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.
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.
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.
Four mechanoreceptors are responsible for the sense of touch.
A cross section of the glabrous skin shows the principal receptors for touch. All of these receptors are innervated by large-diameter myelinated fibers. The Meissner corpuscles and Merkel cells lie in the superficial layers of the skin at the base of the epidermis, 0.5 to 1.0 mm below the skin surface. The Meissner corpuscles border the edges of each papillary ridge, whereas the Merkel cells form dense bands surrounding the sweat gland ducts along the center of the ridges. The RA1 and SA1 fibers that innervate these receptors branch at their terminals so that each fiber innervates several nearby receptor organs. The Pacinian and Ruffini corpuscles lie within the dermis (2–3 mm thick) and deeper tissues. The RA2 and SA2 fibers that innervate these receptors each innervate only one receptor organ. (RA1, rapidly adapting type 1; RA2, rapidly adapting type 2; SA1, slowly adapting type 1; SA2, slowly adapting type 2.)
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.
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).
Table 23–1Cutaneous Mechanoreceptor Systems ||Download (.pdf) Table 23–1 Cutaneous Mechanoreceptor Systems
| ||Type 1 ||Type 2 |
| ||SA1 ||RA11 ||SA2 ||RA22 |
|Receptor ||Merkel cell ||Meissner corpuscle ||Ruffini ending ||Pacinian corpuscle |
|Location ||Tip of epidermal sweat ridges ||Dermal papillae (close to skin surface) ||Dermis ||Dermis (deep tissue) |
|Axon diameter (m) ||7–11 ||6–12 ||6–12 ||6–12 |
|Conduction velocity (ms) ||40–65 ||35–70 ||35–70 ||35–70 |
|Best stimulus ||Edges, points ||Lateral motion ||Skin stretch ||Vibration |
|Response to sustained indentation ||Sustained with slow adaptation ||None ||Sustained with slow adaptation ||None |
|Frequency range (Hz) ||0–100 ||1–300 || ||5–1,000 |
|Best frequency (Hz) ||5 ||50 || ||200 |
|Threshold for rapid indentation or vibration (best) (m) ||8 ||2 ||40 ||0.01 |
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).
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.
The skin of the finger.
A. Scanning electron micrograph of the fingerprints in the human index finger. The glabrous skin of the hand is structured as arrays of papillary ridges and intervening sulci (limiting ridges) that recur at regular intervals. Globules of sweat exude from ducts at the center of the papillary ridges, forming a grid-like pattern along each ridge. The Merkel cells are located in dense clusters along the center of the papillary ridges between the ducts (see Figure 23–1). (Adapted, with permission, from Quilliam 1978.)
B. Histological section of the glabrous skin cut parallel to the skin surface. The cholinesterase-stained Meissner corpuscles form regularly spaced chains along both sides of each papillary ridge. Thus Meissner corpuscles and Merkel cells form alternating bands of RA1 and slowly adapting type 1 (SA1) touch receptors that span each fingerprint ridge. (Adapted, with permission, from Bolanowski and Pawson 2003.)
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).
Innervation pattern of Meissner corpuscles and Merkel cells in hairy and glabrous skin.
A. A confocal transverse section of a papillary ridge in the human glabrous skin shows the innervation of mechanoreceptors in the fingertip. Meissner corpuscles are located just below the epidermis (blue) at the apex of the ridge, and are innervated by two or more rapidly adapting type 1 (RA1) fibers. The fibers lose their myelin sheaths (orange) when entering the receptor capsule, exposing broad terminal bulbs at which sensory transduction occurs (green). Individual slowly adapting type 1 (SA1) fibers innervate groups of Merkel cells clustered at the base of the intermediate ridge, providing localized signals of pressure applied to that ridge. Scale bar = 50 m. (Adapted, with permission, from Nolano et al. 2003.)
B. A higher-magnification micrograph portrays antibody-labeled Merkel cells (red) innervated by an SA1 fiber (green). Each fiber extends multiple branches parallel to the surface of the skin that allow it to integrate tactile information from multiple receptor cells in a small zone of skin. The diameter of each Merkel cell is approximately 10 m. (Adapted, with permission, from Snider 1998.)
