Tactile Afferent Fibers Research Articles

Tactile innervation densities across the whole body.

The skin is our largest sensory organ and innervated by afferent fibers carrying tactile information to the spinal cord and onto the brain. The density with which different classes of tactile afferents innervate the skin is not constant but varies considerably across different body regions. However, precise estimates of innervation density are only available for some body parts, such as the hands, and estimates of the total number of tactile afferent fibers are inconsistent and incomplete. Here we reconcile different estimates and provide plausible ranges and best estimates for the number of different tactile fiber types innervating different regions of the skin, using evidence from dorsal root fiber counts, microneurography, histology, and psychophysics. We estimate that the skin across the whole body of young adults is innervated by ∼230,000 tactile afferent fibers (plausible range: 200,000-270,000), with a subsequent decrement of 5-8% every decade due to aging. Fifteen percent of fibers innervate the palmar skin of both hands and 19% the region surrounding the face and lips. Slowly and fast-adapting fibers are split roughly evenly, but this breakdown varies with skin region. Innervation density correlates well with psychophysical spatial acuity across different body regions, and, additionally, on hairy skin, with hair follicle density. Innervation density is also weakly correlated with the size of the cortical somatotopic representation but cannot fully account for the magnification of the hands and the face.

Effects of Fusion between Tactile and Proprioceptive Inputs on Tactile Perception

Tactile perception is typically considered the result of cortical interpretation of afferent signals from a network of mechanical sensors underneath the skin. Yet, tactile illusion studies suggest that tactile perception can be elicited without afferent signals from mechanoceptors. Therefore, the extent that tactile perception arises from isomorphic mapping of tactile afferents onto the somatosensory cortex remains controversial. We tested whether isomorphic mapping of tactile afferent fibers onto the cortex leads directly to tactile perception by examining whether it is independent from proprioceptive input by evaluating the impact of different hand postures on the perception of a tactile illusion across fingertips. Using the Cutaneous Rabbit Effect, a well studied illusion evoking the perception that a stimulus occurs at a location where none has been delivered, we found that hand posture has a significant effect on the perception of the illusion across the fingertips. This finding emphasizes that tactile perception arises from integration of perceived mechanical and proprioceptive input and not purely from tactile interaction with the external environment.

Encoding of tangential torque in responses of tactile afferent fibres innervating the fingerpad of the monkey

Torsional loads are ubiquitous during everyday dextrous manipulations. We examined how information about torque is provided to the sensorimotor control system by populations of tactile afferents. Torsional loads of different magnitudes were applied in clockwise and anticlockwise directions to a standard central site on the fingertip. Three different background levels of contact (grip) force were used. The median nerve was exposed in anaesthetized monkeys and single unit responses recorded from 66 slowly adapting type-I (SA-I) and 31 fast adapting type-I (FA-I) afferents innervating the distal segments of the fingertips. Most afferents were excited by torque but some were suppressed. Responses of the majority of both afferent types were scaled by torque magnitude applied in one or other direction, with the majority of FA-I afferent responses and about half of SA-I afferent responses scaled in both directions. Torque direction affected responses in both afferent types, but more so for the SA-I afferents. Latencies of the first spike in FA-I afferent responses depended on the parameters of the torque. We used a Parzen window classifier to assess the capacity of the SA-I and FA-I afferent populations to discriminate, concurrently and in real-time, the three stimulus parameters, namely background normal force, torque magnitude and direction. Despite the potentially confounding interactions between stimulus parameters, both the SA-I and the FA-I populations could extract torque magnitude accurately. The FA-I afferents signalled torque magnitude earlier than did the SA-I afferents, but torque direction was extracted more rapidly and more accurately by the SA-I afferent population.

MECHANOSENSORY PERCEPTION: ARE THERE CONTRIBUTIONS FROM BONE‐ASSOCIATED RECEPTORS?

