Ricco's law and absolute threshold for foveal detection of black holes

Visual detection of small black objects surrounded by a light background depends on background luminance, pupil size, optical blur, and object size. Holding pupil and optics fixed, we measured the minimum background luminance needed for foveal detection of small black targets as a function of target size. For all three observers, absolute threshold varied inversely with target area when disk diameter subtended less than 10^' of visual angle. For target diameter ≥10^', threshold remained constant at about 0.3 Td, which was also the absolute threshold for detecting light spots 10^' or larger in diameter on a black background. These results are consistent with Ricco's law of spatial summation: a "black hole" is just detectable when the background luminance is sufficiently high for its absence inside the Ricco area to reduce 555 nm photon flux by 7500 photons/s, which is the same change needed to detect light spots on a black surround. These results can be accounted for by a differential pair of Ricco detectors, each about the size of the receptive field center of magocellular retinal ganglion cells when projected into object space through the eye's weakly aberrated optical system. Statistical analysis of the model suggests the quantum fluctuations due to internal, biological noise (i.e., "scotons") are a greater handicap than the photon fluctuations inherent in the light stimulus at absolute foveal threshold.

Role of extrinsic noise in the sensitivity of the rod pathway: rapid dark adaptation of nocturnal vision in humans

Rod-mediated 500 nm test spots were flashed in Maxwellian view at 5 deg eccentricity, both on steady 10.4 deg fields of intensities (I) from 0.00001 to 1.0 scotopic troland (sc td) and from 0.2 s to 1 s after extinguishing the field. On dim fields, thresholds of tiny (5') tests were proportional to √I (Rose-DeVries law), while thresholds after extinction fell within 0.6 s to the fully dark-adapted absolute threshold. Thresholds of large (1.3 deg) tests were proportional to I (Weber law) and extinction thresholds, to √I.

CONCLUSIONS

rod thresholds are elevated by photon-driven noise from dim fields that disappears at field extinction; large spot thresholds are additionally elevated by neural light adaptation proportional to √I. At night, recovery from dimly lit fields is fast, not slow.

Understanding disability glare: light scatter and retinal illuminance as predictors of sensitivity to contrast

The presence of a bright light in the visual field has two main effects on the retinal image: reduced contrast and increased retinal illuminance because of scattered light; the latter can, under some conditions, lead to an improvement in retinal sensitivity. The combined effect remains poorly understood, particularly at low light levels. A psychophysical flicker-cancellation test was used to measure the amount and angular distribution of scattered light in the eye for 40 observers. Contrast thresholds were measured using a functional contrast sensitivity test. Pupil-plane glare-source illuminances (i.e., 0, 1.35, and 19.21  lm/m2), eccentricities (5°, 10°, and 15°), and background luminances (1, 2.6, and 26  cd/m2) were investigated. Visual performance was better than predicted, based on a loss of retinal image contrast caused by scattered light, particularly in the mesopic range. Prediction accuracy improved significantly when the expected increase in retinal sensitivity in the presence of scattered light was also incorporated in the model.

Effects of conditioning peripheral repetitive magnetic stimulation in patients with complex regional pain syndrome

OBJECTIVES

We tested whether repetitive magnetic stimulation (rMS) induces an afferent input to the spinocerebral tract in patients with complex regional pain syndrome (CRPS).

METHODS

Cortical and spinal motor evoked potentials (cMEP and sMEP), as well as the contra- and ipsilateral silent period (cCSP and iCSP), were recorded in patients with CRPS type I before and after conditioning rMS, applied at cervical nerve roots innervating affected muscles. Patients were compared with a group of healthy subjects.

RESULTS

In the group of patients we found that cMEP amplitudes were always significantly smaller for both hemispheres. In the group of healthy subjects we found a significant prolongation of the cCSP and iCSP after rMS. This was absent in patients. SMEP were always unchanged in both groups.

DISCUSSION

This led us to the explanation that the afferent input to the motor cortical system in CRPS patients is less effective than in healthy subjects.

