they help reveal which brain areas are
affected by anesthetic agents and when.
Images studied so far suggest that
anesthetic drugs wield their effects by
altering connections between nerve
cells, making it difficult for different
brain areas to talk. Areas highly affected
by the communication breakdown
include the cerebral cortex, the wrinkled
layer of “gray matter” at the surface of
the brain; the thalamus, a ball of tissue
at the center of the brain; and the brain’s
arousal centers, located at the top of the
brain stem, the intersection between the
brain and the spinal cord.
Studies looking at how various knockout drugs break up the organized patterns
of activity among these brain regions
reveal disruptions in several circuits. A
key circuit involves the cortex and thalamus. The cortex plays a role in attention,
language and information processing,
and the thalamus acts as a relay station
for sensory information flowing into the
brain. By passing signals to each other
via nerve cells, the two areas help people
make sense of what they see, hear and feel.
Studies in animals and humans on
anesthetics show that blood flow to the
thalamus is reduced, disrupting this
crucial connection. But last summer, sci-
entists directed by Irene Tracey of
Oxford University in England reported
that another brain structure, the puta-
men, is actually the first to unhook from
the rest of the brain under anesthesia.
Located within the basal ganglia, a bundle
on the right or quite where on the left,”
Tracey says. By pairing that info with
fMRI, the scientists hope to pick up on
subtle connectivity changes as they occur.
Cerebral kick-start The stages that the brain passes through as a person slides into
anesthesia may parallel the stages seen among brain-injured patients recovering from coma
(cerebral metabolism and EEG readings represented below). Minimally conscious patients receiving a sedative sometimes exhibit a temporary burst of excitability called “paradoxical excitation.”
Researchers have seen similar excitations as people move toward an anesthetized state.
Minimally conscious Paradoxical excitation
Scientists see a similar series of disconnections in brain areas when
people catch their daily dose of z’s.
Sights, sounds and other distractions
often melt away as the chatter between
the thalamus and cortex quiets and sleep
ensues. But even at the deepest stage of
sleep, a barking dog or a good shake can
rouse a person. Most certainly, the slice
of a knife will do the job.
In contrast, people under general
anesthesia can’t be stirred. As the drugs
take effect, patients are rendered senseless, numb to the world around them and
within. Though the anesthetized brain
uses some of the same neural machinery
as sleep, studies of brain activity show
that the two states are in fact entirely
different. The steady, slow-rolling EEG
patterns induced under general anesthesia more closely resemble those seen in
coma. Brown, Nicholas Schiff of Weill
Cornell Medical College in New York
City and Ralph Lydic of the University
of Michigan reviewed the similarities