Page 346 - Color Atlas Of Pathophysiology (S Silbernagl Et Al, Thieme 2000)

The Electroencephalogram (EEG)
The neurons of the cerebral cortex, when their fibers are more likely to lead to positive poten-
membrane potential is changed, generate tials at the surface (→ A2) because they act
varying electrical fields on the surface of the near the cell body, i.e., deep in the cerebral cor-
skull that can be recorded with suitable leads. tex. Inhibition in the area of the cell body the-
The EEG can provide valuable clues to neuro- oretically results in a negative potential at the
Systems nal functions and as a result has gained great surface, but it is not strong enough to be regis-
clinical importance. Like the electrocardio-
tered at the surface of the scalp (→ A3).
The neurons in the thalamus that excite the
gram ([ECG] → p.184), the EEG registers the
cortical pyramidal cells undergo a rhythmical
summated activity of the cells that, projected
Neuromuscular and Sensory similarly directed dipoles. rhythm is transmitted by the thalamocortical
activity due to negative feedback (→ A4). This
onto the area of the recording lead, generates
tracts to the pyramidal cells, with one thalamic
The potential changes on the cortical sur-
neuron simultaneously exciting several py-
face largely depend on the postsynaptic poten-
tials at dendrites of the pyramidal cells (→ A).
ramidal cells. Because of this, subcortical le-
Although the postsynaptic potentials have a
sions are better registered in the EEG than
small cortical ones.
lower amplitude than the action potentials,
they last significantly longer. Because the py-
The frequency of the recorded waves (de-
the cortical surface, their local activity gener-
on when analysing the EEG (→ B1). In adults
who are awake with their eyes open it is pre-
ates dipoles in the direction of the surface
10 ramidal cells are positioned at right angles to flections) is a diagnostically significant criteri-
dominantly β-waves (14–30 Hz) that are reg-
much more easily than other cells in the cor-
tex. They thus have a much greater impact on istered. With their eyes closed the somewhat
the surface potential than other neurons. Fur- slower α-waves (8–3 Hz) dominate. Yet slower
thermore, they are all orientated in parallel to waves such as the ϑ-waves (4–7 Hz) and the δ-
one another, so that equidirectional potential waves (0.5–3 Hz) are not normally recorded in
changes of neighboring pyramidal cells are waking adults but only in children and adoles-
summated. EEG deflections are to be expected cents. However, in adults the latter slow waves
only if (around the lead electrode) several py- are recorded during the phases of deep sleep
ramidal cells are simultaneously depolarized, (→ p. 340). Some diseases of the brain can re-
i.e., there is a synchronized event. sult in slowing (sleeping-drug overdose, de-
During an excitatory postsynaptic potential, mentia, schizophrenia) or acceleration (alco-
+
Na enters the cell and thus leaves behind a lo- holism, manic-depressive illness) of the re-
cal negative extracellular potential (→ A1). corded frequency.
The depolarization promotes an efflux of K + The EEG is of particular importance when
ions along the remaining cell membrane, this diagnosing epilepsy, which is characterized by
efflux in turn generating a local positive extra- massive synchronized excitation of cortical
cellular potential. If an excitatory synapse at neurons (→ p. 338). It causes “spike” activity
the apical end of a dendrite is activated, the ex- (“seizure spikes”; → B2) or “spike and wave”
tracellular space in the area is relatively nega- complexes (→ B3).
tive, but relatively positive at the base of the In destruction of the cerebral cortex (brain
+
dendrite (→ A1; to simplify matters the K ef- death) all electrical activity will have ceased
flux has been entered at only one site). As a re- and the EEG tracing will therefore be isoelec-
sult a dipole is generated that creates a nega- tric (“flat”), i.e., there will be no deflections.
tive potential at the surface. Commissural fi-
bers from the other cortical hemisphere and
nonspecific parts of the thalamus form excit-
atory synapses mainly at the surface; excit-
ation via these fibers thus leads to a negative
336 potential at the surface electrode (→ A1). Con-
versely, activation of specific thalamocortical
Silbernagl/Lang, Color Atlas of Pathophysiology © 2000 Thieme
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