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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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 , 2010

Beta Waves : Gamma 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 Saturday, April 10 , 2010

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

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

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 . Saturday, April 10 , 2010

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

The Utility ( use ) of the EEG in medicine Saturday, April 10 , 2010

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

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

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

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

Thanks Saturday, April 10 , 2010