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The softness and compliance of the skin plays a major role in the sense of touch. When an object contacts the hand, the skin conforms to its contours, forming a mirror image of the object surface. The resultant displacement and indentation of the skin stretches the tissue, thereby stimulating the sensory endings of mechanoreceptors at or near the region of contact.
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These receptors are highly sensitive and are continually active as we manipulate objects and explore the world with our hands. They provide information to the brain about the position of the stimulus on the skin, its shape and surface texture, the amount of force applied at the contact point, and how these features change over time when the hand or the object moves.
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Tactile sensations in the human hand arise from four kinds of mechanoreceptors: Meissner corpuscles, Merkel cells, Pacinian corpuscles, and Ruffini endings (Figure 23–1). The sense of touch can be understood as the combined result of the information provided by these four systems acting in concert.
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Each receptor responds in a distinctive manner depending on its morphology, innervation pattern, and depth in the skin. Receptors are innervated by either slowly adapting or rapidly adapting axons. Slowly adapting (SA) fibers respond to steady skin indentation with a sustained discharge, whereas rapidly adapting (RA) fibers stop firing as soon as the indentation is stationary. Sustained mechanical sensations from the hand must accordingly arise from the SA fibers. The sensation of motion across the skin is signaled by RA fibers. The receptors are further subdivided into two types based on size and location in the skin; each type includes both rapidly and slowly adapting fibers.
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Thus tactile sensation in the hand is mediated by four functional units: rapidly adapting type 1 (RA1), slowly adapting type 1 (SA1), rapidly adapting type 2 (RA2), and slowly adapting type 2 (SA2). Each unit consists of an afferent fiber, the fiber's distal branches, and the receptor organ(s) that surround the axon terminals (Table 23–1).
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Type 1 fibers terminate in clusters of small receptors in the superficial layers of the skin at the margin between the dermis and epidermis (Figure 23–1). RA1 fibers are the most numerous tactile afferents in the hand, reaching a density of approximately 2 per mm2 at the fingertip in man and monkey. The RA1 receptor organ, the Meissner corpuscle, is a globular, fluid-filled structure that encloses a set of flattened, lamellar cells originating from the myelin sheath (see Figure 21–6). The lamellae are coupled mechanically to the edge of the papillary ridge by collagen fibers, a relationship that confers fine mechanical sensitivity to frictional forces as the hand is moved across surfaces (Box 23–1). An RA1 axon typically innervates 10 to 20 Meissner corpuscles, integrating information from several adjacent papillary ridges. Each Meissner corpuscle is innervated by 2 to 5 RA1 axons (Figure 23–3A).
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Box 23–1 Fingerprint Structure Enhances Touch Sensitivity in the Hand
The histological structure of glabrous skin—the smooth, hairless skin of the palm and fingertips—plays a crucial role in the hand's sensitivity to touch. The fingerprints are formed by a regular array of parallel ridges in the epidermis, the papillary ridges (Figure 23–2).
Each ridge is bordered by epidermal folds—the limiting ridges—that are visible as thin lines on the fingers and palm border. The limiting ridges increase the stiffness and rigidity of the skin, protecting it from damage when contacting objects or when walking barefooted.
The fingerprints give the glabrous skin a corrugated, rough structure that increases friction, allowing us to grasp objects without slippage. Frictional forces are augmented further when these ridges contact the textured surfaces of objects. Smooth surfaces slide easily underneath the fingers and thus require greater grip force to maintain stability in the hand; the screw caps on bottles are often ridged to make them easy to turn. Frictional forces between the limiting ridges and objects also amplify surface features when we palpate them, allowing us to detect small irregularities such as the grain of wood.
The regular spacing of the papillary ridges—and the precise localization of specific receptors within this grid—allow us to repeatedly scan surfaces with back and forth hand movements while preserving a constant spatial alignment of adjacent surface features.
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SA1 fibers are also widely distributed in the skin, particularly in the fingertips. The SA1 receptor organs, the Merkel cells, consist of small epithelial cells that surround the terminal branches of an axon. Each Merkel cell encloses a semirigid structure that transmits compressive strain to the sensory nerve ending. Because there are synapse-like junctions between the Merkel cells and the SA1 axon terminals, it has been proposed that the mechanosensitive ion channels reside in the Merkel cells rather than in the nerve endings. Merkel cells are densely clustered in the center of each papillary ridge in glabrous skin (Figure 23–3A), placing them in an excellent position to detect deformation of the overlying skin, either from pressure above or lateral stretch. In hairy skin Merkel cells are localized in small clusters called touch domes (Figure 23–3B).