Mouse ganglion-cell photoreceptors are driven by the most sensitive rod pathway and by both types of cones

Intrinsically photosensitive retinal ganglion cells (iprgcs) are depolarized by light by two mechanisms: directly, through activation of their photopigment melanopsin; and indirectly through synaptic circuits driven by rods and cones. To learn more about the rod and cone circuits driving ipRGCs, we made multielectrode array (MEA) and patch-clamp recordings in wildtype and genetically modified mice. Rod-driven ON inputs to ipRGCs proved to be as sensitive as any reaching the conventional ganglion cells. These signals presumably pass in part through the primary rod pathway, involving rod bipolar cells and AII amacrine cells coupled to ON cone bipolar cells through gap junctions. Consistent with this interpretation, the sensitive rod ON input to ipRGCs was eliminated by pharmacological or genetic disruption of gap junctions, as previously reported for conventional ganglion cells. A presumptive cone input was also detectable as a brisk, synaptically mediated ON response that persisted after disruption of rod ON pathways. This was roughly three log units less sensitive than the rod input. Spectral analysis revealed that both types of cones, the M- and S-cones, contribute to this response and that both cone types drive ON responses. This contrasts with the blue-OFF, yellow-ON chromatic opponency reported in primate ipRGCs. The cone-mediated response was surprisingly persistent during steady illumination, echoing the tonic nature of both the rod input to ipRGCs and their intrinsic, melanopsin-based phototransduction. These synaptic inputs greatly expand the dynamic range and spectral bandpass of the non-image-forming visual functions for which ipRGCs provide the principal retinal input.

Intense focused ultrasound can reliably induce sensations in human test subjects in a manner correlated with the density of their mechanoreceptors

Sensations generated by intense focused ultrasound (iFU) can occur cutaneously and/or at depth, in contrast to other forms of stimulation (e.g., heat, electricity), whose action usually occurs only at the skin surface, or mechanical stimulation (e.g., von Frey hairs, calibrated forceps, tourniquets) that compress and thus stimulate all tissue. Previous work on iFU stimulation has led to the hypothesis that the tactile basis of iFU stimulation should correlate with the density of mechanoreceptors at the site of iFU stimulation. Here we tested that hypothesis, correlating a "two-point" neurological examination-a standard measure of superficial mechanoreceptor density-with the intensity of superficially applied iFU necessary to generate sensations with high sensitivity and specificity. We applied iFU at 1.1 MHz for 0.1 s to the fingertip pads of 17 test subjects in a blinded fashion and escalated intensities until they consistently observed iFU-induced sensations. Most test subjects achieved high values of sensitivity and specificity, doing so at values of spatially and temporally averaged intensity measuring <100 W/cm(2). Moreover, the test subjects' sensitivity to iFU stimulation correlated with the density of mechanoreceptors as determined by a standard two-point discrimination neurological examination, consistent with earlier hypotheses.

Comparison of LEP and QST and their contribution to standard sensory diagnostic assessment of spinal lesions: a pilot study

This study evaluates the additional use of laser-evoked potentials (LEP) and quantitative sensory testing (QST) in the sensory assessment of spinal lesions. Four consecutive patients with spinal lesions verified by MRI and clinical evidence for mild spinothalamic tract involvement were included. The electrophysiological workup [somatosensory evoked potentials (SEP) and LEP] was compared to QST. Electrophysiology and QST were reassessed after about 6 months. LEP detected impaired spinothalamic tract function in 7/8 examinations. QST pointed to spinothalamic tract lesions by loss of thermal function (3/8); most frequent positive sensory signs (3/8) were paradoxical heat sensations. LEP and QST results were concordant in 6/8 examinations. SEPs were abnormal in 2/8 examinations. Congruent results between SEP and both LEP and QST were obtained in 3/8 examinations. LEP detected more deficits than any single QST parameter or their combination but additional QST allows the detection of positive sensory signs. The diagnostic gain of SEP was limited.

Combined optical and electrical stimulation of neural tissue in vivo

Low-intensity, pulsed infrared light provides a novel nerve stimulation modality that avoids the limitations of traditional electrical methods such as necessity of contact, presence of a stimulation artifact, and relatively poor spatial precision. Infrared neural stimulation (INS) is, however, limited by a 2:1 ratio of threshold radiant exposures for damage to that for stimulation. We have shown that this ratio is increased to nearly 6:1 by combining the infrared pulse with a subthreshold electrical stimulus. Our results indicate a nonlinear relationship between the subthreshold depolarizing electrical stimulus and additional optical energy required to reach stimulation threshold. The change in optical threshold decreases linearly as the delay between the electrical and optical pulses is increased. We have shown that the high spatial precision of INS is maintained for this combined stimulation modality. Results of this study will facilitate the development of applications for infrared neural stimulation, as well as target the efforts to uncover the mechanism by which infrared light activates neural tissue.

