BIO 132BIO 132NeurophysiologyNeurophysiology

Lecture 38Lecture 38

Rhythms of the Rhythms of the BrainBrain

An electroencephalogram (EEG) is a An electroencephalogram (EEG) is a measurement of the activity of the brain, measurement of the activity of the brain, recorded from the surface of the scalp.recorded from the surface of the scalp.

Recordings made as early as 1875Recordings made as early as 1875 Setup:Setup:

24 or so electrodes taped to scalp at standard positions24 or so electrodes taped to scalp at standard positions Output of electrodes amplifiedOutput of electrodes amplified Differences between the charges recorded at each Differences between the charges recorded at each

electrode are made and display on a graph versus electrode are made and display on a graph versus time.time.

Measurements of individual neurons is not possible Measurements of individual neurons is not possible from the scalp, but the activity of collections of from the scalp, but the activity of collections of neurons is possible.neurons is possible.

Electroencephalogram

All cortical neurons have the same orientation; dendrites near the All cortical neurons have the same orientation; dendrites near the surface and axons projecting inward.surface and axons projecting inward.

NaNa++ entering dendrites during neuronal firing leaves the outside of entering dendrites during neuronal firing leaves the outside of the dendrites negatively charged.the dendrites negatively charged.

If enough neurons beneath an electrode are activated at the same If enough neurons beneath an electrode are activated at the same time, the resulting electric field they produce can be detected time, the resulting electric field they produce can be detected through the tissue of the scalp.through the tissue of the scalp.

EEG Setup

EEG Setup

Scalp

Skull

Dura materArachnoid mater

Pia mater

Cortex

Subarachnoid space

Pyramidal neuron

Axon

Dendrites

- ---

+ +

+ +

Input from another area

Electrode

Collective synchronous activity of thousands Collective synchronous activity of thousands of neurons are needed to create an EEG wave.of neurons are needed to create an EEG wave.

More synchronous activity leads waves with More synchronous activity leads waves with larger amplitudes and slower frequencies.larger amplitudes and slower frequencies.

Less synchronous activity indicates more Less synchronous activity indicates more active brain activity.active brain activity.

Waves are categorized into four general types:Waves are categorized into four general types: Alpha – fast and small; awake statesAlpha – fast and small; awake states Beta – fast and small; REM statesBeta – fast and small; REM states Theta – slow and large; Non-REM statesTheta – slow and large; Non-REM states Delta – very slow and large; Non-REM statesDelta – very slow and large; Non-REM states

EEG Patterns

EEG Patterns

Alpha

Beta

Theta

Delta

0 5Time (sec)

There are two hypotheses about what There are two hypotheses about what underlies EEG rhythms:underlies EEG rhythms: A. Pacemaker cells (perhaps in the thalamus) A. Pacemaker cells (perhaps in the thalamus)

have a constant rhythmic output and can have a constant rhythmic output and can influence other brain areasinfluence other brain areas

Analogy: A conductor waving his baton influences Analogy: A conductor waving his baton influences the rhythm of an orchestrathe rhythm of an orchestra

B. Connections between neighboring neurons B. Connections between neighboring neurons cause collective firingcause collective firing

Analogy: A crowd of people clapping out of synch Analogy: A crowd of people clapping out of synch will cue off each other and begin clapping in synch.will cue off each other and begin clapping in synch.

EEG Wave Sources

It is unclear if brain waves measured as It is unclear if brain waves measured as EEGs serve a useful function or if they are EEGs serve a useful function or if they are just an artifact of normal brain activity.just an artifact of normal brain activity.

One hypothesis is that perhaps the One hypothesis is that perhaps the rhythmic waves are used to code rhythmic waves are used to code information across different brain areas. information across different brain areas. However, as yet, there is no evidence for However, as yet, there is no evidence for this.this.

EEG Wave Functions

Seizures are caused by massive synchronous activity Seizures are caused by massive synchronous activity that spreads.that spreads.

Epilepsy is when repeated occurrences of seizures Epilepsy is when repeated occurrences of seizures happen.happen.

Many causesMany causes GABA agonists sometimes useful treatmentGABA agonists sometimes useful treatment

General seizure – entire cortex involved as synchronous General seizure – entire cortex involved as synchronous firing spreads all overfiring spreads all over Symptoms: loss of consciousness, muscles contract, odd Symptoms: loss of consciousness, muscles contract, odd

sensationssensations Partial seizure – localized in a brain areaPartial seizure – localized in a brain area

Symptoms: (depends on location of seizure) limb movements, Symptoms: (depends on location of seizure) limb movements, odd sensations, hallucinations, déjà vuodd sensations, hallucinations, déjà vu

Seizures

Definition: Definition: A readily reversible state of reduced A readily reversible state of reduced responsiveness and interaction with the responsiveness and interaction with the environment.environment.