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.
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.
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.
Receptive Fields Define the Zone of Tactile Sensitivity
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.
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.
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.
Receptive fields in the human hand are smallest at the fingertips.
Each colored area on the hands indicates the receptive field of an individual sensory nerve fiber. (Adapted, with permission, from Johansson and Vallbo 1983.)
A–B. The receptive fields of receptors in the superficial layers of skin encompass spot-like patches of skin. Those of receptors in the deep layers extend across wide regions of skin (light shading), but responses are strongest in the skin directly over the receptor (dark spots). The arrows indicate the directions of skin stretch that activate SA2 fibers.
C. Pressure sensitivity throughout the receptive field is shown as a contour map. The most sensitive regions are indicated in red and the least sensitive areas in pale pink. The receptive field of an RA1 fiber (above) has many points of high sensitivity, marking the positions of the group of Meissner corpuscles innervated by the fiber. The receptive field of an RA2 fiber (below) has a single point of maximum sensitivity overlying the Pacinian corpuscle. The receptive field of SA1 fibers is similar to that of RA1 fibers. Likewise, the receptive field of SA2 fibers resembles that of RA2 fibers.
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).
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.
Two-Point Discrimination Tests Measure Texture Perception
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).
Tactile acuity in the human hand is highest on the fingertip.
A. The two-point threshold measures the minimum distance at which two stimuli are resolved as distinct. This distance varies for different body regions; it is approximately 2 mm on the fingers, but as much as 10 mm on the palm and 40 mm on the arm, thigh, and back. The two-point perceptual thresholds highlighted in pink match the diameters of the receptive fields of receptors in the pink zones on the body. The greatest discriminative capacity is afforded in the fingertips, lips, and tongue, which have the smallest receptive fields. (Adapted, with permission, from Weinstein 1968.)
B. Spatial acuity is measured in psychophysical experiments by having a blindfolded subject touch a variety of textured surfaces. As shown here, the subject is asked to determine whether the surface of a wheel is smooth or contains a gap, whether the ridges of a grating are oriented across the finger or parallel to its long axis, or which letters appear on raised type used in letterpress printing. The tactile acuity threshold is defined as the groove width, ridge width, or font size that yields 75% correct performance (detectable midway between chance and perfect accuracy). The threshold spacing on the human fingertip is 1.0 mm in each of these tests. (Adapted, with permission, from Johnson and Phillips 1981.)
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.
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.
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.
Responses of touch receptors to Braille dots scanned by the fingers.
The Braille symbols for the letters A through R were mounted on a drum that was repeatedly rotated against the fingertip of a human subject. Following each revolution the drum was shifted upward so that another portion of the symbols was scanned across the finger. Microelectrodes placed in the median nerve of this subject recorded the responses of the mechanoreceptive fibers innervating the fingertip. The action potentials discharged by the nerve fibers as the Braille symbols were moved over the receptive field are represented in these records by small dots; each horizontal row of dots represents the responses of the fiber to a single revolution of the drum. The SA1 receptors register the sharpest image of the Braille symbols, representing each Braille dot with a series of action potentials and falling silent when the spaces between Braille symbols provide no stimulation. RA1 receptors provide a blurred image of the Braille symbols because their receptive fields are larger, but the individual dot patterns are still recognizable. Neither RA2 nor SA2 receptors are able to encode the Braille patterns because their receptive fields are larger than the dot spacing. The high firing rate of the RA2 fibers reflects the keen sensitivity of Pacinian corpuscles to vibration. (RA1, rapidly adapting type 1; RA2, rapidly adapting type 2; SA1, slowly adapting type 1; SA2, slowly adapting type 2.) (Reproduced, with permission, from Phillips, Johansson, and Johnson 1992.)
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.
Slowly Adapting Fibers Detect Object Pressure and Form
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.
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).
Slowly adapting type 1 (SA1) fibers encode the shape and size of objects touching the hand.
