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Cortical Organization & Introduction to the EEG. Dr Taha Sadig Ahmed , Physiology Department. Cortical Organization.

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Cortical organization introduction to the eeg

Cortical Organization & Introduction to the EEG

Dr Taha Sadig Ahmed ,

Physiology Department


Cortical organization
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 :

  • (A) Epithalamus :

  • Which is connected & functionally related to the olfactory system

  • (B) Ventral thalamus :

  • Whose projections are not fully delineated yet

  • (III) Dorsal thalamus :

  • Which is going to be the main object of our present discussion


The dorsal thalamic nuclei

The Dorsal Thalamic Nuclei be divided into :


The Dorsal Thalamic Nuclei can be divided be divided into :

into 

(A) Thalamic Sensory Relay Nuclei ,

which include 

(1) Specific Sensory Nuclei , &

(2) Non-specific Sensory Nuclei .

(B) Thalamic Nuclei mainly concerned with motor control



1 specific sensory nuclei
(1) Specific sensory nuclei : be divided into :

  • These are thalamic nuclei which project to specific & discrete areas of the cerebral cortex .

  • They include 

  • (i) the MedialGeniculateBodies , which relay auditory impulses from the Cochlea to the Auditory Cortices in the Superior Temporal Gyri .

  • (ii) the Lateral GeniculateBodies , which relay visual impulses from the Retina to  the Visual Cortices in the Occipital Lobes .

  • (iii) the Ventrobasal group of nuclei ( Ventrobasal Complex ) which relay somatosensory information from the body surface ( pain , touch & temp ) & joints

    ( proprioception ) to the postcentral gyrus .


2 non specific sensory nuclei reticular thalamic nuclei
(2) Non-specific sensory nuclei be divided into : (Reticular Thalamic Nuclei )

  • These are the Midline & Intralaminar nuclei that project diffusely to the whole neocortex 

  • they are an important constituent component of RAS ( Reticular Activating System ) .


The course of ascending sensory pathways in the midbrain
The Course of Ascending Sensory Pathways in the Midbrain be divided into :

  • The Specific Sensory Pathways  occupy the Superior & Lateral regions of the Midbrain .

  • The Non-Specific Sensory Pathways  occupy the Midbrain Tegmentum ( Central , midline part )



Ii thalamic nuclei related to motor
(II) Thalamic Nuclei Related to Motor be divided into :

  • These receive inputs from

  • (1) the Basal Ganglia and Cerebellum  & project to the Motor Cortex. .

  • (2) Also included in this group are the Anterior Thalamic Nuclei which receive fibers from the Mamillary Bodies , & project to the Limbic Cortex


Lateral Geniculate Bodies (Vision) ) be divided into :

Medial Geniculate Bodies (Hearing )

Specific Sensory

Thalamic nuclei

Ventrobasal Nuclei ( Somatic Sensations )

Non-Specific Sensory

Intralaminar & Midline Nuclei RAS

Somatomotor :Receive inputs from BG & Cerebellum

Project to M1

Motor-related

Project to Limbic Cortex

Limbic (Anterior Thalamic Nuclei ): Receive inputs from Mamillary Bodies

Thus , there are two ascending sensory systems : (1) Specific Sensory System , & (2) Non-Specific Sensory System


The reticular formation rf

The Reticular Formation be divided into : ( RF)


The reticular formation rf1
The Reticular Formation ( RF) be divided into :

  • The RF is , phylogenetically speaking , the old core of the brain )

  • It occupies the mid-ventral parts of the Medulla and Midbrain .

  • It is primarily made up of various loose clusters of cells and fibers

  • These interconnected circuits of neurons are present in the :

    (1) Brainstem Tegmentum ( brainstem core ) , &

    (2) Thalamic reticular nuclei .

  • It is related to discrete and diverse functions that have to do with respiratory , CVS , endocrine , muscle tone , & autonomic control .

  • It contains the cell-bodies & fibers of many of the Serotonergic , Noradrenergic , & Adrenergic.


Functional divisions of the reticular formation
Functional divisions of the Reticular Formation be divided into :

  • Functionally , It has activating ( excitatory) and inhibitory components .

  • The excitatory component is termed “ The Retiocular Activating System ”(RAS) .

  • The RAS is comprises :

    (1) Ascending RAS : that connects to areas in theThalamus , Hypothalamus and Cerebral Cortex , &

    (2) Descending RAS : connects to the Cerebellum and Sensory Nerves and Pathways.


  • The descending fibers of the reticular formation to the spinal cord are in the Reticulospinal tract . This tract 

  • (1) influences gamma efferent activity , thereby modulating the excitability of the spinal (somatic) stretch reflex & hence  modulating the degree of muscle tone

  • (2) influences activity of Spinal Preganglionic Autonomic Nerves , thereby modulating the excitability of the spinal autonomic reflex arc .