Electric current perception of the general population including children and the elderly

Although the 50 Hz electric current perception threshold is a key parameter for limiting electric touch currents in electrical technology and for limiting indirect effects of external electromagnetic fields, the data available mainly reflect men's perception ability; with only sparse data for women and almost none for children or the elderly. Measurements with 240 children aged 9 - 16 years, and 123 elderly people, allow this gap of knowledge to be filled. Taking into account the demographic age distribution, it was possible to generate a probability distribution representing the perception ability of the overall general population, and thus to provide a more established basis for deriving safety limits. The results show that the existing limit values for electric touch currents are considerably too high if compared with results derived from the new data with the same criteria. On the other hand, it appeared that children do not exhibit such a high sensitivity to electricity as assumed to date. Therefore, former rule-of-thumb estimates to account for higher sensitivities of children lie on the safe side. The presented assessment of the general population's electric current perception ability should stimulate a critical review of the existing regulations.

ROD AND CONE SIGNALING TRANSMISSION IN THE RETINA OF ZEBRAFISH: AN ERG STUDY

It has been suggested that in lower vertebrates, visual signals are propagated in the retina using mixed bipolar cell pathways. In this article, we examined rod and cone signaling transmission in the retina of teleost zebrafish. Full-field electroretinograms (ERGs) were recorded from the cornea of control or 2-amino-4-phosphonobutyric acid (APB)-treated zebrafish. During dark adaptation following bright light adaptation, the intensity-response (V-log I) curve for both the ERG b- and d-waves shifted. In control animals, the b-wave response displayed a typical visual Purkinje shift and became more sensitive. By contrast, the d-wave became less sensitive. In the early dark adaptation, cone-dominant signals were readily recorded in both b- and d-waves. In the late dark adaptation, rod signals became dominant. However, no obvious d-wave was recorded. Intraocular injections of APB selectively blocked the b-wave, but revealed the d-wave in both early and late dark adaptation.

Cellular dynamics of cholinergically induced alpha (8-13 Hz) rhythms in sensory thalamic nuclei in vitro

Although EEG alpha (8-13 Hz) rhythms are traditionally thought to reflect an "idling" brain state, they are also linked to several important aspects of cognition, perception, and memory. Here we show that reactivating cholinergic input, a key component in normal cognition and memory operations, in slices of the cat primary visual and somatosensory thalamus, produces robust alpha rhythms. These rhythms rely on activation of muscarinic receptors and are primarily coordinated by activity in the recently discovered, gap junction-coupled subnetwork of high-threshold (HT) bursting thalamocortical neurons. By performing extracellular field recordings in combination with intracellular recordings of these cells, we show that (1) the coupling of HT bursting cells is sparse, with individual neurons typically receiving discernable network input from one or very few additional cells, (2) the phase of oscillatory activity at which these cells prefer to fire is readily modifiable and determined by a combination of network input, intrinsic properties and membrane polarization, and (3) single HT bursting neurons can potently influence the local network state. These results substantially extend the known effects of cholinergic activation on the thalamus and, in combination with previous studies, show that sensory thalamic nuclei possess powerful and dynamically reconfigurable mechanisms for generating synchronized alpha activity that can be engaged by both descending and ascending arousal systems.

A rat in vitro model for the measurement of multiple excitability properties of cutaneous axons

OBJECTIVE

To establish an in vitro model for measurement of the excitability properties of cutaneous sensory axons.

METHODS

We used a saphenous skin-nerve preparation from adult rat in combination with computerized threshold tracking. We measured strength-duration time constant, the recovery of excitability after a supramaximal stimulus and the accommodation to conditioning subthreshold polarizing stimuli (threshold electrotonus, current-threshold relationship) and compared these with previously published recordings from sensory axons in human median nerve.

