This is the fourth in a series examining senses. The previous articles were: an overview; chemosensation; and thermosensation. This week, the sense examined is mechanosensation – the sense of touch, which is basically the ability to sense changes in force. Of all the senses so far examined, mechanoreception is the one that makes the most intuitive sense because it connects us to our physical reality. It is used to sense pressure changes in touch and smoothness, stretching of muscles and ligaments, percussive vibrations and smooth flowing sensations. We apply it when we enjoy the contrasting textures of the creaminess of brie on a crisp cracker or slam on the brakes to stop the car; when we inhale deeply of the freshness of a spring morning and when we receive the first sting of the season.
Our skin and cell membranes contain “receptors” which are receivers that sense physical forces. These receptors are embedded inside nerves. Two receptors (Pacinian corpuscles, Ruffini endings) are deeply embedded in the dermis, and the other two (Meissner’s corpuscles and Merkel cells) are close to the surface of the skin (Figure 1). The deep Pacinian corpuscles sense vibrations and Ruffini endings sense stretching of the skin; Meissner corpuscles sense skin movement and Merkel cells sense sharp objects. There is also a distinction in whether they are slow-adapting for response to sustained pressure (Merkel cells and Ruffini endings) or fast-adapting for acute changes (Meissner’s corpuscles and Pacinian corpuscles).
Figure 1: The different mechanoreceptor types are located in different regions of the skin and are responsible for perception of different characteristics of a touch stimulus. Pacinian corpuscles and Ruffini endings are located deep in the dermis. Meissner corpuscles are located in the dermis near the epidermis, and Merkel cells are located in the epidermis, near the surface of the skin. ‘Mechanoreceptors’ by Casey Henley is licensed under a Creative Commons Attribution Non-Commercial Share-Alike (CC BY-NC-SA) 4.0 International License. Figure and legend reproduced from: openbooks.lib.msu.edu/neuroscience/chapter/touch-the-skin/
Ion channels as sensors was the subject of work for the 2021 Nobel Prize in Physiology or Medicine. [1] One half of the work, by Ardem Patapoutian’s laboratory at the Scripps Research Institute in La Jolla, California, was to identify a novel channel that responds to pressure, called piezo channels after the Greek word meaning pressure. They come in two types. Piezo1 channels are found in vasculature and participate in blood pressure and flow monitoring, whereas Piezo2 channels are associated with Merkel cells, which are enriched in highly sensitive rodent whiskers and human fingertips. The other half of the prize was awarded to David Julius for his discovery of TRPV1, the first Transient Receptor Potential (TRP) channel discovered. TRPs are evolutionarily conserved but functionally diverse channels which are capable of perceiving several stimuli, including heat and pain, light, chemicals and pressure. The TRPV4 (lung stretch and kidney osmosensation) and TRPC1 (stretch activation) channels are thought to be activated by direct pressure, although a host of other TRP channels seem to activate in response to pressure only secondarily to another stimulus. These include sensing cell migration, bone mineralization and blood flow. In summary, the Piezo1 and Piezo2 channels are clearly directly activated by pressure, whereas clarity on whether TRP channels are directly or indirectly activated is still awaited. [2]
So how do pressure-activated ion channels work? When these mechanoreceptors are deformed by pressure, they open, like a clothes peg would when the handles are squeezed. This distortion transiently opens channels in the membrane to allow the movement of cations, such as Ca2+, K+, Na+ and small amounts of Mg2+. The movement of these ions then triggers the nerves they are associated with to activate and send a signal to the brain. Sensations such as touch or vibration which reach the somatosensory cortex are consciously perceived. Proprioception (when the body senses its orientation in space) is dual – it can be consciously perceived and controlled, such as when you are balancing on a log, but is often subconscious and processed in the cerebellum and spinal cord. Blood pressure, lung stretch receptors and kidney osmosensation are subconscious and trigger responses from the brainstem or hypothalamus. Finally, durotaxis, which is the migration of cells towards stiffer areas, may be important for tissue repair following inflammation (after that first sting of spring), atherosclerosis, or the movement of cancer cells during metastasis. This is an active area of investigation for therapeutics for liver fibrosis after a deeply depressing winter and too much alcohol. [3]
Insects operate with some similar TRP channels but, for obvious reasons, lack the types of skin mechanosensors that mammals have. But they also have fast- and slow-adapting bristles which sense pressure and distinguish acute from sustained pressure respectively, campaniform sensilla which sense strain changes in the cuticle of their appendages, hair plates as proprioceptors, and chordotonal organs which sense stretching (see Figure 2). [4] In terms of the ion channels, a TRP family member called NOMPC functions as a direct mechanosensory ion channel in bristles and in the chordotonal organ. The piezo channel is implicated in nociception (pain) but not specifically in other mechanosensory factors, although it may contribute. [5] Frankly, this is so complicated, and there are so many homologs, paralogs and orthologs [6] that I couldn’t process it all to be clear here. Reference [5] provides such detail if you are interested.
