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Introduction to Cortical Organization & EEG. Dr Taha Sadig Ahmed Consultant , Clinical Neurophysiology. Cortical Organization.
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Introduction to Cortical Organization & EEG Dr TahaSadig Ahmed Consultant , Clinical Neurophysiology
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 ) .
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 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 .
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-specificafferent 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 .
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 .
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
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 .
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 .
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
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 ”)
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 .
