Sleep - A R Gardner-Medwin
Refs:Relevant chapters in Kandel & Schwartz Principles of Neural Science or other neurobiology texts. There are many interesting books on this puzzling topic.
Clinical Problems: principally insomnia due to shiftwork/jetlag, sleep apnoea, psychological/psychiatric problems, narcolepsy. Consequences are profound, but surprisingly not very well characterised. Studies by sleep deprivation. Increased tendency to fall asleep is most obvious: poor performance, but only on long boring tasks. Irritability, paranoia, bizarre statements and hallucinations in extreme states.
Three brain states not two: Waking (W), slow wave sleep (SWS) and rapid eye movement (REM) or paradoxical sleep (PS) are 3 profoundly different states of brain activity.
SWS: Slow waves (1-3 Hz, 100V) in EEG, diminished muscle tone, occasional body movements, reports of thinking or dreaming on awakening. 60% (infants) - 85% (elderly) of sleep time. Always first phase of sleep except in narcoleptics. Sleep walking occurs in SWS. Increased % following sleep deprivation.
REM: EEG as in waking. Theta rhythm in hippocampus. Total relaxation of postural muscles. Strong descending inhibition of motorneurones. Phasic bursts of activity in pons, lateral geniculate and cortical cells (including motor cortex): “PGO” waves. 80-90% reports of dreams in awakening, even in those who claim not to dream. Duration of 10-45 min typically every 90 min during sleep. Increased % following deprivation of REM sleep.
Sleep is not a state of brain inactivity, rather of altered neuronal firing patterns (commonly with synchronous bursts), cutting off of sensory inflow and motor outflow and altered ‘connectivity’ of the brain: the visual pathway conveys activity from the brainstem not from the eyes. Memory for externally or internally generated experiences is poor. Is a sleeping person conscious? How can you tell if they can’t talk and can’t remember?
Approaches to the Question: What is sleep for? What are the effects of sleep deprivation? What stimuli increase sleep? What happens in the body during sleep? What processes can we suggest might in theory be beneficial? What are the evolutionary consequences of sleep behaviour? No definite conclusions yet about the functions of sleep.
Control of Sleep. Early concept of reticular activity (or arousal) system is still valid, based in effects of sections (encéphale isolé and cerveau isolé), stimulation and lesions. Sensory pathways send collaterals (‘non-specific’) contributing to arousal rather than to discrimination. Now known that there are sleep promoting centres (e.g. raphé nuclei: on the midline). Diffuse projection systems employing serotonin (5HT) are involved in sleep promotion and in descending inhibition, noradrenaline (from locus coeruleus) in arousal and acetylcholine in phasic events of REM. These specific neurochemical roles are not simple or always essential however.