Rhythms of the Brain Sleep & Circadian Rhythms
Brain Rhythms • Electrical rhythms • EEGs • Behavioral Rhythms • Sleep-wake cycles • Circadian rhythms
Electrical Rhythms • Known since 1929 that groups of neurons can be monitored with probes (electrodes) placed on the scalp • Electroencephalograms (EEGs) • Used clinically to help diagnose brain disorders such as epilepsy and sleep disturbances. • Gives information about the general mental state of the individual. • The EEG is present from before birth until death. In fact, in some places death itself is defined by the absence of an EEG.
How an EEG Works • The on-going electrical activity of the brain is measured from scalp electrodes • A number of electrodes are attached to the scalp of the subject either by glue or by wearing an electro-cap
Recording an EEG • The EEG is usually monitored using a polygraph • The electrodes are plugged into the polygraph and the EEG is displayed on a moving sheet of paper or on a computer screen. • Signals from the body are small (low voltage) • Amplified with high gain physiological amplifiers
Normal EEG Time is on the X-axis and voltage is on the Y-axis.
What an EEG Records • The EEG displays continuous changes in voltage over time. • Records current flow in the brain • Largely from the activity of the pyramidal cells in layer IV of the neocortex • Principally synchronous activity • Any waveform recorded at the scalp is the summation and cancellation of neural activity from a large number of neural generators
Types of EEG Rhythms • Alpha Waves • 8-13 Hz • large amplitude EEG waves that are associated with a relaxed but awake state • Beta Waves • >14 Hz - lower amplitude than alpha • associated with a more alert or "active" mental state • activated cortex, both awake & REM sleep
More Wave Forms • Delta Waves • <4 Hz - stage 3, non-REM sleep • Theta Waves • 4-7 Hz - stage 1, non-REM sleep • Spindles • episodic rhythms from thalamus - stage 2, non-REM sleep
Brain Waves and State Changes in state are associated with a change in the amplitude and frequency distribution of the EEG.
How Rhythms Are Generated • Pacemaker • has a rhythm itself and other neurons follow • Circadian pacemaker • Thalamic neurons • Cochlear nucleus neurons • ‘Choppers' have a rhythm because of membrane properties • Network Rhythm • How neurons are interconnected • Two-neuron excitation and inhibition feedback loop • Networks may also drive other neurons, e.g. thalamus and cortex
EEGs and Information Processing • EEGs can be used to generate information about normal brain function • Time-lock the recording of the EEG to the onset of an event of interest, such as viewing a picture • The resulting activity is an “event related potential" (ERP) • Can be distinguished from the background EEG • ERPs are of relatively small amplitude • EEG is made up of all brain activity (visible at the scalp) at a point in time, while the ERP is that part of the activity associated with the processing of a specific event.
Event Related Potentials • ERPs can be recorded from all of the primary sensory modalities (visual, auditory, somatosensory and gustatory) and from motor events • They can be recorded from multiple locations on the scalp. • challenges to determining the location within the brain from which ERPs emanate
Components of ERP Waveforms • Components are the positive and negative fluctuations in any ERP waveform. • Components that occur prior to 100 ms are thought to reflect information processing in the early sensory pathway. • For example, the auditory brain stem ERP arises from neural impulses traveling from the cochlea through auditory brain stem centers • Middle latency components reflect activity in the thalamus • possibly the earliest cortical processing.
Long Latency ERPs • Considered most interesting by neuroscientists • Include: P1, P2, N1, N2, N400 and P3 components. • Named by there polarity • N for negative • their ordinal position after stimulus onset (P1 is the first positive peak) • their latency after stimulus onset (the N400 is a negative-going component peaking at 400 msec). • In general, the long-latency components occurring prior to 200 msec are thought to reflect late sensory and early perceptual processes • Those after 250 milliseconds thought to reflect higher level cognitive processes (memory and language).
Functions EEG Rhythms • Synchrony Theory • All parts of the cortex involved in a concept or action are tied together • Example: trying to catch a basketball will cause basketballness synchrony • Artifact Theory • Rhythms have no direct function, but are by-products of the necessarily strong interconnections of neurons in the brain • We don't know...
Seizures • Most extreme form of synchronous activity = seizure • General seizure - whole neocortex • Partial seizure - limited region of neocortex • Absence seizures - occur in childhood, 30 sec periods of unconsciousness • Seizures in sensory areas cause auras - smells, sparkling lights, etc. • Déja vu results from a partial seizure often involving the temporal lobe
Epilepsy • Repeated seizures = epilepsy • a symptom, not a disease itself • Affects about 1% of population = 25,000,000 people in USA • Causes: tumors, trauma, metabolic abnormalities, infection, vascular disease, genetic predisposition
Epilepsy MRI • 3D MRI of a patient presenting partial complex seizures of the right temporal origin: the difference image (ROI) between demonstrating areas of increased perfusion, is shown superimposed onto the corresponding MR image
Drugs & Epilepsy • Anticonvulsants • Counter excitability • Can Prolong inhibitory actions of GABA • barbiturates & benzodiazepines • Increase refractory period of neurons • phenytoin & carbamazepine • Convulsants • Drugs which increase neural excitability • Also used in withdrawal from depressants like alcohol or barbiturates
Circadian Rhythms • “Circadian rhythm", comes from the Latin circa diem, meaning "about a day". • Works roughly on a 24 hour cycle. • The circadian rhythm regulates all the rhythms of the body from the digestion and elimination process, to the growth and renewal of cells, as to the rise and fall of body temperature.
