Rhythms of the Brain

1. Rhythms of the Brain

2. Types of Rhythms • Circadian – fluctuate daily • Sleep-wake, temperature, hormones, urine production, gastrointestinal activity • Cognitive and motor performance levels • Infradian – less than once a day • Hibernation, ovulation • Ultradian – more than once a day • Sleep cycles (REM and other sleep stages)

3. How do they do it? • Aplysia (sea slug) – neurons discharge faster in light than in dark to regulate feeding. • Willow warblers – show migratory behavior even when raised in the lab. • Squirrels – hibernate even when kept at a constant temperature (below body). • Cycles appear to be genetic.

4. Environmental Cues • Biological clock overrides most environmental cues. • Some cues (Zeitgebers) are used: • Temperature • Light • Temperature doesn’t make squirrels hibernate.

5. Three Steps to Regulation • Input – senses light or temperature. • Pacemaker – generates and regulates the rhythm – e.g., thalamic pacemaker. • Also, rhythmic activity may arise from the collective behavior of inhibitory and excitatory neurons. • Output – permits pacemaker to affect tissues and organs.

6. Bird Biological Clock • Input = pineal gland – in birds and some animals, located at the top of the skull to sense light changes. • Pacemaker – tryptophan is converted to melatonin (same process as serotonin). • N-acetyltransferase is key to this process • Output = melatonin released to bloodstream to affect organs.

7. Human Biological Clock • Pineal gland unimportant to humans, but melatonin may be important. • Information about light comes directly from the retina to the suprachiasmatic nucleus (SCN) in the hypothalamus. • SCNs generate rhythms (spontaneous firing).

8. Multiple Pacemakers • SCN cycles (affected by environment): • Sleep-wake cycles • Skin temperature • Hormones in blood, calcium in urine. • Cycles independent of SCN: • Sleep cycles (REM) • Internal body temperature • Cortisol in blood, potassium in urine.

9. Ultradian Rhythms • Humans cycle through an alertness and cognitive performance cycle every 90 to 100 minutes throughout the day. • Research into such rhythms is just beginning.

10. Sleep • A readily reversible state of reduced responsiveness to, and interaction with, the environment. • Lack of sleep produces unpleasant symptoms. • Irritability, impaired performance on cognitive tasks, no lasting effects on physical health. • Sleep debt • Why we sleep: • Restoration or adaptation?

11. Human Sleep-Wake Cycles • Urine production decreases at night due to fluctuations in vasopressin. • Sleep occurs instantaneously, not gradually. • Different people need different amounts of sleep. -- no obvious relation to mental or physical activity. • Free of environmental cues (free running) people adhere to a 24.8 hr day – mutant hamsters. • Jet lag occurs when sleep-wake cycles are out of phase with environment. • Change to local schedule immediately

12. How Long People Sleep • Body temperature is highest in the afternoon when people are active. • People sleep when body temperature is low and wake when it is high. • Going to sleep when body temperature is high results in longer sleeping times. • Feeling “dull” results from desynchronization of sleep cycles.

13. Types of EEG Rhythms • Frequency is measured in Hz • Four types: • Beta(>14 Hz) – thinking (active cortex) • Alpha (8-13 Hz) – quiet waking states • Theta (4-7 Hz) – present during first stage of sleep. • Delta (<4 Hz) – present during deep sleep

14. Sleep Cycles • Five stages: • Stage 1 – alpha rhythms (sitting quietly) • Stage 2 – theta rhythms (random neural activity) • Stage 3 – sleep spindles and K complexes (synchronized bursts or neural activity) • Stage 4 – delta rhythms (marked slowing) • Stage 5 – REM sleep (rapid eye movement)

15. Stages (Cont.) • Non-REM sleep: • Less dreaming • Ability to move muscles • Sympathetic ANS inactive • No impact on learning with deprivation • REM sleep: • Most dreaming • Atonia – inability to move muscles • Sympathetic ANS active • Learning is affected by deprivation

16. Control of Sleep Cycles • Diffuse modulatory systems control sleep and waking (locus coeruleus). • Also control thalamus to synchronize brain waves during sleep. • NE and Serotonin active during waking enhance activity. • Different ACh neurons active in Pons during REM sleep.

17. Control of Sleep (Cont.) • Sleep-related rhythms from the thalamus block sensory information to the cortex. • Activity in descending modulatory systems inhibits motor neurons during dreaming (REM sleep). • Sleep-promoting substances in blood related to immune system stimulation – this is why we sleep more when sick.

18. What is Dreaming? • Activation-synthesis hypothesis: • Dreams are associations and memories elicited by pontine neurons via thalamus • The cortex tries to synthesize this random activity into something meaningful. • Consolidation hypothesis: • REM sleep aids integration and consolidation of memories. • Sleep learning is bogus.

19. Rhythms and Disturbance • Epilepsy • Delayed sleep-phase insomnia • Seasonal depression

20. Epileptic Seizures • Extreme synchronous behavior in which many neurons fire at once. • Localized or global • Upsets balance of excitation and inhibition among neurons • Anticonvulsants – drugs that counter excitability of neurons. • Convulsants – drugs that block GABA.

21. Delayed Sleep-Phase Insomnia • Sleep soundly for 8 or more hours but have trouble getting to sleep in the first place. • Wake with difficulty and feel sleepy if forced to conform to a normal schedule. • Goal is to resynchronize internal clock with other people’s schedules.

22. Seasonal Depression • Desynchronization between circadian rhythms, sleep and emotion states may result in depression. • Depression is almost invariably cyclic. • Many depressed people enter REM sleep earlier than normal. • Sleep deprivation may ease depression temporarily.