Figure 2: Anatomy of mechanoreceptor organs on the fly leg. (A) Schematic of the Drosophila leg, illustrating the four classes of mechanoreceptor organs. (B) Mechanosensory bristles are the primary exteroceptive organs. In other insects, bristles are known as tactile hairs. (C) Campaniform sensilla are small domes which detect tension and compression in the surrounding cuticle. (D) Hair plates are tightly packed groups of small, stiff, parallel hairs, each of which is innervated by a single sensory neuron. They function as proprioceptors, sensing movements of one joint segment relative to the adjoining segment. (E) Chordotonal organs are stretch-sensitive mechanoreceptors that contain many individual sensory neurons with diverse mechanical sensitivities. They are found at leg joints, where they encode the angle and movement of the leg, as well as in Johnston’s organ in the fly antenna, where they encode auditory signals. Image and (shortened) legend from Reference [4].
Some of the applications to which understanding mechanosensation has been applied are developing intelligent tactile skin [7] and self-regulating locomotion [8] for robots. After all, it may be necessary to have little mechanical bees flying around pollinating our crops if we continue to destroy the environment. In fact, biomimetic robots with a hivemind are being developed to teach real bees to avoid dangerous food patches by doing waggle dances. [9] Hopefully there will be real bees around to lead to safe forage in a few decades.
References
[1] Earley, S., Santana, L.F. and Lederer, W.J. (2022). The physiological sensor channels TRP and piezo: Nobel Prize in Physiology or Medicine 2021. Physiological Reviews, 102(2), pp.1153–1158. doi:10.1152/physrev.00057.2021.
[2] Cox, C.D., Poole, K. and Martinac, B. (2024). Re-evaluating TRP channel mechanosensitivity. Trends in Biochemical Sciences, 49(8), pp.693–702. doi:10.1016/j.tibs.2024.05.004.
[3] Wikipedia Contributors (2025). Durotaxis. Wikipedia. https://en.wikipedia.org/wiki/Durotaxis
[4] Tuthill, J.C. and Wilson, R.I. (2016). Mechanosensation and adaptive motor control in insects. Current Biology, 26(20), pp.R1022–R1038. doi:10.1016/j.cub.2016.06.070.
[5] Goodman, M.B., Haswell, E.S. and Vásquez, V. (2023). Mechanosensitive membrane proteins: usual and unusual suspects in mediating mechanotransduction. The Journal of General Physiology, 155(3). doi:10.1085/jgp.202213248.
[6] Homolog = shared ancestry; ortholog = homologs that have diverged; paralog = homologs that arose from a gene duplication event.
[7] Hong, S.J., Lee, Y.R., Bag, A., Kim, H.S., Trung, T.Q., Sultan, M.J., Moon, D.-B. and Lee, N.-E. (2025). Bio-inspired artificial mechanoreceptors with built-in synaptic functions for intelligent tactile skin. Nature Materials, 24(7), pp.1100–1108. doi:10.1038/s41563-025-02204-y.
[8] Dallmann, C.J., Dickerson, B.H., Simpson, J.H., Wyart, C. and Jayaram, K. (2023). Mechanosensory control of locomotion in animals and robots: moving forward. Integrative and Comparative Biology, 63(2), pp.450–463.
[9] Lazic, D. and Schmickl, T. (2023). Will biomimetic robots be able to change a hivemind to guide honeybees’ ecosystem services? Bioinspiration & Biomimetics. doi:10.1088/1748-3190/acc0b9.
Acknowledgement: I used Claude AI (Anthropic) as a research assistant. But I found the papers cited, read them, and the article was written entirely by me.