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Type 2 fibers innervate the skin sparsely and terminate in single large receptors in the deeper layers of the dermis or in the subcutaneous tissue (Figure 23–1). The receptors are larger and less numerous than the receptor organs of the type 1 fibers. The large size of these receptors allows them to sense mechanical displacement at some distance from the sensory nerve endings.
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The RA2 fibers terminate in Pacinian corpuscles located in the subcutaneous tissue (Figure 23–1). Each RA2 axon terminates without branching in a single Pacinian corpuscle, and each Pacinian corpuscle receives but a single RA2 axon. Pacinian corpuscles are large onion-like structures in which successive layers of connective tissue are separated by fluid-filled spaces. These layers surround the unmyelinated RA2 ending and its myelinated axon up to one or more nodes of Ranvier. The capsule amplifies high-frequency vibration, a role that is important for tool use. Estimates of the number of Pacinian corpuscles in the human hand range from 2,400 in the young to 300 in the elderly.
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The SA2 fibers innervate Ruffini endings concentrated at the finger and wrist joints and along the skin folds in the palm; they are relatively rare in the fingertips. The Ruffini endings are elongated fusiform structures that enclose collagen fibrils extending from the subcutaneous tissue to folds in the skin at the joints, in the palm, or in the fingernails. The SA2 nerve endings are intertwined between the collagen fibers in the capsule, and are excited by stimuli that stretch the receptor along its long axis.
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Receptive Fields Define the Zone of Tactile Sensitivity
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Individual mechanoreceptor fibers convey information from a limited area of skin called the receptive field. Tactile receptive fields have been determined in the human hand using microneurography.
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Åke Vallbo and Roland Johansson inserted microelectrodes through the skin into the median or ulnar nerves in the human hand and recorded the responses of individual afferent fibers. They found that in humans, as in other primate species, there are important differences between touch receptors, both in their physiological responses and in the structure of their receptive fields.
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Type 1 fibers have small, highly localized receptive fields with multiple spots of high sensitivity that reflect the branching patterns of their axons in the skin (Figure 23–4). Receptive fields on the fingertips are the smallest on the body, averaging 11 mm2 for SA1 fibers and approximately 25 mm2 for RA1 fibers. The fields are small because of the high density of receptors in the fingertips. Receptive fields become progressively larger on the proximal phalanges and the palm, consistent with the lower density of mechanoreceptors in these regions.
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In contrast, type 2 fibers innervating the deep layers of skin are connected to only a single Pacinian corpuscle or Ruffini ending. Because these receptors are large they collect information from a broader area of skin. Their receptive fields contain a single "hot spot" where sensitivity to touch is greatest; this point is located directly above the receptor (Figure 23–4).
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The receptive fields of type 1 fibers are significantly smaller than most objects that contact the hand. Thus RA1 and SA1 fibers detect small pieces of an object, signaling the properties of only a portion of its surface. As in the visual system, the spatial features of objects are distributed across a population of stimulated receptors with responses that are integrated in the brain to form a unified percept.
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Two-Point Discrimination Tests Measure Texture Perception
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The ability of humans to resolve spatial details of textured surfaces depends on which part of the body is contacted. Tactile acuity is highest on the fingertips and the lips, where receptive fields are smallest (Figure 23–5A). The separation that defines performance midway between chance and perfect discrimination, the threshold for spatial acuity, is approximately 1 mm on the fingertips of young adults; by the sixth or seventh decade of life it declines on average to approximately 2 mm. When we grasp an object we can discriminate features of its surface separated by as little as 0.5 mm. Humans are able to distinguish horizontal from vertical orientation of gratings with remarkably narrow spacing of the ridges (Figure 23–5B).
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Tactile acuity is slightly greater in women than in men and varies between fingers but not between hands. The distal pad of the index finger has the keenest sensitivity; spatial acuity declines progressively from the index to the little finger and declines rapidly at locations proximal to the distal finger pads. Tactile spatial resolution is 50% poorer at the distal pad of the little finger and six to eight times coarser on the palm.
