
当我们触摸一个物体时，施加在皮肤上的表面负荷会传递给嵌入皮肤内的数千个专门的神经末梢（机械感受器）。这些机械感受器将施加在它们身上的机械信号转换为入射刺激的神经编码，使我们能够感知物体。为了理解触觉的机制，关键是要了解施加的表面负荷、机械感受器位置的机械状态以及转换后的神经编码之间的关系。在本文中，我们描述了灵长类动物指垫的整体粘弹性特性，并展示了其与SA-1机械感受器动态放电率的关系。我们开发了灵长类动物指尖的两个三维（3D）有限元粘弹性模型，一个均匀模型和一个多层模型，并用来自对灵长类动物指垫进行微压痕实验的一系列力响应数据进行了校准。我们通过模拟线载荷压痕并将表面挠度与文献数据（Srinivasan，1989年，《线载荷下灵长类动物指尖的表面挠度》，《生物力学杂志》，22(4)，第343 - 349页）进行比较来测试这些模型的有效性。我们表明，具有弹性表皮和粘弹性核心的多层模型能够预测灵长类动物指垫的空间和时间生物力学响应。最后，为了展示该模型的实用性，模拟了平板的斜坡和保持压痕。多层模型预测，在平板压痕时，机械感受器位置处的应变能密度将以与SA-1机械感受器的平均动态放电率相同的速率衰减（先前由Srinivasan和LaMotte观察到，1991年，《皮肤机械感受器反应中形状的编码》，《躯体感觉系统中的信息处理》（文纳 - 格伦国际研讨会系列），O. Franzen和J. Westman编，麦克米伦出版社，英国伦敦），这表明SA-1机械感受器的适应速率受其周围组织的粘弹性性质支配。
