Skin Biomechanics and Mechanoreceptor Firing during Manual Tactile Contact

The human hand is a complex system, with over 20 degrees of freedom. Perhaps even more impressive is how we control it, often relying on subconscious processes to accomplish daily tasks. Central to this control is our sensory feedback, particularly our sense of touch, at the forefront of our interaction with the environment. This tactile feedback stems from thousands of sensory afferents whose intricate workings remain only partially understood. Numerous aspects remain to be explored; from the molecular mechanisms of mechanotransduction to the integration of mechanoreceptors within the dermal layers, the specific skin deformations that trigger their response, and ultimately how this tactile feedback is used in the control loops governing hand movements. This thesis uses three approaches to study the biomechanics of the skin, how skin deformations elicit neural responses, and how neural signals might be used for motor control. In particular, we focus on impending slips, which could be an important signal of grip safety. The first approach centres on skin biomechanics of the human index finger under torsion. Through applying rotating stimuli to immobilised fingers, this study investigates the occurrence of partial slips and the resulting deformations of the finger pad. Our findings reveal a distinct annulus of shear-dominated strains propagating from the periphery of the contact area to its centre, contrasting with the local compression and dilation observed with translational loading. The second approach introduces an active manipulation setup to study both fingers of a pinch grasp. The thumb is often overlooked despite its critical role in forming a grasp by opposing the other fingers. Additionally, we pioneer using thimbles to alter tactile feedback that is less invasive than anaesthesia and more effective than gloves in preventing partial slips. We used the object to investigate how grip force (GF) adapts to different object weights without visual cues, aiming to understand the role of surface skin deformations in this adaptation. While participants wearing thimbles could still adjust their GF to object weight, their response was slower than when using their bare fingers. However, we found no evidence in the surface skin deformations to explain the rapid GF adjustments observed without thimbles. Notably, we observed a difference in the response between the two fingers, with the thumb showing more local net compressions and the index finger more local net dilations. The third approach develops an ex vivo animal model using rat forepaws to study the responses of cutaneous afferents to mechanical stimuli. We applied indentations and sliding stimuli to isolated forepaws, recording responses from cutaneous afferents with a glass suction electrode. The afferent subjected to slip showed firing activity during sliding but remained quiet during static contact. We used histological imaging to investigate mechanoreceptors’ end-organs, successfully visualizing two Meissner corpuscles by targeting neurofilament heavy chain with immunohistochemistry. Additionally, the role of the ion channels ENaC and ASIC was explored by administering amiloride to two afferents, which significantly altered their response to indentations in opposite ways. As these protocols cannot be performed in in vivo human research, our approach opens broad possibilities for studies on tactile feedback. Overall, this thesis introduces innovative methods that enable deeper studies into the sense of touch across species and different tactile scenarios. This provides a framework for comprehensive investigations into touch and facilitates comparative analyses across this spectrum of different conditions. We also make key findings such as new strain patterns under torque and for two-finger pinch grip. These results allow us to better understand GF adjustment and to formulate new ways to probe this topic in the future.