  • (3) modulates sensory inputs to the CNS by setting gain (amplification or deamplification ) to synapses within the Substantia Gelatinose in the spial cord .

  • The reticular formation also influences endocrine glands hormone production  such as ADH & ACTH


The reticular activating system ras

The Reticular Activating System ( RAS ) spinal cord are in the Reticulospinal tract . This tract


  • The RAS is a complex polysynaptic pathway that receive collateral fibers from

    the Somatosensory ascending tracts ,

    Trigeminal afferents, and

    visual and auditory afferents

  • All these afferents are equally excitatory to RAS

  • Hence , whereas the classic sensory pathways are specific ( i.e., their afferent fibers are activated by only one specific type of sensory stimulus,

  • In contrast , the RAS is a non-specificafferent system to all parts of the cerebral cortex .


  • In terms of its cortical projections , the RAS has 2 parts : collateral fibers from

  • (1) One part of it bypasses the Thalamus and  projects diffusely to the cortex

  • (2) The other part of the RAS terminates in the Intralaminar & related nuclei . Then , from there , it projects again diffusely & non-specifically to all parts of the cerebral cortex .


  • EEG ( Electroencephalogram ) collateral fibers from recording of cortical activity from the cortical (or 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 ) .

  • The EEG patterns are largely age-dependent ( change with age ).


  • Alpha Rhythm : collateral fibers from

  • Frequeny = 8-12 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 is called Alpha Block or Alpha Reactivity .


  • Beta Waves : collateral fibers from

  • 18-30 Hz , lower amplitude than alpha .

  • In awake subject : frontal regions

  • Gamma Waves :

  • 30 -80 Hz .

  • Often seen in a subject who is , on being aroused , focuses his attention on something


  • Theta Waves : collateral fibers from

  • 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 .


Age-Dependent Changes in Posterior Rhythm collateral fibers from

  • In the neonate , the occipital 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 .


  • Other Causers of EEG Variations collateral fibers from

  • The frequency of alpha rhythm is decreased by 

    (1) Low Blood Glucose Level ( Hypoglycemia )

    (2) low body temperature ( Hypothermia ) ,

    (3) Low Level of Adrenal Glucocorticoids , and

    (4) High Arterial CO2 (PaCO2).

  • It is increased by the reverse conditions

  • Forced breathing ( hyperventilation , HV ) is clinically used to bring out EEG abnormalities

    .


The physiologic basis of the eeg

The Physiologic Basis collateral fibers fromof the EEG


  • The EEG is a record of cortical neural units in a volume conductor

  • It is usually recorded through the skull and is therefore of much lower voltage than it would be if recorded directly from the cortex .

  • Recording from the scalp or cortical surface registers 

  • a positive wave when the net current flow is towards rhe electrode, &

  • a negative wave when the net current flow is away from the surface .


The Cortical Dipole conductor

  • If all cortical activity was random , the net activity recorded from the cortical surface would be zero

  • Because , in that case , the –ve signals would cancel +ve signals , and their resultant would be zero ; and consequently we will have no EEG waves .

  • Therefore , presence of waves in almost all situations in the EEG indicates that activity is waxing and waning in the cortical area sampled by the EEG .

  • At present , there is solid evidence that the EEG waves are due to oscillations 

    (A) Intracortical oscillations : within the cortex itself , and

    (B) Oscillations in feedback circuits between the thalamus and cortex .


A intracortical oscillations
A/ Intracortical Oscillations conductor

  • The dendrites of Pyramidal cortical cells look like a forest : because they are similarly oriented and densely packed in the superficial layers of the cortex .

  • The relationship between dendrites and their cell-body

    ( soma ) 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 .

  • The flow of current ijn the soma-dendrites’ dipole produces the EEG waves .


B thalamocortical oscillations
B/ Thalamocortical Oscillations relative to soma

  • 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 .


Clinical uses of the eeg
Clinical Uses of the EEG responsible for the EEG alerting response following sensory stimulation

  • The value of the EEG in localizing a subdural hematoma or a cerebral tumor has been superseded in modern neuroimaging ( CT , MRI , fMRI , 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 patterns during seizures ; and also ,frequently , characteristic interictal patterns between attacks .


Clinical uses of the eeg1
Clinical Uses of the EEG responsible for the EEG alerting response following sensory stimulation

  • Seizures can be divided int

  • 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) Generlaized Tonic-Clonic Seizure ( Grand –mal )

  • (2) Absence ( Petit-Mal ) seizures:


Generalized tonic clonic seizures grand mal gtc
Generalized Tonic-Clonic seizures ( Grand-mal , GTC) responsible for the EEG alerting response following sensory stimulation

  • 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 .


Absence petit mal seizures
Absence ( Petit-Mal ) seizures responsible for the EEG alerting response following sensory stimulation

  • (2) Absence ( petit-Mal ) 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 .


  • END responsible for the EEG alerting response following sensory stimulation


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