Finger movement responses of cutaneous mechanoreceptors in the dorsal skin of the human hand

1. The movement sensitivity of dorsal skin mechanoreceptors in the human hand was studied by the use of single afferent recording techniques. 2. Units were classified as slowly (SA) and fast adapting (FA) and further characterized by thresholds to vertical indentation and by receptive-field sizes. Whereas SA units were evenly distributed within the supply area of the superficial branch of the radial nerve. FA units were usually situated near joints. 3. The proportion of different receptor types (32% SAI, 32% SAII, 28% FAI, 8% FAII; n = 107) compared favorably with previous electrophysiological and anatomic data, arguing for minimal sampling bias. The majority of the skin mechanoreceptive units were SA, largely due to a relative scarcity of FAII [Pacinian corpuscles (PC)] units. 4. A large majority (92%) of the afferents responded to active hand or finger movements. Responses in all unit types were consistent with observed movement-induced deformations of their receptive fields. 5. FAI units responded bidirectionally, albeit usually with somewhat higher discharge frequencies for finger flexion, which in most cases were associated with skin stretch. FAI units showed meager responses to remote stimuli, typically responding to one or, at the most, two adjacent joints. 6. SA units typically showed simple directional responses to joint movements with an increased discharge during flexion and a reduced discharge during extension. Joint movement that influenced the skin within the receptive field of SA units elicited graded responses even if the field, as assessed by perpendicular indentations, was minute. This finding suggests that definition of cutaneous receptive fields by classical perpendicular indentations may be inappropriate for the receptors in the hairy, nonglabrous skin. 7. The interpretation of the data from these recordings suggests that cutaneous mechanoreceptors in the dorsal skin can provide the CNS with detailed kinematic information, at least for movements of the hand.     

Quantitative analyses of dynamic strain sensitivity in human skin mechanoreceptors

Microneurographical recordings from 24 slowly adapting (SA) and 16 fast adapting (FA) cutaneous mechanoreceptor afferents were obtained in the human radial nerve. Most of the afferents innervated the hairy skin on the back of the hand. The afferents' receptive fields were subjected to controlled strains in a ramp-and-hold fashion with strain velocities from 1 to 64%.s(-1), i.e., strain velocities within most of the physiological range. For all unit types, the mean variation in response onset approached 1 ms for strain velocities >8%.s(-1). Except at the highest strain velocities, the first spike in a typical SAIII unit was evoked at strains <0.5% and a typical SAII unit began to discharge at <1% skin strain. Skin strain velocity had a profound effect on the discharge rates of all classes of afferents. The "typical" peak discharge rate at the highest strain velocity studied was 50-95 imp/s(-1) depending on unit type. Excellent fits were obtained for both SA and FA units when their responses to ramp stretches were modeled by simple power functions (r2 > 0.9 for 95% of the units). SAIII units grouped with SAII with respect to onset latency and onset variation but with SAI units with respect to dynamic strain sensitivity. Because both SA and FA skin afferents respond strongly, quickly, and accurately to skin strain changes, they all seem to be able to provide useful information about movement-related skin strain changes and therefore contribute to proprioception and kinesthesia.     

Vibrotactile thresholds in non-Pacinian mechanoreceptive afferents: the importance of temporal parameters

Vibrotactile thresholds of non-Pacinian mechanoreceptive afferents (RA, SAI, SAII) were determined as a function of stimulus duration (100–800 ms) at three frequencies (20–240 Hz). Altogether 64 units innervating the hairy skin of the cat were tested. At all frequencies SAII units had the lowest thresholds whereas SAI units had the highest thresholds. At all frequencies and in all unit populations the absolute thresholds (1 impulse/stimulus train) were independent of stimulus duration, whereas the tuning thresholds (1 impulse/cycle) were elevated as a function of increasing stimulus duration. The present results suggest that the detection threshold decreases as a function of increasing stimulus duration are not based on peripheral properties of non-Pacinian mechanoreceptors.     