RESULTS

Threshold electrotonus and the amplitude of superexcitability were indistinguishable between human median nerve in vivo and rat saphenous nerve in vitro, but several excitability parameters were significantly different in the rat: strength-duration time constant was significantly shorter (0.19+/-0.01 vs. 0.53+/-0.02 ms); the refractory period was shorter (1.9+/-1.1 ms vs. 3.5+/-1.0 ms) and late subexcitability was smaller (6.3+/-0.3% vs. 11.3+/-0.5%); thirdly, during recording of current-threshold relationship, rat nerves displayed more inward rectification to strong hyperpolarizing currents. Parameters were stable over more than 3h.

CONCLUSIONS

Excitability changes of sensory Abeta-fibres can be reliably studied in the rat in vitro and are qualitatively similar to humans.

SIGNIFICANCE

This rat model will facilitate pharmacological studies of nerve excitability and work on models of neuropathy.

Factors influencing the magnitude and reproducibility of corticomotor excitability changes induced by paired associative stimulation

Several paired-associative stimulation (PAS) protocols induce neuroplastic changes in human motor cortex (M1). To understand better the inherent variability of responses to PAS, we investigated the effectiveness and reproducibility of two PAS paradigms, and neurophysiological and experimental variables that may influence this. Motor evoked potentials (MEPs) were elicited by transcranial magnetic stimulation (TMS) of right M1, and recorded from surface EMG of left abductor pollicis brevis (APB) and first dorsal interosseous before and after PAS. PAS consisted of electrical stimulation of left median nerve paired with TMS over right M1 25 ms later. Twenty subjects were given one of two PAS protocols: short (132 paired stimuli at 0.2 Hz) or long (90 paired stimuli at 0.05 Hz), and were re-tested with the same protocol on 3 separate occasions, with 11 subjects tested in the morning and 9 in the afternoon. Neurophysiological variables assessed included MEP amplitude, resting and active motor threshold, short-interval intracortical inhibition, intracortical facilitation and cortical silent period duration. The short PAS protocol produced greater APB MEP facilitation (51%) than the long protocol (11%), and this did not differ between sessions. The neurophysiological variables did not consistently predict responses to PAS. Both PAS protocols induced more APB MEP facilitation, and greater reproducibility between sessions, in experiments conducted in the afternoon. The mechanism for this is unclear, but circadian rhythms in hormones and neuromodulators known to influence neuroplasticity warrant investigation. Future studies involving PAS should be conducted at a fixed time of day, preferably in the afternoon, to maximise neuroplasticity and reduce variability.

Median-radial sensory nerve comparative studies in the detection of median neuropathy at the wrist in diabetic patients

OBJECTIVE

Median-ulnar comparative studies (MUCS) play an important role in the electrodiagnosis of carpal tunnel syndrome, but in diabetes concomitant involvement of Guyon's canal (ulnar nerve compression at the wrist) would reduce the sensitivity of MUCS. This study tested the utility of median-radial comparative studies (MRCS) in diabetic patients.

METHODS

Anti-dromic MUCS and MRCS were prospectively performed in 120 patients with diabetes, and 64 normal controls. In 28 diabetic patients, latent addition using threshold tracking was performed in superficial radial sensory axons to estimate persistent nodal sodium currents.

RESULTS

MUCS was abnormal in 49% of the diabetic patients, and MRCS was abnormal in 58%. Median motor distal latencies were prolonged in 38%, and median sensory nerve conduction velocities were slowed in 40%. The longer latency differences in MRCS were associated with smaller persistent sodium currents, suggesting that intra-axonal sodium accumulation mediated by hyperglycemia enhances nerve compression.

CONCLUSIONS

MRCS appears to be the most sensitive electrodiagnostic test in the detection of median neuropathy at the wrist in diabetic patients. Nerve conduction slowing across the carpal tunnel may be associated with metabolic abnormalities under hyperglycemia.

SIGNIFICANCE

Assessment of nerve conduction across the common entrapment sites could provide new insights into the pathophysiology of diabetic neuropathy related to metabolic factors.