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Tactile acuity on proximal parts of the body decreases in parallel with the growing size of receptive fields of SA1 and RA1 fibers (Figure 23–5A). When a pair of probes is spaced several millimeters apart on the hand, each of them is perceived as a distinct point because they produce separate dimples in the skin and stimulate nonoverlapping populations of SA1 and RA1 receptors. As the probes are moved closer together, the two sensations become blurred because both probes are contained within the same receptive field. The spatial interactions between tactile stimuli form the basis of neurological tests of two-point discrimination and texture recognition.
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Blind individuals use the fine spatial sensitivity of SA1 and RA1 fibers to read Braille. The Braille alphabet represents letters as simple dot patterns that are easy to distinguish by touch (Figure 23–6). A blind person reads Braille by moving the fingers over the dot patterns. This hand movement enhances the sensations produced by the dots.
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Because the Braille dots are spaced approximately 3 mm apart, a distance greater than the receptive field diameter of an SA1 fiber, each dot stimulates a different set of SA1 fibers. An SA1 fiber fires a burst of action potentials as a dot enters its receptive field and is silent once the dot leaves the field (Figure 23–6). Specific combinations of SA1 fibers that fire synchronously signal the spatial arrangement of the Braille dots. RA1 fibers also discriminate the dot patterns, enhancing the signals provided by SA1 fibers.
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Slowly Adapting Fibers Detect Object Pressure and Form
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The most important functional feature of the slowly adapting fibers (SA1 and SA2) is their ability to signal skin deformation and pressure. The sensitivity of an SA1 receptor to edges, corners, points, and curvature provides information about object shape, size, surface texture, and compliance. We perceive an object as hard or rigid if it indents the skin, and soft if the skin surface instead deforms the object.
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Paradoxically, as an object's diameter increases, the responses of individual SA1 fibers become weaker and the sensation less distinct. For example, the tip of a pencil pressed 1 mm into the skin feels sharp, unpleasant, and highly localized at the contact point, whereas a 1 mm indentation by the eraser feels blunt and broad. The weakest sensation is evoked by a flat surface pressed against the finger pad (Figure 23–7).
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To understand why these objects evoke different sensations, we need to consider the physical events that occur when the skin is touched. When a pencil tip is pressed against the skin it dimples the surface at the contact point and forms a shallow, sloped basin in the surrounding region (approximately 4 mm in radius). Although the indentation force is concentrated in the center, the surrounding region is also perturbed by local stretch, called tensile strain. SA1 receptors at both the center and the surrounding "hillsides" of skin are stimulated, firing spike trains proportional to the degree of local stretch.
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If a second probe is pressed close to the first one, more SA1 fibers are stimulated but the neural response of each fiber is reduced because the force needed to displace the skin is shared between the two probes. Ken Johnson and his colleagues have shown that as more probes are added within the receptive field, the response intensity of each fiber becomes progressively weaker because the displacement forces on the skin are distributed across the entire contact zone. Thus the skin mechanics results in a case of "less is more." Individual SA1 fibers respond more vigorously to a small object than to a large one because the force needed to indent the skin is concentrated at a small contact point. In this manner each SA1 fiber integrates the local skin indentation profile within its receptive field.
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The sensitivity of SA1 receptors to local stretch of the skin also enables them to detect edges, the places where an object's curvature changes abruptly. SA1 firing rates are many times greater when a finger touches an edge than when it touches a flat surface.
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The indentation force of a flat or gently curved surface is distributed symmetrically within the central contact zone, whereas the force applied by an object boundary displaces the skin asymmetrically, beyond the edge as well as at the edge. This asymmetric distribution of force produces enhanced responses from receptive fields located along the edges of an object. As edges are often perceived as sharp, we tend to grasp objects on flat or gently curved surfaces rather than by their edges.
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The SA2 fibers that innervate Ruffini endings respond more vigorously to stretch of the skin than to indentation because the receptors are located along the palmar folds or at the finger joints. The SA2 fibers therefore provide information about the shape of large objects grasped with the entire hand, the "power grasp" in which all five fingers press an object against the palm. They also provide information about hand shape and finger movements when the hand is empty. If the fingers are fully extended and abducted we feel the stretch in the palm and proximal phalanges as the glabrous skin is flattened. Similarly, if the fingers are fully flexed, forming a fist, we feel the stretch of the skin on the back of the hand, particularly over the metacarpal-phalangeal and proximal interphalangeal joints.
