Abstract
In vertebrates like mammals and birds, two types of sleep have been identified: rapid eye movement and non-rapid eye movement sleep. Each one is associated with specific electroencephalogram patterns and is accompanied by variations in cardiac and respiratory frequencies. Sleep has been demonstrated only in a handful of invertebrates, and evidence for different sleep stages remains elusive. Previous results show that crayfish sleeps while lying on one side on the surface of the water, but it is not known if this animal has sleep phases. Heart rate and respiratory frequency are modified by diverse changes in the crayfish environment during wakefulness, and previously, we showed that variations in these variables are present during sleep despite that there are no autonomic anatomical structures described in this animal. Here, we conducted experiments to search for sleep phases in crayfish and the relationships between sleep and cardiorespiratory activity. We used the wavelet transform, grouping analysis with k-means clustering, and principal component analysis, to analyze brain and cardiorespiratory electrical activity. Our results show that (a) crayfish can sleep lying on one side or when it is motionless and (b) the depth of sleep (measured as the power of electroencephalographic activity) changes over time and is accompanied by oscillations in cardiorespiratory signal amplitude and power. Finally, we propose that in crayfish there are at least three phases of sleep.
Highlights
Sleep is generally defined as a rapidly reversible state of immobility and greatly reduced sensory responsiveness, characterized by a species-specific posture (Flanigan et al, 1974)
rapid eye movement (REM) sleep is accompanied by increases in blood pressure, heart rate (HR), respiratory frequency (RF), and metabolism to levels almost as high as those found in the awake state (Foulkes and Schmidt, 1983; Trinder et al, 2001)
Our results show that in crayfish, the heart rate and respiratory frequency are regulated during wakefulness (Figures 6, 7) and during the different sleep phases (Figure 11), as it occurs in vertebrates
Summary
Sleep is generally defined as a rapidly reversible state of immobility and greatly reduced sensory responsiveness, characterized by a species-specific posture (Flanigan et al, 1974). Non-rapid eye movement EEG activity varies depending on sleep depth, but it is generally slower and of higher amplitude than the EEG activity observed during either wake or REM sleep (Rechtschaffen and Kales, 1968). NREM sleep is characterized by decreases in muscle tone, body movements, heart rate (HR), respiratory frequency (RF), blood pressure (BP), metabolic rate, and temperature. These parameters reach their lowest values during SWS. REM sleep is accompanied by increases in blood pressure, HR, RF, and metabolism to levels almost as high as those found in the awake state (Foulkes and Schmidt, 1983; Trinder et al, 2001). No definitive answer has yet been found to the main question: what is the function of sleep?
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