Adult Brain Holds a Hidden Reserve of Silent Synapses for New Memories

MIT researchers found a large reserve of silent synapses in the adult cortex, located on filopodia, making up about 30% of synapses in adult mouse brains. These NMDA-receptor–only connections are normally inactive until activity unlocks AMPA recruitment, turning them into active synapses within minutes. This mechanism lets the brain store new memories without erasing existing ones, suggesting a balance between stability and flexibility in learning and aging. Filopodia act as ready-to-use slots for new information, and the team used eMAP and electrophysiology to demonstrate activation.