Touch

Abstract

Touch is defined as direct contact between two physical bodies. In neuroscience, touch describes the special sense by which contact with the body is perceived in the conscious mind. Touch allows us to recognise objects held in the hand and use them as tools. Because the skin is elastic, it forms a mirror image of object contours, allowing us to perceive their size, shape and texture. Four classes of mechanoreceptors use Piezo2 protein complexes to distinguish the form, weight, motion, vibration and hand posture that define each object. Parallel messages from approximately 20 000 nerve fibres are integrated by neurons in the cerebral cortex that detect specific object classes. Some touch involves active movement – stroking, tapping or pressing – whereby a limb is moved against another surface. The sensory and motor components of touch are connected anatomically in the brain and are important functionally in guiding skilled behaviours.

Key Concepts

  • The sense of touch is mediated by four mechanoreceptors located in the skin: the Meissner corpuscles, Merkel cells, Pacinian corpuscles and Ruffini endings. All of these receptors express the protein complex Piezo2 formed by three identical large protein subunits.
  • The most numerous touch receptors—Meissner corpuscles—detect textures and edges as the hand is moved over surfaces because of their location along the margins of the fingerprint ridges and signal the speed and direction of movement with rapidly adapting firing patterns.
  • The Merkel cells—small receptor cells clustered at the centre of the fingerprint ridge and in small domes elsewhere on the body—signal the weight, form and surface features of objects contacting the skin with a continuous, slowly adapting spike train proportional to pressure.
  • The most sensitive touch receptors—Pacinian corpuscles—provide sensory information from tools grasped or moved by the hand because of their high sensitivity to vibration transmitted through the object. They sense vibration through pens or pencils when writing or drawing.
  • Each mechanoreceptor signals information about touch applied to a small patch of skin—called its receptive field—that corresponds to the anatomical location of the receptor in the body.
  • Several thousand mechanoreceptors are stimulated in each finger when an object is grasped in the hand.
  • The information provided by individual touch receptors is transmitted in parallel via the dorsal columns, medial lemniscus and ventral posterior thalamus to the parietal lobe of the cerebral cortex where it is integrated to reconstruct a tactile image of the entire object.
  • The primary somatic sensory (S‐I) cortex—located in the postcentral gyrus—contains a topographic map of the body in which regions that are touched most frequently and densely innervated are magnified so that the majority of somatosensory cortical neurons encode touch information provided from the hands, feet or lips.
  • Responses of neurons in the second somatic sensory (S‐II) cortex—located on the upper bank of the lateral fissure—are modulated not only by touch information from mechanoreceptors in the skin but also by the context, subjective attention, behavioural significance and previous experience of similar stimuli.
  • The posterior parietal cortex integrates tactile, proprioceptive and visual information about object properties with corollary signals from motor centres in the cerebral cortex to guide hand actions when grasping and manipulating objects in skilled tasks.

Keywords: somatosensory system; tactile sense; skin senses; mechanosensation; stereognosis; hand

