Page 345 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Cerebral Cortex, Electro- dendrites is small. Only a portion of the
encephalogram (EEG) rhythms recorded in the EEG are produced
directly in the cortex (α and γ waves in con-
Proper function of the cerebral cortex is essen- scious perception; see below). Lower
tial for conscious perception, planning, action, frequency waves from other parts of the brain,
and voluntary movement (! p. 322ff.). e.g. α waves from the thalamus and θ waves
Cortical ultrastructure and neuronal cir- from the hippocampus, are “forced on” the
cortex (brain wave entrainment).
cuitry (! A). The cerebral cortex consists of six
Central Nervous System and Senses modules (diameter 0.05–0.3 mm, depth tion) of deeper layers of the cortex and hyperpolari-
layers, I–VI, lying parallel to the brain surface.
By convention, downward deflections of the EEG are
Vertically, it is divided into columns and
positive. Generally speaking, depolarization (excita-
1.3–4.5 mm) that extend through all six layers.
zation of superficial layers cause downward deflec-
tion (+) and vice versa.
Input from specific and unspecific areas of the
thalamus terminate mainly on layers IV and on layers
Brain wave types. The electrical activity level
I and II, respectively (! A3); those from other areas
of the cortex is mainly determined by the
of the cortex terminate mainly on layer II (! A2). The
degree of wakefulness and can be distin-
large and small pyramidal cells (! A1) comprise
guished based on the amplitude (a) and
80% of all cells in the cortex and are located in layers
V and III, respectively (glutamate generally serves as
(f ! 10 Hz; a ! 50µV), which predominate
the transmitter, e.g., in the striatum; ! p. 325 D).
when an adult subject is awake and relaxed
The pyramidal cell axons leave the layer VI of their re-
(with eyes closed), are generally detected in
12 spective columns and are the sole source of output frequency (f) of the waves (! B, C). α Waves
from the cortex. Most of the axons project to other
multiple electrodes (synchronized activity).
areas of the ipsilateral cortex (association fibers) or to When the eyes are opened, other sensory or-
areas of the contralateral cortex (commissural fibers) gans are stimulated, or the subject solves a
(! A2); only a few extend to the periphery (! A4
and p. 325 C). Locally, the pyramidal cells are con- math problem, the α waves subside (α
nected to each other by axon collaterals. The princi- blockade) and " waves appear (f ! 20 Hz). The
pal dendrite of a pyramidal cell projects to the upper amplitude of # waves is lower than that of α
layers of its column and has many thorn-like waves, and they are chiefly found in occipital
processes (spines) where many thalamocortical, (! B) and parietal regions when they eyes are
commissural and association fibers terminate. The opened. The frequency and amplitude of #
afferent fibers utilize various transmitters, e.g., waves varies greatly in the different leads
norepinephrine, dopamine, serotonin, acetylcholine
and histamine. Inside the cerebral cortex, informa- (desynchronization). # Waves reflect the in-
tion is processed by many morphologically variable creased attention and activity (arousal activ-
stellate cells (! A1), some of which have stimula- ity) of the ascending reticular activating system
tory effects (VIP, CCK and other peptide transmit- (ARAS; ! p. 322). γ Waves (! 30 Hz) appear
ters), while others have inhibitory effects (GABA). during learning activity. Low-frequency θ
Dendrites of pyramidal and stellate cells project to waves appear when drowsiness descends to
neighboring columns, so the columns are connected sleep (sleep stages A/B/C; ! D); they trans-
by thousands of threads. Plasticity of pyramidal cell form into even slower δ waves during deep
synapses — i.e., the fact that they can be modified in
conformity with their activity pattern — is important sleep (! C, D).
for the learning process (! p. 336). The EEG is used to diagnose epilepsy (local-
ized or generalized paroxysmal waves and
Cortical potentials. Similar to electrocardio- spikes; ! C), to assess the degree of brain
graphy, collective fluctuations of electrical maturation, monitor anesthesia, and to deter-
potentials (brain waves) in the cerebral cortex mine brain death (isoelectric EEG).
can be recorded by electroencephalography
using electrodes applied to the skin over the Magnetoencephalography (MEG), i.e. recording
cranium (! B). The EPSPs contribute the most magnetic signals induced by cortical ion currents,
to the electroencephalogram (EEG) whereas can be combined with the EEG to precisely locate the
site of cortical activity (resolution a few mm).
the share of the relatively low IPSPs (! p. 50ff.)
332
generated at the synapses of pyramidal cell
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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