Functional changes in the temporomandibular joint mechanoreceptors associated with experimentally induced condylar resorption in rats

ABSTRACT Objectives To evaluate the influence of experimentally induced progressive condylar resorption (PCR) on temporomandibular joint (TMJ) mechanoreception. Materials and Methods Twenty 13-week-old male albino Wistar rats were divided equally into control and PCR groups. A compressive force was loaded on the left TMJ of PCR group rats to induce condylar resorption. Single-unit activities of TMJ mechanoreceptors were also induced through passive jaw movement. Recording was performed for the left Gasserian ganglion at 3 days and 1 week after the establishment of PCR group. The effects of PCR on TMJ units were assessed by measuring the firing threshold, maximum instantaneous firing frequency, and average firing frequency. Results Compared with the control group, there were no significant differences in the firing threshold of the PCR group after 3 days. The thresholds were significantly higher 1 week after compressive force loading on the condyle. The maximum instantaneous firing frequencies and the average firing frequencies showed no significant differences after 3 days. However, these were significantly lower 1 week after compressive force loading. Conclusions The findings suggest that compressive force loading on the condyle may influence the function of TMJ mechanoreceptors.

Reduction of spike generation frequency by cooling in brain slices from rats and from patients with epilepsy

This study aimed to understand the mechanism by which brain cooling terminates epileptic discharge. Cortical slices were prepared from rat brains (n = 19) and samples from patients with intractable epilepsy that had undergone temporal lobectomy (n = 7). We performed whole cell current clamp recordings at approximately physiological brain temperature (35℃) and at cooler temperatures (25℃ and 15℃). The firing threshold in human neurons was lower at 25℃ (−32.6 mV) than at 35℃ (−27.0 mV). The resting potential and spike frequency were similar at 25℃ and 35℃. Cooling from 25℃ to 15℃ did not change the firing threshold, but the resting potential increased from −65.5 to −54.0 mV and the waveform broadened from 1.85 to 6.55 ms, due to delayed repolarization. These changes enhanced the initial spike appearance and reduced spike frequency; moreover, spike frequency was insensitive to increased levels of current injections. Similar results were obtained in rat brain studies. We concluded that the reduction in spike frequency at 15℃, due to delayed repolarization, might be a key mechanism by which brain cooling terminates epileptic discharge. On the other hand, spike frequency was not influenced by the reduced firing threshold or the elevated resting potential caused by cooling.

The Creation of A Negative Slope Conductance Region by the activation of The Persistent Sodium Current Prolongs Near-Threshold Synaptic Potentials

ABSTRACTA change of the input resistance (Rin) of the neuron involves a change in the membrane conductances by opening and closing of ion channels. In passive membranes, i.e., membranes with only linear leak conductances, the increase or decrease of these conductances leads to a decrease or increase of the Rin and the membrane time constant (τm). However, the presence of subthreshold voltage dependent currents can produce non-linear effects generating deviations from this relationship, especially the contradictory effect of negative conductances, as produced by the sodium-persistent current (INaP), on the Rin. In this work we aimed to analyze experimentally and theoretically the impact of the negative conductance produced by INaP on Rin. Experiments of whole-cell patch-clamp conducted in CA1 hippocampus pyramidal cells from brain slices showed a paradoxical voltage-dependent decrease of the Rin and the τm in subthreshold membrane potentials close to the firing threshold after the perfusion with TTX, which inhibits INaP. This effect is postulated to be a result of the negative slope conductance in the subthreshold region produced by this conductance. The analysis of the experimental data, together with simulations found that the slope conductance of INaP is negative for subthreshold membrane potentials and its magnitude is voltage dependent in the same range observed for the voltage-dependence of Rin and τm. The injection of an artificial INaP using dynamic-clamp in the presence of TTX restored the Rin and τm to its original values. Additionally the injection of an artificial leak current with a negative conductance in the presence of TTX restored the Rin and τm as the artificial Inap did. On the other hand, the injection of an artificial leak current with a positive conductance in the presence of TTX had no effect on the Rin and τm. We conclude that INaP increases the Rin and τm by the negative slope conductance observed in its non-monotonic I-V relationship. These results demonstrate that the effect of Inap on Rin and τm is stronger in potentials near the firing threshold, which could potentiate the temporal summation of the EPSPs increasing their temporal integration and facilitating action potential firing. Because of its negative slope conductance, INaP is more effective in increasing excitability near threshold than a depolarizing leak current.

