What’s going on during sleep?
If you are a regular reader you may have noticed the current fixation on sleep. This fixation is due to a number of reasons:
a) I stated in a blog post last week that I would talk about sleep for the next two weeks and;
b) Because I find sleep fascinating.
Before I begin getting into the nitty gritty, I want to point out that my post on Saturday was a very basic overview of how sleep is induced. I did not touch on the many other mechanisms responsible for regulating sleep such as the circadian rhythm which is regulated by our suprachiasmatic nuclei and pineal gland in the brain. I may touch upon this information at some point, but today I will discuss the role sleep has on the consolidation of memories.
OK let’s get started
Stage 2 of sleep is characterized by spindle oscillations. Spindle oscillations consist of 7-14 Hz waxing-and-waning field potentials (i.e. electrical waves). These oscillations are generated in our thalamus (our information relay centre) and sent to the cortex which then returns information to the thalamus via a continuous feedback loop (i.e the thalamacortical loop). Information shared between the thalamus and the cortex is sent via neurons and each electrical signal is referred to as a synapse. Our cortex, specifically our neocortex, is what makes us different from other mammals. It provides us with the capacity to problem-solve, logically-reason, and analyze past, present, and future situations.
It is posited that during Stage 2 of sleep this thalamacortical connection provokes and activates certain intracellular mechanisms in the cortex (i.e. it opens various molecular gateways within the nucleus of pyramidal cells). More specifically, this process likely opens the gateway between synaptic activation from the thalamus and gene expression in the cortex. This “opening of the gateway” sets the stage for the next stage of sleep.
The next stages of sleep (3 & 4) are dominated by delta oscillations. During these stages, researchers posit that if the gateway has been opened (by the spindle oscillations described above) information will reach the nucleus of the pyramidal cell (cell located in the cortex) that, in turn, will stimulate the synthesis of various proteins. The synthesis of such proteins can induce long-term modifications or even morphological changes to the neuronal structure. By changing the morphology of your neurons you are changing how memories are stored and thus, remembered.
In summary, during stages 2,3 and 4, we appear to create strongly connected and interactive neural networks via the recollection of past events and activities. As the expression goes “those that fire together (i.e. neurons), wire together”. Thus, it is the constant communication between the thalamus and cortex via the thalamacortical loop that is largely responsible for the strengthened neural networks and thus, our ability to record, remember and recall our past experiences.
Whew – that’s complex stuff and I haven’t even touched on REM sleep!
My apologies if this is a bit dense and/or confusing. Please email me with questions and I will try very hard to clarify.
This Friday I will talk about the various techniques used by you (my readers!) to get a good sleep.
Sejnowski T., et al. 2000. Why do we sleep? Brain Research; 886:208-223.