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Week 11: Memories—getting wired during sleep
•Studies in invertebrates and mammals have suggested that learning increases the
strength of the connections, or “synapses,” between neurons.
Given that skill learning is often enhanced during sleep, one would expect to see
concomitant increases in synaptic strength.
-However, in many studies, sleep actually decreases synaptic strength.
-In mice, the number of dendritic spines on neurons, which correlates with the number of
synapses, increased during wakefulness and decreased after a period of sleep.
•This led to the idea that sleep is a time for reducing the number of synaptic connections
to enhance the information storage capacity of the brain.
That sleep strengthens learning but weakens synapses presents a seeming paradox.
•Theory suggests that during sleep, the brain replays or “reactivates” neural activity
patterns corresponding to recently learned experiences, thus enabling the modification
of synaptic connections necessary to stabilize memory.
This replay of recent experiences during sleep has indeed been observed in several
areas of the brain in both rodents and monkeys.
-What has been missing is direct evidence that this reactivation is actually tied to
learning rather than being just an epiphenomenon.
•To test the role of sleep in spine formation, Yang et al. repeated their experiment with
and without an 8-hour period of sleep deprivation immediately after training. Sleep
deprivation markedly decreased the number of new spines. This effect also was branch-
specific in that sleep deprivation reduced spine formation primarily on the dendritic
branch with the higher number of new spines. Importantly, sleep had no effect on the
rate of spine elimination. The authors also observed that sleep made newly formed
spines much more likely to still be present 1 day later, consistent with the idea that
consolidated memories are less sensitive to decay. In other words, sleep gives spines
The results show that, at least under some circumstances, sleep can lead to the growth
of new synapses.
used a genetically encoded calcium-sensitive indicator to visualize the firing (i.e.,
spiking electrical activity) of individual neurons in the motor cortex before, during, and
after training on a treadmill running task. Consistent with observations in other cortical
areas, motor cortex cells that fired many spikes during training tended to have elevated
firing rates after training. Not only was there reactivation in the motor cortex after skill
learning, reactivation was also sensitive to the same manipulations that affected
dendritic spine growth. The drug MK801, which interferes with synaptic plasticity,
blocked both task-induced spine growth and reactivation. Similarly, training mice on a
second motor skill midway through the sleep session reduced both spine growth and
reactivation. These manipulations support the supposition that reactivation is the
mechanism underlying dendritic plasticity during sleep and help elucidate the
mechanisms of sleep-dependent memory consolidation.