A full 1/3 of our lives is spent sleepingA full 1/3 of our lives is spent sleeping About 1/12 of our lives is spent dreaming.About 1/12 of our lives is spent dreaming.

Sleep is universal among vertebrates (animals with Sleep is universal among vertebrates (animals with a spine).a spine).

Sleep has two stages that repeat over an over: REM Sleep has two stages that repeat over an over: REM and non-REM sleepand non-REM sleep

Sleep

REM sleep:REM sleep: REM stands for “rapid eye movement”REM stands for “rapid eye movement” EEGs are beta waves during this phase, showing that the EEGs are beta waves during this phase, showing that the

brain is very activebrain is very active Brain is more active (using more OBrain is more active (using more O22) than awake states) than awake states

Paralysis of all muscles except diaphragm (breathing), Paralysis of all muscles except diaphragm (breathing), extra-ocular muscles (eye movement), and muscles of extra-ocular muscles (eye movement), and muscles of inner ear.inner ear.

Muscles have no tone (usually muscle spindles maintain Muscles have no tone (usually muscle spindles maintain some activity of alpha-motor neurons).some activity of alpha-motor neurons).

Usually an increase in heart rate and respiration (both Usually an increase in heart rate and respiration (both somewhat irregular)somewhat irregular)

Temperature control quitsTemperature control quits Dreaming occurs 90-95% of the time in the REM stageDreaming occurs 90-95% of the time in the REM stage

REM vs non-REM

Non-REM sleep:Non-REM sleep: Slow large EEGs (both theta and delta waves)Slow large EEGs (both theta and delta waves) Decreased muscle tension but no paralysisDecreased muscle tension but no paralysis Increased parasympathetic activity:Increased parasympathetic activity:

Decreased H.R., respiration, metabolism and Decreased H.R., respiration, metabolism and increased digestionincreased digestion

Decreased brain activity (O2 consumption by Decreased brain activity (O2 consumption by the brain is decreased)the brain is decreased)

Decreased sensory input to the cortexDecreased sensory input to the cortex

REM vs non-REM

The brain cycles between non-REM and The brain cycles between non-REM and REM sleep 4-5 times per night.REM sleep 4-5 times per night.

Each cycle lasts about 90 minutes and is Each cycle lasts about 90 minutes and is called an called an ultradian rhythmultradian rhythm..

Each cycle consists of about 60 minutes of Each cycle consists of about 60 minutes of non-REM and 30 minutes of REM sleep.non-REM and 30 minutes of REM sleep. The proportion of the cycle spent in non-REM The proportion of the cycle spent in non-REM

sleep is greater at the onset of sleep and sleep is greater at the onset of sleep and diminishes as sleep progresses.diminishes as sleep progresses.

Stages of Sleep

Stages of Sleep

11 pm 6 am5 am4 am3 am2 am1 ammidnight

TIME

Awake(alpha)

REM(beta)

Stage 1(theta)

Stage 2(theta)

Stage 3(delta)

Stage 4(delta)

There is no known function of sleep.There is no known function of sleep. Hypotheses:Hypotheses:

Allows time for regenerationAllows time for regeneration Conserves energyConserves energy

Most animals are nocturnal or diurnal to fill a ecological nicheMost animals are nocturnal or diurnal to fill a ecological niche Allows time for sensory processing and laying down of Allows time for sensory processing and laying down of

memoriesmemories Although it is unknown, sleep must serve some Although it is unknown, sleep must serve some

functionfunction Most animals will die if kept from sleeping for too longMost animals will die if kept from sleeping for too long All vertebrates sleep – evolution would have dropped All vertebrates sleep – evolution would have dropped

sleep if it didn’t serve a useful function.sleep if it didn’t serve a useful function.

Function of Sleep

Circadian rhythm – Biological cycle that lasts one day.Circadian rhythm – Biological cycle that lasts one day. Many systems of the body are affected by circadian Many systems of the body are affected by circadian

rhythms (tough to find one that is not).rhythms (tough to find one that is not).

