|
Sleep Centers
What, and where, are the sleep-inducing centers that act on the RAS
? Our suspicions are now focused on a collection of nerve cells at the
midline of the brainstem that are known as the "nuclei of raphe"
(from a Greek word meaning "seam" and signifying the juncture
of the two halves of the brain). In Sweden, Annica Dahlstrom and Kzell
Fuxe have shown that under ultraviolet light these cells emit a yellow
fluorescence that shows they are rich in the hormone-like substance serotonin,
which is known to have a wide spectrum of powerful effects on the brain
and other organs of the body [see "Serotonin," by Irvine H.
Page; SCIENTIFIC AMERICAN, December, 1957]. Suspecting from various preliminary
pharmacological experiments that serotonin might play a role in sleep,
we decided to test the effects of destroying the raphe cells, which are
the principal source of the serotonin supply in the brain. We found that
when we destroyed 80 percent of these cells at the level of the medulla
in cats (the animals could not have survived destruction of a larger percentage),
the cats became even more sleepless than those on which Moruzzi had performed
his operation. In more than 100 hours of continuous observation with electrical
recording instruments, our animals slept less than 10 percent of the time.
Our results were closely related to those of Moruzzl's. His operation
dividing the brainstem cut through the raphe system. We found that when
we destroyed only the raphe cells on one side or the other of the site
of his cut, our animals were reduced to the same amount of sleep (20 percent)
as those on which he had performed his experiment. This gives us further
reason to believe the raphe system may indeed be the main center responsible
for bringing on sleep in cats.
These new developments bring serotonin into a prominent place in the
research picture and offer an avenue for biochemical attack on the mysteries
of sleep. The fact that the raphe cells are chiefly notable for their
production of serotonin seems to nominate this substance for an important
role in producing the onset of sleep. We have recently been able to demonstrate
a significant correlation between the extent of the lesion of the raphe
system, the decrease in sleep and the decrease in the amount of serotonin
in the brain as measured by means of spectrofluorescent techniques. In
physiological terms we can begin to see the outlines of the system of
brain structures involved in initiating the on set of sleep and maintaining
the first stage of light slumber. At the level of the brainstem, probably
within the raphe system, there are structures that apparently counteract
the RAS and by their braking action cause the animal to fall asleep. Associated
with these structures there presumably are nearby structures that account
for the modulations of electrical activity (notably the slow brain waves)
that have been observed to accompany light sleep. This slow activity seems
to depend primarily, however, on the higher brain structures, particularly
the cortex and the thalamus; in a decorticated animal the pattern characteristic
of light sleep does not make its appearance. We must therefore conclude
that the set of mechanisms brought into play during the process of falling
asleep is a complicated one and that a number of steps in the process
still remain to be discovered.
Next page
|