Non-technical summary What triggers a realignment of sensations, e.g. a stimulus that is perceived as non-painful in intact skin, but evokes pain in sunburned skin, is yet to be ascertained. This phenomenon is clinically termed allodynia. We show that gentle tactile stimulation (vibration and brushing) of the hairy skin can exacerbate the underlying muscle pain (allodynia) evoked by infusion of hypertonic saline into the tibialis anterior muscle. This effect is dependent upon a low-threshold, mechanosensitive class of nerve fibres in the hairy skin known as C-tactile (CT) fibres. Knowledge of the role of CT fibres in allodynia increases our understanding of the mechanisms that underlie sensory-perceptual abnormalities -a common manifestation of clinical-pain states and neurological disorders. AbstractWe recently showed a contribution of low-threshold cutaneous mechanoreceptors to vibration-evoked changes in the perception of muscle pain. Neutral-touch stimulation (vibration) of the hairy skin during underlying muscle pain evoked an overall increase in pain intensity, i.e. allodynia. This effect appeared to be dependent upon cutaneous afferents, as allodynia was abolished by intradermal anaesthesia. However, it remains unclear whether allodynia results from activation of a single class of cutaneous afferents or the convergence of inputs from multiple classes. Intriguingly, no existing human study has examined the contribution of C-tactile (CT) afferents to allodynia. Detailed psychophysical observations were made in 29 healthy subjects (18 males and 11 females). Sustained muscle pain was induced by infusing hypertonic saline (HS: 5%) into tibialis anterior muscle (TA). Sinusoidal vibration (200 Hz-200 μm) was applied to the hairy skin overlying TA. Pain ratings were recorded using a visual analogue scale (VAS). In order to evaluate the role of myelinated and unmyelinated cutaneous afferents in the expression of vibration-evoked allodynia, compression block of the sciatic nerve, and low-dose intradermal anaesthesia (Xylocaine 0.25%) were used, respectively. In addition, the modulation of muscle pain by gentle brushing (1.0 and 3.0 cm s −1 ) -known to excite CT fibres -was examined. Brushing stimuli were applied to the hairy skin with all fibres intact and following the blockade of myelinated afferents. During tonic muscle pain (VAS 4-6), vibration evoked a significant and reproducible increase in muscle pain (allodynia) that persisted following compression of myelinated afferents. During compression block, the sense of vibration was abolished, but the vibration-evoked allodynia persisted. In contrast, selective anaesthesia of unmyelinated cutaneous afferents abolished the allodynia, whereas the percept of vibration remained unaffected. Furthermore, allodynia was preserved in the adjacent non-anaesthetized skin. Conformingly, gentle brushing produced allodynia (at both brushing speeds) that persisted during the blockade of myelinated afferents. Prior to the induction and following cessation of muscle pain, all subj...

We undertook a neurophysiological investigation of the responses of low-threshold mechanoreceptors in the human finger pad to surfaces of differing softness. Unitary recordings were made from 26 slowly adapting type I (SAI), 17 fast-adapting type I (FAI), and 9 slowly adapting type II (SAII) afferents via tungsten microelectrodes inserted into the median nerve at the wrist. A servo-controlled stimulator applied ramp-and-hold forces (1, 2, 4 N) at a constant loading and unloading rate (2 N/s) via a flat silicone disc over the center of the finger pad. Nine discs were used, which linearly increased in stiffness across the range. Population responses of the SAI afferents showed the greatest sensitivity to compliance, with a steep monotonic increase in mean firing rate with increasing stiffness (decreasing compliance) of the surface during the loading and plateau (but not unloading) phases. FAI afferents also showed a linear increase in firing during the loading but not unloading phase, although the slope was significantly lower than that of the SAI afferents at all amplitudes. Conversely, SAII afferents were influenced by object compliance only in certain conditions. Given their high density in the finger pads and their linear relationship between firing rate and object compliance during the loading and plateau phases, SAI afferents (together with FAI afferents during the loading phase) are ideally suited to contributing information on surface compliance to the overall estimation of softness, but the SAII afferents appear to play only a minor role.

Axon order throughout the visual pathway of the quokka wallaby (Setonix brachyurus) was determined after localised retinal applications of the tracers DiI and/or DiASP. Postnatal days (P) 22-90 were studied to encompass the development and refinement of retinal projections. Order was essentially similar at all stages. Axons entered the optic nerve head true to their sector of retinal origin. In the optic nerve, nasal and temporal axons continued to reflect their retinal origin, dominating, respectively, the medial and lateral halves. By contrast, dorsal and ventral axons exchanged locations between the retrobulbar level and one-third the distance along the nerve; thus, the inversion of the dorsoventral retinal axis, imposed by the lens, was corrected. Decussating axons maintained their relative locations through the chiasm. At the base of the optic tract, nasal and temporal axons underwent an axial rotation to lie on the medial and lateral sides, respectively; thus nasal overlapped with ventral axons and temporal with dorsal axons. Axons maintained their alignments throughout the tract, and as a result, nasal and ventral axons invaded the superior colliculus medially, whereas temporal and dorsal axons invaded laterally. Each retinal quadrant terminated preferentially in its retinotopically appropriate sector of the colliculus. The arrangement of axons in the quokka visual pathway displays several novel features. Axon order is distinct throughout, involving a well-demarcated exchange of dorsal and ventral axons in the nerve and an axial rotation of nasal and temporal axons at the base of the tract; these relocations suggest decision regions for growing axons. The organisation presumably underlies the less extensive searching within the developing superior colliculus to generate retinotopic maps in the quokka and also in tammar wallaby [Marotte, J. Comp Neurol. 293:524-539, 1990] than in the rat [Simon and O'Leary, J. Neurosci. 12:1212-1232, 1992].

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Using a multi-channel platinum surface electrode array, recordings from cat primary visual cortex were obtained in response to visual stimuli, and electrical stimuli delivered using the elements of the array itself. Neural responses to electrical stimuli were consistent, regardless of stimulus polarity or leading phase (biphasic), although thresholds were lower for monophasic than biphasic pulses. Both visual and electrical stimuli reliably evoked responses with characteristic components, which interacted with each other in a nonlinear summation showing first facilitation then suppression during the window of interaction. The chronaxie for eliciting threshold cortical responses was about 100 mus, and the charge density with a pulse width of 50-100 mus was around 55 muC cm(-2). These data form the basis of understanding the types of cortical responses to stimuli delivered by devices suitable for chronic implantation.