Processing of Vibrotactile Inputs From Hairy Skin by Neurons of the Dorsal Column Nuclei in the Cat

Nevner at det hårsekk-nerver som reagerer på lavfrekvent vibrasjon (<50 Hz) og at signalene blir sterkere med høyere intensitet. Spesielt ved lave frekvenser <20 Hz er grensen for aktivering av nerveendene lav, med sterkere aktivering ved høyere intensiteter. Lav frekvens, høy amplitude gir sterkest respons! Viser hvordan signalene går opp til hjernen.

Dynamically sensitive tactile neurons of the DCN the input of which came from hairy skin could be divided into two classes, one associated with hair follicle afferent (HFA) input, the other with Pacinian corpuscle (PC) input. The HFA-related class was most sensitive to low-frequency (<50 Hz) vibration and had a graded response output as a function of vibrotactile intensity changes.

In conclusion, the functional capacities of these two classes of cuneate neuron appear to account for behavioral vibrotactile frequency discriminative performance in hairy skin, in contrast to the limited capacities of vibrotactile-sensitive neurons within the spinocervical tract system.

However, tactile information derived from the hairy skin is well represented within the spinocervical pathwayBrown 1981; Brown and Franz 1969) and in the responses of neurons within its target structure, the lateral cervical nucleus (Craig and Tapper 1978; Downie et al. 1988), in addition to its substantial representation within the dorsal column-lemniscal pathway (Brown et al. 1974; Dykes et al. 1982; Golovchinsky 1980; Gordon and Jukes 1964; Perl et al. 1962).

Although tactile information from both the glabrous and hairy skin regions is conveyed over the dorsal column pathway to higher centers, there are known to be marked differences between these two skin regions in human vibrotactile detection thresholds with those on the hairy skin of the forearm being approximately an order of magnitude higher than those for the glabrous finger tips (Merzenich and Harrington 1969; Talbot et al. 1968).

At low vibrotactile frequencies (≤100 Hz), the input from hairy skin comes from afferents associated with hair follicles, the hair follicle afferent (HFA) fibers (Burgess et al. 1968; Merzenich and Harrington 1969), whereas that from glabrous skin arises from rapidly adapting intradermal receptors, known as Meissner corpuscles in primates (Brown and Iggo 1967; Talbot et al. 1968).

As neurons of this class had circumscribed RFs that remained stable on the skin surface, even when it was possible to displace the skin across the underlying tissues, and were most sensitive to vibration at low frequencies (usually <50 Hz), they appeared to derive their input selectively or predominantly from HFA fibers. The remaining dynamically sensitive neurons (15/36) could often be activated with manual tapping stimuli from widespread regions of the limb or even the experimental table, and in circumstances in which the skin could be displaced, it appeared that responsiveness was associated with subcutaneous sources.

The vibration-sensitive neurons with lowest vibrotactile thresholds at frequencies of ≤50 Hz appeared to be activated selectively by HFA inputs and displayed a graded responsiveness as a function of changes in vibration intensity. The impulse traces of Fig. 3 show the range of responsiveness for one HFA neuron and its gradation of output as a function of amplitude increases at vibration frequencies of 10–100 Hz. Responses occur sporadically on some cycles at low-amplitude, become more regular, and, at low vibration frequencies (≤20 Hz), give way to pairs or bursts of spikes on individual cycles at the higher amplitudes, such that the firing rates usually exceed the vibration frequency in this low range of stimulus frequencies.

Quantification of the response (in imp/s) as a function of the vibration amplitude permitted construction of stimulus-response relations (Fig. 4A) which, for a different, but representative neuron of this HFA-related type, show that thresholds are lowest (5–20 μm) in the frequency range 5–50 Hz

Furthermore, the graded relations apparent at 5–100 Hz in Fig. 4Afor this particular neuron, and, at 20 Hz, for seven different HFA-related neurons in Fig. 4B, ensure that, at these low frequencies, individual neurons of this class can contribute a sensitive signal of the changing intensity of vibrotactile perturbations in the hairy skin.

B: stimulus-response relations plotting, for another neuron, the mean response (imp/s) as a function of vibration amplitude at a range of frequencies.

A: values for 12 HFA neurons; B: 10 PC neurons; and C: for the mean ± SE threshold as a function of vibration frequency for the 2 classes.

Phaselocking of this neuron’s response was retained at frequencies ranging ≤75 Hz, but at higher frequencies, the failure rate increased on individual cycles, in particular, at vibration frequencies >30 Hz.

However, a switch-over occurs ∼50 Hz with values for HFA neurons falling below those for PC neurons at the higher frequencies.

The natural tactile stimuli most frequently encountered on the hairy skin of the arms and legs are likely to arise from objects brushing across the skin surface or its projecting hairs (e.g., see discussion in Gynther et al. 1995). These moving stimuli will set up various forms of complex vibrational disturbance in the skin that will generate spatiotemporal patterns of sensory inputs that we interpret as the nature and texture of the contacted object.

One dynamically sensitive class was most sensitive to vibration frequencies <50–80 Hz, had RFs within the hairy skin itself, and appeared to derive its peripheral input from the HFA class of afferent fibers.

The cuneate neurons driven selectively by the HFA class of peripheral afferents were most tightly phaselocked in response to vibratory disturbances in the hairy skin at frequencies ≤50 Hz. Furthermore, their impulse levels enabled them to respond at these frequencies in a cycle-by-cycle manner, thus replicating in their impulse pattern the periodicity inherent in the vibration stimulus.

The HFA-related class of cuneate neuron appeared to have a similar capacity for signaling vibrotactile frequency information to that of the RA class of neurons associated with low-frequency vibrotactile inputs from the glabrous skin (Connor et al. 1984; Douglas et al. 1978; Ferrington et al. 1987a, 1988) as percentage entrainment measures for responses to different vibration frequencies ≤50 Hz had average values in the range, 87–93%, for HFA neurons (Fig. 11) in comparison with values of ∼85–97% for RA neurons examined in an earlier study from our laboratory (Fig. 11B in Douglas et al. 1978).

The capacity to encode information about the frequency parameter of vibrotactile events in the impulse patterns of individual neurons appears to be well retained at the next level of the dorsal column-lemniscal pathway, in the ventralposterolateral (VPL) nucleus of the thalamus (Ghosh et al. 1992). From here, the information is conveyed over a parallel projection network to two principal cerebral cortical target regions, the primary and secondary somatosensory areas of the cortex (SI and SII, respectively) (Bennett et al. 1980; Ferrington and Rowe 1980b; Fisher et al. 1983; Mackie et al. 1996; Rowe et al. 1985; Turman et al. 1992, 1995; Zhang et al. 1996, 2001a,b).

This breakdown of a cycle-by-cycle impulse patterning at the cortical level for higher-frequency vibrotactile disturbances may mean (if impulse patterning is the crucial neural substrate for frequency recognition) that frequency coding in the range above ∼100 Hz may be dependent on a concatenation of thalamo-cortical events that include the presence of patterned activity at the thalamic level (Ghosh et al. 1992; Rowe 1990).

It is clear from both the earlier analyses of glabrous skin vibrotactile coding mechanisms in the DCN and from the present analysis for hairy skin that individual neurons in this tactile sensory pathway, relaying through the gracile and cuneate nuclei have a much greater capacity for signaling reliably the intensive and frequency parameters of vibrotactile stimuli than do their counterparts within the parallel spinocervical ascending system (Sahai et al. 2006).

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