1. The identity of the receptors and afferent nerve fibres that mediate the sense of touch varies somewhat with body location. Those that have been most intensively characterized are associated with the distal glabrous skin of the limbs and, in primates, mediate the sense of touch in the fingertips and palms. In this glabrous skin region, there appear to be three or four principal classes of tactile sensory nerves that fall into two broad groups. One group, the so-called slowly adapting (SA) receptors and afferent fibres, is responsive to static mechanical displacement of skin tissues and is made up of two classes, the type I (SAI) fibres that innervate Merkel receptors and the type II (SAII) fibres that innervate Ruffini endings. The second broad group displays a pure dynamic sensitivity to tactile stimuli and also falls into two principal classes, the rapidly adapting (RA) tactile fibres that are associated with Meissner corpuscle receptors and the Pacinian corpuscle (PC)-associated class of tactile afferent fibres. 2. In other regions of the skin, such as the hairy skin of the arms, legs and trunk, there are similar functional classes of tactile sensory nerves, although the receptor endings differ somewhat from those of the glabrous skin. 3. Receptors in close association with the long bones of the limbs include groups of Pacinian corpuscles distributed along the interosseous membranes. These are highly sensitive to dynamic forms of mechanical stimuli, in particular vibrotactile disturbances. However, despite their close association with bone, these receptors probably cannot be legitimately considered 'osseoreceptors'. 4. Both the periosteum and the bone marrow are richly supplied by nerve fibres. However, much evidence indicates that these are largely or entirely in the fine-diameter category of nerve fibres, whose roles may be confined to either nociception or to the efferent autonomic regulation of bone-associated blood vessels. 5. In conclusion, it remains uncertain whether any aspects of our innocuous touch or kinaesthetic senses, in either the limbs or in orofacial regions, can be ascribed to 'osseoreceptors' located in the periosteum or within the bone marrow itself.

Transmission Security for Single Kinesthetic Afferent Fibers of Joint Origin and Their Target Cuneate Neurons in the Cat

Transmission between single identified, kinesthetic afferent fibers of joint origin and their central target neurons of the cuneate nucleus was examined in anesthetized cats by means of paired electrophysiological recording. Fifty-three wrist joint afferent-cuneate neuron pairs were isolated in which the single joint afferent fiber exerted suprathreshold excitatory actions on the target cuneate neuron. For each pair, the minimum kinesthetic input, a single spike, was sufficient to generate cuneate spike output, often amplified as a pair or burst of spikes, particularly at input rates up to 50-100 impulses per second. The high security was confirmed quantitatively by construction of stimulus-response relationships and calculation of transmission security measures in response to both static and dynamic vibrokinesthetic disturbances applied to the joint capsule. Graded stimulus-response relationships demonstrated that the output for this synaptic connection between single joint afferents and cuneate neurons could provide a sensitive indicator of the strength of joint capsule stimuli. The transmission security measures, calculated as the proportion of joint afferent spikes that generated cuneate spike output, were high (>85-90%) even at afferent fiber discharge rates up to 100-200 impulses per second. Furthermore, tight phase locking in the cuneate responses to vibratory stimulation of the joint capsule demonstrated that the synaptic linkage preserved, with a high level of fidelity, the temporal information about dynamic kinesthetic perturbations that affected the joint. The present study establishes that single kinesthetic afferents of joint origin display a capacity similar to that of tactile afferent fibers for exerting potent synaptic actions on central target neurons of the major ascending kinesthetic sensory pathway.

Transmission security for single, hair follicle-related tactile afferent fibers and their target cuneate neurons in cat.

Transmission from single, identified hair follicle afferent (HFA) nerve fibers to their target neurons of the cuneate nucleus was examined in anesthetized cats by means of paired recording from individual cuneate neurons and from fine, intact fascicles of the lateral branch of the superficial radial nerve in which it is possible to identify and monitor the activity of each group II fiber. Selective activation of individual HFA fibers was achieved by means of focal vibrotactile skin stimulation. Forearm denervation precluded inputs from sources other than the monitored HFA sensory fiber. Transmission characteristics were analyzed for 21 HFA fiber-cuneate neuron pairs in which activity in the single HFA fiber of each pair reliably evoked spike output from the target neuron at a fixed latency. As the cuneate responses to each HFA impulse often consisted of 2 or 3 spikes, in particular at HFA input rates up to approximately 20 imp/s, the synaptic linkage displayed potent amplification and high-gain transmission, characteristics that were confirmed quantitatively in measures of transmission security and cuneate spike output measures. In response to vibrotactile stimuli, the tight phase locking in the responses of single HFA fibers was well retained in the cuneate responses for vibration frequencies up to approximately 200 Hz. On measures of vector strength, the phase locking declined across the synaptic linkage by no more than approximately 10% at frequencies up to 100 Hz. However, limitations on the impulse rates generated in both the HFA fibers their associated cuneate neurons meant that the impulse patterns could not directly signal information about the vibration frequency above 50-100 Hz. Although single HFA fibers are also known to have secure synaptic linkages with spinocervical tract neurons, it is probable that this linkage lacks the capacity of the HFA-cuneate synapse for conveying precise temporal information, in an impulse pattern code, about the frequency parameter of vibrotactile stimuli.

