1. Introduction to Cortical Organization & EEG
Dr Taha Sadig Ahmed
Consultant , Clinical Neurophysiology
Saturday, April 10 , 2010

2. Cortical Organization
The cerebral cortex contains several types of neurons . However , for the purpose of the present discussion , the pyramidal cell may be considered the most important cortical neuron
The cortex is composed of 6 layers , named I, II, III, IV, V, VI
Layers I, II, III contain cortico-cortical fibers
( i.e., intracortical connections ) .
Layer IV = receives inputs from specific thalamic nuclei .
Afferents from non-specific nuclei are distributed in layers 1 to 4 ( I to IV) .
Layers V = provides an output ( sends efferent cortical fibers ) the 
(i) basal ganglia,
(ii) brainstem and
(iii) spinal cord
Layers VI = provides an output to the thalamus ( cortico-thalamic fibers ) .
Saturday, April 10 , 2010

3. On developmental and topographic grounds , the thalamus can be divided into :
(I) Epithalamus :
(II) Ventral Thalamus :
(III) Dorsal Thalamus :
In our present discussion , we will not be concerned with (A) or (B) above .
Saturday, April 10 , 2010

4. The Dorsal Thalamus The Dorsal Thalamic Nuclei can be divided into :
(A) Sensory Relay Nuclei ,
(B) Nuclei related to Motor Functions
These nuclei ( in B) which mediate motor functions receive inputs mainly from from Basal Ganglia & Cerebellum .
Saturday, April 10 , 2010

5. Thalamic Sensory Relay Nuclei
Can be divide into 
(1) Specific Sensory Nuclei : which project ( send efferents ) to specific & discrete areas of the cerebral cortex .
They include the Medial & Lateral Geniculate Bodies , & the Ventrobasal Complex .
(2) Non-Specific Sensory Thalamic Nuclei : ( also called Reticular Thalamic Nuclei , & comprise the Midline & Intralaminar nuclei )
The Non-Specific nuclei project diffusely to the whole neocortex .
They are an important constituents of the Reticular Activating System ( RAS ) .
Saturday, April 10 , 2010

6. RAS is a Part of the Reticular Formation ( RF)
The Reticular Activating System ( RAS ) is
part of the Reticular Formation ( RF)
The RF itself is made of loose clusters of
cell-bodies & fibers of Serotonergic ,
Noradrenergic & Adrenergic neurons that
participate in diverse CNS functions such as
control of respiration , circulation , & regulation of
muscle tone .
The RF has ascending and descending
components .
The ascending component , which is mainly
excitatory , is called “ The Reticular Activating
System , RAS ” , because it palys a crucial role in
maitenance of consciousness .
Saturday, April 10 , 2010

7. The Reticular Activating System ( RAS)
Saturday, April 10 , 2010

8. The RAS is a complex polysynaptic pathway that receivesexcitatory collaterals from all sensory pathways ( afferents of somatic sensations as well as those of special senses )
It projects diffusely & non-specifically to all parts of the cerebral cortex , hence it is a non-specific afferent system
Whereas some of its fibers , on their way to the cortex , bypass the thalamus , many other fibers terminate in the Reticular Thalamic Nuclei ( Intralaminar & Midline nuclei ) ;
Then , from there , they projects diffusely & non-specifically to all parts of the cerebral cortex .
Saturday, April 10 , 2010

9. The Physiologic Basis of the EEG
Saturday, April 10 , 2010

10. This surface ( scalp ) calp or cortical surface registers 
The routine surface EEG is recorded from over the scalp ( through the skull , CSF & meninges ) , and is therefore of much lower voltage than if it were recorded directly from the over the pial surface or cortex .
This surface ( scalp ) calp or cortical surface registers 
A positive wave is registered when the net current flows towards the electrode, &
a negative wave is recorded when the current flow away from the electrode .
Saturday, April 10 , 2010

11. A/ Intracortical Oscillations
The Cortical Dipole
The waxing & waning EEG waves are due to two types of oscillations 
(A) Intracortical oscillations : within the cortex itself , and
(B) Oscillations in feedback circuits between the thalamus and cortex .
A/ Intracortical Oscillations
The dendrites of Pyramidal cortical cells are similarly oriented and densely packed , hence they look like a forest
The relationship between dendrites and their soma ( cell-body ) is that of a constantly shifting dipole .
Saturday, April 10 , 2010

12. A/ Intracortical Oscillations
Excitatory & inhibitory endings ( axon terminals ) on dendrites  continuously create EPSPs and IPSPs , respectively
These lead currents flowing between the soma & dendrites
When the the sum of the dendritic activity is negative relative to soma , the soma becomes depolarized
( hypopolarized )
and , consequently , hyperexcitable .
Conversely , when the sum of the dendritic activity is positive relative to soma , the cell becomes hyperpolarized and less excitable .
Thes current flows between soma & dendrites , when summated from many cells , contribute to production of EEG waves
Saturday, April 10 , 2010

