Touch

Abstract

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 inform the brain about the form, weight, motion, vibration and hand posture that define each object. Parallel messages from ∼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. 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. The Ruffini endings – located along the folds of the palm, over the joints and along the margins of the fingernails – respond to stretch of the skin, thereby signalling posture and movements of the body including the hand and mouth. 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.