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.

Changes in Tactile Spatial Discrimination and Cutaneous Coding Properties by Skin Hydration in the Elderly

Neurosensory tactile functions were investigated in human subjects by two different and complementary experimental approaches. First, a conventional psychophysical method (two-point gap discrimination) was used to determine the tactile discrimination threshold by analyzing the subjects' ability to detect a gap of variable width between two contact points when a series of stimuli was applied to the skin. Using this method we confirmed the marked degradation of tactile spatial acuity with age and showed that skin discriminative function was partially restored after hydration of the skin with a moisturizer. The second approach consisted of a microneurographic recording of tactile afferent fibers in response to two types of mechanical stimuli applied reproducibly to the corresponding receptive fields. With this method, we found that the afferent messages were depressed following hydration of the skin surface. Thus, partial restoration of tactile spatial acuity after hydration appears to be due to both a softening of the stratum corneum permitting better localization of the stimulus and a weaker transfer of the stimulus toward the sensory receptors.

Parallel processing of tactile information in cat cerebral cortex: effect of reversible inactivation of SII on SI responses

1. Responsiveness of neurons in the distal forelimb region of primary somatosensory cortex (SI) was examined in cat in association with the cooling-induced, reversible inactivation of the corresponding region of the second somatosensory area (SII). The aim was to test whether a component of the stimulus-generated tactile input to SI came via an indirect, intracortical path from the thalamus through SII, or whether, when SI responsiveness fell in association with SII inactivation, the effect could be explained by a disfacilitation of the SI neuron; that is, a removal of a tonic facilitatory influence on the SI neuron that arises from within SII. 2. The responses of 33 SI neurons to controlled tactile stimuli, usually 1-s long trains of vibration or rectangular pulses delivered to the skin of the distal forelimb, were examined quantitatively before, during, and after the rapid, reversible inactivation of the SII area. 3. Nineteen of the 33 neurons (approximately 60%) were unaffected in their response level by SII inactivation. These included neurons of several functional classes whose input came from different classes of tactile afferent fibers, including the Pacinian corpuscle (PC) associated fibers, other rapidly adapting (RA) afferents from glabrous skin, and presumed hair follicle afferent (HFA) fibers. The remaining 14 neurons (approximately 40%), which also included different functional classes, displayed a reduction in response level with SII inactivation. Because this was not accompanied by significant prolongation of the SI spike waveforms, it is not attributable to direct spread of cooling from SII to SI. Construction of stimulus-response relations demonstrated that any effect of SII inactivation on individual SI neurons was consistent over the whole response range. 4. The fall in responsiveness for some SI neurons in association with SII inactivation may be attributable to disfacilitation, that is, a loss of tonic facilitation arising in SII, rather than to a block of peripherally generated inputs that traverse an indirect path from the skin to SI, via SII. There are three reasons for suggesting this. First, in the course of SII cooling, the latency and time course of SI evoked potentials were not delayed in a way that might be expected if part of the SI response had come via SII. Second, the SII inactivation could reduce the SI spontaneous activity (as well as the stimulus-related responsiveness). The facilitation from SII is therefore not necessarily dependent on overt tactile stimulation, and its source may therefore be endogenous to SII.(ABSTRACT TRUNCATED AT 400 WORDS)

Submodality segregation and receptive-field sequences in cuneate, gracile, and external cuneate nuclei of the cat.

Submodality segregation and receptive-field sequences in cuneate, gracile, and external cuneate nuclei of the cat.

Functional capacities of tactile afferent fibres in neonatal kittens.