Cutaneous afferents provide a neuronal population vector that encodes the orientation of human ankle movements

The aim of this study was to analyse the directional coding of two-dimensional limb movements by cutaneous afferents from skin areas covering a multidirectional joint, the ankle. The activity of 89 cutaneous afferents was recorded in the common peroneal nerve, and the mean discharge frequency of each unit was measured during the outward phase of ramp and hold movements imposed in 16 different directions. Forty-two afferents responded to the movements in the following decreasing order (SA2, n = 24/27; FA2, n = 13/17; FA1, n = 3/24; SA1, n = 2/21). All the units activated responded to a specific range of directions, defining their 'preferred sector', within which their response peaked in a given direction, their 'preferred direction'. Based on the distribution of the preferred directions, two populations of afferents, and hence two skin areas were defined: the anterior and the external lateral parts of the leg. As the directional tuning of each population was cosine shaped, the neuronal population vector model was applied and found to efficiently describe the movement direction encoded by cutaneous afferents, as it has been previously reported for muscle afferents. The responses of cutaneous afferents were then considered with respect to those of the afferents from the underlying muscles, which were previously investigated, and an almost perfect matching of directional sensitivity was observed. It is suggested that the common movement-encoding characteristics exhibited by cutaneous and muscle afferents, as early as the peripheral level, may facilitate the central co-processing of their feedbacks subserving kinaesthesia.     

Characterization of tactile afferent fibers in the hand of the marmoset monkey

The marmoset monkey, Callithrix jacchus, has increasingly been the subject of experiments for the analysis of somatosensory system function in simian primates. However, as response properties of the mechanoreceptive afferent fibers supplying the skin have not been characterized for this primate, the present study was undertaken to classify fibers innervating the glabrous skin of the marmoset hand and determine whether they resembled those described for other mammalian species, including cat, macaque monkey, and human subjects. Forty-seven tactile afferent fibers with receptive fields (RFs) on the glabrous skin of the hand were isolated in fine median and ulnar nerve strands. Controlled tactile stimuli, including static indentation and skin vibration, were used to classify fibers. Twenty-six (55%) responded to static indentation in a sustained manner and were designated slowly adapting (SA) fibers, while 21 (45%) were selectively sensitive to the dynamic components of the stimulus. The SA fibers had well-defined boundaries to their RFs, lacked spontaneous activity in most cases (23/26 fibers), had an irregular pattern of discharge to static skin indentation, and displayed graded response levels as a function of indentation amplitude, attributes that were consistent with the properties of slowly adapting type I (SAI) fibers described in other species. The dynamically sensitive afferent fibers could be subdivided into two distinct functional classes, based on their responses to vibrotactile stimulation. The majority (15/21) responded best to lower frequency vibration (~10-50 Hz) and had small RFs, whereas the second class responded preferentially to higher frequency vibration (50-700 Hz) with maximal sensitivity at ~200-300 Hz. These two classes resembled, respectively, the rapidly adapting (RA) and Pacinian corpuscle-related (PC) fiber classes found in other species, and like them, responded to vibration with tightly phase-locked patterns of response over a wide range of frequencies. The results demonstrate that the functional classes of tactile afferent fibers that supply the glabrous skin in the marmoset monkey appear to correspond with those described previously for the cat and macaque monkey, and are similar to those supplying the human hand and fingers, although the SA fibers in the human hand appear to fall into two classes, the SAI and SAII fibers. With the increasing use of the marmoset monkey as a primate model for somatosensory system studies, these data now allow tactile neurons identified at central locations, such as the cerebral cortex and thalamus, to be classified in relation to inputs from the peripheral classes identified in the present study.     