Sex-related differences in NMDA-evoked rat masseter muscle afferent discharge result from estrogen-mediated modulation of peripheral NMDA receptor activity

In the present study, the hypothesis that sex-related differences in glutamate-evoked rat masseter muscle afferent discharge may result from estrogen-related modulation of peripheral N-methyl-d-aspartate (NMDA) receptor activity and/or expression was tested by examining afferent fiber discharge in response to masseter injection of NMDA and the expression of NR2A/B subunits by masseter ganglion neurons in male and female rats. The results showed that injection of NMDA into the masseter muscle evoked discharges in putative mechanonociceptive afferent fibers and increased blood pressure that was concentration-dependent, however, a systemic action of NMDA appeared responsible for increased blood pressure. NMDA-evoked afferent discharge was significantly greater in female than in male rats, was positively correlated with plasma estrogen levels in females and was significantly greater in ovariectomized female rats treated with a high dose (5 mug/day) compared with a low dose (0.5 mug/day) of estrogen. Pre-treatment of high dose estrogen-treated-ovariectomized female rats with the Src tyrosine kinase inhibitor PP2 did not affect NMDA-evoked afferent discharge. NMDA-evoked afferent discharge was attenuated by the antagonists ketamine and ifenprodil, which is selective for NR2B containing NMDA receptors. Fewer masseter ganglion neurons expressed the NR2A (16%) subunit as compared with the NR2B subunit (38%), which was expressed at higher frequencies in intact female (46%) and high dose estrogen-treated ovariectomized female (60%) rats than in male (31%) rats. Taken together, these results suggest that sex-related differences in NMDA-evoked masseter afferent discharge are due, at least in part, to an estrogen-mediated increase in expression of peripheral NMDA receptors by masseter ganglion neurons in female rats.

Spontaneous low threshold spike bursting in awake humans is different in different lateral thalamic nuclei

Spontaneous action potential bursts associated with low threshold calcium spikes (LTS) occur in multiple human lateral thalamic nuclei, each with different physiologic characteristics. We now test the hypothesis that different patterns of spontaneous LTS bursting occur in these nuclei during awake surgery in patients with essential tremor and the arm at rest. This protocol was chosen to minimize the effect of the patient's disease upon thalamic activity which is a potential confound in a surgical study of this type. Neuronal activity was studied in the human thalamic nuclei receiving somatic sensory input (Vc, ventral caudal), input from the deep cerebellar nuclei (Vim, ventral intermediate), or input from the pallidum (Vo, ventral oral). In each nucleus the burst rates were significantly greater than zero. Burst rates were higher in Vc than in Vim, while firing rates were lower. These findings suggest that neurons in Vc are hyperpolarized and have more frequent inhibitory events. Pre-burst inter-spike intervals (ISIs) were significantly longer in Vc, but were significantly shorter when corrected for the average ISIs between bursts (burst rate/inverse of the primary event rate). These results suggest that inhibitory events in Vc are of lower magnitude relative to a hyperpolarized resting membrane potential. Studies in many species demonstrate that input from the pallidum to the thalamus is inhibitory, suggesting that input to Vo is predominantly inhibitory. However, neurons in Vo have neither slower firing rates nor more frequent LTS bursts. Previous studies have found that spontaneous LTS is similar between classes of neurons within Vc, as defined by their response to thermal and painful stimuli. The differences in spontaneous LTS between human nuclei but not between functional classes within a nucleus may be a basic organizing principle of thalamic inhibitory circuitry.

Influence of a 902.4 MHz GSM signal on the human visual system: Investigation of the discrimination threshold

The proximity of a mobile phone to the human eye raises the question as to whether radiofrequency (RF) electromagnetic fields (EMF) affect the visual system. A basic characteristic of the human eye is its light sensitivity, making the visual discrimination threshold (VDThr) a suitable parameter for the investigation of potential effects of RF exposure on the eye. The VDThr was measured for 33 subjects under standardized conditions. Each subject took part in two experiments (RF-exposure and sham-exposure experiment) on different days. In each experiment, the VDThr was measured continuously in time intervals of about 10 s for two periods of 30 min, having a break of 5 min in between. The sequence of the two experiments was randomized, and the study was single blinded. During the RF exposure, a GSM signal of 902.4 MHz (pulsed with 217 Hz) was applied to the subjects. The power flux density of the electromagnetic field at the subject location (in the absence of the subject) was 1 W/m(2), and numerical dosimetry calculations determined corresponding maximum local averaged specific absorption rate (SAR) values in the retina of SAR(1 g) = 0.007 W/kg and SAR(10 g) = 0.003 W/kg. No statistically significant differences in the VDThr were found in comparing the data obtained for RF exposure with those for sham exposure.

