Extraocular muscle activity, rapid eye movements and the development of active and quiet sleep.

Rapid eye movements (REMs), traditionally measured using the electrooculogram (EOG), help to characterize active sleep in adults. In early infancy, however, they are not clearly expressed. Here we measured extraocular muscle activity in infant rats at 3 days of age (P3), P8 and P14-15 in order to assess the ontogeny of REMs and their relationship with other forms of sleep-related phasic activity. We found that the causal relationship between extraocular muscle twitches and REMs strengthened during the first two postnatal weeks, reflecting increased control of the extraocular muscles over eye movements. As early as P3, however, phasic bursts of extraocular muscle twitching occurred in synchrony with twitching in other muscle groups, producing waves of phasic activity interspersed with brief periods of quiescence. Surprisingly, the tone of the extraocular muscles, invisible to standard EOG measures, fluctuated in synchrony with the tone of other muscle groups; focal electrical stimulation within the dorsolateral pontine tegmentum, an area that has been shown to contain wake-on neurons in P8 rats, resulted in the simultaneous activation of high tone in both nuchal and extraocular muscles. Finally, when state-dependent neocortical electroencephalographic activity was observed at P14, it had already integrated fully with sleep and wakefulness as defined using electromyographic criteria alone; this finding is not consistent with the notion that active sleep in infants at this age is 'half-activated.' All together, these results indicate exquisite temporal organization of sleep soon after birth and highlight the possible functional implications of homologous activational states in striated muscle and neocortex.