Download Lecture 38 (Rhythms)

BIO 132
Neurophysiology
Lecture 38
Rhythms of the Brain
Electroencephalogram
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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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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.
EEG Setup
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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.
EEG Setup
Electrode
Scalp
Skull
Dura mater
Arachnoid mater
Subarachnoid space
Pia mater
Cortex
Dendrites
Pyramidal neuron
Axon
- - +
+
+
+
Input from another area
EEG Patterns
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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:
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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
EEG Patterns
Alpha
Beta
Theta
Delta
0
Time (sec)
5
EEG Wave Sources
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There are two hypotheses about what underlies EEG
rhythms:
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A. Pacemaker cells (perhaps in the thalamus) have a constant
rhythmic output and can influence other brain areas
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Analogy: A conductor waving his baton influences the rhythm of an
orchestra
B. Connections between neighboring neurons cause collective
firing
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Analogy: A crowd of people clapping out of synch will cue off each
other and begin clapping in synch.
EEG Wave Functions
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It is unclear if brain waves measured as EEGs serve a
useful function or if they are just an artifact of normal
brain activity.
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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.
Seizures
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Seizures are caused by massive synchronous activity that spreads.
Epilepsy is when repeated occurrences of seizures happen.
Many causes
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General seizure – entire cortex involved as synchronous firing
spreads all over
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GABA agonists sometimes useful treatment
Symptoms: loss of consciousness, muscles contract, odd sensations
Partial seizure – localized in a brain area
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Symptoms: (depends on location of seizure) limb movements, odd
sensations, hallucinations, déjà vu
Sleep
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Definition: A readily reversible state of reduced responsiveness
and interaction with the environment.
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A full 1/3 of our lives is spent sleeping
About 1/12 of our lives is spent dreaming.
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Sleep is universal among vertebrates (animals with a
spine).
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Sleep has two stages that repeat over an over: REM and
non-REM sleep
REM vs non-REM
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REM sleep:
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REM stands for “rapid eye movement”
EEGs are beta waves during this phase, showing that the
brain is very active
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Brain is more active (using more O2) than awake states
Paralysis of all muscles except diaphragm (breathing), extraocular 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
REM vs non-REM
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Non-REM sleep:
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Slow large EEGs (both theta and delta waves)
Decreased muscle tension but no paralysis
Increased parasympathetic activity:
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Decreased H.R., respiration, metabolism and increased digestion
Decreased brain activity (O2 consumption by the brain is
decreased)
Decreased sensory input to the cortex
Stages of Sleep
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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.
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The proportion of the cycle spent in non-REM sleep is
greater at the onset of sleep and diminishes as sleep
progresses.
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
TIME
4 am
5 am
6 am
Function of Sleep
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There is no known function of sleep.
Hypotheses:
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Allows time for regeneration
Conserves energy
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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
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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.
Circadian Rhythms
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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
Alertness
Temp
[Growth hormone]blood
[Cortisol]blood
[K+]ICF
TIME
Sleep
Circadian Rhythms
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First evidence of circadian rhythms came from the mimosa plant.
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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.
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Conclusion: Mimosa plant must be sensing the moon. (incorrect)
closet
Evidence of an Internal Clock
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More than 100 years later, Swiss botanist, Camdolle,
showed that a similar plant has a 22-hour cycle when
placed in no-light conditions.
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Organisms with internal clocks entrain (set) those clocks
to the length of a day using external cues.
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Conclusion: The plant must have an internal clock. (correct)
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).
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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.
Night vs. Morning People
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It is hypothesized that individuals with clocks less than
24 hours are “morning people”.
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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”.
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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”.
Location of the Internal Clock
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In mammals, destruction of the suprachiasmic nucleus
(SCN) in the hypothalamus abolishes circadian rhythms.
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Fairly small – 0.3 mm2
Lies atop the optic chiasm, receiving direct light sensory input
Hypothalamus
SCN
Optic chiasm
Pituitary
More Evidence for the SCN
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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.
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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 22hour hamster.
Mechanism of the Internal Clock
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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.
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Hypothesis:
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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.
Mechanism of the Internal Clock
DNA
mRNA
protein
From retina
output
SCN neuron