Figure 1. Cross section of the skin showing the major classes of cutaneous mechanoreceptors. Modified from Gardner EP, Martin JH and Jessell TM () The bodily senses. In: Kandel ER, Schwartz JH and Jessell TM (eds) Principles of Neural Science, 4th edn. pp. 430–450. New York: McGraw‐Hill.
Figure 2. Images of the principal touch receptors in the skin. (a) Meissner corpuscles and Merkel cells are revealed in immunostained confocal images of a papillary (fingerprint) ridge from the human fingertip. Meissner corpuscles (white arrows) are located below the epidermis (blue) along the lateral borders of each ridge; each corpuscle is innervated by at least two RA1 fibres. SA1 fibres innervate clusters of neighbouring Merkel cells (yellow arrow) in the centre of the ridge, providing localised signals of pressure applied to the finger. The fibres lose their myelin sheaths (red) when entering the receptor capsule exposing broad terminal bulbs (green) where sensory transduction occurs. Photograph courtesy of M Nolano; reproduced with permission from Nolano et al. (). (b) Photograph of a Pacinian corpuscle (∼1.6 mm in length) located in the mesentery of the abdominal wall. Each Pacinian corpuscle is innervated by a single RA2 fibre. Reproduced courtesy of S Bolanowski from Bell et al. ().
Figure 3. Receptive fields in the human hand mapped with single fibre recordings from the median nerve. Each coloured area on the hands indicates the receptive field of an individual sensory nerve fibre. Receptive fields of Merkel disk receptors and Meissner corpuscles cover spotlike patches of skin on the hand, and are smaller than those of Ruffini endings and Pacinian corpuscles because of differences in receptor cell size. SA1 and RA1 fibres innervate clusters of mechanoreceptors; SA2 and RA2 fibres innervate only one large receptor cell. The neural responses in the lower panels illustrate responses of the four fibre types to steady pressure on the skin. Modified from Johansson RS and Vallbo AB () Tactile sensory coding in the glabrous skin of the human hand. Trends in Neuroscience 6: 27–32.
Figure 4. The homotrimeric structure of Piezo2 ion channels as modelled from cryoelectron microscopy. Mammalian Piezo2 proteins contain approximately 2800 amino acid residues arranged in 38 transmembrane segments; three Piezo2 protein chains combine to form mechanosensory ion channels in touch receptor membranes. (a) Visualisation of the three‐dimensional structure of Piezo2 ion channels as viewed from three different angles; the homotrimeric structure resembles those previously constructed for Piezo1 channels (Guo and MacKinnon, ). (b) A side view of the surface electrostatic potential, showing the hydrophobic transmembrane region (marked by green dashed lines). The midplane opening diameter, depth, surface area (Adome) and projection area (Aproj) of the illustrated dome are labelled. The colour bar indicates the surface electrostatic potential, ranging from negative (red) to positive (blue). (c) Labelled models of the principal structural features of the ion channel in cell membranes. From Wang L, Zhou H, Zhang M, Liu W, Deng T, Zhao Q, Li Y, Lei J and Li X (2019) Structure and mechanogating of the mammalian tactile channel PIEZO2. Nature 573: 225–229.
Figure 5. Somatosensory areas of the cerebral cortex. (a) Lateral view of the brain showing primary (S‐I), secondary (S‐II) and posterior parietal areas. (b) Coronal section through the postcentral gyrus indicating the cytoarchitectural subdivisions of S‐I cortex, and their relation to S‐II cortex. (c) Schematic outline of the hierarchical connections to and from the S‐I cortex. Neurons projecting from the thalamus send their axons to areas 3a and 3b, but some also project to areas 1 and 2. Neurons in areas 3a and 3b project to areas 1 and 2. Information from the four areas of S‐I cortex is conveyed to neurons in the posterior parietal cortex (area 5) and S‐II cortex. From Gardner EP and Johnson KO (2012) The somatosensory system: receptors and central pathways. In: Kandel ER, Schwartz JH, Jessell TM, Siegelbaum SA and Hudspeth AJ (eds) Principles of Neural Science, 5th edn. pp. 475‐497. New York: McGraw‐Hill.
Figure 6. Spike trains recorded from neurons in Brodmann area 2 of the cerebral cortex in response to motion across their receptive fields; the direction of motion is indicated by upward and downward deflections in the lower trace and by arrows on the hands. (a) A motion‐sensitive neuron responds to stroking the skin in all directions. (b) A direction‐sensitive neuron responds strongly to motion towards the ulnar side of the palm but fails to respond to motion along the same path in the opposite direction. Responses to distal or proximal movements are weaker. (c) An orientation‐sensitive neuron responds better to motion across a finger (ulnar–radial) than to motion along the finger (distal–proximal) but does not distinguish ulnar from radial nor proximal from distal directions. From Gardner EP and Johnson KO (2012) The somatosensory system: receptors and central pathways. In: Kandel ER, Schwartz JH, Jessell TM, Siegelbaum SA and Hudspeth AJ (eds) Principles of Neural Science, 5th edn. pp. 475–497. New York: McGraw‐Hill. Reproduced with permission of McGraw‐Hill.
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Gardner, Esther P(Aug 2020) Touch. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0029142]