neurons (or both) and thereby locking the in-
dividual out of REM sleep.
70,89
Relationship of Coma to Sleep
Because the brain enters a state of quiescence
during sleep on a daily basis, it is natural to
wonder whether coma may not be a pathologic
entrance into the sleep state. In fact, both
impaired states of consciousness and NREM
sleep are characterized by EEG patterns that
include increased amounts of high-voltage slow
waves. Both conditions are due, ultimately, to
lack of activity by the ascending arousal system.
However, in sleep, the lack of activity is due to
an intrinsically regulated inhibition of the arou-
sal system, whereas in coma the impairment of
the arousal system is due either to damage to
the arousal system or to diffuse dysfunction of
its diencephalic or forebrain targets.
Because sleep is a regulated state, it has
several characteristics that distinguish it from
coma. A key feature of sleep is that the subject
can be aroused from it to wakefulness. Patients
who are obtunded may be aroused briefly, but
they require continuous stimulation to main-
tain a wakeful state, and comatose patients may
not be arousable at all. In addition, sleeping
subjects undergo a variety of postural adjust-
ments, including yawning, stretching, and turn-
ing, which are not seen in patients with path-
ologic impairment of level of consciousness.
The most important difference, however, is
the lack of cycling between NREM and REM
sleep in patients in coma. Sleeping subjects
undergo a characteristic pattern of waxing and
waning depth of NREM sleep during the night,
punctuated by bouts of REM sleep, usually
beginning when the NREM sleep reaches its
lightest phase. The monotonic high-voltage slow
waves in the EEG of the comatose patient indi-
cate that although coma may share with NREM
sleep the property of a low level of activity in the
ascending arousal systems, it is a fundamentally
different and pathologic state.
The Cerebral Hemispheres
and Conscious Behavior
The cerebral cortex acts like a massively parallel
processor that breaks down the components of
sensory experience into a wide array of abstrac-
tions that are analyzed independently and in
parallel during normal conscious experience.
42
This organizational scheme predicts many of the
properties of consciousness, and it sheds light
on how these many parallel streams of cortical
activity are reassimilated into a single conscious
state.
The cerebral neocortex of mammals, from
rodents to humans, consists of a sheet of neu-
rons divided into six layers. Inputs from the tha-
lamic relay nuclei arrive mainly in layer IV, which
consists of small granule cells. Inputs from other
cortical areas arrive into layers II, III, and V.
Layers II and III consist of small- to medium-
sized pyramidal cells, arrayed with their apical
dendrites pointing toward the cortical surface.
Layer V contains much larger pyramidal cells,
also in the same orientation. The apical den-
drites of the pyramidal cells in layers II, III, and
V receive afferents from thalamic and cortical
axons that course through layer I parallel to the
cortical surface. Layer VI comprises a varied
collection of neurons of different shapes and
sizes (the polymorph layer). Layer III provides
most projections to other cortical areas, whereas
layer V provides long-range projections to the
brainstem and spinal cord. The deep part of
layer V projects to the striatum. Layer VI pro-
vides the reciprocal output from the cortex back
to the thalamus.
91
It has been known since the 1960s that the
neurons in successive layers along a line drawn
through the cerebral cortex perpendicular to
the pial surface all tend to be concerned with
similar sensory or motor processes.
92,93
These
neurons form columns, of about 0.3 to 0.5 mm
in width, in which the nerve cells share incom-
ing signals in a vertically integrated manner.
Recordings of neurons in each successive layer
of a column of visual cortex, for example, all
respond to bars of light in a particular orien-
tation in a particular part of the visual field.
Columns of neurons send information to one
another and to higher order association areas
via projection cells in layer III and, to a lesser
extent, layer V.
94
In this way, columns of neu-
rons are able to extract progressively more com-
plex and abstract information from an incom-
ing sensory stimulus. For example, neurons in
a primary visual cortical area may be primarily
concerned with simple lines, edges,and corners,
but by integrating their inputs, a neuron in
a higher order visual association area may
Pathophysiology of Signs and Symptoms of Coma
25
respond only to a complex shape, such as a hand
or a brush.
The organization of the cortical column does
not vary much from mammals with the most
simple cortex, such as rodents, to primates with
much larger and more complex cortical devel-
opment. The depth or width of a column, for
example, is only marginally larger in a primate
brain than in a rat brain. What has changed
most across evolution has been the number of
columns. The hugely enlarged sheet of cortical
columns in a human brain provides the mas-
sively parallel processing power needed to per-
form a sonata on the piano, solve a differential
equation, or send a rocket to another planet.
An important principle of cortical organi-
zation is that neurons in different areas of the
cerebral cortex specialize in certain types of
operations. In a young brain, before school age,
it is possible for cortical functions to reorga-
nize themselves to an astonishing degree if one
area of cortex is damaged. However, the orga-
nization of cortical information processing goes
through a series of critical stages during de-
velopment, in which the maturing cortex gives
up a degree of plasticity but demonstrates im-
proved efficiency of processing.
95,96
In adults,
the ability to perform a specific cognitive pro-
cess may be irretrievably assigned to a region
of cortex, and when that area is damaged, the
individual not only loses the ability to perform
that operation, but also loses the very concept
that the information of that type exists. Hence,
the individual with a large right parietal infarct
not only loses the ability to appreciate stimuli
from the left side of space, but also loses the
concept that there is a left side of space. We
have witnessed a patient with a large right pa-
rietal lobe tumor who ate only the food on the
right side of her plate; when done, she would
Figure 1–6. A summary drawing of the laminar organization of the neurons and inputs to the cerebral cortex. The neuronal
layers of the cerebral cortex are shown at the left, as seen in a Nissl stain, and in the middle of the drawing as seen in Golgi
stains. Layer I has few if any neurons. Layers II and III are composed of small pyramidal cells, and layer V of larger pyra-
midal cells. Layer IV contains very small granular cells, and layer VI, the polymorph layer, cells of multiple types. Axons
from the thalamic relay nuclei (a, b) provide intense ramifications mainly in layer IV. Inputs from the ‘‘nonspecific system,’’
which includes the ascending arousal system, ramify more diffusely, predominantly in layers II, III, and V (c, d). Axons
from other cortical areas ramify mainly in layers II, III, and V (e, f). (From Lorente de No R. Cerebral cortex: archi-
tecture, intracortical connections, motor projections. In Fulton, JF. Physiology of the Nervous System. Oxford University
Press, New York, 1938, pp. 291–340. By permission of Oxford University Press.)
26
Plum and Posner’s Diagnosis of Stupor and Coma