Effects of rapid and slow cooling on thermoregulatory reactions in hypertensive rats after administration of calcium

Iontophoretic administration of calcium ions into the skin close to the application site of a cold stimulus decreased the threshold of thermoregulatory reactions in hypertensive rats to a greater extent than in normotensive control animals, which may be evidence that the tissues involved in thermoregulatory reactions to cold have a greater sensitivity to calcium in hypertensive rats. The initially earlier onset of vascular and metabolic reactions and the increase in the vascular reaction seen in hypertensive rats became more marked after administration of calcium. Treatment with calcium, increasing the vascular reaction to cooling, facilitates a more marked discrimination between hyper-and normotensive animals in terms of the appearance of the vasoconstrictor reactions of skin blood vessels in response to cold. The effects of the added calcium on cold-dependent reactions depended on the rate of cooling.

Multiple measures of axonal excitability in peripheral sensory nerves: An in vivo rat model

Excitability measurements on human motor and sensory nerves have provided new insights into axonal membrane changes in peripheral nerve disorders. The aim of this study was to establish an in vivo rat preparation suitable for threshold tracking of sensory nerve action potentials (SNAPs) to model clinical sensory nerve excitability studies. In Sprague-Dawley rats anesthetized with ketamine and xylazine, current stimuli were applied to the base of the tail and SNAPs recorded from distal needle electrodes. Multiple excitability data were obtained as previously described for human nerves and compared to recordings from the motor tail axons and to sensory recordings from human median and ulnar nerves. The pattern of excitability changes in rats was broadly similar to that in humans, although some parameters differed significantly. Individual recordings were stable for at least 3 h. These data show that the rat tail enables excitability properties of sensory as well as motor axons to be studied experimentally, e.g., in models of nerve disease and during pharmacological interventions.

Abnormal plasticity of the sensorimotor cortex to slow repetitive transcranial magnetic stimulation in patients with writer's cramp

Previous studies demonstrated functional abnormalities in the somatosensory system, including a distorted functional organization of the somatosensory cortex (S1) in patients with writer's cramp. We tested the hypothesis that these functional alterations render S1 of these patients more susceptible to the "inhibitory" effects of subthreshold 1 Hz repetitive transcranial magnetic stimulation (rTMS) given to S1. Seven patients with writer's cramp and eight healthy subjects were studied. Patients also received rTMS to the motor cortex hand area (M1). As an outcome measure, short-latency afferent inhibition (SAI) was tested. SAI was studied in the relaxed first dorsal interosseous muscle using conditioning electrical stimulation of the index finger and TMS pulses over the contralateral M1. Baseline SAI did not differ between groups. S1 but not M1 rTMS reduced SAI in patients. rTMS had no effects on SAI in healthy subjects. Because SAI is mediated predominantly at a cortical level in the sensorimotor cortex, we conclude that there is an abnormal responsiveness of this area to 1 Hz rTMS in writer's cramp, which may represent a trait toward maladaptive plasticity in the sensorimotor system in these patients.

Modulation of corticospinal excitability by paired associative stimulation: Reproducibility of effects and intraindividual reliability

OBJECTIVE

Electrical stimulation of the median nerve followed by a magnetic pulse on the primary motor cortex (M1) is effective to cause an increase in the amplitude of motor evoked potential (MEP) registered in the target muscle with the interstimulus interval (ISI) at 25ms (paired associative stimulation, PAS). The aim of this study is to evaluate the reproducibility of PAS with ISI 25 (PAS25), assessed in two separate sessions. Intraindividual reliability of TMS measures was also evaluated.

METHODS

Motor threshold of abductor pollicis brevis (APB), assessed at rest, and MEP amplitude of APB and abductor digiti minimi (ADM) were assessed before and after PAS25 in 18 healthy volunteers (nine males and nine females).

RESULTS

Data showed a significant increase of MEP amplitude in the target muscle (APB) after PAS25 and a reproducibility of group effect in the two sessions, as assessed by ANOVA, but a lack of intraindividual reliability, as assessed by intraclass correlation coefficients (ICC).

CONCLUSIONS

The results underline the reproducibility of mean effects and the need to be careful when comparing the same subject on different days.

SIGNIFICANCE

Electrical stimulation of the median nerve followed by a magnetic pulse delivered on M1 after 25ms causes a reproducible increase in MEP amplitude, without showing an acceptable intraindividual reliability.

