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Art by Ryan Bliss
"I'm just your typical sneering hardnosed opinionated materialistic/naturalistic/quasi-reductionist who thinks all conscious cognitive phenomena are a consequence of complex physical substrates, and you gotta love me for it." -- PZ Myers
Sleep is a bit outside my specialty, but I know that brainstem centers like the RAS play a central role.  One possibly irrelevant coincidence is that I've done a lot of work on a homologous network in the fish hindbrain, and interestingly, it is primarily a motor and sensory integration center in anamniotes.  There might be a tie-in there.  It's old, old circuitry that once played a role in perception and movement, and now in us it's been coopted to be the foundation of wakefulness and sleep.

Q: What is the RAS?

Reticular Activating System.  It's a mysterious meshwork of neurons in the base of the brain that is important in, well, activating things.  It doesn't seem to do anything on its own (in vertebrates, at least), but it seems to work in turning a spotlight on other regions of the brain at appropriate times.  It is apparently extremely important in switching between sleep and wakefulness.

Q: And what is the relationship between the amniotic hindbrain and the human brainstem?

The brainstem is part of the hindbrain.  It's just a very deep part of the brain, important in a lot of basic, primitive functions -- like keeping your heart beating and so forth.

Amniote and anamniote just refers to an important taxonomic distinction within vertebrates.  Amniotes are lizards, birds, mammals; anamniotes are fish and frogs.

Unfortunately for this discussion, I'm most familiar with anamniote brains.  What is a bit provocative to me, though, is that the homologous structure to the RAS in anamniotes isn't this subtle sleep/wake/attention center -- it's a sensorimotor integration center.  That means it processes sensory input (auditory and tactile directly, visual indirectly), and forwards signals directly to spinal motor pathways.  Think of it as a kind of core experience in the brain, which heeds stimuli in the outside world and responds to it in an almost reflex fashion.

You can see it in action pretty easily.  Tap on the side of an aquarium, and the fish startle and flick away.  That entire behavior is modulated by the hindbrain and the fishy version of the RAS -- it can translate perception into motion all by itself.

So it's just kind of interesting that this structure has been adapted in us into this guardian of the transition between waking and sleep.  It may not be too surprising that that transition can be accompanied by phantom perceptions and spurious sensations of movement.

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