Membrane potentials recorded from the mucosa of the toad's tongue during chemical stimulation

An isolated stretch of tongue mucosa was obtained from the Chilean toad (Callyptocephalella gayi). The preparation was observed under transmitted illumination through a binocular microscope. The surface cells were impaled with micro-electrodes and different chemical agents were applied to the area. The following results were obtained.1. The surface cells had resting potentials of -6 to -40 mV (mean of -17.6 mV) with the preparation bathed in Ringer solution.2. The cells underwent depolarization by application of different salts (NaCl, NaF, KCl, Na(2)SO(4), CaCl(2) and MgCl(2)) in concentrations of 0.25-1.0 M. The potentials evoked by the salt solutions often overshot the zero membrane potential level by several millivolts. The positive-going potential change produced by application of salts was increased during hyperpolarization of the membrane by inward current and was decreased by current of the opposite sign. Application of salts during depolarization of the membrane to a certain positive level produced a negative-going potential change. The potentials evoked by different salts were about the same, qualitatively, when recordings were made from different areas of the tongue, i.e. top of the fungiform and filiform papillae, doughnut-shaped folds at the base of the fungiform papillae, areas between papillae and from the side of the tongue totally devoid of papillary structures.3. Quinine applied in concentrations of 2 x 10(-2)M did not change the resting polarization of the surface epithelial cells. However, quinine applied to cells already depolarized by outward currents induced further depolarization. When it was delivered to cells hyperpolarized by inward current the substance induced further hyperpolarization.4. Sucrose (0.5-1.0 M) did not change the membrane potential of lingual cells regardless of the level of cell polarization induced by injected currents.5. Hydrochloric, sulphuric, nitric and acetic acids produced minimal biphasic effects: a brief hyperpolarization followed by a slower secondary depolarization.6. Water increased the membrane potential of the surface cells. Salts applied at the peak of the water-evoked hyperpolarization induced cell depolarization which was much larger than that evoked during application of salts to cells bathed in Ringer solution.     

Effect of tactile stimulation pulse characteristics on sensation threshold and power consumption

The psychophysical responses of human subjects to vibratory tactile stimulation of the skin were investigated experimentally. The parameters, of the waveform important to the minimization of power consumed by the tactile array of electromechanical vibrators and the maximization of the skin sensitivity to the stimulus were explored to develop optimum stimulation. Parameters investigated included the amplitude, frequency, and duty cycle of the current waveform used to drive the vibrators as well as the number of pulses per stimulating burst and the recovery time between bursts. Graphical techniques were used to determine, the optimal combination of the parameters which gave a stimulus that excited the skin to above tactile threshold while maintaining at a relative minimum the power required for the stimulus. The optimal stimulation waveform contains a burst of 10 rectangular pulses of 4% duty cycle separated by a period of nonstimulation of 2 s. Such a waveform can elicit a sensitivity of 29.4 mA⁻¹ consuming only 55 μW of power.     

Role of lingual nerve fibers in the transmission of mechanical sensation from the tongue and mouth in cats

The role of fibers of the anterior, middle, and posterior branches of the lingual nerve in the conduction of mechanical sensory information from receptors of the tongue and mouth was established in cats. Considerable overlapping of areas of the dorsal and ventral surfaces of the tongue supplied by fibers of these nerves is demonstrated. Different types of mechanoreceptor responses from the tongue and mouth are described.     

Behavior of rat fetuses following chemical or tactile stimulation

The behavior of fetal rats was examined on Day 19 of gestation with procedures that enabled chemical stimulation and direct observation of fetuses. Rat fetuses are sensitive to both tactile stroking and intraoral infusion of chemical solutions, but the pattern and amount of activity depend upon the modality of stimulation. Fetal responsiveness is affected by prior experience with chemical stimuli. Repeated exposure within a 10-min period results in a waning of response, and repeated exposure across a delay of 48 hr results in a different pattern of response than is seen to a novel stimulus. Reexposure to a stimulus experienced earlier in gestation also alters fetal responsiveness to other forms of tactile and chemical stimulation. These findings indicate that the rat fetus exhibits olfactory function in utero and suggest central processing of sensory information, including evidence of habituation, a fetal orienting reflex to novel stimuli, and the existence of prenatal behavioral states associated with different patterns of response.     

