Inhibition within the trigeminal nucleus induced by afferent inputs and its influence on stimulus coding by mechanosensitive neurones

Reduced Cerebellar Brain Inhibition and Vibrotactile Perception in Response to Mechanical Hand Stimulation at Flutter Frequency

The cerebellum is traditionally considered a movement control structure because of its established afferent and efferent anatomical and functional connections with the motor cortex. In the last decade, studies also proposed its involvement in perception, particularly somatosensory acquisition and prediction of the sensory consequences of movement. However, compared to its role in motor control, the cerebellum's specific role or modulatory influence on other brain areas involved in sensory perception, specifically the primary sensorimotor cortex, is less clear. In the present study, we explored whether peripherally applied vibrotactile stimuli at flutter frequency affect functional cerebello-cortical connections. In 17 healthy volunteers, changes in cerebellar brain inhibition (CBI) and vibration perception threshold (VPT) were measured before and after a 20-min right hand mechanical stimulation at 25 Hz. 5 Hz mechanical stimulation of the right foot served as an active control condition. Performance in a Grooved Pegboard test (GPT) was also measured to assess stimulation's impact on motor performance. Hand stimulation caused a reduction in CBI (13.16%) and increased VPT but had no specific effect on GPT performance, while foot stimulation had no significant effect on all measures. The result added evidence to the functional connections between the cerebellum and primary motor cortex, as shown by CBI reduction. Meanwhile, the parallel increase in VPT indirectly suggests that the cerebellum influences the processing of vibrotactile stimulus through motor-sensory interactions.

Processing afferent proprioceptive information at the main cuneate nucleus of anesthetized cats

Medial lemniscal activity decreases before and during movement, suggesting prethalamic modulation, but the underlying mechanisms are largely unknown. Here we studied the mechanisms underlying proprioceptive transmission at the midventral cuneate nucleus (mvCN) of anesthetized cats using standard extracellular recordings combined with electrical stimulation and microiontophoresis. Dual simultaneous recordings from mvCN and rostroventral cuneate (rvCN) proprioceptive neurons demonstrated that microstimulation through the rvCN recording electrode induced dual effects on mvCN projection cells: potentiation when both neurons had excitatory receptive fields in muscles acting at the same joint, and inhibition when rvCN and mvCN cells had receptive fields located in different joints. GABA and/or glycine consistently abolished mvCN spontaneous and sensory-evoked activity, an effect reversed by bicuculline and strychnine, respectively; and immunohistochemistry data revealed that cells possessing strychnine-sensitive glycine receptors were uniformly distributed throughout the cuneate nucleus. It was also found that proprioceptive mvCN projection cells sent ipsilateral collaterals to the nucleus reticularis gigantocellularis and the mesencephalic locomotor region, and had slower antidromic conduction speeds than cutaneous fibers from the more dorsally located cluster region. The data suggest that (1) the rvCN-mvCM network is functionally related to joints rather than to single muscles producing an overall potentiation of proprioceptive feedback from a moving forelimb joint while inhibiting, through GABAergic and glycinergic interneurons, deep muscular feedback from other forelimb joints; and (2) mvCN projection cells collateralizing to or through the ipsilateral reticular formation allow for bilateral spreading of ascending proprioceptive feedback information.

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.

Intracellular response properties of neurons in the spinal trigeminal nucleus to peripheral and cortical stimulation in the cat

The responses of the secondary neurons in the spinal trigeminal nucleus oralis (STNo) were recorded intracellularly to peripheral and cortical stimulation in chloralose-anesthetized cats. Electrical stimulation of the trigeminal sensory nerves (the frontal, infraorbital and inferior alveolar nerves) evoked an EPSP superimposed by one or a few spikes followed by a biphasic IPSP in one group of STNo neurons (Type I), and a prolonged EPSP superimposed by a burst of spikes in the other group of STNo neurons (Type II). Nearly half of Type I neurons were trigeminothalamic neurons projecting to the contralateral ventral posteromedial nucleus, while the remaining Type I and all the Type II neurons were non-projection neurons. A majority of Type I neurons responded with spike potentials to stimulation of only one sensory nerve, while most Type II neurons responded to stimulation of more than one nerve. Stimulation of the contralateral primary somatosensory cortex evoked IPSPs in most Type I projection neurons, and EPSPs in all Type II as well as most Type I non-projection neurons. In Type I neurons touch or pressure applied to a circumscribed area in the facial skin evoked an EPSP superimposed by one or a few spikes followed by a biphasic IPSP, and IPSPs were evoked from a wide surrounding area in the face by the same mechanical stimulation. In Type II neurons innocuous mechanical stimulation within a wide area evoked an EPSP, while IPSPs could not be induced from anywhere. The results indicate that postsynaptic inhibition is involved in the surround inhibition as well as corticofugal descending inhibition of sensory transmission in the trigeminal sensory nucleus.

