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Publication of the Association of Polysomnographic Technologists • 2006, Volume 15, Number 4 • www.aptweb.org
They Come From the Cortex
BY WILL ECKHARDT, BS RPSGT CRT, ASSOCIATE EDITOR
here do scalp potentials come from and what produces these
voltages? How do volume conduction, tissue dipoles and geometric orientation affect the electroencephalogram (EEG)? What information can we derive from these waves forms once conducted through
tissue and recorded through our amplifiers? We will explore these issues.
W
We record the EEG from scalp electrodes commonly placed by what
is know as the International 10-20 System of Electrode Placement
(albeit modified generally in sleep studies). Hans Berger recorded the
first human EEG in the1920’s. We have come along way in the equipment used in recording the EEG but the source remains the same. EEG
is a means of looking at voltages derived from our cortex which vary as
a function of time and their spatial distribution in relation to the recording electrode.
EEG can be recorded via scalp electrodes or from intracranial electrodes. Scalp sites sample from a larger area than intracranial placement. Intracranial sites provide more local sampling giving generally
different data from that of the global scalp recordings. Scalp EEG is
now believed to be derived from postsynaptic potentials (postsynaptic
potentials are changes in the electrical potential of the neuron that
receives information at a
neuronal junction or
synapse) from the cortex
that summate and reach
the scalp giving us our
EEG waveforms. Intrinsic
cell currents (produced by
ionic channel activation)
may contribute to the EEG
but is still under investigation. Action potentials
were once thought to
contribute to the EEG but
recently have been dismissed as their temporal
limits are too short.
Fig. 1 Pyramidal Cell
Fig. 2 Dipole
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The cortex is composed of a dense collection of neuron cell bodies
with
myelinated
and
unmyelinated fibers running through it. It is less
then 5 mm thick. The cortex covers both cerebral
hemispheres of the brain.
There are millions of neurons within the cortex,
each having contact with
thousands of other neurons. The cortex has
areas with distinct functions and EEG output. The
neurons receive input
from subcortical areas via
the thalamus. The cerebral cortex and
the thalamus often work together in generating brain rhythms1. These wave
forms are derived from the summation of
different rhythms rather than being a
rhythm generated by a single cell or
group of cells. The cortex also sends
input signals to other areas within the
cortex via association fibers. Efferent
(directed away) signals are sent to many Will Eckhardt
other brain structures e.g. the brainstem, thalamus, cerebellum, the basal nuclei and the spinal cord.
Most of the cortex has six layers of neurons and is called the neocortex. Cytoarchitecture is the distribution of these neurons. Pyramidal
cells (see Fig 2) the most common neurons within the cortex, are
named such due to their cell body shape. Although they are found in all
layers other than layer 1, they are they are most predominant in layers
2, 3, and 5.2
Pyramidal neurons have a cell body, an axon, a single apical dendrite
and a number of basal dendrites. Their axon originating on the base of
the cell body leaves the cortex being the output pathway of the cortex.
Axons can branch many times contacting hundreds of other neurons.
These neurons are layered and project into other areas via their axons
and axon collaterals. Pyramidal neurons are associated with excitatory
neurotransmitters. Other neurons in the cortex are local and stay within the area of their cell body. These are known as interneurons. These
neurons are often inhibitory.
Presently we believe EEG potentials are due to excitatory postsynaptic potentials (EPSP) and inhibitory postsynaptic potentials (IPSP) propagated by the cell body and dendrites of thousands of synchronized
pyramidal neurons3. The summation of these potentials is facilitated buy
the architecture of pyramidal neurons. These neurons are oriented in a
columnar structure with apical dendrites pointing toward the cortical
surface. These very small dipoles (see Fig 2 — a separation of unlike
charges) therefore have similar orientations. The Solid Angle (see Fig 3
— a measure of the apparent cross-sectional area of an object as
viewed from a distance) of the dipole and the actual voltage of the dipole
generated by a single cell is too small to produce recordable EEG at the
surface. It is the summation of solid angles and synchronization of potentials in groups of neuronal synapses that enables the EEG to be recordable at the surface of the head.
There can be a great deal of difference in the recording from two
electrodes spaced only millimeters apart which was previously thought
to imply the activity was from the immediate proximity of the surface
electrode4. Those electrodes far apart and producing the same wave
forms were considered linked to a common source. The solid angle theorem (discussed below) and summation of the potentials is now considered to be the means by which we record postsynaptic potentials at the
surface electrode.
The small area within the cortex created by summated activity in
neighboring active cells has been referred to as a dipole layer (see Fig.