Gating of sensation and evoked potentials following foot stimulation during human gait

To investigate how gait influences the perceived intensity of cutaneous input from the skin of the foot, the tibial or sural nerves were stimulated at the ankle during walking or running on a treadmill. As compared to standing, the detection threshold for these stimuli was raised by more than 30% during the locomotion tasks. During walking, there was a phase-dependent modulation in perceived intensity of suprathreshold stimuli (1.5, 2, or 2.5×PT). Stimuli given just prior to footfall were perceived as significantly above average (Wilcoxon signed-rank test). In contrast there was a significant phasic decrease in sensitivity for shocks delivered immediately after ipsi- and contralateral footfall. The amplitude of somatosensory evoked potentials (P50–N80 complex), simultaneously evoked from pulse trains to the sural nerve and recorded at scalp level, was, on average, 62% of the level during standing. During gait, the amplitude of this complex was significantly smaller just after footfall than the amplitude during late swing (MANOVA). It is suggested that the reduced sensation and the decreased evoked potentials after touchdown may be due to occlusion or masking by concomitant afferent input from the feet. On the other hand, the phasic increase in sensitivity at the end of swing is thought to result from a centrally generated facilitation of sensory transmission of signals in anticipation of foot-placing.     

Formation of the accessory apparatus of chemical sensation in the rat tongue during development: A scanning electron microscopic investigation

The formation of the accessory apparatus for chemical sensation in the rat tongue during ontogeny was studied by scanning electron microscopy. Different chemically sensitive structures in the tongue were found to mature at different times, possibly in connection with changes in the dietary pattern at different stages of postnatal development.Laboratory of Physiology of Reception, P. K. Anokhin Institute of Normal Physiology, Academy of Medical Sciences of the USSR. Department of Anatomy and Physiology of Man and Animals, V. I. Lenin Moscow Pedagogic Institute. (Presented by Academician of the Academy of Medical Sciences of the USSR S. S. Debov.) Translated from Byulleten' Éksperimental'noi Biologii i Meditsiny, Vol. 84, No. 10, pp. 499–502, October, 1977.     

Referral of tactile sensation to the tips of L-shaped sticks

When we touch something with a tool, we feel the touch at the tip of the tool rather than at the hand that holds the tool. We reported previously that judging the temporal order of two successive stimuli delivered to the tips of straight sticks held in each hand was dramatically altered by crossing the sticks without changing hand position. The results suggested that tactile signals are referred to the tip of a tool held in the hand. Here we examined temporal order judgement using L-shaped sticks instead of straight ones to determine whether the shape of a tool affects the way tactile signals are referred. Subjects reported the order of stimuli correctly in most trials when the tip of each L-shaped stick occupied the hemispace ipsilateral to the anatomical laterality of the hand holding the L-shaped stick. The subjects, however, misreported the order of stimuli presented at moderately short intervals (<300 ms) when the tip of the stick occupied the hemispace contralateral to the anatomical laterality of the hand holding it. The judgment reversal occurred irrespective of the number of physical crossings between the sticks and the arms (0, 1, and 3), as long as the tips of L-shaped tools were placed in the contralateral hemispace. Our results suggest that our brain refers tactile signals from the hand directly to the location of the tip without much accounting for the route that connects hand and tip.     

Vestibular modulation of multisensory integration during actual and vicarious tactile stimulation

The vestibular system has been shown to contribute to multisensory integration by balancing conflictual sensory information. It remains unclear whether such modulation of exteroceptive (e.g., vision), proprioceptive, and interoceptive (e.g., affective touch) sensory sources is influenced by epistemically different aspects of tactile stimulation (i.e., felt from within vs. seen, vicarious touch). In the current study, we aimed to (a) replicate previous findings regarding the effects of galvanic stimulation of the right vestibular network in multisensory integration, and (b) examine vestibular contributions to multisensory integration when touch is felt but not seen (and vice versa). During artificial vestibular stimulation (LGVS, i.e., right vestibular stimulation), RGVS (i.e., bilateral stimulation), and sham (i.e., placebo stimulation), healthy participants (N = 36, Experiment 1; N = 37, Experiment 2) looked at a rubber hand while either their own unseen hand or the rubber hand were touched by affective or neutral touch. We found that (a) LGVS led to enhancement of vision over proprioception during visual only conditions (replicating our previous findings), and (b) LGVS (versus sham) favored proprioception over vision when touch was felt (Experiment 1), with the opposite results when touch was vicariously perceived via vision (Experiment 2) and with no difference between affective and neutral touch. We showed how vestibular signals modulate the weight of each sensory modality according to the context in which they are perceived and that such modulation extends to different aspects of tactile stimulation: felt and seen touch are differentially balanced in multisensory integration according to their epistemic relevance.     