Temporal patterning in the responses of gracile and cuneate neurones in the cat to cutaneous vibration

1. Recordings were made in decerebrate cats from gracile and cuneate neurones responding to vibration-induced inputs from Pacinian corpuscle (P.c.) receptors of the hind-limb and forelimb footpads. The two groups of neurones were compared, in particular for their capacities for responding to cutaneous vibration with phase-locked impulse patterns. 2. In both nuclei the P.c. neurones were most sensitive to vibration in the range 80 to greater than 600 Hz. Stimulus-response relations were similar for the two groups, as were measures derived from these relations such as response levels, absolute thresholds and the dynamic range (defined as the vibration amplitude range over which responses were graded). 3. At frequencies up to 300-400 Hz, responses for some neurones in both nuclei remained well phase locked to the vibration; however, quantitative analysis using a factorial analysis of variance indicated that the phase locking was poorer in gracile than cuneate neurones. 4. In both nuclei there was marked variability from neurone to neurone in measures of phase locking which may reflect variations in the extent of convergence of P.c. fibres upon different target neurones. For neurones in either nucleus that had comparatively tight phase locking of responses to vibration it is proposed that their output is functionally dominated by one or a few of their convergent P.c. input fibres.

Mechanoreceptive submodality channel interactions: single unit analysis of afferent inhibition in the primary somatosensory cortex of the cat

This study examined whether afferent inhibition generated by activation of one mechanoreceptive submodality influences the response of neurons of the other submodality tested. Response properties of quickly adapting hair and Pacinian neurons in primary somatosensory cortex of the cat were evaluated to assess afferent inhibition generated by single-cycle sinusoidal mechanical stimuli. Animals were lightly anesthetized with sodium thiopental. Stimulation at 20 Hz (low-velocity stimulus) was used to activate hair cells, the receptive fields of which were located in the skin; stimulation at 200 Hz (high-velocity stimulus) was used to activate Pacinian cells, the receptive fields of which were located in the deeper tissues. The skin was partially dissected from the deeper tissue in order to uncouple mechanically the effective receptive surfaces and to achieve greater selectivity. Hair and Pacinian cell test responses were paired with single-cycle 20 and 200 Hz conditioning stimuli. A 20 Hz stimulus, more effective in activating hair cells than Pacinian cells, strongly inhibited the test response of hair cells only and a 200 Hz stimulus, which is more effective in activating Pacinian cells than hair cells, markedly inhibited the test response of Pacinian cells only. Our data indicate that afferent inhibition generated by activation of one submodality channel is largely confined to that submodality channel and is not distributed to the other.

Convergence in the somatosensory pathway between cutaneous afferents from the index and middle fingers in man

Average short-latency cerebral potentials were recorded from the parietal scalp to mechanical stimulation of the index and middle fingers and to electrical stimulation of the digital nerves in normal subjects. The early components of the cerebral potential, representing the arrival of the afferent volley at the sensorimotor cortex, were studied during stimulation of the fingers separately and together. When strong or moderate stimuli were used there was a suppressive interaction between the afferent input from the two fingers with either electrical or mechanical stimulation. During simultaneous stimulation of both fingers the size of the early component of the cerebral potential was less than predicted by simple addition of the potentials produced by stimulation of the fingers individually. When very weak stimuli, close to the level necessary for detection by the subject, the input from the two fingers produced additive or facilitatory interactions in the early components of the cerebral potential. These results suggest that there is convergence between the afferent inputs from the index and middle fingers along the somatosensory pathway. At levels of stimulation comparable to those which produced facilitation in the electrophysiological studies, simultaneous stimulation to both fingers was detected significantly more frequently than would be expected from the detection of stimulation to individual fingers.

