|I'll Never Forget: Go to Sleep||Memory|
Many students stay up all night to prepare for exams. NEVER DO THAT! You need sleep to consolidate the memory you have acquired while awake. Most memories are first stored in the hippocampus which is a small brain region with limited capacity. To store new memories, some old memories in the hippocampus must be erased. Important memories are transferred to the neocortex, which is the newest part of the cerebral cortex in the evolution history. The memory transfer is carried out mainly during sleep, in particular, the slow wave sleep (SWS).
Stages of Sleep
Sleep is divided into two types: rapid eye movement (REM) and non-rapid eye movement (NREM). The NREM sleep is further divided into three stages: N1, N2, and N3. Sleep begins with the N1 stage, and becomes deeper at N2 stage. The deepest sleep occurs at N3, which is called the slow wave sleep. Evidence suggests that NREM sleep consolidates hippocampus-dependent memory (Marshall and Born, 2007; Born and Wilhelm, 2012), whereas REM sleep may improve the consolidation of emotional memories in the amygdala (Menz et al., 2013; Groch et al., 2013).
Slow Wave Sleep
In electroencephalography (EEG), SWS is characterized by slow oscillations in the range 0.5 - 1 Hz. The slow oscillation reflects synchronized neuronal activities alternating between an UP state and a DOWN state (Crunelli and Hughes, 2010, Figure 1). The UP state is associated with high-frequency neuronal firing (bursts) while the DOWN state corresponds to the silent period. In the neocortex, the high-frequency bursts may arise from synaptic reactivation described in Chapter 8. The strength of reactivation reflects the importance of a particular synapse. In the hippocampus, the bursts are known as sharp wave ripples (SWRs), which may emerge without extra-hippocampal inputs. However, during SWS the neocortical bursts have been found to precede SWRs, suggesting that the exact timing of SWRs can be influenced by the neocortex (Sirota et al., 2003).
According to the MTT Hypothesis, long-term memory is encoded in microtubule tracks for the transport of PSD-95. Therefore, to create a long-lasting memory trace in the neocortex is to construct or modify microtubule tracks so that PSD-95 can be delivered from the soma to target synapses, namely, those with strong reactivation. The next chapter will show how this can be achieved by the dialogue between the neocortex and hippocampus.
Author: Frank Lee