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BIO 132

BIO 132. Lecture 38 Rhythms of the Brain. Neurophysiology. Electroencephalogram. An electroencephalogram (EEG) is a measurement of the activity of the brain, recorded from the surface of the scalp. Recordings made as early as 1875 Setup:

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BIO 132

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  1. BIO 132 Lecture 38 Rhythms of the Brain Neurophysiology

  2. Electroencephalogram • An electroencephalogram (EEG) is a measurement of the activity of the brain, recorded from the surface of the scalp. • Recordings made as early as 1875 • Setup: • 24 or so electrodes taped to scalp at standard positions • Output of electrodes amplified • Differences between the charges recorded at each electrode are made and display on a graph versus time. • Measurements of individual neurons is not possible from the scalp, but the activity of collections of neurons is possible.

  3. EEG Setup • All cortical neurons have the same orientation; dendrites near the surface and axons projecting inward. • Na+ entering dendrites during neuronal firing leaves the outside of the dendrites negatively charged. • If enough neurons beneath an electrode are activated at the same time, the resulting electric field they produce can be detected through the tissue of the scalp.

  4. - - - - + + + + EEG Setup Electrode Scalp Skull Dura mater Arachnoid mater Subarachnoid space Pia mater Input from another area Cortex Dendrites Pyramidal neuron Axon

  5. EEG Patterns • Collective synchronous activity of thousands of neurons are needed to create an EEG wave. • More synchronous activity leads waves with larger amplitudes and slower frequencies. • Less synchronous activity indicates more active brain activity. • Waves are categorized into four general types: • Alpha – fast and small; awake states • Beta – fast and small; REM states • Theta – slow and large; Non-REM states • Delta – very slow and large; Non-REM states

  6. EEG Patterns Alpha Beta Theta Delta 0 5 Time (sec)

  7. EEG Wave Sources • There are two hypotheses about what underlies EEG rhythms: • A. Pacemaker cells (perhaps in the thalamus) have a constant rhythmic output and can influence other brain areas • Analogy: A conductor waving his baton influences the rhythm of an orchestra • B. Connections between neighboring neurons cause collective firing • Analogy: A crowd of people clapping out of synch will cue off each other and begin clapping in synch.

  8. EEG Wave Functions • It is unclear if brain waves measured as EEGs serve a useful function or if they are just an artifact of normal brain activity. • One hypothesis is that perhaps the rhythmic waves are used to code information across different brain areas. However, as yet, there is no evidence for this.

  9. Seizures • Seizures are caused by massive synchronous activity that spreads. • Epilepsy is when repeated occurrences of seizures happen. • Many causes • GABA agonists sometimes useful treatment • General seizure – entire cortex involved as synchronous firing spreads all over • Symptoms: loss of consciousness, muscles contract, odd sensations • Partial seizure – localized in a brain area • Symptoms: (depends on location of seizure) limb movements, odd sensations, hallucinations, déjà vu

  10. Sleep • Definition: A readily reversible state of reduced responsiveness and interaction with the environment. • A full 1/3 of our lives is spent sleeping • About 1/12 of our lives is spent dreaming. • Sleep is universal among vertebrates (animals with a spine). • Sleep has two stages that repeat over an over: REM and non-REM sleep

  11. REM vs non-REM • REM sleep: • REM stands for “rapid eye movement” • EEGs are beta waves during this phase, showing that the brain is very active • Brain is more active (using more O2) than awake states • Paralysis of all muscles except diaphragm (breathing), extra-ocular muscles (eye movement), and muscles of inner ear. • Muscles have no tone (usually muscle spindles maintain some activity of alpha-motor neurons). • Usually an increase in heart rate and respiration (both somewhat irregular) • Temperature control quits • Dreaming occurs 90-95% of the time in the REM stage

  12. REM vs non-REM • Non-REM sleep: • Slow large EEGs (both theta and delta waves) • Decreased muscle tension but no paralysis • Increased parasympathetic activity: • Decreased H.R., respiration, metabolism and increased digestion • Decreased brain activity (O2 consumption by the brain is decreased) • Decreased sensory input to the cortex

  13. Stages of Sleep • The brain cycles between non-REM and REM sleep 4-5 times per night. • Each cycle lasts about 90 minutes and is called an ultradian rhythm. • Each cycle consists of about 60 minutes of non-REM and 30 minutes of REM sleep. • The proportion of the cycle spent in non-REM sleep is greater at the onset of sleep and diminishes as sleep progresses.

