Journal of neurophysiology
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The vasoactive mediator, endothelin-1, elicits a novel form of hyperalgesia, stimulation-dependent hyperalgesia. Acting on its cognate receptor on the vascular endothelial cell, endothelin-1 produces a state in which mechanical stimulation now elicits release of pronociceptive mediators from endothelium that, in turn, acts at receptors on sensory neurons. The only evidence that octoxynol-9, a surface-active agent that attenuates both endothelial cell function and stimulus-dependent hyperalgesia, does not affect nociceptors is indirect (i.e., octoxynol-9 treatment did not affect behavioral nociceptive threshold or hyperalgesia induced by agents that act directly on nociceptors). ⋯ Consistent with our previous behavioral observations, we observed no significant effect of octoxynol-9 on mechanical threshold in nociceptors, their response to sustained suprathreshold mechanical stimulation, conduction velocity, and change in mechanical threshold in response to the direct-acting hyperalgesic agent, PGE2. Although octoxynol-9 did not produce a biologically meaningful change in parameters of nociceptor function, we cannot exclude the possibility of a type II error. However, our data provide preliminary evidence of no effect of octoxynol-9 on nociceptors and are consistent with the suggestion that the primary action of octoxynol-9 in our studies is due to its action on the endothelium.
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Serotonin (5-HT), and its 5-HT1A receptor (5-HT1AR) subtype, is a powerful modulator of the cardiorespiratory system and its sensory reflexes. The nucleus tractus solitarii (nTS) serves as the first central station for visceral afferent integration and is critical for cardiorespiratory reflex responses. However, the physiological and synaptic role of 5-HT1ARs in the nTS is relatively unknown. ⋯ On the other hand, GABAergic nTS-evoked inhibitory postsynaptic currents did not change by activation of the 5-HT1ARs, but spontaneous inhibitory nTS network activity decreased. Blocking 5-HT1ARs tended to increase nTS-evoked inhibitory postsynaptic currents and inhibitory network activity. Taken together, 5-HT1ARs in the caudal nTS decrease breathing, likely via attenuation of afferent transmission, as well as overall nTS network activity.
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Although splitting of food items between the incisors often requires high bite forces, rarely do the teeth harmfully collide when the jaw quickly closes after split. Previous studies indicate that the force-velocity relationship of the jaw closing muscles principally explains the prompt dissipation of jaw closing force. Here, we asked whether people could regulate the dissipation of jaw closing force during food splitting. ⋯ We conclude that control of jaw closing force dissipation following food splitting depends on task demands. Consistent with our hypothesis, converging neurophysiological and morphometric data indicated that this control involved a differential activation of the jaw closing masseter muscle along the anteroposterior axis. These latter findings suggest that the regulation of jaw closing force after sudden unloading of the jaw exploits masseter muscle compartmentalization.
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The discovery that spontaneous fluctuations in blood oxygen level-dependent (BOLD) signals contain information about the functional organization of the brain has caused a paradigm shift in neuroimaging. It is now well established that intrinsic brain activity is organized into spatially segregated resting-state networks (RSNs). Less is known regarding how spatially segregated networks are integrated by the propagation of intrinsic activity over time. ⋯ Our data reveal that intrinsic activity propagates through and across networks on a timescale of ∼1 s. Variations in the latency structure of this activity resulting from sensory state manipulation (eyes open vs. closed), antecedent motor task (button press) performance, and time of day (morning vs. evening) suggest that BOLD signal lags reflect neuronal processes rather than hemodynamic delay. Our results emphasize the importance of the temporal structure of the brain's spontaneous activity.
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Randomized Controlled Trial
A dissociation between propriospinal facilitation and inhibition after bilateral transcranial direct current stimulation.
Propriospinal premotoneurons (PN) are essential for accurate control of the upper limb. They receive bilateral input from premotor (PM) and primary motor (M1) cortices. In humans, excitability of PNs can be estimated from motor-evoked potentials (MEPs) by pairing a descending volley using transcranial magnetic stimulation (TMS) to summate with an ascending volley from peripheral nerve stimulation at the C3-C4 level of the spinal cord. ⋯ Contrary to an earlier study with cathodal tDCS, INH was unchanged across all sessions. The difference between these and earlier findings may relate to dual- vs. single-hemisphere M1 stimulation. M1-M1 tDCS may be a useful adjuvant to techniques that aim to reduce upper limb impairment after stroke.