: abundant horizontal (with relation to the cortical surface) fibers. Few cells.
II – External granular layer (association and commissural)
: tightly packed pyramidal and stellate cells.
III – Pyramidal layer (cortico-cortical)
: medium size pyramidal cells.
IV – Internal granular layer (sensory)
: numerous, primarily stellate cells plus dense band of horizontal fibers.
V – Ganglionic layer (corticospinal)
: largest pyramidal cells in cortex.
VI – Multiform layer
: spindle cells.
Cells of the cortex:
Pyramidal cells
.
Long apical dentrite perpendicular to the surface of the cortex.
Short lateral dendrites.
Synaptic connections on protruding spines of apical dendrites are excitatory (EPSP's).
Synaptic connections on cell body (soma) and proximal apical dendrite are inhibitory (IPSP's) and derived from stellate cells. The inhibitory synapses are thus near the axon hillock and can act as a bottleneck for incoming EPSP's.
Axons run perpendicular to the cortical surface.
Stellate cells
: multiple dendrites with short axons terminating on other cells in the cortex Þ inhibit pyramidal cells
Spindle cells
: short dendrites each end of cell. Axons often leave cortex.
Columnar organization.
All cell bodies lie one above the other so as to form columns.
Pyramidal cell apical dendrites run perpendicular to the cortical surface and axons of pyramidal and stellate cell project in a vertical axis to deeper and more superficial structures. Incoming axons also run vertically before terminating.
Architecture of the Thalamus
Two groups of nuclei.
Specific
: project to relatively small areas of cortex.
Non-specific
(centromedian, parafascicular, nuclei of the midline, anterior ventral nucleus): project to large areas of cortex. These nuclei appear to be of particular importance in modulation of electrical activity of the brain.
Architecture of the Reticular Formation
Ascending axons terminate in non-specific thalamic nuclei, etc.
Role in activation and deactivation of cerebral structures has major effect on cortical electrical activity.
Origins of electrical activity (the neural basis of the EEG)
Routine scalp EEG activity is recorded at some distance from the surface of the brain with large surface area electrodes. Thus the summed activity of large numbers of neurons is being recorded.
How does the above "anatomy" contribute to the formation of the electrical activity seen on the EEG?
The "generator"
Pyramidal cells are primarily responsible because only they are uniformly oriented with apical dendrites perpendicular to the cortical surface which are long enough to form effective dipoles
.
Scalp EEG is a summation of non-propagating dendritic and somatic post-synaptic potentials (EPSP's/IPSP's). (Why not axon potentials? axon and axon hillock potentials are too short in duration and too "deep" to contribute to EEG. Also, synchronous firing unlikely given short duration preventing summation of potentials over scalp.)
Apical dendrite EPSP's and soma IPSP's = negative field over scalp.
Apical dendrite IPSP's and soma EPSP's = positive field over scalp.
The "pacemaker"
Pyramidal cells discharge synchronously and rhythmically over large areas of the cortex. Therefore, there has to be a pacemaker, and it appears to be the thalamus.
Pacemakers thought to be in non-specific thalamic nuclei and composed of reverberating circuitry driven by synaptic bombardment from other sites.
Transection of midbrain in cat does not eliminate rhythmical cortical activity.
Reticular activating system
: has an impact on thalamic pacemaker.
Alerting reduces cortical rhythmicity.
Sleep alters EEG rhythmical pattern.
Summary of basis of the EEG:
Repetitive waves recorded from the surface of the brain/scalp are summated synaptic potentials generated by cortical pyramidal cells.
Cortical pyramidal synaptic potentials occur in response to rhythmic discharges from thalamic nuclei.
Frequency and amplitude of thalamic discharges are determined by a network of excitatory and inhibitory interconnections among thalamic cells.
During "activation," inputs from the reticular formation abolish the rhythmic discharges in the thalamic nuclei and cause the cortical potentials to become desynchronized (awake or REM)
Recording electrical activity of the brain
Recording devices (EEG machine) consists of banks of AC differential amplifiers capable of amplifying and recording fluctuating activity in microvolt range. Often 16 – 21 electrode pairs are recorded simultaneously.
EEG: normal waves:
Alpha
(8 – l3 Hz) seen in the posterior head regions. Present when individual awakens with eyes closed.
Beta
(14 Hz or more) central head regions.
Theta
(4 – 7 Hz)seen in small amounts
Delta
(3 Hz or less) frontocentrally in awake adults, prominent during sleep
EEG can be used to look for:
Focal abnormalities – loss of normal rhythms and predominance of focal delta activity; sometimes epileptiform activity.
General cerebral disturbance – loss of normal rhythms and predominance of theta and delta frequencies in a generalized distribution.
Epilepsy – spike and sharp waves interictally. Routine EEG normal greater than 50% of time.