Have you ever seen a puppy or infant twitching at night and wondered if he/she was having a nightmare? As worrisome as they may be, these sleep twitches, termed “myoclonic twitches”, are not exclusive to our bad dreams. In fact, many species exhibit these twitches, and an increasing amount of evidence suggests that they may play a more important role during development than occasionally disturbing our sleep.
Although it is not fully understood what causes these mycolonic twitches, developmental neuroscientists are beginning to understand their importance for young brains. In particular, these twitches may have significant impact on the developing cerebellum - a lobe on the back of our brain that, despite its small size, has as many cells as the rest of the brain combined and is extremely crucial for movement and coordination. The cerebellum is composed of incredibly complex and well-organized neurons, however how these neurons organize themselves during development is poorly understood, and whether these myoclonic twitches have any impact on cerebellar function is even more of an enigma.
A recent study published in Journal of Neurophysiology from Greta Sokoloff and colleagues at the University of Iowa directly addressed these questions. They had previously discovered that neurons called “Purkinje cells” in the cerebellum increase their activity in response to myoclonic twitching; in this new paper, the authors investigated whether this increase in activity has any substantial impact on cerebellar development. To do so, the authors recorded neural activity from Purkinje cells from rats at different stages of development while simultaneously recording muscle activity to measure myoclonic twitches. They discovered that Purkinje cells increased nearly doubled their firing rate immediately following twitches across all age groups. Moreover, twitches evoked a very large “complex response” (burst of activity) in these cells, which may serve as a strong “wake up call” for re-wiring (what neuroscientists call “activity dependent plasticity”) . Purkinje cells were also more active during sleep than wakefulness, arguing that their activity is highly state-dependent, at least early during development.
Moreover, the authors reported that, independent of twitching, the cerebellar Purkinje cells exhibit other age-dependent forms of activity. When the animal was 8 days old, both “simple” and “complex” activity in Purkinje cells were very rhythmic, which may reflect a synchronization process across large regions of the cerebellum. It is known that rhythmicity and synchronization can promote communication across distant neural circuits . Importantly this rhythmicity, and the rate and number of myoclonic twitches, decreased substantially as the animals aged. Thus the authors conclude that cerebellar development may be crucially dependent on both early myoclonic twitches and rhythmic activity of the cerebellar network. Given that the cerebellum is crucial for motor coordination and error correction , these twitches may serve to connect developing cerebellar circuits to developing muscles: they may bridge the brain and the body together! Furthermore, perhaps the rhythmicity observed at 8 days old may be necessary to unify cerebellar circuits together for proper coordination.
Although much about cerebellar development remains unknown, Sokoloff and colleagues have demonstrated that it is highly sensitive to muscular movements and goes through dramatic changes across early development. Although many questions remain, one thing is certain: don’t worry if your puppy or kitten twitches at night, they may not be having a nightmare after all.
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