1. Responses were recorded from individual tactile afferent fibres isolated by microdissection from the median nerve of pentobarbitone-anaesthetized neonatal kittens (1-5 days post-natal age). Experiments were also conducted on adult cats to permit precise comparisons between neonatal and adult fibres.2. Neonatal fibres with receptive fields on the glabrous skin of the foot pads were classified into two broad groups, a slowly adapting class (40%) which responded throughout a 1 sec period of steady indentation and a rapidly adapting or dynamically sensitive class comprising 60% of units. Fibres in these two groups had overlapping conduction velocities in the range 4.3 to 7.5 m/sec and were believed to be the developing Group II afferents of the adult.3. Neonatal slowly adapting fibres qualitatively resembled their adult counter-parts. They displayed graded stimulus-response relations which, over the steepest segment of the curves, had mean slopes of 15.7 impulses/100 mum of indentation. Plateau levels of response were often reached at amplitudes of skin indentation of < 0.5-0.7 mm.4. Dynamically sensitive fibres with receptive fields on the glabrous skin were studied using sinusoidal cutaneous vibration which in the adult enables them to be divided into two distinct classes. However, in the neonate, they formed a continuum whether criteria of sensitivity or responsiveness were used.5. In response to vibration neonatal fibres differed from adult ones according to the following quantitative indices: (i) sensitivity as measured by both absolute thresholds and thresholds for a 1: 1 pattern of response, both of which were higher in the neonate than in the adult at all frequencies > 50 Hz and differed by an order of magnitude at frequencies >/= 200 Hz; (ii) responsiveness based on the mean impulse rate evoked at a fixed amplitude of cutaneous vibration; (iii) band width of vibratory sensitivity which in the neonate was confined to approximately 5-300 Hz whereas in the two classes of adult units it covered the range 5-800 Hz; (iv) capacity for coding information about vibration frequency. Impulse activity of neonatal fibres was less tightly phase-locked to the vibratory stimulus and showed a poorer reflection of the periodic nature of the vibratory stimulus than impulse patterns of adult units.6. The results reveal that tactile receptors and afferent fibres in the neonate are functionally immature. Their restricted coding capacities suggest that peripheral tactile sensory mechanisms impose limits on the ability of the new-born animal to derive information about its tactile environment.

Differential contributions to coding of cutaneous vibratory information by cortical somatosensory areas I and II.

Differential contributions to coding of cutaneous vibratory information by cortical somatosensory areas I and II.

Reconstruction of population response to a vibratory stimulus in quickly adapting mechanoreceptive afferent fiber population innervating glabrous skin of the monkey.

Reconstruction of population response to a vibratory stimulus in quickly adapting mechanoreceptive afferent fiber population innervating glabrous skin of the monkey.

Phase-locking in response of single neurons in cochlear nucler complex of the cat to low-frequency tonal stimuli.