Clustering of Pacinian corpuscle afferent fibres in the human median nerve

 To further study the functional organisation of human peripheral nerves, the intrafascicular arrangement of afferent fibres supplying Pacinian corpuscles (PCs) was explored by percutaneous microneurography using thin-calibre, concentric needle electrodes. In normal adults, 20 PC afferents were identified in 13 recording sites. Low-amplitude (less than 30 μm) vibratory stimuli to the skin were applied with tuning forks oscillating at 128 Hz or 256 Hz and response patterns of individual PC units were studied. In many recording sites, two, sometimes even three, PC afferents with adjacent or overlapping receptive fields in the hand were clustered in the nerve. The observed incidence in the records containing a certain number of PC units was compared with the expected probability calculated according to the hypothesis that all nerve fibres are randomly organised in peripheral nerves. The results suggested that PC afferents are partially segregated in the nerve. In addition, PC afferents were neighbouring on slowly adapting type II (SAII) units and skin sympathetic activity in individual fascicles. SAII units often innervated the same skin area as PC units, but did not respond to vibration. The data provided additional information regarding the functional organisation of the human peripheral nerve and the mechanisms underlying the sense of vibration in man with special regard to population behaviour of neighbouring PC mechanoreceptors. Received: 25 September 1998 / Accepted: 14 December 1998     

Cutaneous receptors contribute to kinesthesia at the index finger, elbow, and knee

The neural mechanisms underlying the sense of joint position and movement remain controversial. While cutaneous receptors are known to contribute to kinesthesia for the fingers, the present experiments test the hypothesis that they contribute at other major joints. Illusory movements were evoked at the interphalangeal (IP) joints of the index finger, the elbow, and the knee by stimulation of populations of cutaneous and muscle spindle receptors, both separately and together. Subjects matched perceived movements with voluntary movements of homologous joints on the contralateral side. Cutaneous receptors were activated by stretch of the skin (using 2 intensities of stretch) and vibration activated muscle spindle receptors. Stimuli were designed to activate receptors that discharge during joint flexion. For the index finger, vibration was applied over the extensor tendons on the dorsum of the hand, to evoke illusory metacarpophalangeal (MCP) joint flexion, and skin stretch was delivered around the IP joints. The strong skin stretch evoked the illusion of flexion of the proximal IP joint in 6/8 subjects (12 +/- 5 degrees, mean +/- SE). For the group, strong skin stretch delivered during vibration increased the perceived flexion of the proximal IP joint by eight times with a concomitant decrease in perceived flexion of the MCP joint compared with vibration alone (P < 0.05). For the elbow, vibration was applied over the distal tendon of triceps brachii and skin stretch over the dorsal forearm. When delivered alone, strong skin stretch evoked illusory elbow flexion in 5/10 subjects (9 +/- 4 degrees). Simultaneous strong skin stretch and vibration increased the illusory elbow flexion for the group by 1.5 times compared with vibration (P < 0.05). For the knee, vibration was applied over the patellar tendon and skin stretch over the thigh. Skin stretch alone evoked illusory knee flexion in 3/10 subjects (8 +/- 4 degrees) and when delivered during vibration, perceived knee flexion increased for the group by 1.4 times compared with vibration (P < 0.05). Hence inputs from cutaneous receptors, muscle receptors, and combined inputs from both receptors likely subserve kinesthesia at joints throughout the body.     

Finger joint movement sensitivity of non-cutaneous mechanoreceptor afferents in the human radial nerve

Summary The responses of non-cutaneous receptors in the human hand to normal digit movements were studied using single afferent recordings from the radial nerve. Eight joint-related afferents had thresholds of 50 mN or less. All responded to passive flexion movements within the physiological range of joint rotation and showed predominantly static response sensitivity; none increased its discharge during passive extension. However, only two of these eight afferents showed the same response pattern during active movements; three discharged only during the extension phase whereas the other three discharged both during extension and flexion. No highthreshold, joint-related mechanoreceptive afferents were encountered in a population of 148 afferents recorded from the cutaneous portion of the radial nerve indicating a scarcity of such afferents on the dorsal aspect of finger joints. Seven high-threshold, subcutaneous mechanoreceptive units not related to joints had thresholds for indentations of 50 mN or more and lacked responses to finger movements. Low-threshold mechanoreceptive afferents related to joints in the human hand may thus provide kinematic information in the physiological midrange of both passive and active movements. Joint position cannot, however, be derived unambiguously from their discharge since the receptor responses may be dramatically altered by muscle activity.     

Proteolysis of the 185,000 MW streptococcal cell wall antigen generating 4000 and 6000 MW peptides with distinct antigenic determinants.