  14. Stages of Sleep Awake (alpha) REM (beta) Stage 1 (theta) Stage 2 (theta) Stage 3 (delta) Stage 4 (delta) 11 pm midnight 1 am 2 am 3 am 4 am 5 am 6 am TIME

  15. Function of Sleep • There is no known function of sleep. • Hypotheses: • Allows time for regeneration • Conserves energy • Most animals are nocturnal or diurnal to fill a ecological niche • Allows time for sensory processing and laying down of memories • Although it is unknown, sleep must serve some function • Most animals will die if kept from sleeping for too long • All vertebrates sleep – evolution would have dropped sleep if it didn’t serve a useful function.

  16. Circadian Rhythms • Circadian rhythm – Biological cycle that lasts one day. • Many systems of the body are affected by circadian rhythms (tough to find one that is not). Sleep Sleep Sleep Alertness Temp [Growth hormone]blood [Cortisol]blood [K+]ICF TIME

  17. Circadian Rhythms • First evidence of circadian rhythms came from the mimosa plant. • Day – leaves are extended; Night – leaves retracted • In 1729 French physicist, Mairan, placed mimosa plants in a dark closet with no possible sunlight exposure and the plants continued to extend and retract leaves on a 24-hour cycle. • Conclusion: Mimosa plant must be sensing the moon. (incorrect) closet

  18. Evidence of an Internal Clock • More than 100 years later, Swiss botanist, Camdolle, showed that a similar plant has a 22-hour cycle when placed in no-light conditions. • Conclusion: The plant must have an internal clock. (correct) • Organisms with internal clocks entrain (set) those clocks to the length of a day using external cues. • Light sensory information is the most influential cue • Humans fall back on their internal clock in the absence of all external daily cues (called a free-running state). • While some humans have internal clocks that are 24 hours, some have clocks that are less than 24 hours and some have clocks that are more than 24 hours.

  19. Night vs. Morning People • It is hypothesized that individuals with clocks less than 24 hours are “morning people”. • Instead of having about 16 hours of awake time and 8 hours of asleep time, morning people are ready for bed sooner and sleep shorter than “24-hour people”. • It is hypothesized that individuals with clocks more than 24 hours are “night people”. • Instead of having about 16 hours of awake time and 8 hours of asleep time, night people are ready for bed later and would sleep longer than “24-hour people”.

  20. Hypothalamus SCN Optic chiasm Pituitary Location of the Internal Clock • In mammals, destruction of the suprachiasmic nucleus (SCN) in the hypothalamus abolishes circadian rhythms. • Fairly small – 0.3 mm2 • Lies atop the optic chiasm, receiving direct light sensory input

  21. More Evidence for the SCN • Circadian studies on Golden hamsters showed they normally have a 24-hour internal clock. • One male hamster showing a 22-hour internal clock was bred with 24-hour females. • The pups fell into two groups: 24-hour and 22-hour clocks • Further breeding between 22-hour hamsters resulted in some of the offspring having 20-hour clocks. • Gene identified and called the tau gene • If a SCN of a 22-hour hamster is transplanted into the SCN of a 24-hour hamster, the hamster becomes a 22-hour hamster.

  22. Mechanism of the Internal Clock • Latest evidence suggests that the internal clock is controlled by neurons in the SCN that change their output on a cycle that is close to 24 hours. • Hypothesis: • These neurons have a gene that codes for mRNA that codes for a protein. • The protein then changes the output of the neuron and inhibits further synthesis of the mRNA that created it. • This cycle of expression/inhibition takes about 24 hours.

  23. - Mechanism of the Internal Clock DNA mRNA protein From retina output SCN neuron

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