Circadian Rhythms

TIME

Sleep Sleep Sleep

Alertness

Temp

[Growth hormone]blood

[Cortisol]blood

[K+]ICF

First evidence of circadian rhythms came from the mimosa plant.First evidence of circadian rhythms came from the mimosa plant. Day – leaves are extended; Night – leaves retractedDay – leaves are extended; Night – leaves retracted

In 1729 French physicist, Mairan, placed mimosa plants in a dark In 1729 French physicist, Mairan, placed mimosa plants in a dark closet with no possible sunlight exposure and the plants continued closet with no possible sunlight exposure and the plants continued to extend and retract leaves on a 24-hour cycle.to extend and retract leaves on a 24-hour cycle. Conclusion: Mimosa plant must be sensing the moon. (incorrect)Conclusion: Mimosa plant must be sensing the moon. (incorrect)

Circadian Rhythms

closet

More than 100 years later, Swiss botanist, Camdolle, More than 100 years later, Swiss botanist, Camdolle, showed that a similar plant has a 22-hour cycle when showed that a similar plant has a 22-hour cycle when placed in no-light conditions.placed in no-light conditions. Conclusion: The plant must have an internal clock. (correct)Conclusion: The plant must have an internal clock. (correct)

Organisms with internal clocks entrain (set) those Organisms with internal clocks entrain (set) those clocks to the length of a day using external cues.clocks to the length of a day using external cues. Light sensory information is the most influential cueLight sensory information is the most influential cue

Humans fall back on their internal clock in the Humans fall back on their internal clock in the absence of all external daily cues (called a free-absence of all external daily cues (called a free-running state).running state). While some humans have internal clocks that are 24 hours, While some humans have internal clocks that are 24 hours,

some have clocks that are less than 24 hours and some some have clocks that are less than 24 hours and some have clocks that are more than 24 hours.have clocks that are more than 24 hours.

Evidence of an Internal Clock

It is hypothesized that individuals with clocks It is hypothesized that individuals with clocks lessless than 24 hours are than 24 hours are “morning people”“morning people”.. Instead of having about 16 hours of awake time and Instead of having about 16 hours of awake time and

8 hours of asleep time, morning people are ready for 8 hours of asleep time, morning people are ready for bed sooner and sleep shorter than “24-hour people”.bed sooner and sleep shorter than “24-hour people”.

It is hypothesized that individuals with clocks It is hypothesized that individuals with clocks moremore than 24 hours are than 24 hours are “night people”“night people”.. Instead of having about 16 hours of awake time and Instead of having about 16 hours of awake time and

8 hours of asleep time, night people are ready for 8 hours of asleep time, night people are ready for bed later and would sleep longer than “24-hour bed later and would sleep longer than “24-hour people”.people”.

Night vs. Morning People

In mammals, destruction of the suprachiasmic nucleus (SCN) in the In mammals, destruction of the suprachiasmic nucleus (SCN) in the hypothalamus abolishes circadian rhythms.hypothalamus abolishes circadian rhythms. Fairly small – 0.3 mmFairly small – 0.3 mm22

Lies atop the optic chiasm, receiving direct light sensory inputLies atop the optic chiasm, receiving direct light sensory input

Location of the Internal Clock

Hypothalamus

SCN

Optic chiasm

Pituitary

Circadian studies on Golden hamsters showed Circadian studies on Golden hamsters showed they normally have a 24-hour internal clock.they normally have a 24-hour internal clock.

One male hamster showing a 22-hour internal One male hamster showing a 22-hour internal clock was bred with 24-hour females.clock was bred with 24-hour females. The pups fell into two groups: 24-hour and 22-hour The pups fell into two groups: 24-hour and 22-hour

clocksclocks Further breeding between 22-hour hamsters resulted Further breeding between 22-hour hamsters resulted

in some of the offspring having 20-hour clocks.in some of the offspring having 20-hour clocks. Gene identified and called the Gene identified and called the tautau gene gene If a SCN of a 22-hour hamster is transplanted If a SCN of a 22-hour hamster is transplanted

into the SCN of a 24-hour hamster, the hamster into the SCN of a 24-hour hamster, the hamster becomes a 22-hour hamster.becomes a 22-hour hamster.

More Evidence for the SCN

Latest evidence suggests that the internal clock Latest evidence suggests that the internal clock is controlled by neurons in the SCN that change is controlled by neurons in the SCN that change their output on a cycle that is close to 24 hours.their output on a cycle that is close to 24 hours.

Hypothesis:Hypothesis: These neurons have a gene that codes for mRNA These neurons have a gene that codes for mRNA

that codes for a protein.that codes for a protein. The protein then changes the output of the neuron The protein then changes the output of the neuron

andand inhibits further synthesis of the mRNA that inhibits further synthesis of the mRNA that created it.created it.

This cycle of expression/inhibition takes about 24 This cycle of expression/inhibition takes about 24 hours.hours.

Mechanism of the Internal Clock

Mechanism of the Internal Clock

From retina

SCN neuron

DNA mRNA

protein

output

-