The part of our sleep cycle involving Rapid Eye Movement (REM), otherwise known as paradoxical sleep, is a crucial phase of sleep identified in mammals and birds involving the random rapid movement of the eyes. This REM is characterized by relaxed muscle tone throughout the whole body and vivid dreams accompanied by rapid, low-voltage desynchronized brain waves. The reason the body is relaxed during this time is so the person does not act out their dreams in this heightened state, despite a great deal of energy being used by the brain, two contrasting levels of activity in the body in the same state forming the electrical potential-based paradox. If this failsafe is overridden a REM sleep disorder could manifest, with the underlying neuroanatomy regulating paradoxical sleep still unclear. Now, a study from researchers led by the University of Bern identifies neurons responsible for REM during sleep. The team states their discovery provides the first evidence for a brainstem premotor command, where the brain plans for voluntary movements, helping to coordinate the control of eye movements whilst deactivating muscles during REM sleep. The opensource study is published in the journal Nature Communications.
Previous studies have indicated the REM sleep phase, the shortest part of the sleep cycle, originates in the brain stem where it causes a sharp profusion of the neurotransmitter acetylcholine, and markedly, an almost complete absence of the monoamine neurotransmitters histamine, serotonin, and norepinephrine. Notably, the networks of neurons utilizing monoamine neurotransmitters are involved in the regulation of emotion, arousal, and memory. Recent studies from the group investigated neurons in the brain stem and found they form a structure resembling butterfly wings, which they dubbed the ‘Nucleus Papilio.’ This neural bundle associates heavily with the nerve centers responsible for eye movement, and those involved in sleep control, suggesting they play a part in paradoxical sleep. The current study investigates whether neurons in the Nucleus Papilio play a part in the control of eye movements during REM sleep.
The current study utilizes optogenetics to illustrate how the artificial activation of the Nucleus Papilio in mice causes rapid eye movement, particularly during the REM sleep phase. Results show the inhibition or elimination of these neurons blocks the movement of the eyes. Data findings identify a cluster of neurons known to bind to calcium in the Nucleus Papilio, specifically located in the dorsal paragigantocellular nucleus, a bundle of nerves already established in the regulation of REM sleep.
The lab states these calcium-binding neurons within the Nucleus Papilio are found in both rodents and primates, including humans, in the same anatomical location. They go on to add they have provided evidence in a mouse model that these Nucleus Papilio neurons and their projections to the oculomotor nuclei help to regulate eye movements during REM sleep. They conclude the new capability to induce eye movement during paradoxical sleep presents a potent apparatus for gaining knowledge regarding these neurons and their associated roles.
The team surmises they have identified the neurons responsible for rapid eye movement during paradoxical or REM sleep. For the future, the researchers state they now plan to confirm the activation of Nucleus Papilio neurons during REM sleep in humans.
Source: University of Bern
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