Electrophysiological Effects of Temporary Deafferentation on Two Characterized Cell Types in the Nucleus Gracilis of the Rat

Abstract

Single- and multiunit recordings were made in the nucleus gracilis of anaesthetized rats in order to study the characteristics of the responses to natural cutaneous stimulation before and during local anaesthetic-induced deafferentation. Two types of cells were found which exhibited different electrophysiological features at rest and in response to stimulation of their receptive fields (RFs). Low-frequency (LF) neurons (77%) had very low spontaneous activity, and most could be antidromically activated from the medial lemniscus. High-frequency (HF) cells (23%) had a much higher spontaneous discharge rate, with shorter spike duration, and did not project through the lemniscus. Both cell types generated phasic responses with similar latencies following cutaneous stimulation. Recordings of lemniscal axons had similar characteristics to those of LF neurons. Within minutes after anaesthetizing the functional centre of the RF, the LF and HF cells displayed new RFs, and enhanced responses to stimuli delivered at the periphery of the original fields. Firing rates increased during anaesthesia, but only in LF cells. Using a paired-stimulation paradigm, many LF neurons displayed during anaesthesia a decrease of the normal inhibition that the conditioning stimulus evoked on the responses to the test stimulus; the opposite effect was observed in all HF cells. These results suggest that (i) LF neurons correspond to thalamic projection cells, and HF neurons may be inhibitory interneurons; (ii) by disinhibiting LF (but not HF) cells, temporary deafferentation may increase neuronal responsiveness to peripheral stimulation, and thus contribute to reveal new RFs, and (iii) these changes in the nucleus gracilis may partly account for the reorganization of representational maps at higher levels of the somatosensory pathways.

Corticofugal Modulation of the Tactile Response Coherence of Projecting Neurons in the Gracilis Nucleus

“…The criteria for the antidromic identification were fixed latency with the mean 2.0 Ϯ 0.08 ms (range 1.2-2.8 ms) and the ability to follow fast (Ͼ100 Hz) stimuli. The analyzed neurons showed a low spontaneous activity with the mean 1.1 Ϯ 0.4 spikes/s (range 0 -10 spikes/s) whose pattern coincided with the firing characteristics of projecting neurons previously described (Nuñez et al 2000;Panetsos et al 1997). Also, selected neurons had a RF that overlapped (Ͼ50%) the stimulated cortical area (matching condition; Nuñez 1998, 2004).…”

Sensorimotor cortical influences on cuneate nucleus rhythmic activity in the anesthetized cat

“…Previous results demonstrated the existence of spontaneous oscillatory activity in the cuneate nucleus of chloralose-anesthetized cats, 7,28 in the gracile nucleus of pentobarbital-and urethane-anesthetized rats, 32 and in DCN slices from young rats. 31 This study further shows that, excepting the slow and spindle oscillations, the remaining rhythms survived lesion of the PT or bilateral removal of the frontoparietal cortex.…”

“…The CLs discharged preferentially within the d band, while the nCLs tended to fire at higher frequencies presenting a more tonic behavior, as was reported for gracile neurons in barbiturate-and urethane-anesthetized rats. 32 Furthermore, some of the rhythmic cuneate neurons showed the d activity grouped into slow rhythms (Table 1, Fig. 3A1).…”

Spike Timing, Spike Count, and Temporal Information for the Discrimination of Tactile Stimuli in the Rat Ventrobasal Complex

“…Primary afferents could thus exploit the spike timing precision of their responses only to discriminate between close locations within the body area they represent, e.g., within a fingertip (Johansson and Birznieks, 2004), but not to discriminate between separate locations such as different digits or different whiskers. Temporal information conveyed by spike timing about stimulus location between digits or between whiskers in the ventrobasal complex is thus a product of the basic transformations that occur in the brainstem (Panetsos et al, 1997;Aguilar et al, 2002Aguilar et al, , 2003Fernández de Sevilla et al, 2006;Soto et al, 2006), which determine the enlargement of receptive fields at thalamic level. Importantly, the receptive field size of thalamocortical neurons is particularly large in active or awake states (Nicolelis et al, 1993;Nicolelis and Chapin, 1994;Friedberg et al, 1999;Aguilar and Castro-Alamancos, 2005).…”