Cortical Organization & Introduction to the EEG

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

2. 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 ) .

3. 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

4. The Dorsal Thalamic Nuclei

5. The Dorsal Thalamic Nuclei can be divided 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

6. (A) Thalamic Sensory Relay Nuclei

7. (1) Specific sensory nuclei : • 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 .

8. (2) Non-specific sensory nuclei (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 ) .

9. The Course of Ascending Sensory Pathways in the Midbrain • The Specific Sensory Pathways  occupy the Superior & Lateral regions of the Midbrain . • The Non-Specific Sensory Pathways  occupy the Midbrain Tegmentum ( Central , midline part )

10. (B) Thalamic Nuclei Related to Motor Control

11. (II) Thalamic Nuclei Related to Motor • 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

12. Lateral Geniculate Bodies (Vision) ) 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

13. The Reticular Formation ( RF)

14. The Reticular Formation ( RF) • 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.

15. Functional divisions of the Reticular Formation • 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.

16. 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

17. The Reticular Activating System ( RAS )

18. 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 .

19. In terms of its cortical projections , the RAS has 2 parts : • (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 .

20. EEG ( Electroencephalogram ) 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 ).

21. Alpha Rhythm : • 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 .

22. Beta Waves : • 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

23. 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 .

24. Age-Dependent Changes in Posterior Rhythm • 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 .

25. Other Causers of EEG Variations • 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 .

26. The Physiologic Basis of the EEG

27. 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 .

28. The Cortical Dipole • 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 .

29. A/ Intracortical Oscillations • 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

30. 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 .

31. 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 .

32. 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 .

33. Clinical Uses of the EEG • 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 .

34. Clinical Uses of the EEG • 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:

35. Generalized Tonic-Clonic seizures ( Grand-mal , 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 .

36. Absence ( Petit-Mal ) seizures • (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 .

37. END