Journal of neurophysiology
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We investigated neural coding of sinusoidally modulated tones (sAM and sFM) in the primary auditory cortex (A1) of awake marmoset monkeys, demonstrating that there are systematic cortical representations of embedded temporal features that are based on both average discharge rate and stimulus-synchronized discharge patterns. The rate-representation appears to be coded alongside the stimulus-synchronized discharges, such that the auditory cortex has access to both rate and temporal representations of the stimulus at high and low frequencies, respectively. Furthermore, we showed that individual auditory cortical neurons, as well as populations of neurons, have common features in their responses to both sAM and sFM stimuli. ⋯ The discharge synchrony-based best modulation frequency (tBMF) was typically lower than the rBMF measured from the same neuron. The distribution of rBMF over the population of neurons was approximately one octave higher than the distribution of tBMF. 4) There was a high degree of similarity between cortical responses to sAM and sFM stimuli that was reflected in both discharge rate- or synchrony-based response measures. 5) Inhibition appeared to be a contributing factor in limiting responses at modulation frequencies above the rBMF of a neuron. And 6) neurons with shorter response latencies tended to have higher tBMF and maximum discharge synchrony frequency than those with longer response latencies. rBMF was not significantly correlated with the minimum response latency.
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Descending inhibitory and facilitatory influences from the rostroventral medulla (RVM) on responses of lumbosacral spinal neurons to noxious colorectal distension (CRD, 80 mmHg, 20 s) were studied. At 25 sites in the RVM, electrical stimulation produced biphasic effects, facilitating responses of spinal neurons to CRD at lesser intensities of stimulation (5-25 microA) and inhibiting responses of the same neurons at greater intensities of stimulation (50-100 microA). At 38 other sites in the RVM, electrical stimulation produced only intensity-dependent inhibition of neuron responses to CRD. ⋯ Microinjection of glutamate into the RVM at a low dose (5 nmoles) facilitated responses of spinal neurons to CRD and inhibited responses of the same neurons at a greater dose (50 nmoles). In some experiments, microinjection of lidocaine (0.5 microl of 4% solution) or the neurotoxin ibotenic acid (0.5 microl, 10 microg) into the RVM produced reversible or long-lasting, respectively, decreases in spontaneous activity and responses of spinal neurons to CRD. These results reveal that spinal visceral nociceptive transmission is subject to a tonic descending excitatory influence from the RVM and that descending modulatory effects from the RVM on visceral nociceptive transmission are qualitatively similar to modulation of cutaneous nociceptive transmission.