Electroacupuncture (EA) modulates the expression of NMDA receptors in primary sensory neurons in relation to hyperalgesia in rats

N-methyl-D-aspartate (NMDA) receptor on the central terminals of the dorsal root ganglion (DRG) appears to be playing an important role in the development of central sensitization related to persistent inflammatory pain. Acupuncture analgesia has been confirmed by numerous clinical observations and experimental studies to be a useful treatment to release different kinds of pains, including inflammatory pain and hyperalgesia. However, the underlying mechanisms of the analgesic effect of acupuncture are not fully understood. In the present study, using a rat model of inflammatory pain induced by complete Freund's adjuvant (CFA), we observed the effect of electroacupuncture (EA) on animal behavior with regard to pain and the expression of a subunit of NMDA receptor (NR1) and isolectin B4 (IB4) in the neurons of the lumbar DRG. Intraplantar injection of 50 microl CFA resulted in considerable changes in thermal hyperalgesia, edema of the hind paw and "foot-bend" score, beginning 5 h post-injection and persisting for a few days, after which a gradual recovery occurred. The changes were attenuated by EA treatment received on the ipsilateral "Huan Tiao" and "Yang Ling Quan" once a day from the first day post-injection of CFA. Using an immunofluorescence double staining, we found that the number of double-labeled cells to the total number of the IB4 and NR1-labeled neurons increased significantly on days 3 and 7 after CFA injection. The change was attenuated by EA treatment. These results suggest that EA affects the progress of experimental inflammatory pain by modulating the expression of NMDA receptors in primary sensory neurons, in particular, IB4-positive small neurons.

Modulation of intracortical facilitatory circuits of the human primary motor cortex by digital nerve stimulation

We investigated the effect of electrical digit stimulation on two different intracortical facilitatory phenomena. Paired-pulse transcranial magnetic stimuli (TMS) with different conditioning stimulus (CS) intensities were applied over the primary motor cortex (M1). Electromyographic (EMG) recordings were made from the relaxed right abductor digiti minimi muscle (ADM). The effect of preceding sensory stimulation applied to the ipsilateral digit V on the conditioning magnetic stimulus was examined. Changing the CS intensity affected the influence of peripheral electrical stimulation on motor evoked potential (MEP) amplitudes evoked by paired pulse TMS. Inhibition induced by ipsilateral digit stimulation was strongest with the lowest CS intensity if MEP amplitudes were evoked by a subthreshold CS followed by a suprathreshold test stimulus (TS) at an interstimulus interval (ISI) of 10 ms. In contrast, inhibition induced by digit stimulation in a paired-pulse paradigm with a suprathreshold first and a subthreshold second stimulus at ISI of 1.5 ms was strongest with the highest CS intensity. These findings suggest that appropriately timed peripheral electrical stimuli differentially modulate facilitatory interactions in the primary motor cortex. They further support the hypothesis that intracortical facilitation (ICF) and short-interval intracortical facilitation (SICF) are evoked through different mechanisms.

Evaluation of Na+/K+ pump function following repetitive activity in mouse peripheral nerve

After conduction of prolonged trains of impulses the increased Na+/K+ pump activity leads to hyperpolarization. The aim of this study was to develop a mouse model to investigate the Na+/K+ pump function in peripheral nerve by measuring the decrease in excitability during activity-dependent hyperpolarization. Acute electrophysiological investigations were carried out in seven adult mice. Nerve excitability was evaluated by tracking the change in threshold current after 5 min of 100 Hz stimulation of the tibial nerve at ankle. We developed a threshold tracking system that allowed us to follow several excitability measures simultaneously from the evoked plantar compound muscle action potential (CMAP) and sciatic compound nerve action potential (CNAP). Three minutes after repetitive supramaximal stimulation maximal CMAP and CNAP amplitudes recovered but the threshold was increased approximately 40% for motor axons approximately 34% for axons generating CNAP. The threshold recovered with a rate of 3.8%/minute that was similar for nerve and motor responses. By tracking the effect of polarizing currents we found evidence of activity dependent hyperpolarization, and our data suggest that the observed threshold change after repetitive stimulation of the mouse tibial nerve is an indicator of the Na+/K+ pump function in vivo. Evaluation of activity-dependent hyperpolarization may be an important indicator of axonal ability to cope with Na+ load.