Afferent inhibition on various types of cats cuneate neurons induced by dynamic and steady tactile stimuli

Afferent inhibition in several distinct types of cuneate neurons was studied using controlled natural stimuli in 35 lightly anesthetized cats. Mechanoreceptive cuneate neurons were recorded extracellularly with microelectrodes from the middle and caudal divisions of the main nucleus. They were classified into several modality subtypes based on their response to adequate mechanical stimuli. Emphasis was laid on the neurons which had their receptive fields (RFs) in the forepaw. Afferent inhibition was induced by conditioning tactile stimuli in 31 out of 168 neurons (18%) tested. There were particular combinations between the neuron types inhibited and conditioning stimulus modalities. Dynamic stimuli such as high frequency vibration and hair movement by air-jet stimuli applied to areas beyond the excitatory RFs induced inhibition on touch (T), hair (H) and slowly adapting pad (SA) units predominantly in the paw region. In contrast, steady pressure stimulation on the skin adjacent to the excitatory RFs induced inhibition in exclusively slowly adapting neurons receiving afferent inputs from hairy skin such as touch (T), joint (J) and subcutaneous (Deep) units in the paw, elbow and shoulder regions. Most of the inhibitory RFs were organized laterally or eccentrically rather than concentrically around the excitatory RF. Two J units were found to be inhibited by steady pressure applied to the shoulder region of the contralateral forelimb. Functional significance of the intramodality and cross modality inhibition of cuneate neurons is discussed.

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.

Spatio-temporal patterns of pre- and postsynaptic inhibition induced by primary afferent activation in the trigeminal sensory nucleus in cats

Spatio-temporal patterns of pre- and postsynaptic inhibition were studied in the trigeminal spinal nucleus oralis of cats by means of systematic electrical stimulation of the facial skin. Stimulation of the facial skin induced an EPSP-IPSP sequence in trigemino-thalamic relay cells (TRC). The IPSP was depressed by picrotoxin but was resistant to strychnine. The largest IPSP was evoked from the center of the excitatory area, where stimulation induced the largest EPSP and spike potentials at the lowest intensity in the same TRC. The amplitude of the IPSP decreased with increasing distance from the center in parallel with that of the EPSP. In the great majority of trigeminal primary afferent fibers, the largest primary afferent depolarization (PAD) was not evoked from the center of the excitatory area, where the threshold for spike generation was lowest, but from the adjacent points on the face. Spike activities in a trigeminal primary afferent fiber did not evoke any detectable PAD in itself. The duration of the PAD was definitely longer than the IPSP in TRC. However, the temporal distribution of the peak of PADs was very similar to that of the EPSP in TRC. Inhibition was evoked in glutamate-induced spike discharges of TRC by stimulation of the points on the face, which were located close to the center of the excitatory area of the TRC. However, the afferent inhibition of both spontaneous and peripherally induced spike discharges of TRC outlasted the postsynaptic inhibition. Thus, the late phase of the afferent inhibition is most probably due to presynaptic inhibition. Presynaptic inhibition, together with postsynaptic inhibition, would be involved also in the early phase of afferent inhibition through its mutual inhibitory organization.

Coding of information about tactile stimuli by neurones of the cuneate nucleus

1. The responses of cuneate neurones to controlled tactile stimulation of the foot pads were examined in unanaesthetized, decerebrate cats. The neurones were divided into three functional classes; one sensitive to steady tactile stimuli, and two dynamically sensitive classes which could be readily differentiated by their responsiveness to cutaneous vibration. Each class appeared to receive an exclusive input from only one of the three known groups of tactile receptors associated with the foot pads, namely the Pacinian corpuscles, the Merkel endings and the intradermal, encapsulated endings known as Krause or Meissner corpuscles. 2. Cuneate neurones responsive to steady indentation of the skin displayed approximately linear or sigmoidal stimulus-response relations over indentation ranges up to approximately 1.5--2 mm. Response variability at a fixed stimulus intensity was relatively low and showed little systematic change over the full range of the stimulus-response curves. 3. One class of dynamically sensitive cuneate neurones responded to cutaneous vibration over a range of approximately 5-80 Hz with maximal responsiveness around 30 Hz. The other class, the Pacinian neurones, responded over a range of approximately 80- greater than 600 Hz with maximal responsiveness at 200-400 Hz. The thresholds and combined band width of vibratory sensitivity of these populations were comparable with known subjective thresholds and range of cutaneous vibratory sensibility. 4. Responses of cuneate neurones were phase-locked to the vibratory stimulus suggesting that information about vibration frequency could be coded by the patterns of impulse activity. Quantitative measures indicated that maximal phase-locking occurred in responses to vibration frequencies of 10-50 Hz with a progressive decline at higher frequencies. Above 400 Hz, impulse activity occurred almost randomly throughout the vibratory stimulus cycle and therefore carried little further signal of vibratory frequency. The decline, with increasing frequency, in the ability of cuneate neurones to signal information about vibratory frequency parallels the known subjective capacities for frequency discrimination. 5. A switch-over occurred at approximately 80 Hz in the population of cuneate neurones able to provide the more reliable signal of vibratory frequency; above 80 Hz, the Pacinian neurones; below 80 Hz, the neurones receiving intradermal, rapidly adapting receptor input from the pads. 6. The observed properties of cuneate neurones are compatible with a role in signalling information which could contribute to subjective tactile abilities.