A 185,000 MW glycoprotein antigen derived from Streptococcus mutans was digested with subtilisin. Purification by reversed phase high-powered liquid chromatography (HPLC) resulted in a homogeneous 20,000 MW protein which possesses the streptococcal antigen (SA) I and II determinants. This protein was immunogenic in mice both for the 20,000 MW and the native 185,000 MW SA. Further proteolysis with subtilisin generated four peptides of 18,000, 10,000, 6000 and 4000 MW. Whereas the 20,000, 18,000 and 10,000 MW peptides contained both SAI and SAII determinants, the 4000 MW peptide possessed only the SAI and the 6000 MW peptide the SAII determinant. The 4000-SAI peptide is of special significance, as the smallest SAI material separated in the past was 150,000 MW. This was difficult to purify and proved to be protective against dental caries on immunization of rhesus monkeys.     

Mechanoreceptive units in the human infra-orbital nerve

Eighty-four low-threshold mechanoreceptive afferents innervating facial hairy skin or the red zone of the lip were recorded with micro-electrodes from the human infra-orbital nerve. Based on their responses to skin indentations, the units were classified as slowly or fast-adapting, with small or large receptive fields. The responses to hair movement, skin stretching and contraction of facial muscles were also studied. Both hairy skin and the red zone were innervated by slowly and by fast-adapting units. The innervation density was found to be highest at the corner of the mouth and on the upper lip. Slowly adapting units with small fields in hairy skin were most common and included units responding to sustained hair displacement. These units are suggested to have two types of end-organs, either pilo-Ruffini endings or Merkel cell-neurite complexes. The slowly adapting units with large fields were spontaneously active stretch receptors and may have corresponded to Ruffini corpuscles, although the possibility of other, intramuscular, receptors could not be ruled out. Only one afferent possibly innervated a Pacinian corpuscle. Most mechanoreceptors were also activated by skin stretching or contraction of facial muscles. Many of the slowly adapting units with small fields responded to the onset and release of stretch, whereas their discharge in response to sustained stretching adapted more or less completely. Spontaneously active units had the most sustained stretch response. It is concluded that several types of cutaneous mechanoreceptors can operate as sensitive proprioceptors of importance for facial kinaesthesia and motor control.     

Mechanoreceptive afferents in the human sural nerve

Microneurography was used to characterize 104 low-threshold mechanoreceptive afferents in the human sural nerve. The afferents were readily classified into four types using criteria developed for the glabrous skin of the hand: SA I (31%), SA II (11%), FA I (49%), and FA II (9%). The distribution of fascicle fields and receptive fields of individual afferents on the lateral side of the foot indicates that the glabrous skin portion of the innervation territory of the sural nerve is more densely innervated than the non-glabrous skin portion. The different populations in the glabrous and non-glabrous skin regions were similar regarding proportion of unit types, receptive field sizes, and force thresholds. The receptive field sizes of the type I units of the present sample were about twice the size compared to those of the glabrous hand units, and the force threshold were at least three times higher for three of the unit types (SA I, FA I, and FA II). Given their receptive properties, it is likely that mechanoreceptive afferents in the sural nerve provide rich information about contact patterns between the foot and environment during stance and locomotion.     

Population estimates for responses of cutaneous mechanoreceptors to a vertically indenting probe on the glabrous skin of monkeys

