introduction to cortical organization & eeg dr taha sadig ahmed consultant, clinical...
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Introduction to Cortical Organization
& EEGDr Taha Sadig Ahmed
Consultant , Clinical Neurophysiology
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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 .
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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 .
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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 ) .
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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
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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
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• 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 .
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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 .
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• 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
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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 .
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• 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 ) .
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• 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
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• 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
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Causes of Changes in
EEG Patterns
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• 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 ”)
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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 .
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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:
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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 .
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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 .
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