13. B/ Thalamocortical Oscillations
The other source of the EEG waves is the reciprocal oscillating activity between Midline Thalamic nuclei and cortex
In the awake state , these thalamic nuclei are partially depolarized and fire tonically at rapid rates .
This is associated with more rapid firing of cortical neurons
During NREM sleep , they are hyperpolarized and discharge only spindle-like bursts .
Saturday, April 10 , 2010

14. passes up the specific sensory systems to the Midbrain ,
The ascending activity ( impulse traffic ) in RAS responsible for the EEG alerting response following sensory stimulation 
passes up the specific sensory systems to the Midbrain ,
entering the RAS via collaterals ,
and continues through the Interlaminar Nuclei of the Thalamus and the Non-Specific Projection system to the cortex .
Saturday, April 10 , 2010

15. Introduction to The Electroencephalogramalm ( a,b,c of the EEG )
Saturday, April 10 , 2010

16. EEG ( Electroencephalogram )  recording of cortical activity from the scalp surface .
ECoG ( Electrocorticogram ) : recording of cortical activity from the pial surface.
Bipolar EEG recording : shows fluctuations in potential between 2 recording scalp electrodes .
Unipolar ( Referential ) EEG recording: shows fluctuations in potential between a scalp exploring electrode and an indifferent electrode on some part of the body distant from the scalp ( or cortex ) .
Saturday, April 10 , 2010

17. Alpha Rhythm ( Waves ) : Frequeny = 8-13 Hz ,
amplitude uV , usually.
Observed in relaxed wakefulness with eyes closed
Usually , it is most prominent in the occipital region , less frequently in parietal region , & still less frequently in the temporal region .
It is reactive to eye-opening and increased alertness : when the subject is asked to open his eyes , alpha waves become replaced by beta waves .
This reactivity to eye-opening or alerting stimuli is called Alpha Block or Alpha Reactivity .
Saturday, April 10 , 2010

18. Beta Waves : Gamma Waves : 13-30 Hz ,
Have lower amplitude than alpha waves .
Seen In awake subject : frontal regions
Gamma Waves :
Hz .
Often seen in a subject who is , on being aroused , focuses his attention on something
Saturday, April 10 , 2010

19. Theta Waves : Delta Waves :
Large amplitude , regular , 4-7 Hz activity .
Present in awake state in children and adolescents
Present during sleep .
Delta Waves :
Large amplitude , < 4 Hz waves
Seen in deep sleep and in coma
Saturday, April 10 , 2010

20. Causes of Changes in EEG Patterns
Saturday, April 10 , 2010

21. Effect of Age ( in particular in children ) :
The EEG pattern is to a great extent age-dependent
In the neonate , the occipital dominant rhythm (called Posterior Dominant Rhythm , PDR) is a slow Hz pattern.
As the child grows , the occipital dominant rhythm becomes faster .
Saturday, April 10 , 2010

22. The frequency of the alpha rhythm is decreased by : (2) Hypoglycemia
(3) Hypothermia
(4) Low level of Glucocorticoids
(5) Hypercapnea ( High PaCO2 , high arterial CO2 )
(6) Lowered PaCO2 during hyperventilation. This is used as a clinical test .
Epilepsy causes changes in EEG patterns
( discussed in the following slides relating to the use of “ EEG Usefulness in Medicine ”)
Saturday, April 10 , 2010

23. The Utility ( use ) of the EEG in medicine
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24. Clinical Uses of the EEG
The value of the EEG in localizing a subdural hematoma or a cerebral tumor has been superseded by modern neuroimaging techniques
( CT , MRI , etc ) .
These lesions may be irritative to cortical tissue & can be epileptogenic ( can cause unprovoked seizures ).
Epileptogenic foci sometimes generate
high-voltage waves that can be localized.
Epilepsy is a syndrome with many causes . In some forms it has characteristic clinical and EEG patterns .
Saturday, April 10 , 2010

25. Epileptic Seizures Epileptic seizures can be divided into
I/ Partial Onset Seizures :
Arising from a specific , localized cortical focus .
II/ General-onset seizures :
Involve both cerebral hemispheres simultaneously . This category is further subdivided into :
(1) Grand – mal (Generlaized Tonic-Clonic Seizure ( GTC )
(2) Petit-Mal ( Absence ) seizures:
Saturday, April 10 , 2010

26. Grand-mal ( Generalized Tonic-Clonic seizures GTC)
Are Characterized by
Loss of consciouness , which usually occurs without warning .
This is followed by a tonic phase with sustained contraction of limb muscles ; & then 
a clonic phase characterized by symmetric jerking of the limbs as a result of alternating contraction and relaxation .
There is fast EEG activity during the tonic phase
Slow waves , each preceded by a spike , occurs at the time of each clonic jerk .
For a while after the attack , slow waves are present .
Saturday, April 10 , 2010

27. Petit-Mal ( Absence ) seizures
Characterized by momentary loss of responsiveness .
They are associated with 3 Hz ( 3 per second ) doublets , each consisting of a typical spike and rounded wave .
Saturday, April 10 , 2010

28. Thanks
Saturday, April 10 , 2010