Neuroscience 18a – Sleep
Anil Chopra
- Describe the stages of sleep according to electrophysiological and behavioural criteria and explain how they contribute to an average night’s sleep
- List the structures in the brainstem and hypothalamus which regulate the sleep/wake cycle and explain how they interact to modulate cortical activity
- Outline briefly how the endogenous sleep/wake cycle is synchronised to the day-night cycle
- Describe briefly the proposed functions of sleep and the evidence for them.
- Describe the main causes of insomnia and how it may be treated.
All mammals (and most vertebrae) sleep, as humans we spend 1/3 of our lives sleeping. It is characterised by a specific posture, minimal movement, reduced response to external stimuli and it is reversible.
Stages of aSleep Cycle
In order to assess the stages of sleep, 3 measurement methods are used.
-Electroencephalogram (EEG)
-Electrooculogram (EOG)
-Electromyogram (EMG)
- Theta rhythm
- Theta + spindles + K complexes
- Theta + delta rhythms
- Theta + delta rhythms
- Beta rhythm + eye movements (REM or paradoxical sleep)
Muscle tone is moderately reduced during stages 1-4 and is very low during REM (Rapid eye movement) sleep. Dreams may occur during any stage of sleep but are most prominent and most easily recalled during REM sleep. During dreams the limbic system is more active and the frontal cortex less active. Descending through stages 1-4 takes about 1 hour, followed by several minutes of REM sleep. In a normal period of 8 hours of sleep there can be a number sleep cycles.
Mechanism of Sleep
Normal arousal is maintained by
- Midbrain cholinergic nuclei (Chol. N)
- Raphe nuclei (RN)
- N coeruleus (NC)
- Tuberomammillary nucleus (TMN)
In order to initiate sleep, activity from the ventral preoptic nucleus(of the hypothalamus) increases and activity from the lateral hypothalamus decreases resulting in GABA mediated inhibition of the Tuberomammillary nucleus and the nucleus coeruleus. The caudal pontine reticular formation (CPRF) is active during REM sleep.
Circadian Rhythm
This is the control of the sleep-awake cycle. It is controlled by light levels. Cells in the retina detect the decrease in light level and activate the suprachiasmatic nucleus in the hypothalamus. This causes stimulation of the pinealgland to increase production in melatonin which increases drowsiness. In the absence of this mechanism, endogenous cycle lengthens by several hours.
Necessity of Sleep
- Most animals sleep
- The process of sleep has been highly conserved during evolution
- Sleep deprivation has detrimental effects on life (mainly slow wave/NREM sleep)
- Sleep is regulated very accurately
Functions of Sleep
- Restoration and recovery
- Energy conservation
- Predator avoidance
- Memory consolidation
- Other effects on brain function
- Dreams: possibly as a safety valve for antisocial emotions or a method of disposal of unwanted memories.
It has been shown that NREM (slow wave) sleep plays a role in declarative memory and that REM sleep is associated with procedural memory.
Sleep Deprivation
Sleep deprivation has a number of negative effects:
•Sleepiness, irritability
•Performance decrements/ increased risk of errors and accidents
•Concentration/learning difficulties
•Hallucinations (after long sleep deprivation)
•Glucose intolerance
•Reduced leptin/increased appetite
•Death – rats (14-40 days), humans (fatal familial insomnia).
After sleep loss, the latency period of sleep onset decreases as do the lengths of the stages of sleep.
Insomnia
Insomnia is the inability to sleep and has a very high prevalence in the western world. Causes of chronic cases include:
-Physiological e.g. sleep apnoea
-Brain dysfunction e.g. depression, fatal familial insomnia.
It can be treated with hypnotics which enhance GABAergic circuits.
Narcolepsy
Narcolepsy is characterised by falling asleep repeatedly (cataplexy). It is most often caused by an orexin deficiency. Orexin is produced by the lateral hypothalamus and causes wakefulness. It causes dysfunction in the control of REM sleep.