The Normal EEG Flashcards
Cortical layers
Six horizontal layers - layer I most superficial underneath the pial surface vs layer VI deepest overlying subcortical white matter:
Layer I Molecular layer containing dendrites and axons from other layers
Layer II External granular layer containing cortico-cortical connections
Layer III External pyramidal layer containing cortico-cortical connections
Layer IV Internal granular layer receiving input from thalamus
Layer V Internal pyramidal layer sending output to subcortical structures
Layer VI Multiform layer sending output to thalamus
Cortico-cortical connections are in which layers?
Extensive horizontal cortico-cortical connections in layers I, II, and III make up the vast majority of the cortical synapses.
Thalamo-cortical connections are in which layers? What type of regulation and thalamic nucleus is involved?
Layers IV (input from thalamus) and VI (output to thalamus)
Thalamocortical projections have an important role in modulating inhibition via thalamic fibers from the reticular nucleus of the thalamus.
Where does the electrical activity from the EEG arise from?
The extracellular field potential generated by changes in membrane potential of neurons for the most part with some contribution from glial cells.
What is the equilibrium potential for an ion?
The membrane potential at which there is no net movement of that ion across the cell membrane.
The resting membrane potential of a neuron…
…Is the membrane potential at which there is no net flow of ions across the cell membrane and typically −70mV, the inside of the neuron being negative in relation to the outside.
The resting membrane potential is determined by the movement of potassium, sodium, and chloride ions along their electrochemical gradient across the cell membrane.
The major contributor of resting membrane potential is the potassium leak channels with a net outward flow of potassium ions (K+) under resting conditions.
An action potential is generated…
…when the negativity in the interior of the neuron, i.e., the resting membrane potential, decreases to a critical point (typically around −40mV). The voltage-gated sodium channels play a major role in the generation and propagation of action potential by allowing sodium to enter into the soma. Once generated, the action potential—a short duration (usually less than 2ms) high-amplitude wave of depolarization—travels through the neuronal processes and reaches synapse, a specialized contact between neurons usually between axons and dendrites.
Neurotransmitters are released from the presynaptic terminal when…
…an action potential causes sufficient change in the voltage (depolarization) at the presynaptic terminal to activate voltage-gated calcium channels allowing calcium to enter into the presynaptic terminal.
This triggers a cascade of events leading to the fusion of presynaptic vesicles with the membrane of the presynaptic terminal, thereby releasing neurotransmitter molecules into the synaptic cleft.
Binding of neurotransmitters to the receptors on the postsynaptic terminal activates ion channels associated with them, allowing the passage of ions leading to local changes in membrane potential
Postsynaptic potential
Local changes in membrane potential following activation of ion channel secondary to biding of neurotransmitters on the postsynaptic terminal
Non-propagated small-amplitude potentials lasting 10–100ms.
Can be excitatory (excitatory postsynaptic potential (EPSP)) or inhibitory (inhibitory postsynaptic potential (IPSP)) depending on the type of ion channel activated and the electrochemical gradient for ions that can pass through the channel.
EPSP
EPSPs generally result from an inward flow of positive ions such as sodium or calcium and cause depolarization (excitation), thus decreasing the threshold for triggering an action potential in the postsynaptic terminal.
IPSP
On the other hand, IPSPs result from an inward flow of negative ions (e.g., chloride) or outward flow of positive ions (e.g., potassium) and cause hyperpolarization (inhibition), thus increasing the threshold for triggering an action potential in the postsynaptic terminal.
A single EPSP or IPSP…
…is not sufficient enough to move the membrane potential of the postsynaptic terminal to or away from the threshold for triggering of action potential.
Summation of several PSPs is necessary for that purpose.
Summation of PSPs
Such summation can be spatial (summation of several PSPs in the vicinity) or temporal (summation of several PSPs occurring in quick succession).
Extracellular field potentials from PSPs and the EEG
A large number of EPSPs and IPSPs generated in a complex network of neurons alter the overall excitability of the neurons in the network. Such PSPs generate an extracellular field potential that changes over time which is believed to be the basis of potentials recorded on EEG.
The extracellular field potential is a secondary phenomenon resulting from the development of potential gradients between areas of localized membrane potential change and the remaining areas of the neuronal membrane.
EPSP and a “sink’
IPSP and a “source’
Referring to flow of positive ions
A ‘sink’ is generated at the site of an EPSP because of an inflow of positive ions into the localized area of the neuron, and there is a corresponding ‘source’ at a distance where positive ions come out of the neuron; current flows from the ‘source’ to the ‘sink’ in the extracellular space giving rise to the extracellular field potential.
EPSP and IPSP recording negative and positive potentials respectively
Thus, a recording electrode close to the synapse receiving an excitatory input (EPSP) would record a negative potential because of an inward flow of positive ions causing negativity in the extracellular space nearby, whereas a deep recording electrode at a distance would record positivity because of an outflow of positive ions associated with the current flowing through the extracellular space.
The reverse is true for an inhibitory input (IPSP): A recording electrode close to the synapse receiving an inhibitory input (IPSP) would record a positive potential because of an inward flow of negative ions (or an outward flow of positive ions), whereas a recording electrode at a distance would record negativity because of an outflow of negative ions (or an inflow of positive ions) associated with the current flowing through the extracellular space.
Therefore, polarity of extracellular field potentials recorded by surface electrodes on EEG depends on the direction of current flow as well as on the position of the electrode relative to the location of the generator.
This translates to the fact that superficial EPSPs and deep IPSPs will show the same polarity (negative) on a surface recording electrode. Likewise, superficial IPSPs and deep EPSPs will show the same polarity (positive) on a surface recording electrode
Pyramidal neurons in the cerebral cortex and vertical dipoles
Pyramidal neurons in the cerebral cortex are arranged in vertical columns with their cell bodies typically in the layer III (external pyramidal) or V (internal pyramidal) and their processes (dendrites and axons) spanning the entire column and receiving thousands of synaptic contacts. This allows for summated potentials with a vertical dipole or a dipole oriented at an angle to the recording electrodes, which can be recorded on the EEG.
Horizontal dipoles
On the other hand, summated potentials resulting in horizontal dipoles (oriented parallel to the recording electrodes) cannot be generally recorded on EEG.
To produce a scalp EEG signal, ______ is required.
To produce a scalp EEG signal, 6cm2 or more of synchronously active area of cortex is required.
Distance and scalp EEG
Scalp electrodes record volume-conducted potentials. Signal decreases proportionally to the square of the distance between the source and the electrode.
Electrode view and the solid angle
The potential recorded at the electrode is proportional to the solid angle subtended by the dipole layer.
Vertical vs horizontal dipoles
As the orientation of the dipole becomes progressively less radial and more tangential to a recording electrode, the electrode records a voltage field of lesser amplitude. If the dipole is directly below the electrode but it is perfectly tangential, the electrode records no potential because of its location on the zero isopotential line of the source scalp field.
Recording of fluctuating field potentials
Can be recorded via conventional EEG
When an afferent fiber forming an excitatory synapse on an apical dendrite near the surface produces bursts of action potentials interrupted by periods of quiescence, EPSPs sum up during the bursts giving rise to fluctuating field potentials.
When recording from surface is done with an amplifier with a finite time constant (as in conventional EEG), such fluctuations in field potential are recorded as waveforms.
Recording of depolarization shift
Sustained firing of the afferent fiber leads to sustained depolarization of the apical dendrites causing a depolarization shift which is not reflected on the surface in conventional EEG recorded with an amplifier with a finite time constant. Sustained changes in field potential (baseline shifts) can be recorded using a direct current or DC amplifier that has an infinite time constant.