Human tactile detection thresholds: modification by inputs from specific tactile receptor classes

1. Human detection thresholds for a vibratory stimulus applied to the volar surface of the index finger were examined under conditions where afferents from specific tactile receptor classes were simultaneously activated from the thenar eminence. The experiments were designed to test whether stimuli which have been shown previously to induce afferent inhibition of ;tactile' neurones in the cuneate nucleus of the cat could modify human subjective performance in a tactile detection task. Conditioning stimuli to the thenar eminence were usually of three forms; steady indentation to engage slowly adapting tactile receptors; 300 Hz vibration to engage Pacinian corpuscles; and 30 Hz vibration to engage the intradermal, rapidly adapting tactile receptors which are thought to be Meissner's corpuscles.2. In ten subjects the mean detection threshold for a 30 Hz test stimulus in the absence of conditioning stimulation was 8.6 +/- 1.0 mum (S.E.). Detection thresholds were increased substantially in the presence of a 300 Hz, 100 mum conditioning stimulus (mean increase 11.1 +/- 2.0 mum), whereas minor or insignificant effects were seen with conditioning stimuli consisting of (a) 30 Hz, 100 mum (mean increase 1.4 +/- 0.8 mum), (b) steady indentation, 1.5 mm in amplitude (mean increase 1.3 +/- 0.7 mum) or (c) 300 Hz, 100 mum to the contralateral thenar eminence (mean increase 0.4 +/- 0.5 mum).3. The 300 Hz conditioning stimulus to the ipsilateral thenar eminence caused a marked increase in detection thresholds at all test stimulus frequencies over the range 10-450 Hz. The effects of the conditioning stimulation therefore operated on inputs from Pacinian corpuscles, which are responsible for vibration detection at 80-450 Hz, and on inputs from the intradermal, rapidly adapting receptors which are responsible for vibration detection at 10-80 Hz.4. The band width of conditioning vibratory frequencies which was effective at amplitudes of 100 mum in bringing about increases in detection threshold extended from 50-80 Hz to 300 Hz, the maximum tested.5. Whereas amplitudes of 1-2 mum produced clear increases in detection thresholds with conditioning stimuli of 300 Hz, amplitudes of > 200 mum were needed at 30 Hz.6. The observed elevations in detection threshold are consistent with an afferent-induced inhibitory action exerted at synaptic relays of the sensory pathway by tactile inputs arising exclusively or predominantly from Pacinian corpuscles.

Properties of different functional types of neurones in the cat's rostral trigeminal nuclei responding to sinus hair stimulation

1. Properties of neurones in the trigeminal nuclei principalis and oralis responding to movements of facial sinus hairs were studied in cats anaesthetized by I. V. infusion of pentobarbitone.2. Using electrophysiological methods trigeminal neurones were classified into primary afferent fibres, trigeminothalamic relay neurones, interneurones and other unspecified higher order neurones.3. When receptive fields of synaptically activated neurones were compared with those of primary afferent fibres, an often extensive convergence from first order on to higher order neurones was established. Out of 119 relay neurones six received input from one sinus hair only. Spontaneous activity was encountered about twice as often in synaptically activated neurones than in primary afferent fibres.4. The responsiveness of single neurones was unstable over time in about one fifth of the population and then the total number of impulses discharged in successive responses could vary by as much as 500%. Unstable responsiveness occurred sometimes alone but was often accompanied by marked changes in the size or the configuration of the receptive field. Such instabilities were observed in all kinds of synaptically activated neurones but not in primary afferent fibres.5. Afferent inhibition in relay neurones could be elicited from within the excitatory receptive field and appeared to be related to the activation of distinct receptor populations responding to specific stimulus parameters. Inhibition was also seen in interneurones following both mechanical stimulation of the skin and electrical stimulation of lemniscal fibre terminals in the contralateral ventromedial thalamus.6. The results are discussed and compared with previous findings about sinus hair representation in the trigeminal nucleus and the ascending lemniscal projection. The findings indicate that the concept of the ;static properties' of relay neurones is not adequate for all trigeminothalamic relay neurones and may require a critical reconsideration.7. It is suggested that the afferent input from sinus hairs is effectively controlled at the level of the rostral trigeminal nuclei. This control may affect the spatial input to relay neurones, the temporal components of their responses and the intensity dimension of their transmission capacity. It is postulated that by these mechanisms tactile information from the sinus hair system is modulated according to the instantaneous sensory requirements of the behaving cat.