ß
Publication of the Association of Polysomnographic Technologists • 2006, Volume 15, Number 4 • www.aptweb.org
2). A dipole layer can have infinite orientations with respect to scalp
electrodes. The electrodes on the scalp “see” only the potentials and
polarity of the potential pointed at them. Each orientation will produce
a unique result because of the effect on the solid angle (see Fig. 3) the
dipole presents to the recording electrodes. The surface area of the
dipole layer and the orientation of the layer with respect to the electrodes have profound effects on the recording of electrical potentials.
Due to volume conduction (the process of current flow through the tissues between the electrical generator and the electrode) and summation of solid angles we see data from sources of at least several centimeters, independent of our electrode size, and potentially generated
by many local sources. Due to the solid angle theorem and volume conduction the closest electrode to the neuronal generator may not always
record the largest potential5. Differing placement of the recording electrode around the circumference of the area will result in marked
changes in the solid angle even though the event itself remains
unchanged. The polarity of the event also depends on electrode placement not whether the event is due to EPSP or IPSP, the former being
a positive potential and the latter being negative.
Sleep is a normal function of our brains. There are regions of the
brain and brainstem that promote wakefulness. As the influence of “the
wakefulness generators decreases, neurons that promote sleep
become active. Sleep ensues as a light transitional stage and becomes
a more synchronized form (within the bandwidth that we view in
polysomnography) as more neuronal networks are involved.
Transmission between neurons is enhanced during wake and REM
whereas during NREM sleep a blocking of afferent information is seen in
the thalamus1. The brains activity, during wake and REM, are nearly the
same. Although the afferent information stops during NREM sleep, the
cortex remains active. The corticothalamic conection remains active as
do the corticocortical communications. Brainstem stimulation and the
response of the thalamocortical cells on the other hand are associated
with EEG activation and neuronal excitability that creates an activated
state vs. a sleep state.
In conclusion what is it that the EEG shows me? As you know we can
determine NREM, REM, and wake. We can also determine normal EEG,
being a lack of clinically significant patterns associated with disorders.
Abnormal EEG can also be determined but does not necessarily mean a
clinically significant disorder. This is why MRI is an often utilized diagnostic tool in relation to brain function.
Electrophysiologists study potentials generated by just one neuron or
even small groups recorded with microelectrodes or mesoelectrodes.
We, in sleep, are dealing with oscillating macroscopic potentials recorded from the scalp6. To name a few illnesses that EEG may be utilized in
the diagnosis and treatment of: strokes, brain tumors, infectious diseases, severe head injury, and brain death. The EEG is merely one of
many tools in assessment of brain function but remains the gold standard in evaluation of sleep state.
Now I shall block the afferent information from my brainstem reticular formation and let sleep ensue. H
References:
1. Mircea Steriade. Principles and Practice of Sleep Medicine 2006 Elsevier Chapter 9 Brain
Electrical Activity and Sensory Processing During Waking and Sleep States:101
2. Duane E. Haines. Fundamental Neuroscience, Second Edition:508
3. Bruce J. Fisch. Fisch & Spehlmann’s EEG Primer Basic Principles of Digital and Analog
EEG, Third Revised and Enlarged Edition: 4-9
4. R Cooper, J.W. Osselton, J.C. Shaw. EEG Technology Second Edition: 8-13
5. Volume Conduction Principles in Clinical Neurosurgery. February 2005. Veterinary
Neurology and Neurosurgery.
http://www.neurovet.org/Electrophysiology/VolumeConduction/VolCondPartABcite.htm
6. Paul L. Nunez, Ramesh Srinivasan Electric Fields of the Brain The Neurophysics of EEG
Second Edition: 3-4
About the Author
Will Eckhardt, RPSGT, CRT, is a member of the APT Board of Directors and serves as the
APT Board Liaison for the APT Standards and Guidelines Committee. He is a board member
of the New England Polysomnographic Society (NEPS) and is NEPS Education Committee
Chair. His full time position is with Sleep HealthCenters where he is the Director of Education.
Eckhardt also is a faculty member at Northern Essex Community College where he teaches
in the polysomnography program and is a member of the advisory board. He is a member of
the A2Zzz Magazine editorial board and a recipient of the APT Dr. Allen DeVilbiss Literary
Award in 2004. He is also a member of the American Academy of Sleep Medicine Committee
on Polysomnographic Technologists Issues.
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Fig. 3 Solid Angle — The voltage recorded by each electrode is proportional to
the product of the solid angle and the actual voltage of the dipole. Even though
the cross-sectional area of the dipole layer is the same the voltages measured
by the two electrodes would differ from one another in amount because the solid
angles are different.
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