A structural fingertip model for simulating of the biomechanics of tactile sensation

Tactile performance of human fingertips is associated with activity of the nerve endings and sensitivity of the soft tissue within the fingertip to the static and dynamic skin indentation. The nerve endings in the fingertips sense the stress/strain states developed within the soft tissue, which are affected by the material properties of the tissues. The vibrotactile sensation and tactile performance are thus believed to be strongly influenced by the nonlinear and time-dependent properties of the soft tissues. The purpose of the present research is to simulate the biomechanics of tactile sensation. A two-dimensional model, which incorporates the essential anatomical structures of a finger (i.e. skin, subcutaneous tissue, bone, and nail), has been used for the analysis. The skin tissue is assumed to be hyperelastic and viscoelastic. The subcutaneous tissue is considered to be a nonlinear, biphasic material composed of a hyperelastic solid and an inviscid fluid phase. The nail and bone are considered to be linearly elastic. The advantages of the proposed fingertip model over the previous "waterbed" and "continuum" fingertip models include its ability to predict the deflection profile of the fingertip surface, the stress and strain distributions within the soft tissue, and most importantly, the dynamic response of the fingertip to mechanical stimuli. The proposed model is applied to simulate the mechanical responses of a fingertip under a line load, and in one-point (1PT) and two-point (2PT) tactile discrimination tests. The model's predictions of the deflection profiles of a fingertip surface under a line load agree well with the reported experimental data. Assuming that the mechanoreceptors in the dermis sense the stimuli associated with normal strains (the vertical and horizontal strains) and strain energy density, our numerical results suggest that the threshold of 2PT discrimination may lie between 2.0 and 3.0 mm, which is consistent with the published experimental data. The present study represents an effort to develop a structural model of the fingertip that incorporates its anatomical structure, and the nonlinear and time-dependent properties of the soft tissues.     

An experimental device to provide substitute tactile sensation from the anaesthetic hand

Anaesthesia of the hand can result from injury or disease. It severely impairs the normal use of the hand and brings with it the danger of accidental self-inflicted injury due to the absence of pain. A prosthetic device intended to overcome this non-correctable handicap will have to measure the pressure at the finger of the hand and produce a suitable stimulus that is a function of this pressure. The experimental device described uses flexible pressure sensors, based on the principle of the mercury strain gauge. An RC beat-frequency oscillator emits an audible sound whose frequency is a function of this pressure. The considerations for the design of the device are discussed and first results of the practical use are reported.     

Cholinergic gating of improvement of tactile acuity induced by peripheral tactile stimulation

As shown in many animal experiments, cholinergic mechanisms participate in N-methyl-d-aspartate (NMDA) receptor-dependent neuroplasticity. Acetylcholine is thought to play a similar role in humans, where it modulates attention and learning. Here, we tested the cholinergic action on non-associative learning in the tactile domain. We studied the influence of scopolamine, a cholinergic antagonist, on changes in tactile acuity as induced by peripheral tactile coactivation. Coactivation is a non-associative tactile learning protocol and has been shown to improve tactile two-point discrimination of the stimulated finger in addition to selective changes of cortical processing. Under placebo conditions, tactile two-point discrimination was improved on the stimulated index finger. After application of scopolamine, tactile improvement was completely eliminated and tactile acuity was even impaired. No drug effects were found on the left index finger indicating that the drug had no effect on performance per se. The current results provide further evidence that in humans cholinergic mechanisms are also involved in non-associative learning induced by passive stimulation protocols.     

