Higher activity does not necessarily mean easier awakening

EEG and muscle recordings distinguish sleep stages: non-rapid-eye-movement (NREM) sleep features slow waves, while rapid-eye-movement (REM) sleep has a different EEG rhythm and low muscle tone. Sound encounters a changing background, making comparison between these stages especially informative.

Calcium activity in TeA CaMKIIα-positive neurons is detectable during wakefulness, NREM and REM, and is highest in REM. Yet activating these cells rapidly awakens mice from NREM without a comparable effect during REM. Activity level and the ability to drive awakening are distinct.

Natural NREM-to-wake transitions also lack a clear population calcium increase. The study therefore moves from asking whether all awakenings share one activity change to a more specific question: do these cells play a different role when sound interrupts sleep?

Original figure 1Activity and arousal capacity diverge

Awakening noise accompanies a stronger cortical response

During NREM, noise trials that awaken mice accompany larger TeA calcium responses. Relatively quiet noise can awaken a mouse when the response is high, while loud noise with a low response is less likely to do so. Intensity and cortical response together describe this gate.

In awake mice, single-cell electrical activity and population calcium recordings both show responses generally increasing with sound intensity, while tuning to pure-tone frequencies is weak. The focus is therefore closer to how intensity engages arousal than to fine discrimination of pitch.

Inhibiting TeA neurons during sound delivery reduces arousal probability, delays awakening and shortens the ensuing wake period. Recording and intervention together establish their contribution to sound-evoked arousal.

Original figure 2Sound responses and arousal probability

The gate is already changing

For quieter sounds, trials followed by awakening already show higher TeA baseline calcium activity before sound onset. This baseline difference is not clear for louder sounds. With weak external input, the ongoing brain state is particularly closely related to whether sleep is interrupted.

EEG adds another observation: higher pre-sound power in part of the delta range associates with a smaller ensuing TeA response. This relationship in simultaneous recordings connects sleep slow waves with sensory responsiveness and helps explain why volume alone cannot predict every awakening.

Original figure 3Baseline activity and sleep slow waves

Driving awakening versus responding to sound

TeA sends outputs to several regions. Activating its terminals in either amygdala BLA/La or thalamic LPMR induces NREM arousal, whereas the other tested targets show no clear effect. Recording sound responses then distinguishes the two effective targets.

BLA/La cells receiving TeA input show noise-evoked calcium responses, while the corresponding LPMR population does not show a clear response. The ability to induce arousal artificially does not mean noise recruits that connection in the same way, directing further investigation toward BLA/La.

Original figure 4Different responses among downstream targets

Excitatory input from cortex

Both ends are predominantly glutamatergic: TeA cells projecting to BLA/La mainly occupy cortical layers two through five, and most BLA/La input-recipient cells are excitatory. The connection thus supports an excitatory relay from cortex to amygdala.

In brain slices, activating TeA terminals evokes excitatory synaptic responses in BLA/La glutamatergic neurons, including action potentials. The anatomical route therefore has functional support: cortical output can drive downstream cells.

Original figure 5An excitatory TeA–BLA/La connection

Reducing this cortical population makes arousal less likely

Finally, the study selects TeA cells by their projection to BLA/La and reduces their activity. Sound-evoked arousal becomes less likely, later and shorter. Interventions in TeA more broadly and in this projection-defined population produce convergent behavioral changes.

The intervention changes not only whether awakening occurs but also how quickly it begins and how long wakefulness lasts. Together these changes describe the course of a sound-triggered interruption rather than reducing cortical participation to a yes-or-no outcome.

Sound does not simply travel from ear to awakening in one uninterrupted step. Before engaging arousal networks, it encounters a cortical component whose effects depend on sleep state; the TeA–BLA/La connection is one segment that can alter the outcome.

Original figure 6Projection-defined inhibition and sound arousal

A gate between hearing and awakening

The study uses adult male mice and primarily noise and pure tones. It reveals how sound intensity, ongoing brain state and a cortical–amygdala connection relate to arousal. Whether natural sounds with different meanings pass through the same gate remains a separate question.