Abstract
Mechanical algesia is an important process for the preservation of living organisms, allowing potentially life-saving reflexes or decisions when given body parts are stressed. Yet, its various underlying mechanisms remain to be fully unraveled. Here, we quantitatively discuss how the detection of painful mechanical stimuli by the human central nervous system may, partly, rely on thermal measurements. Indeed, most fractures in a body, including microscopic ones, release some heat, which diffuses in the surrounding tissues. Through this physical process, the thermo-sensitive TRP proteins, that translate abnormal temperatures into action potentials, shall be sensitive to damaging mechanical inputs. The implication of these polymodal receptors in mechanical algesia has been regularly reported, and we here provide a physical explanation for the coupling between thermal and mechanical pain. In particular, in the human skin, we show how the neighboring neurites of a broken collagen fiber can undergo a sudden thermal elevation that ranges from a fraction to tens of degrees. As this theoretical temperature anomaly lies in the sensibility range of the TRPV3 and TRPV1 cation channels, known to trigger action potentials in the neural system, a degree of mechanical pain can hence be generated.
Highlights
ON RUPTURE AND ENERGY DISSIPATIONThe growth of mechanical damages through a body is an irreversible thermodynamic process [1]
The perception of pain arises from the bio-electrical signals that sensory neurons send from the aggressed body part to the nervous system (e.g, [26])
Before discussing further the preceding results, and their implication for the feeling of pain, let us acknowledge the simplicity of the model we have considered
Summary
The growth of mechanical damages through a body is an irreversible thermodynamic process [1]. In most engineering materials (e.g., [2]), this quantity, denoted G, is well studied, since it characterizes the loading necessary for a crack to propagate [3] It is in the order of 10 J m-2 in weak glasses [4] and can reach 100 kJ m-2 in the strongest media, as titanium [5] or steel [6]. There are many possible ways for it to be transformed, ranging from its storage as surface potential energy on the walls of the new fractures [8] to its emission to the far field as mechanical [9] or electromagnetic [10] waves, that is, sound and luminescence It was, in particular, shown that a significant part of the mechanical input is converted into heat close to the damage [11,12,13,14], as the rupture of stretched atomic and molecular bonds is prone to generate a local and incoherent -thermal- atomic motion. The related elevations in temperature have been measured in various synthetic solids (e.g., [12, 13, 15]), and are believed to be more than only a side effect of the
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