Phase-locking in response of single neurons in cochlear nucler complex of the cat to low-frequency tonal stimuli.R A LavineR A LavinePublished Online:01 May 1971https://doi.org/10.1152/jn.1971.34.3.467MoreSectionsPDF (3 MB)Download PDF ToolsExport citationAdd to favoritesGet permissionsTrack citations ShareShare onFacebookTwitterLinkedInWeChat Previous Back to Top Download PDF FiguresReferencesRelatedInformation Cited ByInfluence of developmental nicotine exposure on spike-timing precision and reliability in hypoglossal motoneuronsGregory L. Powell, Richard B. Levine, Amanda M. Frazier, and Ralph F. Fregosi15 March 2015 | Journal of Neurophysiology, Vol. 113, No. 6Phase-Locked Responses to Pure Tones in the Auditory ThalamusMark N. Wallace, Lucy A. Anderson, and Alan R. Palmer1 October 2007 | Journal of Neurophysiology, Vol. 98, No. 4Processing of Vibrotactile Inputs From Hairy Skin by Neurons of the Dorsal Column Nuclei in the CatV. Sahai, D. A. Mahns, L. Robinson, N. M. Perkins, G. T. Coleman, and M. J. Rowe1 March 2006 | Journal of Neurophysiology, Vol. 95, No. 3Phase-Locked Responses to Pure Tones in the Inferior ColliculusLiang-Fa Liu, Alan R. Palmer, and Mark N. Wallace1 March 2006 | Journal of Neurophysiology, Vol. 95, No. 3Temporal Representation of the Delay of Iterated Rippled Noise in the Dorsal Cochlear NucleusVeronika Neuert, Jesko L. Verhey, and Ian M. Winter1 May 2005 | Journal of Neurophysiology, Vol. 93, No. 5Temporal Properties of Responses to Broadband Noise in the Auditory NerveDries H. G. Louage, Marcel van der Heijden, and Philip X. Joris1 May 2004 | Journal of Neurophysiology, Vol. 91, No. 5Neural Processing of Amplitude-Modulated SoundsP. X. JORIS, C. E. SCHREINER, and A. REES1 April 2004 | Physiological Reviews, Vol. 84, No. 2Integrin α2β1 affects mechano-transduction in slowly and rapidly adapting cutaneous mechanoreceptors in rat hairy skinNeuroscience, Vol. 129, No. 2Tactile sensory function in the forearm of the monotremeTachyglossus aculeatus10 March 2003 | The Journal of Comparative Neurology, Vol. 459, No. 2Simple Networks for Spike-Timing-Based Computation, with Application to Olfactory ProcessingNeuron, Vol. 37, No. 5Transmission Security for Single, Hair Follicle–Related Tactile Afferent Fibers and Their Target Cuneate Neurons in CatM. K. Zachariah, G. T. Coleman, D. A. Mahns, H. Q. Zhang, and M. J. Rowe1 August 2001 | Journal of Neurophysiology, Vol. 86, No. 2Characterization of Tactile Afferent Fibers in the Hand of the Marmoset MonkeyG. T. Coleman, H. Bahramali, H. Q. Zhang, and M. J. Rowe1 May 2001 | Journal of Neurophysiology, Vol. 85, No. 5Hierarchical Equivalence of Somatosensory Areas I and II for Tactile Processing in the Cerebral Cortex of the Marmoset MonkeyH. Q. Zhang, M. K. Zachariah, G. T. Coleman, and M. J. Rowe1 May 2001 | Journal of Neurophysiology, Vol. 85, No. 5Signalling of static and dynamic features of muscle spindle input by external cuneate neurones in the cat7 September 2004 | The Journal of Physiology, Vol. 519, No. 2Signalling of static and dynamic features of muscle spindle input by cuneate neurones in the cat22 September 2004 | The Journal of Physiology, Vol. 510, No. 3The human frequency-following response (FFR): Normal variability and relation to the click-evoked brainstem responseHearing Research, Vol. 59, No. 2Glycine response in isolated dorsal cochlear nucleus of C57BL/6J mouseBrain Research, Vol. 574, No. 1-2Centrally synthesized maps of sensory spaceTrends in Neurosciences, Vol. 9Responses from AVCN units in the cat before and after inducement of an acute noise traumaHearing Research, Vol. 11, No. 3Neural phase-locking properties in the absence of cochlear outer hair cellsHearing Research, Vol. 4, No. 3-4Transmission delay of phase-locked cells in the medial geniculate bodyHearing Research, Vol. 3, No. 1Phase-locked responses to low frequency tones in the medial geniculate bodyHearing Research, Vol. 1, No. 3Response Characteristics of Cochlear Nucleus Neurons to Vowel Sounds29 June 2016 | Annals of Otology, Rhinology & Laryngology, Vol. 86, No. 1Intracellular study of synaptic events related to phase-locking responses of cat cochlear nucleus cells to low frequency tonesBrain Research, Vol. 112, No. 2The effect of sound pressure waveform on human brain stem auditory evoked responsesBrain Research, Vol. 92, No. 3The physiology of auditory frequency analysisProgress in Biophysics and Molecular Biology, Vol. 28Discharge charateristics of neurons in anteroventral and dorsal cochlear nuclei of catBrain Research, Vol. 64Classification of subpopulations of neurons in the cochlear nuclei of the kangaroo ratExperimental Neurology, Vol. 37, No. 1 More from this issue > Volume 34Issue 3May 1971Pages 467-83 https://doi.org/10.1152/jn.1971.34.3.467PubMed5560042History Published online 1 May 1971 Published in print 1 May 1971 Metrics

IDENTIFICATION OF RELAY CELLS AND INTERNEURONS IN THE CUNEATE NUCLEUS.

IDENTIFICATION OF RELAY CELLS AND INTERNEURONS IN THE CUNEATE NUCLEUS.