Introduction to Cortical Organization & EEG Dr Taha Sadig Ahmed Consultant, Clinical Neurophysiology Saturday, April 10, 20101.

Introduction to Cortical Organization

& EEGDr Taha Sadig Ahmed

Consultant , Clinical Neurophysiology

Saturday, April 10 , 20101

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 ) .

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• 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 .


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 .


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 ) .


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 ofcell-bodies & fibers of Serotonergic ,Noradrenergic & Adrenergic neurons thatparticipate in diverse CNS functions such ascontrol of respiration , circulation , & regulation ofmuscle tone .• The RF has ascending and descendingcomponents .• The ascending component , which is mainlyexcitatory , is called “ The Reticular Activating System , RAS ” , because it palys a crucial role in maitenance of consciousness . Saturday, April 10 , 20106


The Reticular Activating

System ( RAS)

• 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 .

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The Physiologic Basis

of the EEG



• 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 .


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 .


• 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 .

A/ Intracortical Oscillations

Thes current flows between soma & dendrites , when summated from many cells , contribute to production of EEG waves


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 .


• 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 .

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Introduction to The

Electroencephalogramalm

( a,b,c of the EEG )

• 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 ) .


• Alpha Rhythm ( Waves ) :• Frequeny = 8-13 Hz , • amplitude 50-100 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 , 201017

• Beta Waves :• 13-30 Hz , • Have lower amplitude than alpha waves .• Seen In awake subject : frontal regions

• Gamma Waves :• 31 -80 Hz .• Often seen in a subject who is , on being aroused ,

focuses his attention on something


• Theta 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



Causes of Changes in

EEG Patterns


• 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 0.5-2.0 Hz pattern.

• As the child grows , the occipital dominant rhythm becomes faster .

• 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 ”)



The Utility ( use ) of the

EEG in medicine

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 .


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:


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 .


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 .



Thanks Thanks

Introduction to Cortical Organization & EEG Dr Taha Sadig Ahmed Consultant, Clinical Neurophysiology Saturday, April 10, 20101.

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