Download Student Cortical Organization

Introduction to Cortical
Organization
& EEG
Dr 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 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 .
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The Reticular Activating
System ( RAS)
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• 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 nonspecific 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 ( cellbody ) is that of a constantly
shifting dipole .
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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 .
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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 )
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• 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
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Alpha
Block or Alpha Reactivity .
• 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 .
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• 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
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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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Thanks
Saturday, April 2010 ,10
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