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Physiology 3120 Lecture Notes - Evoked Potential, Axon Hillock, Somatotopic Arrangement

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Tom Stavraky

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Human Physiology
Monday, November 23, 2009
“Neuro XIV”
Consciousness, Evoked Potential, EEG, and Sleep
Two components of consciousness
Content of consciousness; cerebral cortex functions (memory, perception, etc.)
Level of consciousness; reticular system in the brainstem
Evoked potentials
Primary & secondary (only worry about primary)
Only observed in the leg area of the somatosensory cortex; activation of different
populations of neurons will produce different patterns
Change in voltage; results from summation of extracellular currents associated with post-
synaptic potentials; not local currents associated with APs (see figure)
Current from DCML axons doesn’t contribute to evoked potential
Depolarization of cortical neurons occurs in the cell body; current flows out to the
dendrite and out into the extracellular space (most of it goes back down to the nerve, but
some is recorded by the electrodes on the surface of the skull)
APs originating at axon hillock contribute nothing to the evoked potential
Used to (I) determine integrity of reticular pathway, and (II) elucidate somatotopic organization
of cortex
Electroencephalogram (EEG)
Neurons fluctuating between EPSPs and IPSPs at about 10 Hz during one form of sleep
Interneurons in the thalamus inhibit themselves, so that when EPSPs occur, more
inhibition occurs, and vice-versa
When subject is awake & alert, subject shows a desynchronized pattern
High frequency (13-30 Hz), low amplitude
Beta waves
Each night, go through 4-6 sleep cycles
REM sleep
Rapid-eye movements = saccades
Follows slow-wave sleep
Paradoxical sleep; brain looks as though it’s awake, but the body is inhibited (i.e. postural
20-25% of sleep per night
Desynchronized EEG (looks the same as the awake state); post-synaptic inhibition to
postural muscles, but fingers twitch, eyes move, and skeletal muscles associated with
ossicles still work, whiskers in animals, genital erection
Can have too much or too little post-synaptic inhibition; too little produces REM
behaviour disorder; too much produces cataplexy (can occur during the waking state)
Wild fluctuations in heart rate and BP
Non-REM sleep
70-80% of sleep per night
First part of sleep cycles
4 stages in EEG: desynchronized, but increasingly synchronized until you get to deep
sleep, which are slow waves (1-2 Hz, but bigger amplitude)
Associated with sleepwalking, nightmares/night terrors
Decrease in BP, heart rate, and respiratory rate
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