Imaging Sleep and Wakefulness
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Sleep is thought to be universal in the animal kingdom, and essential for maintaining life. Sleep is related to metabolism and brain function. Despite the importance of sleep, what happens in the brain during sleep is still unclear. While sleep may seem to be a simple phenomenon, the brain exhibits unique activity during sleep. In this chapter, we summarize current progress in sleep research, focusing on physiological aspects of the brain during sleep detected using various approaches, including classical electrophysiology and recent imaging techniques.
KeywordsSleep Imaging Animal behavior Fluorescence Electrophysiology
17.1 Introduction: Behavioral Definition of Sleep
Sleep is considered to be a unique and essential phenomenon for life in all animals (Siegel 2008; Cirelli and Tononi 2008; Campbell and Tobler 1984). In fact, animals whose sleep is reduced for long periods eventually die under certain conditions (Rechtschaffen and Bergmann 2002; Driver et al. 2013; Shaw et al. 2002). The development of novel techniques for neuroscience research has rapidly increased our knowledge about the functions and mechanisms of sleep. First of all, what is the state of sleep? To answer this question, we must first define sleep. That is, a strict definition of sleep is the answer in and of itself. There are two major difficulties in defining sleep. First, while sleep seems to be a relatively simple behavior, it is not just a state of rest or immobility. For example, squirrels sleep immediately after arousal from hibernation, indicating that sleep is a distinct state with distinct functions from the energy-conserving rest state of hibernation. To distinguish between sleep and “non-sleeping” rest, sleep has been defined according to the following behavioral criteria: behavioral quiescence, characteristic stationary posture, rapid state reversibility, and elevated threshold to arousing stimuli (Piéron 1913). These criteria distinguish sleep not only from “non-sleeping” rest, but also from coma and anesthesia. Second, the sleep/wake cycle is linked with, but can be dissociated from, circadian rhythm. The sleep/wake cycle is controlled not only by the circadian rhythm, but also by homeostatic mechanisms (Borbély 1982). Homeostatic regulation is an additional criterion for defining sleep to distinguish between sleeping and circadian behaviors. In particular, animals exhibit recovery sleep after extended wakefulness. Based on these criteria, sleep can be defined behaviorally in diverse animal species, including genetic animal models such as the roundworm Caenorhabditis elegans (Raizen et al. 2008; Hill et al. 2014), fruit fly Drosophila melanogaster (Shaw et al. 2000; Hendricks et al. 2000), and zebrafish Danio rerio (Zhdanova et al. 2001; Yokogawa et al. 2007; Prober et al. 2006). Interestingly, according to these definitions, the jellyfish Cassiopea spp, which has a diffuse nervous system, also exhibits sleep-like behavior (Nath et al. 2017). On the other hand, some animals cannot be definitely judged to have sleep because they do not show behaviorally typical compensatory rebound sleep after sleep deprivation or reduced responsivity to stimuli during sleep-like behavior (Cirelli and Tononi 2008). Thus, the behavioral definition of sleep is highly variable, and could overlook sleep in certain animal species and several features of sleep, particularly the neurophysiological properties of sleep, as described below.
17.2 Oscillations in Sleep
17.3 Electrophysiological Insights into the Sleeping Brain
Oscillations associated with sleep are also observed in other electrophysiological events in the cortex, such as local electric potentials (LFP), extracellular multi- and single-unit activity, and membrane potentials. Cortical neurons exhibit changes in their firing patterns that correlate with the state of sleep/wakefulness: a tonic pattern during wakefulness, a burst-and-pause pattern during NREM sleep, and a rhythmic bursting pattern during REM sleep (Evarts 1964; Hubel 1959). In addition, short-term and synchronous cessation of firing occurs in cortical neurons during NREM sleep, which correlates with slow waves in EEG and LFP (Vyazovskiy et al. 2009, 2011; Destexhe et al. 1999; Nir et al. 2011). The active and silent periods of the cortical neuronal population in NREM sleep are referred to as ON and OFF periods, respectively (Vyazovskiy et al. 2009). Slow waves are detected not only in LFP, but also in the membrane potential and EEG during NREM sleep (Steriade et al. 1993a; Achermann and Borbély 1997). Surprisingly, slow waves in LFP appear during both NREM sleep and REM sleep in the middle layer of the primary cortices (Funk et al. 2016). Intracellular recordings in the cortex reveal that oscillatory activity, which consists of periodic depolarizing (UP) and hyperpolarizing (DOWN) states, appears in both EEG and the membrane potential under anesthesia and during NREM sleep (Steriade et al. 1993a, b, 2001; Timofeev et al. 2001; Metherate et al. 1992). UP/DOWN oscillations in membrane potentials disappear during wakefulness and REM sleep (Steriade et al. 2001; Timofeev et al. 2001).
17.4 Imaging Techniques Show Novel Aspects of Sleep
17.5 Future Directions
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