Recordings were obtained from low-threshold mechanoreceptive afferents during stimulation with a 0.5-mm-diameter probe at the receptive field (RF) center and at different distances from the point of maximal sensitivity. At each location, force-controlled stimuli of 0.5–4.0 g were ramped on to a plateau and then off at rates of 1, 10, and 100 g/s. The properties of rapidly adapting (RA) and slowly adapting type I (SAI) mechanoreceptors, when stimulated at the RF center, were similar in many respects to those reported in previous studies. Controlled stimulation away from the RF centers revealed that RF size for RAs was primarily dependent upon ramp rate, and for SAIs the size of the RF was primarily dependent upon load (force). The action potentials from individual afferents during stimulation at each location were binned in time and assigned to spatial segments of 1 mm. These responses were multiplied by: (A) an annular area of the receptive field and (B) the innervation density for the afferent type and skin region. The calculations provided estimates of overall rates of activity among the population of cutaneous afferents that respond to indentation by a small probe. Important differences were obtained between the responses of the population of afferents activated by the trapezoidal stimulus and the responses of afferents stimulated only at the RF center. Populations of tactile afferents provide more information for rate and intensity (force) discriminations than is available from units stimulated at the RF center. For RA afferents, the exponent of the power function describing relationships between stimulus rate and the population discharge (in impulses per second) was 0.3 times greater than the exponent for responses to on-center stimulation. For SAI mechanoreceptors, the exponent of the power functions for static responses to force was 0.22 times greater for the population responses than for on-center activation. Population functions for RA responses to the rate of force application and for SAI responses to static load saturated less than comparable responses to stimuation of the RF center. Thus, the coding capacity of the population extends the range of tactile discriminability. The slope and range of stimulus-response functions for populations was enhanced relative to responses to oncenter stimulation. This occurs because of recruitment of afferents with RF centers adjacent to and remote from the stimulus, depending upon thresholds and receptive field sizes for different stimulus parameters. With stimulation at increasing rates and forces, there is a progressive spatial recruitment of receptors. Over 90% of the activity elicited by suprathreshold punctate stimuli originated from mechanoreceptors with RF centers 1 mm or more away from the stimulus site. When the population response of SA afferents was calculated for different intensities of plateau stimulation, ranging from 1 to 4 g, the slope of the power function corresponded well to psychophysical estimates in the literature on the growth of touch intensity. Recruitment of afferents stimulated off the RF center shaped the temporal pattern of discharge. For RA afferents, the population response reached peak rates toward later portions of the onset and offset response than for on-center stimulation. For SAI afferents, the population discharge during slow onsets accelerated more positively than the responses to on-center stimulation. Variations in the rate, amplitude, and duration of stimulation were demonstrated to be useful in assessing the contribution of SAI and RA afferents to different tactile sensations. At very slow rates of stimulus application, the RA response was so minimal that the population response can be considered to arise from SAI afferents. At high stimulus rates, the population response was greatly accentuated during the onset (indentation) and offset (removal) of a trapezoidal ramp-and-hold stimulus, relative to firing rates during maintained indentation. Ratios of dynamic to static discharge were 3–4 times greater for the population than for on-center stimulation, reaching values as high as 60.21. The ratios of dynamic to static population responses were greatest for stimuli presented to the palm and were least for stimuli presented to the base and middle phalanges of the fingers. Therefore, the relative magnitudes of onset, offset, and steadystate sensations elicited by stimulation at different rates and locations should vary systematically, according to the absolute and relative densities of each receptor type.     

Transmission characteristics for the 1:1 linkage between slowly adapting type II fibers and their cuneate target neurons in cat

Transmission from single, identified, slowly adapting type II (SAII) tactile fibers to their target neurons in the cuneate nucleus was examined in anesthetized cats. Simultaneous recordings were made from cuneate neurons and from fine, intact fascicles of the superficial radial nerve in which it was possible to identify and monitor the activity of each group II fiber. Selective activation of individual SAII fibers was achieved by means of skin stimulation with fine probes, in conjunction with extensive forelimb denervation. Responses were studied for seven SAII-driven cuneate neurons. For three there was unequivocal monitoring of the identified SAII input fiber. However, in six of the seven there was evidence that just one SAII fiber provided suprathreshold input to the cuneate neuron, and neither temporal nor spatial summation was required for reliable transmission. Cuneate impulse rates, in response to SAII inputs lasting 1 s, were less than 250 impulses per second, even though the SAII impulse rates could be 500 s^-1. Responses to individual SAII impulses consisted of a burst of 2–3 impulses at low SAII input rates, but burst responses disappeared at high SAII rates. In all three SAII-cuneate pairs studied, the transmission security (the percentage of SAII impulses that evoked cuneate spike output) exceeded 80% in response to static skin displacement and in response to certain frequencies of skin vibration, in particular, at 100–200 Hz, exceeded 98% when the SAII fiber responded near the 11 level (one impulse per vibration cycle). Transmission characteristics for the SAII-cuneate linkage resulted in the cuneate neuron showing tight phaselocking of responses to high-frequency (>100 Hz) vibrotactile stimuli and higher impulse rates than its SAII input (up to input rates of 50 impulses s^-1). Security of transmission across the SAII-cuneate synapse is similar to that demonstrated previously for tactile fibers of the SAI and Pacinian corpuscle (PC)-related classes, which suggests that there is no marked differential specialization in transmission characteristics for dorsal column nuclei neurons that receive input from different tactile fiber classes. Furthermore, it means that the reported failure of individual SAII fiber inputs to generate conscious sensation in man following intraneural microstimulation is not related to transmission failure at the first central relay.     