Thresholds of cutaneous afferents related to perceptual threshold across the human foot sole

Perceptual thresholds are known to vary across the foot sole, despite a reported even distribution in cutaneous afferents. Skin mechanical properties have been proposed to account for these differences; however, a direct relationship between foot sole afferent firing, perceptual threshold, and skin mechanical properties has not been previously investigated. Using the technique of microneurography, we recorded the monofilament firing thresholds of cutaneous afferents and associated perceptual thresholds across the foot sole. In addition, receptive field hardness measurements were taken to investigate the influence of skin hardness on these threshold measures. Afferents were identified as fast adapting [FAI ( n = 48) or FAII ( n = 13)] or slowly adapting [SAI ( n = 21) or SAII ( n = 20)], and were grouped based on receptive field location (heel, arch, metatarsals, toes). Overall, perceptual thresholds were found to most closely align with firing thresholds of FA afferents. In contrast, SAI and SAII afferent firing thresholds were found to be significantly higher than perceptual thresholds and are not thought to mediate monofilament perceptual threshold across the foot sole. Perceptual thresholds and FAI afferent firing thresholds were significantly lower in the arch compared with other regions, and skin hardness was found to positively correlate with both FAI and FAII afferent firing and perceptual thresholds. These data support a perceptual influence of skin hardness, which is likely the result of elevated FA afferent firing threshold at harder foot sole sites. The close coupling between FA afferent firing and perceptual threshold across foot sole indicates that small changes in FA afferent firing can influence perceptual thresholds.

Memory Nearly on a Spring: A Mean First Passage Time Approach to Memory Lifetimes

We study memory lifetimes in a perceptron-based framework with binary synapses, using the mean first passage time for the perceptron's total input to fall below firing threshold to define memory lifetimes. Working with the simplest memory-related model of synaptic plasticity, we may obtain exact results for memory lifetimes or, working in the continuum limit, good analytical approximations that afford either much qualitative insight or extremely good quantitative agreement. In one particular limit, we find that memory dynamics reduce to the well-understood Ornstein-Uhlenbeck process. We show that asymptotically, the lifetimes of memories grow logarithmically in the number of synapses when the perceptron's firing threshold is zero, reproducing standard results from signal-to-noise ratio analyses. However, this is only an asymptotically valid result, and we show that extending its application outside the range of its validity leads to a massive overestimate of the minimum number of synapses required for successful memory encoding. In the case that the perceptron's firing threshold is positive, we find the remarkable result that memory lifetimes are strictly bounded from above. Asymptotically, the dependence of memory lifetimes on the number of synapses drops out entirely, and this asymptotic result provides a strict upper bound on memory lifetimes away from this asymptotic regime. The classic logarithmic growth of memory lifetimes in the simplest, palimpsest memories is therefore untypical and nongeneric: memory lifetimes are typically strictly bounded from above.

Neuronal Responses Below Firing Threshold for Subthreshold Cross-Modal Enhancement

Multisensory integration (such as somatosensation-vision, gustation-olfaction) could occur even between subthreshold stimuli that in isolation do not reach perceptual awareness. For example, when a somatosensory (subthreshold) stimulus is delivered within a close spatiotemporal congruency, a visual (subthreshold) stimulus evokes a visual percept. Cross-modal enhancement of visual perception is maximal when the somatosensory stimulation precedes the visual one by tens of milliseconds. This rapid modulatory response would not be consistent with a top-down mechanism acting through higher-order multimodal cortical areas, but rather a direct interaction between lower-order unimodal areas. To elucidate the neuronal mechanisms of subthreshold cross-modal enhancement, we simulated a neural network model. In the model, lower unimodal (X, Y) and higher multimodal (M) networks are reciprocally connected by bottom-up and top-down axonal projections. The lower networks are laterally connected with each other. A pair of stimuli was presented to the lower networks, whose respective intensities were too weak to induce salient neuronal activity (population response) when presented alone. Neurons of the Y network were slightly depolarized below firing threshold when a cross-modal stimulus was presented alone to the X network. This allowed the Y network to make a rapid (within tens of milliseconds) population response when presented with a subsequent congruent stimulus. The reaction speed of the Y network was accelerated, provided that the top-down projections were strengthened. We suggest that a subthreshold (nonpopulation) response to a cross-modal stimulus, acting through interaction between lower (primary unisensory) areas, may be essential for a rapid suprathreshold (population) response to a congruent stimulus that follows. Top-down influences on cross-modal enhancement may be faster than expected, accelerating reaction speed to input, in which ongoing-spontaneous subthreshold excitation of lower-order unimodal cells by higher-order multimodal cells may play an active role.