Quantitative aspects of responses in trigeminal relay neurones and interneurones following mechanical stimulation of sinus hairs and skin in the cat

1. Stimulus-response relationships in discharges of trigeminal relay- and interneurones were investigated in the barbiturate anaesthetized cat using controlled sinus hair or skin displacements.2. In comparison with discharges in slowly adapting primary afferent fibres the responses in all higher order neurones were considerably reduced in firing rate and often revealed modifications suggesting the interaction of mechanisms actively modulating the afferent input.3. In relay neurones with or without a tonic discharge component the ;dynamic on' response during a trapezoidal displacement of sinus hairs was found to be determined entirely or predominantly by the movement velocity and to be independent of the deflexion angle of a stimulus. In contrast, the static response in tonic relay neurones was determined by both the movement velocity and the displacement amplitude.4. Spatial summation of afferent input caused either only quantitative changes in the responses of relay neurones leaving the general discharge properties unaltered or caused both qualitative and quantitative changes in the responses.5. Interneurones consisted of two functional groups. In about 25% of them the responses were not or only slightly dependent on the intensity of the applied stimulus, often burstlike and of an all or nothing character. In the second group of interneurones the responses showed a quantitative dependence on the applied stimuli. In this group of interneurones responses often increased with the spatial extension of the peripheral stimulus revealing spatial summation of the afferent input.

Inhibition of cuneate neurones: its afferent source and influence on dynamically sensitive "tactile" neurones

1. Responses were recorded in decereberate, unanaesthetized cats from individual cuneate neurones in order to determine firstly, the afferent sources of inhibition on cuneate neurones and secondly, the influence of afferent-induced inhibition on those response features of dynamically sensitive tactile neurones which determine their capacity to code information about parameters of tactile stimuli.2. For all cuneate neurones which displayed afferent-induced inhibition from areas surrounding or within their excitatory receptive field (71% of the sample) it was consistently found that 300 Hz vibration at low amplitudes (< 25-50 mum) which selectively engages Pacinian corpuscles was an effective source of inhibition. In contrast, steady indentation which activates slowly adapting tactile afferents was quite ineffective, as was low frequency vibration (30 Hz) at amplitudes of < 50-100 mum. The latter stimulus can be used to engage rapidly adapting receptors either within glabrous skin (presumed to be Meissners corpuscles) or in association with hair follicles. It is concluded that afferents from Pacinian corpuscles are the dominant or exclusive source of afferent-induced inhibition of cuneate neurones.3. For dynamically sensitive neurones responsive to low frequency cutaneous vibration (30 Hz) there was a reduction in the slope of stimulus-response relations with afferent-induced inhibition, but no expansion of the range of stimulus amplitudes over which the neurone responded.4. The influence of afferent-induced inhibition on the phase-locking of impulse activity to a cutaneous vibratory wave form was examined by constructing post-stimulus time histograms and cycle histograms. Measures of dispersion of impulse activity around the preferred point of firing in the vibratory waveform indicated that the capacity of individual cuneate neurones to code information about the frequency of the cutaneous vibration was not systematically changed in the presence of afferent-induced inhibition.

A comparison of primary afferent and cortical neurone activity coding sinus hair movements in the cat

Responses in the somatosensory cortical area S I to stimulation of facial sinus hairs were recorded in the anaesthetized cat and compared with activity in primary afferent fibres innervating vibrissae follicles. The specific cortical vibrissa area is somatotopically organized; 39% of the cortical units in that area responded to stimulation of only a single sinus hair but in some cases all maxillary vibrissae activated a single cortical neurone. The responses consisted of three major groups; either a phasic discharge in response to the movement part of a stimulus, or an additional tonic discharge related to the steady period of vibrissa deflection, or a tonic discharge. On the basis of a comparison of response and excitability characteristics of primary afferent and cortical neurones it is concluded that all four kinds of peripheral units innervating sinus hair follicles project to the somatosensory cortical area S I. It appears from these findings that some cortical neurones receive a specific input related to a particular component of the complex primary afferent response in fibres innervating sinus hair follicles. The results are discussed with respect to previous reports on the central representation of facial sinus hairs in different species.