Analysis of activity of muscle spindles of the jaw-closing muscles during normal movements in the cat.

Recordings have been made of afferent activity from spindles of the jaw-closing muscles, together with jaw movement and e.m.g. from temporalis and masseter in conscious, unrestrained cats. 2. In the twenty-nine units studied, the pattern of spindle behaviour observed during eating and lapping was generally what might be expected of stretch receptors. Maximal firing frequencies were found during opening of the mouth (lengthening), while during active closing the discharge was progressively reduced or abolished. Nevertheless, changes in the relation of stretch to firing frequency in different movements indicated that fusimotor drive was not constant. 3. spindle afferents could be divided into two groups on the basis of their maximal firing frequency during eating. "High-frequency" units (range 240-600 impulses/sec) showed pronounced velocity sensitivity, which supports the proposal that they correspond to spindle primaries. 'Low-frequency" units (range 80-200 impulses/sec) showed predominantly length sensitivity and probably correspond to secondary endings. 4. Length sensitivity of low-frequency units was considerably greater in lapping movements than in eating, indicating increased static fusimotor drive in the former. Sensitivity in the opening phase of eating was indistinguishable from that recorded in deeply anaesthetized animals. 5. High-frequency units were generally silenced immediately active shortening commenced. 6. No simple relationship existed between temporalis or masseter e.m.g. and spindle firing. 7. These results imply that normal masticatory movements are not initiated or driven to any appreciable extent via the fusimotor route. Close alpha-gamma co-activation is not a feature of this situation. On the other hand, in some other movements, such as licking the lips, fusimotor drive could fluctuate so as largely to cancel the unloading effects of active muscle shortening.     

Mechanical sensitivity of regenerating myelinated skin and muscle afferents in the cat

These experiments describe the responses of myelinated skin and muscle afferent nerve fibres at a neuroma to stretch, local pressure and vibration in the anaesthetised cat. The sural nerve and the nerve supplying the medial gastrocnemius were studied. Neuroma formation was encouraged by placing the cut end of the nerve in a cuff made of synthetic material (Gore-tex). By 6 days after nerve section, the two nerves contained mechanically sensitive afferents. No motor fibres appeared to be mechanically sensitive. Mechanically sensitive sural afferents responded to ramp stretch of the nerve, applied at the cuff, with a single impulse or brief burst of impulses. The majority of gastrocnemius afferents responded to stretch with slowly adapting trains of impulses. Many muscle group II afferents exhibited a steady resting discharge, while group I afferents had an intermittent or bursting resting discharge or were silent. Those group I axons which showed resting activity had a low stretch threshold and were probably Ia fibres. Many of the silent units were also stretch sensitive. It is proposed that the spontaneously active units and silent units with low stretch thresholds were Ia fibres, while silent units with high stretch thresholds were Ib fibres. Both sural and gastrocnemius afferents responded to locally applied vibration. The mean peak response frequency for sural units was 170 Hz (± 70 Hz SD). For gastrocnemius units it was 325 Hz (± 86 Hz SD). Group I muscle afferents responded to higher frequencies of vibration than group II afferents. In four experiments the nerve was treated at a site a few millimetres proximal to the point of section with the axonal transport blocker colchicine. Twenty-five millimolar colchicine blocked impulse conduction at its point of application. Nevertheless, mechanically sensitive areas developed in the nerve just proximal to the treated region. Ten millimolar colchicine did not block impulse conduction, but led to dispersion of mechanosensitive areas to more proximal regions of the nerve. This result suggests that the disruption of orthograde axonal transport by colchicine leads to development of mechanically sensitive areas in axons further back from their cut ends. Local application of the drugs succinyl choline, tetra-ethyl ammonium and gadolinium had no effect on levels of resting activity or on mechanical sensitivity of afferents in the cuff. The potassium channel blocker 4-aminopyridine, on the other hand, produced an increase in the levels of resting activity and in the stretch responses of afferents. None of these drugs induced any activity in motor axons. It is proposed that mechanical sensitivity is induced at the sprouting tips of sensory axons by a substance or substances transported down the axon from the cell body. Such a conclusion implies that some of the response properties of normal mechanoreceptors in skin and muscle may be the result of influences exerted by the cell body on the peripheral terminal membranes. This conclusion has important implications for understanding transduction mechanisms and the development of somatic receptors, and for interpretation of receptor responses following nerve section and reinnervation or cross-reinnervation.     