A. The area of contact on the skin determines the firing rate and total number of SA1 fibers stimulated. The pink region on the fingertip shows the spread of excitation when probes of different diameters are pressed upon the skin with constant force. The intensity of color is proportional to the firing rates of the stimulated receptors. A small-diameter, sharply pointed probe (left) activates a small population of SA1 receptors. The activated fibers fire intensely because all of the force is concentrated in a small area. A medium-size probe (middle) excites more receptors, but the spread of the force reduces peak firing rates. The probe does not feel as sharp as the small-diameter probe. A large round probe (right) stimulates a large population of SA1 receptors spread across the width of the finger. These fibers fire at low rates because the force is spread over a wide area of skin. The sensation of pressure is diffuse. (Adapted, with permission, from Goodwin, Browning, and Wheat 1995.)
B. The firing rate of an individual SA1 fiber is determined by the probe diameter. When a probe first contacts the skin, the SA1 response is strong regardless of probe diameter. During steady pressure the firing rate is proportional to the curvature of each probe. The highest firing rates are evoked by the smallest probe, while the weakest responses are produced by flat surfaces and gently rounded (large diameter) probes. The tip's curvature is expressed as the inverse of its spherical radius. (Adapted, with permission, from Srinivasan and LaMotte 1991.)
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.
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.
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.
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.
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.
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.
Rapidly Adapting Fibers Detect Motion and Vibration
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.
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.
Rapidly adapting type 2 (RA2) fibers encode vibration.
Vibration is the sensation produced by sinusoidal stimulation of the skin, as by the hum of an electric motor, the strings of a musical instrument, or the tuning fork used in the neurological examination.
A. 1. The Pacinian corpuscle consists of concentric, fluid-filled lamellae of connective tissue that encapsulate the terminal of an RA2 fiber. This structure is uniquely suited to the detection of motion. Sensory transduction in the RA2 fiber occurs in stretch-sensitive cation channels linked to the inner lamellae of the capsule. 2. When steady pressure is applied to the skin the RA2 fiber fires a burst at the start and end of stimulation. In response to sinusoidal stimulation (vibration) the fiber fires at regular intervals such that each action potential signals one cycle of the stimulus. Our perception of vibration as a rhythmically repeating event results from the simultaneous activation of many RA2 units, which fire in synchrony.
B. 1. Psychophysical thresholds for detection of vibration depend on the stimulation frequency. As shown here, humans can detect vibrations as small as 10 nm at 200 Hz when grasping a large object; the threshold is higher at other frequencies and when tested with small probes. (Adapted, with permission, from Brisben, Hsiao, and Johnson 1999.) 2. The neural threshold for detection of vibration is defined as the lowest stimulus intensity that evokes one action potential per cycle of the sinusoidal stimulus. Each type of mechanosensory fiber is most sensitive to a specific range of frequencies. Slowly adapting type 1 (SA1) fibers are most sensitive between 0.3 Hz and 3 Hz, rapidly adapting type 1 (RA1) fibers between 2 Hz and 50 Hz, and RA2 fibers between 30 Hz and 500 Hz. Human thresholds for vibration match those of the most sensitive touch fibers in each range. (Adapted, with permission, from Mountcastle, Lynch, and Carli 1972; Bolanowski et al. 1988.)
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.
Both Slowly and Rapidly Adapting Fibers Are Important for Grip Control
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.
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.
Sensory information from the hand during grasping and lifting. (Adapted, with permission, from Johansson 1996.)
A. The subject grasps and lifts a block between the thumb and fingertips, holds it above a table, and then returns it to the resting position. The normal (grip) force secures the object in the hand, and the tangential (load) force overcomes gravity. The grip force is adapted to the surface texture and weight of the object.
B. The grip and load forces are monitored with sensors in the object. These forces are coordinated following contact with the object, plateau as lift begins, and relax in concert after the object is returned to the table.
C. All four mechanoreceptors detect hand contact with the object but each monitors a different aspect of the action as the task progresses. SA1 fibers encode the grip force and SA2 fibers the hand posture. RA1 fibers encode the rate of force application and movement of the hand on the object. RA2 fibers sense vibrations in the object with each movement: at hand contact, lift-off, table contact, and release of grasp. (RA1, rapidly adapting type 1; RA2, rapidly adapting type 2; SA1, slowly adapting type 1; SA2, slowly adapting type 2.)
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.
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.