Touch

TACTILE EXPERIENCE

Vibration, Touch, Pressure, and Pain

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 recognize 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 fibers 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 behaviors.

Tactile Experience: Vibration, Touch, Pressure, and Pain

Our human sense of touch is most highly developed in the hand where it serves a cognitive function when experiencing objects in the world and guides the skilled movements of the surgeon, the sculptor, and the musician. Tactile information teaches us the physical properties of objects, allowing us to identify them even in the dark. Sensory receptors in the skin provide information to the brain about the size and shape of objects held in the hand. These receptors allow us to perceive whether objects appear hard or soft, smooth or rough in texture, heavy or light in weight, hot, cold or neutral in temperature, and whether the overall sensation produces pain or pleasure.

The tactile sense is one of the several submodalities of the somatic sensory system – the sense of one’s own body. When the body is contacted by an external stimulus, its surface is indented or stretched because the skin is flexible rather than rigid. The mechanical deformation is detected by receptors that signal where contact is made, the amount of force that is exerted, and the speed of motion against the surface. Contact is experienced as light touch or pressure, or even pain, depending on how much force is exerted. When the stimulus moves on the skin, touch is perceived as stroking, tapping, or vibration.

Sensations of touch are often accompanied by temperature sensations of warmth or cold, or by painful or itching sensations because the receptors for touch are intermixed in the skin with other sense organs that detect thermal energy or chemicals released by tissue damage or applied to the skin. We experience these as distinct sensory modalities because the information is processed by different sets of neurons in the central nervous system, and conveyed to the cerebral cortex in separate anatomical pathways.

Primary Afferent Terminals Specialised for the Transduction of Tactile Information

The neurobiological processes that underlie sensations of touch begin with the sensory transduction mechanisms by which physical deformation of the skin is transformed into electrical signals.

The intensity of contact force and speed of motion are detected by special sense organs in the skin called mechanoreceptors, so-called because they detect mechanical energy applied to the skin.

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The elasticity of the skin enables these receptors to detect the shape, texture, and pressure exerted by the object because the skin can conform to the contours of the object, forming a mirror image of its shape. This information is translated into a pulse (action potential) code that is conveyed to the central nervous system by the peripheral nerves. The mechanoreceptors of the skin, like other somatic sense organs, comprise the distal terminals of the dorsal root ganglion neurons or trigeminal sensory neurons. Trigeminal sensory neurons innervate the face and head; the dorsal root ganglion neurons innervate the other parts of the body. These sensory neurons, called primary afferents, have three major components:

  • A cell body that lies in a ganglion on the dorsal root of a spinal or trigeminal nerve
  • A peripheral branch that terminates in a specialised receptor ending.
  • A central branch that projects to the central nervous system.

Primary afferent fibers originating in specific regions of the body are gathered in small bundles or fascicles that join to form the peripheral nerves. They enter the central nervous system through the dorsal roots or the sensory branches of the fifth cranial nerve.

Types of mechanoreceptors in the skin

Individual primary afferent fibers respond selectively to specific types of stimuli because of the morphological and molecular specialization of their peripheral terminals. Unlike other sensory afferents in the skin, mechanoreceptors have a non-neural capsule that surrounds the distal endings. Some of the primary afferent fibers branch and have separate capsular receptors on each end; others have a single large capsule that surrounds the entire nerve terminal. The capsular structures link the nerve terminal to the surface of the body and therefore play a crucial role in transducing the tissue deformation that occurs when something contacts the skin. Although the sensitivity of the receptors to mechanical displacement is a property of ionic channels in the nerve terminal membrane, their response to touch is also shaped by the capsule.

Several major classes of mechanoreceptors have been identified in the human body The principal touch receptors in the glabrous (hairless) skin of the lips, palm, fingers, and sole of the foot are the Meissner corpuscle and the Merkel cell–neurite complex. These receptors are located close to the surface of the skin, at precise locations in the papillary ridges that form the fingerprint patterns. The anatomical arrangement of these receptors in the fingertip skin provides a precise grid for the detection of spatial features such as Braille dots. The hairy skin of the hand dorsum and the other parts of the body senses touch with hair follicle afferents, field receptors, and Merkel cells.

Touch is also detected in both the skin types by Pacinian corpuscles and Ruffini endings in the subcutaneous tissue. Each hand has approximately 150 000 mechanoreceptors which are connected to the central nervous system by 30 000 primary afferent fibers. The density of these receptors is highest on the fingertips (2500 per cm2). Each fingertip is innervated by 250–300 mechanoreceptive fibers. This large number of nerves confers fine tactual acuity to the fingertips, enabling them to read Braille and to discriminate surface texture.

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Figure | Images of the principal touch receptors in the skin. 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 fibers. SA1 fibers innervate clusters of neighboring Merkel cells (yellow arrow) in the center of the ridge, providing localized signals of pressure applied to the finger. The fibers lose their myelin sheaths (red) when entering the receptor capsule exposing broad terminal bulbs (green) where sensory transduction occurs.

Mechanoreceptors are specialized for pressure and motion

A morphological specialization of touch receptors allows them to discriminate the amount of force applied to their receptive field, the speed of motion during stroking or pressing on an object, the fine details of the surface texture, the local curvature in the region of contact, and the posture and shape of the hand when an object is grasped. These intensive and temporal properties are reflected in the

physiological responses to pressure. Meissner corpuscle (Figure) is the principal rapidly adapting receptor (RA) in the hand. These receptors respond to initial contact and to move, but not to steady pressure. The capsule is linked by collagen filaments to the lateral edge of the fingerprint ridge, positioning it to sense the tangential shearing forces induced when the hand is moved across a textured surface or over the edges.

The surface irregularities are signaled by bursts of action potentials in the sensory nerve innervating the Meissner corpuscle (RA1 fiber). The Meissner corpuscle also senses the motion of an object grasped by the hand when it slips unexpectedly, signaling us to tighten the grip to prevent the object from falling Hair follicle afferents and field receptors serve a similar physiological role in hairy skin. The Merkel cell receptor is the principal slowly adapting (SA) receptor in the hand. It resides at the center of the fingerprint ridge where skin elasticity is greatest (Figure). This small epithelial cell transmits compressive strain to the sensory nerve ending, evoking sustained responses that are proportional to the pressure applied to the skin.

Thus, when an object is placed in the hand, the frequency of firing conveys information about its weight; the heavier the object, the higher the firing rate of the Merkel cell afferents (SA1 fibers). Similarly, these receptors sense the grip forces applied by the fingers as an object is enclosed and held by the hand. Because individual Merkel cells are the smallest touch receptors, they also provide high-fidelity information about the spatial structure of objects that is the basis of form and texture perception.

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