Taste qualities elicited by electric stimulation of single human tongue papillae

Summary Single human taste papillae in 5 young test subjects were electrically stimulated 84 times earch with a silver wire (0.4 mm tip diameter)versus a 5 × 3.5 cm² reference electrode on the left wrist of the subject. The electrical pulses were positive (54 papillae) or negative (45 papillae) of 0.5 msec duration and had a frequency of 50, 100, 200, 400, 600 or 800 Hertz. For control 45 papillae were merely touched by the electrode without current stimulation. Significant gustatory and tactile results (²-test,P 0.05) signaled by the subjects were: a) on anodic stimulation 50 ± 5% of the touched papillae gave overall taste responses, namely 22.2% sour, 3.8% bitter and 1.8% salty; the rest were mixed sensations. Sweet was not reported in these final experiments; b) on cathodic stimulation 42.2 ± 5% of the papillae responded with taste, which again were reported as 22.2% sour, 2.2% salty, the rest mixed; c) on mere touching there were hardly any gustatory sensations reported.—These results indicate only a relative specifity for each single human taste papilla and its taste fibres.Supported by the Bundesminister des Inneren der Bundesrepublik Deutschland.     

Facilitation of voluntary swallowing by chemical stimulation of the posterior tongue and pharyngeal region in humans

In this study, we investigated the functional difference between chemical stimulations of the posterior tongue (PT) and pharyngeal region (PR) for facilitation of voluntary swallowing in humans. The PT or PR stimulation consisted of infusion of water (distilled water), 0.3 M NaCl solution or olive oil (non-chemical stimulant) into the PT or the PR through a fine tube at a very slow infusion rate (0.2 ml/min). Water was used as a stimulant of water receptors. A solution of 0.3 M NaCl was used as an inhibitor of the response of water receptors and as a stimulant of salt taste receptors. Excitation of the mucosal receptors would facilitate voluntary swallowing and diminution of sensory inputs from the oral mucosa would induce difficulty in swallowing. Swallowing intervals (SIs) during voluntary swallowing were measured by submental electromyographic activity. Infusion of water into the PR shortened SI (facilitation of swallowing) and infusion of 0.3 M NaCl or olive oil into the same region prolonged it (difficulty in swallowing). On the other hand, infusion of water into the PT prolonged SI and infusion of 0.3 M NaCl into the same region shortened it. The results suggest that water receptors are localized in the PR and that salt taste receptors are almost absent in the PR and present in the PT. With diminution of sensory inputs from the oral mucosa, central inputs would play a dominant role in initiating swallowing voluntarily, and SI would be prolonged. With weak stimulation (infusion of 0.3 M NaCl into the PR or infusion of water into the PT), SI was prolonged and inter-individual variation in SI was pronounced, suggesting that the ability of the central regulation of swallowing to perform repetitive voluntary swallowing varies among subjects. With stimulation of water receptors or salt taste receptors, SI was shortened and inter-individual variation in SI was moderate, suggesting that sensory inputs are important for performing voluntary swallowing smoothly and that the sensory inputs compensate for the difficulty in performing swallowing caused by the central mechanism.     

Effectiveness of sensory stimulation on tactile extinction

 Eleven brain-damaged patients with extinction were asked to report double tactile stimuli before, during, and after optokinetic stimulation and transcutaneous electrical stimulation of the posterior neck region. The goal of the study was to test whether tactile extinction is sensitive to these experimental manipulations in order to better understand the nature of the disorder. Both of these sensory stimulations are known to be effective in modulating only higher-order (cognitive) disorders of spatial coding, such as visual hemineglect, deficit of position sense, hemianesthesia, etc. When applied to the side contralateral to the cerebral lesion, both optokinetic and transcutaneous electrical stimulation significantly affected patients’ performances, increasing the amount of detections of contralesional double stimuli. A tendency towards worse performance was observed when sensory stimulation was applied to the ipsilesional side. The reported effectiveness in reducing tactile extinction suggests that the deficit can not be fully ascribed to a peripheral sensory disorder and that it reflects damage to a higher-order cognitive function involved in contralesional space representation or in the deployment of attention to that side of space. The nature of the close relationship between extinction and hemineglect is also discussed from the point of view of extinction as a deficit of space coding. Received: 19 September 1998 / Accepted: 19 February 1999