Biological Clocks • Timing of physiological events in your circadian schedule • The importance of processing various body functions efficiently • Clocks free-run under constant environmental conditions • Types of biological clocks: • Circadian = about 24 hr • Circannual = about 1 year • Tidal = about 7 days (= a lunar cycle) • Ultradian = in hours
SCN is the Biological Clock • Supracharasmatic nucleus (SCN) is the location of the ‘clock’ • Located inside the hypothalamus at the base of the brain and above the pituitary gland. • Experimental evidence • SCNx makes animals arhythmic • 2-DG metabolism is greatest during the day • SCN neural activity expresses a 24 hr rhythm • Fetal SCN transplants reinstate rhythm in SCNx animals
Other Biological Clocks • Food Entrainable Oscillator in rats • Pineal gland in birds • Pineal Transplant studies • Interaction with SCN
Melatonin • The effect of light on the pineal gland: • Light enters the eye through the retina. • The nerves relay a message via the SCN to the pineal gland about the amount of light • The presence of light inhibits signals from the SCN to the pineal gland and therefore blocks melatonin • This reminds the body that it is time to start slowing down in preparation for sleep.
Melatonin and Hibernation • The role of melatonin and the pineal gland • The ability of hibernating animals to sleep throughout the winter hinges on how the pineal gland responds to light. • The pineal gland is a pea sized gland that secretes a hormone called melatonin during darkness. • The action of melatonin, which is sometimes called the sleep hormone, helps to control body rhythms and sleep wake cycles.
Sleep • Functional states of the brain: • Awake • Non-REM Sleep • Rapid Eye Movement (REM) Sleep
The Awake State • Low amplitude, fast EEG • Vivid externally generated sensation • Logical, progressive thought • Continuous voluntary movement • Frequent rapid eye movement
Non-REM Sleep • High amplitude, slow EEG • No sensation • Logical, repetitive thought • Muscles are relaxed (although capable of moving) • Only rare rapid eye movement • Occasional involuntary movement • Brain energy consumption is minimum • Body temperature and energy consumption lowered • Parasympathetic activity - lowered heart rate, respiration, kidney function and increased digestion
REM Sleep • Low amplitude, fast EEG • Vivid, internally generated sensation & illogical, bizarre thought • Also called paradoxical sleep, due to the similarity between the this brain activity and wakefulness. • Muscle paralysis • Atonia: total loss of skeletal muscle tone • Frequent rapid eye movement • Sympathetic activity: • increased heart rate, respiration; engorgement of clitoris & penis (but not related to sexual dreams) • Temperature control system off line • temperature free falls
The Importance of REM Sleep • REM sleep is less than a third of the total time we sleep per night but may be most important • Brain energy consumption is higher than when awake. • Most dreaming happens during REM sleep • Deprivation of this kind of sleep disturbs normal functioning in a way similar to full sleep deprivation, even if total sleeping time is normal or greater than normal.
A Model of REM Sleep • Interaction of the Cholinergic (+) and Aminergic (-) pathways
Sleep Cycle • Ultradian rhythm • rhythm with period less than 24 hours • 90 minute cycle: • Stage 1 - Non-REM • Stage 2 - Non-REM • Stage 3 - Non-REM • Stage 4 - Non-REM • REM • Awake • Four or five cycles each night
Arousal and Sleep • Role of the Reticular Activating System (RAS): • RAS deactivates the cerebral cortex via multiple ascending tracts • Involvement of serotonin, norepinephrine, and acetylcholine tracts
Why Do We Sleep? • Restoration Theories • Something is being synthesized that we need during wakefulness, or toxin buildup is being reduced, or growth and repair are occurring, etc. • Example: growth hormone is secreted during sleep. This hormone is important for growth in children & is important throughoutadulthood in rebuilding tissues. • Adaptation Theories • Sleep keeps you safe during periods of danger • We don’t know . . . • During sleep many things happen that suggest that sleep is less a resting state and more an active part of the functioning of the organism.
A Recent Hypothesis • Non-REM sleep is required for neurons in the brain to replace glycogen stores (other tissues use fat, but the brain only has glycogen as an energy reserve) • REM sleep is required to maintain the basic ‘software programs’ of neural activity • This hypothesis makes predictions that can be tested, but... • Whatever the essential purpose of sleep is, we still don't know what it is.
Animal Adaptations • All animals seem to require sleep • Some have weird adaptations • Birds and aquatic mammals are capable of unihemispheric slow-wave sleep (USWS) • they can sleep with one eye open and one hemisphere of the brain awake. • Some dolphins sleep in 4-6 sec. ‘microsleeps’ • Some dolphins sleep with one cerebral hemisphere at a time • Sleep deprivation can cause serious side effects, even death in other mammals • effects not so severe in humans
Dreams • Dreams largely occur during REM sleep • You typically remember them only when you wake up during an episode • Theory : • episodic activity in the pons stimulates parts of the cortex to activate and synthesize the bizarre plots of dreams • if this is true, how do some of them seem to seamless? • Some indication that memory storage may be enhanced by REM sleep
Dreaming MRI • MRI shows blood flow during REM sleep • Extrastriate cortex and limbic system are active • Prefrontal cortex is quiet
Palm Biorhythm Program • Biorhythms is a freeware program that calculates and displays the main biorhythm cycles for any person. • http://www.brianapps.net/palmbio/