Activity in receptor afferents from the anterior cruciate ligament evokes reflex effects on fusimotor neurones

Responses from 2-4 muscle spindle afferents from triceps surae and/or posterior biceps and semitendinosus muscles were simultaneously recorded in cats anaesthetized with alpha-chloralose. It was demonstrated that stretch of the anterior cruciate ligament (ACL) of the ipsilateral knee causes changes in dynamic and/or static sensitivity of primary and secondary spindle afferents to sinusoidal stretching. The changes were due to reflex actions of stretch/tension-sensitive receptor afferents from ACL on dynamic and static fusimotor neurones. The findings support the hypothesis that ACL-afferents contribute to the regulation of muscular stiffness around the knee, and thereby also to dynamic knee joint stability.     

Contributions of skin and muscle afferent input to movement sense in the human hand

In the stationary hand, static joint-position sense originates from multimodal somatosensory input (e.g., joint, skin, and muscle). In the moving hand, however, it is uncertain how movement sense arises from these different submodalities of proprioceptors. In contrast to static-position sense, movement sense includes multiple parameters such as motion detection, direction, joint angle, and velocity. Because movement sense is both multimodal and multiparametric, it is not known how different movement parameters are represented by different afferent submodalities. In theory, each submodality could redundantly represent all movement parameters, or, alternatively, different afferent submodalities could be tuned to distinctly different movement parameters. The study described in this paper investigated how skin input and muscle input each contributes to movement sense of the hand, in particular, to the movement parameters dynamic position and velocity. Healthy adult subjects were instructed to indicate with the left hand when they sensed the unseen fingers of the right hand being passively flexed at the metacarpophalangeal (MCP) joint through a previously learned target angle. The experimental approach was to suppress input from skin and/or muscle: skin input by anesthetizing the hand, and muscle input by unexpectedly extending the wrist to prevent MCP flexion from stretching the finger extensor muscle. Input from joint afferents was assumed not to play a significant role because the task was carried out with the MCP joints near their neutral positions. We found that, during passive finger movement near the neutral position in healthy adult humans, both skin and muscle receptors contribute to movement sense but qualitatively differently. Whereas skin input contributes to both dynamic position and velocity sense, muscle input may contribute only to velocity sense.     

