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SECTION III
Central & Peripheral Neurophysiology
primary evoked potential;
the second is the
diffuse secon-
dary response.
The primary evoked potential is highly specific in its loca-
tion and can be observed only where the pathways from a par-
ticular sense organ end. An electrode on the pial surface of
the cortex samples activity to a depth of only 0.3–0.6 mm. The
primary response is negative rather than positive when it is
recorded with a microelectrode inserted in layers II–VI of the
underlying cortex, and the negative wave within the cortex is
followed by a positive wave. The negative–positive sequence
indicates depolarization on the dendrites and somas of the
cells in the cortex, followed by hyperpolarization. The posi-
tive–negative wave sequence recorded from the surface of the
cortex occurs because the superficial cortical layers are posi-
tive relative to the initial negativity, then negative relative to
the deep hyperpolarization. In unanesthetized animals or
humans, the primary evoked potential is largely obscured by
the spontaneous activity of the brain, but it can be demon-
strated by superimposing multiple traces so that the back-
ground activity is averaged out. It is somewhat more diffuse in
unanesthetized animals but still well localized compared with
the diffuse secondary response.
The surface-positive diffuse secondary response, unlike the
primary, is not highly localized. It appears at the same time
over most of the cortex and is due to activity in projections
from the midline and related thalamic nuclei.
PHYSIOLOGIC BASIS OF THE
ELECTROENCEPHALOGRAM
The background electrical activity of the brain in unanesthetized
animals was first described in the 19th century. Subsequently, it
was analyzed in systematic fashion by the German psychiatrist
Hans Berger, who introduced the term
electroencephalogram
(EEG)
to denote the record of the variations in brain potential.
The EEG can be recorded with scalp electrodes through the un-
opened skull or with electrodes on or in the brain. The term
electrocorticogram (ECoG)
is used for the record obtained
with electrodes on the pial surface of the cortex.
EEG records may be
bipolar
or
unipolar.
Bipolar records
show fluctuations in the potential difference between two corti-
cal electrodes; unipolar records show the potential difference
between a cortical electrode and a theoretically indifferent elec-
trode on some part of the body distant from the cortex.
CORTICAL DIPOLES
The EEG recorded from the scalp is a measure of the summa-
tion of dendritic postsynaptic potentials rather than action po-
tentials (Figure 15–4). The dendrites of the cortical cells are a
forest of similarly oriented, densely packed units in the superfi-
cial layers of the cerebral cortex (Figure 15–1). Propagated po-
tentials can be generated in dendrites. In addition, recurrent
axon collaterals end on dendrites in the superficial layers. As ex-
citatory and inhibitory endings on the dendrites of each cell be-
come active, current flows into and out of these current sinks
and sources from the rest of the dendritic processes and the cell
body. The cell–dendrite relationship is therefore that of a con-
stantly shifting dipole. Current flow in this dipole produces
wave-like potential fluctuations in a volume conductor (Figure
15–4). When the sum of the dendritic activity is negative rela-
tive to the cell, the cell is depolarized and hyperexcitable; when
it is positive, the cell is hyperpolarized and less excitable. The
cerebellar cortex and the hippocampus are two other parts of
the central nervous system (CNS) where many complex, paral-
lel dendritic processes are located subpially over a layer of cells.
In both areas, characteristic rhythmic fluctuations occur in sur-
face potential similar to that observed in the cortical EEG.CLINICAL USES OF THE EEG
The EEG is sometimes of value in localizing pathologic pro-
cesses. When a collection of fluid overlies a portion of the cor-
tex, activity over this area may be damped. This fact may aid
in diagnosing and localizing conditions such as subdural he-
matomas. Lesions in the cerebral cortex cause local formation
of irregular or slow waves that can be picked up in the EEG
leads. Epileptogenic foci sometimes generate high-voltage
waves that can be localized.
Epilepsy is a syndrome with multiple causes. In some forms,
characteristic EEG patterns occur during seizures; between
attacks, however, abnormalities are often difficult to demon-
strate. Seizures are divided into
partial (focal) seizures
and
generalized seizures.FIGURE 15–4
Diagrammatic comparison of the electrical
responses of the axon and the dendrites of a large cortical
neuron.
Current flow to and from active synaptic knobs on the den-
drites produces wave activity, while all-or-none action potentials are
transmitted along the axon.100 μV200 mVWave activityDendritic treeAxon
Action potentials