cholinergic and noncholinergic neurons in the
magnocellular basal forebrain nuclei.
76
These
large cholinergic neurons receive afferents from
virtually all of the hypothalamic and monoamin-
ergic brainstem ascending systems and accom-
pany them to diffusely innervate the cerebral
cortex.
77,78
However, the pattern of termination
of the cholinergic neurons is more specific than
the monoamine inputs to the cortex. Whereas
axons from individual monoaminergic neurons
typically ramify widely in the cerebral cortex,
axons from basal forebrain cholinergic neurons
each innervate a patch of cortex of only a few
millimeters in diameter.
42,54
Recordings from
basal forebrain neurons in rats across the wake-
sleep cycle indicate that they have a wide range
of activity patterns. Many are most active dur-
ing wakefulness or during slow-wave sleep, and
they fire in bursts that correlate with EEG wave
patterns.
79
Interestingly, in behaving monkeys,
basal forebrain neuron firing correlates best
with the reward phase of complex behaviors,
suggesting that these neurons may be involved
in some highly specific aspect of arousal, such as
focusing attention on rewarding tasks, rather
than in the general level of cortical activity.
80,81
Thus, the ascending arousal system consists
of multiple ascending pathways originating in
the mesopontine tegmentum, but augmented
by additional inputs at virtually every level
through which it passes on its way to the basal
forebrain, thalamus, and cerebral cortex. These
Figure 1–2. A summary diagram of the ascending arousal system. The cholinergic system, shown in yellow, provides the
main input to the relay and reticular nuclei of the thalamus from the upper brainstem. This inhibits the reticular nucleus and
activates the thalamic relay nuclei, putting them into transmission mode for relaying sensory information to the cerebral
cortex. The cortex is activated simultaneously by a series of direct inputs, shown in red. These include monoaminergic inputs
from the upper brainstem and posterior hypothalamus, such as noradrenaline (NA) from the locus coeruleus (LC), sero-
tonin (5-HT) from the dorsal and median raphe nuclei, dopamine (DA) from the ventral periaqueductal gray matter (vPAG),
and histamine (His) from the tuberomammillary nucleus (TMN); peptidergic inputs from the hypothalamus such as orexin
(ORX) and melanin-concentrating hormone (MCH) both from the lateral hypothalamus (LH); and both cholinergic (ACh)
and gamma-aminobutyric acid (GABA)-ergic inputs from the basal forebrain (BF). Activation of the brainstem yellow path-
way in the absence of the red pathways occurs during rapid eye movement (REM) sleep, resulting in the cortex entering a
dreaming state. LDT, laterodorsal tegmental nuclei; PPT, pedunculopontine. (From Saper, CB, Scammell, TE, Lu J. Hypo-
thalamic regulation of sleep and circadian rhythms. Nature 437:1257–1263, 2005. By permission of Nature Publishing Group.)
Pathophysiology of Signs and Symptoms of Coma
19
Box 1–4 Orexin and Narcolepsy
From its first description by Gelineau in 1880,
66
narcolepsy had puzzled clinicians
and scientists alike. Although Gelineau included within his definition a wide range
of disorders with excessive daytime sleepiness, Gowers has been credited with lim-
iting the term to cases with brief periods of sleep that interrupt a normal waking
state. Kinnier Wilson firmly identified it with attacks of cataplexy, during which
‘‘the patient’s knees give way and he may sink to the ground, without any loss of
consciousness.’’
24
Wilson pointed out that narcolepsy had been considered a very
rare condition of which he had seen only a few cases during the first 20 years of his
practice, but that in the mid-1920s there was a sudden increase in the number of
cases, so that he had seen six within a year in 1927; Spiller reported seeing three
within a year in 1926. Wilson opined that the epidemic of new cases of narcolepsy
in those years was due to the worldwide epidemic of encephalitis from about 1918
to 1925. However, the prevalence of narcolepsy has remained relatively high, with
a current rate of one per 2,000 population, and it has its peak incidence during
the second and third decades of life.
38
Over the years, additional features of narcolepsy were described. About half of
patients reported sleep paralysis, a curious state of inability to move during the
transition from sleep to wakefulness or from wakefulness to sleep.
38
However, up
to 20% of normal individuals may also experience this condition occasionally.
More characteristic of narcolepsy, but occurring in only about 20% of cases, are
episodes of hypnagogic hallucinations, during which the patient experiences a
vivid, cartoon-like hallucination, with movement and action, against a background
of wakefulness. The patient can distinguish that the hallucination is not real. EEG
and EMG recordings during sleep and wakefulness show that narcoleptic patients
fall asleep more frequently during the day, but they also awaken more frequently
at night, so that they get about the same amount of sleep as normal individuals.
However, they often enter into REM sleep very soon after sleep onset (short-onset
REM periods [SOREMPs]), and during cataplexy attacks they show muscle atonia
consistent with intrusion of a REM-like state into consciousness. On a multiple
sleep latency test (MSLT), where the patient lies down in a quiet room five times
during the course of the day at 2-hour intervals, narcoleptics typically fall asleep
much faster than normal individuals (often in less than 5 minutes on repeated oc-
casions) and show SOREMPs, which normal individuals rarely, if ever, experience.
There is a clear genetic predisposition to narcolepsy, as individuals with a first-
degree relative with the disorder are 40 times more likely to develop it them-
selves.
38
However, there are clearly environmental factors involved, even among
monozygotic twins; if one twin develops narcolepsy, the other will develop it only
about 25% of the time. HLA allele DQB1*0602 is found in 88% to 98% of indi-
viduals with narcolepsy with cataplexy, but only in about 12% of white Americans
and 38% of African Americans in the general population.
Scientists worked fruitlessly for decades to unravel the pathophysiology of this
mysterious illness, until in 1999 two dramatic and simultaneous findings suddenly
brought the problem into focus. The previous year, two groups of scientists,
Masashi Yanagisawa and colleagues at the University of Texas Southwestern Med-
ical School, and Greg Sutcliffe and coworkers at the Scripps Institute, had simul-
taneously identified a new pair of peptide neurotransmitters made by neurons in
the lateral hypothalamus, which Yanagisawa called ‘‘orexins’’ (based on the pre-
(continued)
20