Human beings are provided with a variety of senses that help them navigate the world around them, including five basic ones: sight, hearing, touch, smell, and taste. The organs associated with each of the latter report on certain sensations to the brain, which then translates them into understandable information, via a complex, yet fascinating process. Bright colours, a loud thud, an acute pain, a familiar flavour on the tongue or a sweet smell that tickles the nose – all these stimuli are put together into one big picture for us to identify our surroundings. However, while these systems are remarkably sophisticated ones in humans, some animals have super sensors. Felines are well-known for their nocturnal vision, elephants have the most powerful nose of the animal world, while bats rely on sound waves to hunt. Magnetoreception – the ability to detect the Earth’s magnetic field, is even considered a sixth sense that birds, along with certain mammals, reptiles, and fish, are gifted with.
The Physics of the Senses 101 series offers to explore the physical processes that make up each sense, including the so-called sixth sense, and to explain the extent to which they grant certain species 'super-capacities'.
3. Physics of the Senses 101: Haptic Perception or Touch
4. Physics of the Senses 101: Olfaction or Smell
5. Physics of the Senses 101: Gustatory Perception or Taste
6. Physics of the Senses 101: Magnetoreception or a Sense without a Receptor
As discussed in earlier articles of this series, vision and hearing are often considered the primordial senses when it comes to experiencing the world. As Hutmacher (2019) points out: ‘[a]s long as you are awake, it is hard to prevent the visual impressions and changes in your environment from entering your consciousness’ (p.4). Third in line in this so-called hierarchy of the senses is touch – or haptic perception (Hutmacher, 2019, p.1). While not fully understood yet, this sense stands alone in having no defined medium: there is indeed ‘no articulated functional equivalent to air and water […] that make possible the transmissions and diffusions underpinning the other perceptual systems’ (Turvey & Fonseca, 2013, p.1). What is known, however, is that touch is a proximal sense, i.e., stimulated directly rather than distantly, that functions through an array of somatic receptors in the skin. The signals travel along sensory nerves to the brain to be translated into vital information (Sybesma, 1989). This article will therefore delve into details of the mechanism involved in this intriguing sense, explore the consequences of its impairment, before studying the abilities that animals with hypersensitivity to touch exhibit, notably spiders and moles.
To start with, Kern (2004) defines haptic perception as the ‘sum of signals from a large number of measurement points distributed among the human body, consisting of at least 6 types of sensors’ (p.36). More generally, the somatosensory system, i.e., the ‘elements of the peripheral nervous system (PNS) and the central nervous system (CNS)’ (Arezzo et al., 1982, p.1), subserves various modalities. Touch is indeed a shorthand for a group of senses, including, but not limited to, contact with neighbouring objects, vibrations, pressure, pain, temperature, and kinaesthesia. The haptic system uses sensory information derived from tactile receptors, located ‘in the outer areas of the skin in exposed positions like the fingertips (Kern, 2004, p.36), in combination with kinaesthetic receptors embedded in muscles, tendons, and joints (Lederman & Klatzky, 2009).
The perception of actual touch, that is contact with the skin, is conveyed by cutaneous inputs from four different types of mechanoreceptors – sensory transducers that transform mechanical energy into electrical pulses, which ‘are referred to collectively as low-threshold (or high-sensitivity) because even weak mechanical stimulation of the skin induces them to produce action potentials’ (Purves et al., 2004, p.192). To begin, Meissner’s corpuscles are the most common mechanoreceptors of glabrous skin, i.e., smooth and hairless. They lie ‘just beneath the epidermis of the fingers, palms and soles, are elongated receptors formed by a connective tissue capsule’ (p.192), at the centre of which several afferent nerve fibres generate ‘rapidly adapting action potentials following minimal skin depression’ (p.192). These fibres represent around 40% of the human hand’s sensory innervation and are particularly efficient at relaying information from low-frequency vibrations ‘that occur when textured objects are moved across the skin’ (p.192).
Next in order, Pacinian corpuscles, large, encapsulated endings with an onion-like capsule, allow transient disturbances at high frequencies to activate nerve endings. They have a lower response threshold, suggesting they are involved in the discrimination of fine surface textures. To continue, Merkel’s disks, particularly dense in the fingertips, lips, and external genitalia, are slowly adapting mechanoreceptors that produce a sensation of light pressure. They are supposed to ‘play a major role in the static discrimination of shapes, edges, and rough textures’ (Purves et al., 2004, p.193). Lastly, Ruffini’s corpuscles, ‘although structurally similar to other tactile receptors, are not well understood’ (p.193). Located deep in the skin, as well as the ligaments and tendons, they are especially sensitive to cutaneous stretching caused by digit or limb movements. They ‘probably respond primarily to internally generated stimuli’ (p.193), thus being in part responsible for kinaesthesia – or proprioception, that is the sense of self-movement, force and body position.
