Physiology Lab - EEG Flashcards
Describe what an electroencephalogram (EEG) records.
Records electric potentials (which is the electrical activity produced by neurons of the brain)
Use electrodes on the surface of the scalp to record this
What electrical processes are responsible for the generation of the EEG wave?
Slow changes in the membrane potentials of cortical neurons (excitatory and inhibitory postsynaptic potentials - EPSPs and IPSPs)
How far away are the electrodes from the EEG currents in the cortex?
Far from the currents
EEG amplitude is smaller by a factor of 100 to 1000 than that of the membrane potentials recorded intracellulary
Describe the EEG under normal wakeful conditions.
Brain activity is poorly synchronized
Low voltage potentials of high but irregular frequency
= BETA RHYTHM
What is alpha rhythm?
Cerebral activity is decreased - awake, but quiet/relaxed with closed eyes
Greater synchronicity
Higher amplitude, lower frequency
Where does the synchronous rhythmic activity arise and what modifies it?
Arises in the thalamus
Inputs from the brainstem reticular formation modify the activity
What are the clinical applications of EEGs?
Diagnose epilepsy, sleep disorders, encephalopathy, encephalitis and brain death.
Enables discrimination between the various types of epilepsy (generalized and partial)
Evidence of seizure focus
Describe how the leads are placed on the scalp.
Odd on the left
Even on the right
Letters indicate which lobe of the brain it is placed over
How do you interpret an EEG with regards to leads? What does it tell you about?
Which hemisphere is affected (either left or right based on odd/even numbers) W
Which lobe is affected (based on letter)
What is an evoked potential?
Sensory stimuli (light, sound, touch) activate the sensory cortex The electrical activity arising in the cortex is known as the evoked potential
What is a Visual Evoked Potential?
Potentials arising from a visual stimulus
What do we use Visual Evoked Potentials for?
Test the integrity of the sensory pathways
Time (latency) between the stimulus and the evoked potential response
This informs us about conduction within the pathway
What can VEPs diagnose and why?
Multiple Sclerosis - this causes demyelination in the CNS which will slow conduction
Increase in VEP latency is consistent with demyelination in the optic pathways
How do we see the effects of a stimulus on an EEG?
Repeat the stimulus over and over again allowing the same neurons to be activated again and again
This produces a small change in the EEG
EEGs are averaged and evoked potentials become clearer
VEP measurements, the stimulus is a checker-board pattern that is moved side to side several times a second
Describe the VEP of a person with MS.
Increase in latency due to demyelination and thus a delay in conduction
