Active sleep-related depolarization of feline trigemino-thalamic afferent terminals

Neural representations during sleep: From sensory processing to memory traces

In the course of a day, the brain undergoes large-scale changes in functional modes, from attentive wakefulness to the deepest stage of sleep. The present paper evaluates how these state changes affect the neural bases of sensory and cognitive representations. Are organized neural representations still maintained during sleep? In other words, despite the absence of conscious awareness, do neuronal signals emitted during sleep contain information and have a functional relevance? Through a critical evaluation of the animal and human literature, neural representations at different levels of integration (from the most elementary sensory level to the most cognitive one) are reviewed. Recordings of neuronal activity in animals at presentation of neutral or significant stimuli show that some analysis of the external word remains possible during sleep, allowing recognition of behaviorally relevant stimuli. Event-related brain potentials in humans confirm the preservation of some sensory integration and discriminative capacity. Behavioral and neuroimaging studies in humans substantiate the notion that memory representations are reactivated and are reorganized during post-learning sleep; these reorganisations may account for the beneficial effects of sleep on behavioral performance. Electrophysiological results showing replay of neuronal sequences in animals are presented, and their relevance as neuronal correlates of memory reactivation is discussed. The reviewed literature provides converging evidence that structured neural representations can be activated during sleep. Which reorganizations unique to sleep benefit memory representations, and to what extent the operations still efficient in processing environmental information during sleep are similar to those underlying the non-conscious, automatic processing continually at work in wakefulness, are challenging questions open to investigation.

Brainstem glycinergic neurons and their activation during active (rapid eye movement) sleep in the cat

It is well established that, during rapid eye movement (REM) sleep, somatic motoneurons are subjected to a barrage of inhibitory synaptic potentials that are mediated by glycine. However, the source of this inhibition, which is crucial for the maintenance and preservation of REM sleep, has not been identified. Consequently, the present study was undertaken to determine in cats the location of the glycinergic neurons, that are activated during active sleep, and are responsible for the postsynaptic inhibition of motoneurons that occurs during this state. For this purpose, a pharmacologically-induced state of active sleep (AS-carbachol) was employed. Antibodies against glycine-conjugated proteins were used to identify glycinergic neurons and immunocytochemical techniques to label the Fos protein were employed to identify activated neurons. Two distinct populations of glycinergic neurons that expressed c-fos were distinguished. One population was situated within the nucleus reticularis gigantocellularis (NRGc) and nucleus magnocellularis (Mc) in the rostro-ventral medulla; this group of neurons extended caudally to the ventral portion of the nucleus paramedianus reticularis (nPR). Forty percent of the glycinergic neurons in the NRGc and Mc and 25% in the nPR expressed c-fos during AS-carbachol. A second population was located in the caudal medulla adjacent to the nucleus ambiguus (nAmb), wherein 40% of the glycinergic cells expressed c-fos during AS-carbachol. Neither population of glycinergic cells expressed c-fos during quiet wakefulness or quiet (non-rapid eye movement) sleep. We suggest that the population of glycinergic neurons in the NRGc, Mc, and nPR participates in the inhibition of somatic brainstem motoneurons during active sleep. These neurons may also be responsible for the inhibition of sensory and other processes during this state. It is likely that the group of glycinergic neurons adjacent to the nucleus ambiguus (nAmb) is responsible for the active sleep-selective inhibition of motoneurons that innervate the muscles of the larynx and pharynx.