Physiology III
Neuroscience
Exam III - Essay #4

That which is in red was updated after input from Dr. Winterson...See below

Describe the cellular mechanisms underlying the electroencephalogram. List the main types of normal EEG patterns and relate these to particular brain states. Describe the EEC patterns associated with the main types of epilepsy. Describe the neural circuitry and activity proposed to explain partial seizures vs. generalized seizures.

Cellular Mechanisms underlying EEG

The EEG is reading the current from verticle apical dendrites (pyramidal neurons in the cerebral cortex).  The discharge of a single neuron or single nerve fiber in the brain is too small to be recorded on the surface of the head.  Instead, many thousands or even millions of neurons or fibers must fire synchronously.  Thus the intensity of the brain waves from the scalp is determined mainly by the number of neurons and fibers that fire in synchrony with one another, not by the total level of electrical activity in the brain.  In fact, strong nonsynchronous (Beta Waves) nerve signals often nullify one another in the recorded brain waves because they are of opposing polarities.

The AP’s (electrical discharge) of many thousands or even millions of neurons is manifest as a synchronous interictal spike as recorded by an extracellular electrode all the way through the skull as ‘brain waves’ on an EEG.

There are four different types of normal EEG waves:

• Beta – replace alpha when a person begins mental activity
• asynchronous -- Least Synchronized
• frequency of 13-30 per sec
• parietal and frontal regions
• alert wakefulness, REM sleep
• High Frequency -- Low Amplitude
• Alpha – rhythmical waves
• frequency 8-13 per sec
• awake, quiet, resting state
• occipital region mostly, but also parietal and frontal
• 50 microvolts
• disappear during sleep
• Theta – occur during emotional stress in adults esp. frustration and disappointment
• normal in infants and children
• frequency 4-7 per sec
• parietal and temporal regions
• occurs in many brain disorders, often in degenerative brain states
• Delta – 
• Most Synchronized
• frequency 0.5-4 sec
• deep sleep
• surgical anesthesia
• serious organic brain disease
• Low Frequency -- High Amplitude

EEG patterns with epilepsy

• Grand Mal – extreme neuronal discharge in all areas of the brain transmitted down to spinal
• Cord and cause generalized tonic seizures of entire body
• lasts few sec to 3-4 minutes
• post seizure fatigue of neurons when NT is all gone
• high voltage
• synchronous discharge of entire cortex
• Petit Mal (one type of partial) – involves basic thalamocortical brain activating system
• 3-30 sec of unconsciousness with some twitch like contractions
• resumption of previous activities ‘absence syndrome’
• appears in late childhood, disappears by age 30
• most of cortex involved
• Focal (the other type of partial) – localized involvement of any part of the brain – cortex or deeper structures
• usually result of localized organic lesion or functional abnormality such as scar tissue, tumor, or congenital abnormality of local circuit
• Jacksonian variant – wave of excitation spreads over motor cortex causing progressive march of muscle contraction over opposite side of body
• Psychomotor – involves short period of amnesia, attack of abnormal rage, sudden fear, incoherent speech and a motor act to attack someone
• Limbic involvement

Circuitry
Each pyramidal neuron when it is active can activate another pyramidal cell.  If this was to occur unchecked, you would constantly have problems like seizures.  So...when a pyramidal neuron fires it can also activate an inhibitory neighbor cell that releases GABA which inhibits the activation of nearby Pyramidal cells.  This can contain seizures to a local area.  Without GABA, the brain is far more susceptible to seizures because there is no inhibition available.

In a Focal seizure a large amount of pyramidal cells are activated in the cortex.  With the help of GABA inhibition, the activation area doesn't spread too far (not to the thalamus for sure) and remains in a specific spot.  Some excitatory neurons may travel to other places in the cortex or the rest of the brain.  For example, some excitatory neurons may travel to the thalamus and excite it and then the thalamus sends out excitation to the rest of the cortex.  This could cause a Petit Mal seizure if it just activates the cortex.  It the entire brain is activated all the way to the spinal cord, this would be a Grand Mal.  You can see how important GABA is. 

Last Updated 04/10/00 12:27:17 PM
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