The proprioceptors are indeed a distinct major class of receptors, that provide detailed and continuous information about the spatial position of the limbs, drawing on stimuli ‘arising from the body itself, the musculoskeletal system in particular’ (Purves et al., 2004, p.197), which is essential to performing complex movements accurately. Such sensory knowledge is provided by low-threshold mechanoreceptors, such as muscle spindles, Golgi tendon organs, and joint receptors. Muscle spindles are located in most of the skeletal muscles. They consist of four to eight encapsulated intrafusal fibres, that inform about muscle length, i.e., the degree of stretching. The density of spindles relates to the importance and difficulty of the muscles’ tasks. For example, large muscles associated with coarse movements are more poorly supplied with spindles than extraocular muscles or the hand and neck’s intrinsic muscles, allowing for accurate eye, head, and finger movements. Besides, Golgi tendon organs tell the central nervous system about changes in muscle tension, while ‘rapidly adapting mechanoreceptors in and around joints gather dynamic information about limb position and joint movement’ (Purves et al., 2004, p.199).
On the other hand, pain and temperature perceptions, respectively nociception and thermoception, are sensed by receptors referred to as free nerve endings that ramify vastly in the upper regions of the dermis, epidermis, along with some deeper tissues. The relatively unspecialized nociceptors initiate the sensation of pain when mechanical force or chemicals released by damage or inflamed tissue activate the nerve endings. Like other receptors, they transform such stimuli into potentials, which in turn trigger afferent action potentials, then register them as sharp, dull, or aching pain, that are harder to ignore than other types of sensory information. Moreover, visceral nociceptors in the internal organs, convey sensations typical of diseases (Purves et al., 2004). Further experiments demonstrated that ‘nociception involves specialized neurons, not simply greater discharge of the neurons that respond to normal stimulus intensities’ (Purves et al., 2004, p.210).
Lastly, ‘[t]he two additional peripheral receptor populations known as thermoreceptors […] respond to increases or decreases in skin temperature, and mediate the human experiences of warmth and cold, respectively’ (Lederman & Klatzky, 2009, p.1440). They are also non-specialized receptors, that give rise to sensations of cooling or heating by detecting absolute and relative changes in temperature, generally in the innocuous range. They both contribute to the ‘total sensory information about an object making contact with the skin’ and assess ‘body heat loss or gain, that occurs over a large part of the body surface’ (Darian-Smith & Johnson, 1977, p.146). Recent breakthroughs in Physiology and Medicine include the 2021 Nobel Prize awarded to Julius and Patapoutian for their discovery of channels explaining the initiation of nerve impulses by temperature and mechanical force, allowing humans to perceive and adapt to the world around them (The Nobel Prize, 2021).
Subsequently, the information processed by the somatosensory systems travels along different anatomical pathways. For example, discriminative touch and proprioceptive signals from the body follow the posterior column-medial lemniscus pathway, while the spinothalamic pathways carry crude touch, pain, and temperature (Dougherty, 2020). Nerve impulses carry somatic sensations through fibres to their respective neurons near the spinal cord. Neurotransmitters are then released to pass the signal on to the spinal cord itself, which in turn runs up to the brain. Some signals reach the somatosensory cortex, whereas others connect at the thalamus (Arezzo at al., 1982). Through connections with other brain regions, the various sensations combine to form a coherent experience: the perception of the physical self along with the contact with its surroundings, commonly referred to as touch.
Given the variety of sensations received through touch, this sense is essential to human survival. Hutmacher (2009) even argues that haptic abilities seem to be even more vital than visual or auditory perceptions. Indeed, a hypothesis has been formulated since the 1970s that skin-to-skin contact between a baby and its parents in the first hour after birth had long-term health advantages, implying the necessity of physical contact for the development of any individual (Widström et al., 2019). Moreover, Hutmacher (2009) explains that, since the human visual system is severely underdeveloped at birth, new-borns haptically explore objects that they are later able to visually recognize, suggesting that the sense of touch plays an essential role in developing the sense of sight. Lastly, when considering the very rare condition of congenital insensitivity to pain, Hutmacher (2009) points out that although ‘these people have no cognitive defects, they often die in childhood and generally have a decreased life expectancy’ (p.4). Severely haptically impaired individuals are indeed put at very high risk, as they are not
able to determine if a bone is broken or if they have bitten off the tip of their tongue unless they see the swelling of the surrounding tissue or taste blood in their mouth. Because of this inability to sense pain, it is common for patients with congenital insensitivity to pain to have unseen infections as well as have a multitude of bruises, wounds, and broken bones over their lifetime (Hellier, 2016, p.118).