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The SA2 system may play a central role in stereognosis—the recognition of three-dimensional objects using touch only—as well as other perceptual tasks in which skin stretch is a major cue. Benoni Edin has shown that SA2 innervation in the hairy skin plays a substantial role in the perception of hand shape and finger position. The SA2 fibers aid the perception of finger joint angle by detecting skin stretch around the knuckles. The Ruffini endings near these joints are aligned such that different groups of receptors are stimulated as the fingers move in specific directions (Figure 23–4A). In this manner the SA2 system provides a neural representation of skin stretch over the entire hand, a proprioceptive rather than exteroceptive function.
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Rapidly Adapting Fibers Detect Motion and Vibration
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The RA1 receptor organ, the Meissner corpuscle, detects events that produce low-frequency, low-amplitude skin motion. This includes hand motion over the surface of objects, the detection of microscopic surface features, and low-frequency vibration. RA1 fibers contribute to detection of Braille patterns because they sense the change in skin indentation as individual dots pass over their receptive fields (Figure 23–6). They can detect irregularities and bumps as small as 10 m. We use the sensitivity of RA1 fibers to motion to adjust grip force when we grasp an object.
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The RA2 receptor, the Pacinian corpuscle, is the most sensitive mechanoreceptor in the somatosensory system. It is exquisitely responsive to high-frequency (30–500 Hz) vibratory stimuli, and can detect vibration of 250 Hz in the nanometer range (Figure 23–8). The buzzing sensation experienced when a tuning fork is pressed against the skin in a neurological examination is mediated by the synchronized firing of RA2 units. It is a useful measurement of dynamic sensitivity to touch, particularly in cases of localized nerve damage.
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The Pacinian corpuscle's filtering and amplifying of high-frequency vibration allows us to feel conditions at the working surface of a tool in our hand as if our fingers themselves were touching the object under the tool. The clinician uses this exquisite sensitivity to guide a needle into blood vessel and to probe tissue stiffness. The auto mechanic can use vibratory sense to position wrenches on unseen bolts. We can write in the dark because we feel the vibration of the pen as it contacts the paper and transmits the resulting frictional forces from the surface roughness to our fingers.
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Both Slowly and Rapidly Adapting Fibers Are Important for Grip Control
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In addition to their role sensing the size, shape, and texture of objects, mechanoreceptors provide important information concerning the actions of the hand during skilled movements. Roland Johansson and Gören Westling used microneurography to determine the role of mechanoreceptors when objects are grasped in the hand. By placing microelectrodes in the median nerve, they were able to record the firing patterns of individual mechanosensitive fibers as an object was initially contacted by the fingers, grasped between the thumb and index finger, lifted, held above a table, lowered, and returned to the rest position.
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They found that all four classes respond to grasp and that each fiber type monitors a particular function. The RA1, RA2, and SA1 fibers detect contact when an object is first touched (Figure 23–9). The SA1 fibers signal the amount of grip force applied by each finger, and the RA1 fibers sense how quickly the grasp is applied. The RA2 fibers detect the small shock waves transmitted through the object when it is lifted from the table and placed on another surface. We know when an object makes contact with the table top because of these vibrations and therefore can manipulate the object without looking at it. The RA1 and RA2 fibers cease responding after grasp is established. The SA2 fibers signal flexion or extension of the fingers during grasp or release of the object and thereby monitor the hand posture as these movements proceed.
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Signals from the hand that report on the shape, size, and texture of an object are important factors governing the application of force during grasping. Johansson and his colleagues have shown that we lift and manipulate an object with delicacy—with grip forces that just exceed the forces that result in overt slip—and that the grip force is adjusted automatically to compensate for differences in the friction coefficient between surfaces. Subjects predict how much force is required to grasp and lift an object and modify the grip force based on the tactile information provided by SA1 and RA1 afferents. Objects with smooth surfaces are grasped more firmly than those with rough textures, properties coded by RA1 afferents during initial contact of the hand with an object. The significance of the tactile information in grasping is seen in cases of nerve injury or during local anesthesia of the hand; patients apply unusually high grip forces, and the coordination between the grip and load forces applied by the fingers is poor.
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The information supplied by the RA1 receptors to monitor grasping actions is critical for grip control, allowing us to hold on to objects when perturbations cause them to slip unexpectedly. RA1 fibers are silent during steady grasp and usually remain quiet until the object is returned to rest and the grasp released. However, if the object is unexpectedly heavy or jolted by external forces and begins to slip from the hand, the RA1 fibers fire in response to the small tangential movements of the object. The net result of this RA1 activity is that grip force is increased by signals from the motor cortex.