Responses in glabrous skin mechanoreceptors during precision grip in humans

Summary Impulses in single tactile units innervating the human glabrous skin were recorded percutaneously from the median nerve using tungsten electrodes. The units were classified as belonging to one of the four categories: fast adapting with small receptive fields (FA I), fast adapting with large receptive fields (FA II), slowly adapting with small fields (SA I), and slowly adapting with large fields (SA II). A small test object was lifted, positioned in space and replaced using the precision grip between fingers and thumb. The grip force, the load force (vertical lifting force), the vertical movements of the object and vibrations (accelerations) in the object were recorded. After being virtually silent between lifts, the FA I units whose fields contacted the object became highly active during the initial period of grip force increase (initial response). This was also true for most SA I units. Accordingly, most of the skin deformation changes took place at low grip forces (below ca. 1 N). Later, while the load and grip forces increased in parallel during isometric conditions, the FA I and SA I units continued firing but generally at declining impulse rates. As long as the object was held in the air, the SA I units generally maintained firing with a tendency to adaptation. A minority of the FA I unit also discharged, especially during periods of pronounced physiological muscle tremor. The SA I units usually became silent when the grip and load forces in parallel declined to zero during isometric conditions after the object had contacted the table. However, during the very release of the grip the FA I units and some SA I units showed brief burst discharges (release response). The FA II units responded distinctly to the mechanical transients associated with the start of the vertical movement and especially with the sudden cessation of movement at the terminal table contact. FA II units whose end organs were remotely located in relation to the skin areas in contact with the object also responded. Most FA II units also discharged at the initial touch and at the release of the object, albeit less reliably than the type I units. In addition to weak dynamic responses during the phase of isometric force increase, the SA II units showed comparatively strong tonic responses while the object was held during static conditions. High firing rates also were maintained during long-lasting lifts. Moreover, it was established that the signals in SA II afferents were related to the three dimensional force profile in the grip. The results are discussed with regard to the possible implications for the control of precise manipulative movements.     

Sensitization of articular afferents to mechanical stimuli by bradykinin

In 18 cats anaesthetized with alpha-chloralose, we recorded from thin myelinated and unmyelinated articular afferents of the medial articular nerve of the knee joint. Bradykinin was injected intra-arterially close to the knee, alone and in combination with prostaglandin E₂ (PGE₂), and changes of the responses of single afferents to movements of the knee were monitored. Bradykinin changed the mechanosensitivity in 20 of 28 afferents inducing movement sensitivity in initially unresponsive units, lowering the threshold for movements in high-threshold afferents and/ or enhancing pre-existing responses to innocuous and/or noxious joint movements in low and high threshold units. Also the application of PGE₂ and bradykinin within a short interval sensitized the majority of these afferents, and in about 50% of the afferents the effect of the combination was superior to those induced by the single substances. We conclude that the inflammatory mediator bradykinin is able to sensitize articular afferents for movement stimuli and that PGE₂ may enhance this effect. It is suggested that in arthritis inflammatory mediators act synergistically in the initiation and stabilization of the increased mechanosensitivity of slowly conducting articular afferents.     

Tactile directional sensitivity and postural control

People are good at telling the direction of a moving tactile stimulus and this capacity provides a sensitive clinical test of somatosensory disturbances. Tactile directional sensitivity depends on two different kinds of somatosensory information, i.e. spatiotemporal information and information about friction-induced changes in skin stretch. The objective of this study was to compare the relative contribution to postural control of these two types of information for both glabrous and hairy skin. Postural sway amplitudes and sway paths were recorded, with or without access to tactile and/or visual stabilizing stimuli. Subjects were standing on two types of surface, either solid metal or 50 mm foam plastic. Two types of stimulus were used to generate sway-related tactile information. One was a thin air-stream that was used to assess the contribution by spatiotemporal information, and the second was a narrow steel rod that was glued to the skin to assess the contribution by skin-stretch information. The stimuli were applied to the hairy skin of the forearm and to the glabrous skin of the fingertip. In addition, we studied the ability to tell the direction of movement of an air-stream stimulus on glabrous and hairy skin. The air-stream caused significant sway reductions when applied to glabrous, but not hairy skin. The weak effect on hairy skin reflected the perceptually poor directional sensitivity for the air-stream stimulus in this cutaneous area. In contrast, the glued rod reduced sway when applied to both glabrous and hairy skin reflecting the tactile afferents high sensitivity to skin stretch in these areas. Both types of tactile stimulus reduced sway amplitudes more than sway paths for both hairy and glabrous skin. The visual cue, on the other hand, tended to reduce sway paths more than amplitudes. The two types of tactile receptive surface seem to influence postural control in the same manner, despite anatomical and physiological differences. The results invite speculation that patients with poor directional sensitivity might have reduced postural stability compared with healthy individuals.