Conversely, hypersensitivity to touch leads to surprising abilities that only a few species exhibit. Spiders are an example of those, being ‘densely covered by an intriguingly large number of mechanoreceptive hairs on their exoskeleton’ (Barth, 2016, p.1). These hairs represent first-order lever arms – the most sensitive of which are the trichobothria. They respond to the slightest movement of air, and their deflection triggers nervous impulses in the cell endings at their base. Moreover, ‘the sensory cells in the [spider’s] body´s periphery by far outnumber the neurons in the central nervous system’ (Barth, 2020, p.9). As a result, spiders are able to detect vibrations caused by surrounding objects or animals, and trace their origin, making them formidable ‘sit and wait’ hunters (Barth, 2020, p.11):
Being alerted by vibrations of the substrate (a plant) they commonly wait motionlessly until the prey (such as a cockroach or an earwig) producing them comes within the reach of a jump. The spiders change from being motionless to moving like a flash within milliseconds’ (Barth, 2020, p.11).
Nevertheless, it’s the star-nosed mole’s snout that is considered one of the world's most sensitive sensory organs. The twenty-two appendages ringing its nostrils, that are in constant motion as the animal explores its environment, are covered with sensory domes referred to as Eimer’s organs (Catania, 2011), and are used for touching rather than smelling. Each is ‘associated with a Merkel cell–neurite complex, a lamellated corpuscle, and a series of 5–10 free nerve endings that form a circle of terminal swellings’ (Catania, 2011, p.1). Studies suggest that the purpose of these organs is to detect small shapes and textures. Remarkably, the ‘centre of the star is a tactile fovea used for detailed exploration of objects and prey items’ (Catania, 2011, p.1), meaning the moles, which are nearly blind mammals, are able to see through touch. Not only do star-nosed moles put their noses to the ground approximately 10 to 15 times per second (Catania, 2012), they are also the ‘fastest known foragers among mammals, able to identify and consume a small prey item in 120 ms’ (Catania, 2011, p.1).
To summarize, haptic perception is a highly complex sense whose features are yet to be fully understood. While skin can be considered the primary sensory organ, touch is, in fact, a conglomerate of perceptions, including physical contact, pressure, temperature changes, varying degrees of pain, kinaesthesia, and even internal stimuli. Each of these rely on specific receptors that transduce mechanical forces into electrical impulses that travel along the spinal cord to be reconstructed in the brain as a multidimensional experience. Touch is therefore the most essential sense in allowing for physical self-perception as well as communion with the environment, and risk assessment. Haptic impairment has indeed been linked to many diseases and lower life expectancy. On the other hand, hypersensitivity to touch, although scarce, leads to some incredible abilities in the animal world, such as for spiders and star-nosed moles. While touch remains relatively less undestood than the other sensory modalities due to its unique complexity, the very recent Nobel Prize awarded discoveries have sparked intensive research, focusing on elucidating the functions of thermoreceptors in a variety of physiological processes. Such knowledge will hopefully lead to significant breakthroughs in the development of treatments for wide-ranging health conditions, including chronic pain (The Nobel Prize, 2021).
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Figure 1: Devenice, L. (2021). The Creation of Adam Hands Painting [Image] https://www.saatchiart.com/art/Painting-The-Creation-of-Adam-Hands/1065333/8892881/view.
Figure 2: Purves et al. (2004). Characteristics of Tactile Mechanoreceptors [Image]
Figure 3: Sinauer Associates, Inc. (2016). Sensation and Perception, Figure 12.18 [Image] https://documen.site/download/il-tatto_pdf.
Figure 4: The Nobel Prize. (2021). Figure 4 - Seminal discoveries by laureates have explained how heat, cold and touch can initiate signals in our nervous system [Image]. https://www.nobelprize.org/prizes/medicine/2021/press-release/.
Figure 5: Clark, L. (2019). Somatosensory system, running from brain to hand, extends imaginatively outward and into the stick the woman holds [Image] https://www.scientificamerican.com/article/the-brain-senses-touch-beyond-the-body/.
Figure 6: Nightmaresyrup. (2020). Work Doddle, A Star-Nosed Mole Working From Home [Image] https://nightmaresyrup.tumblr.com/post/617259758085341184/work-doodle-a-star-nosed-mole-working-from-home.