BIO 132

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BIO 132

• Neurophysiology

Lecture 38 Rhythms of the Brain
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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.

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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.

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

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

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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.

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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

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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

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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

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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

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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.

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

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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 hormoneblood
Cortisolblood
KICF
TIME
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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
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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.

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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.

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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

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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.

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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.

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