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Fig. 1 | BMC Biology

Fig. 1

From: What is memory? The present state of the engram

Fig. 1

Synaptic connectivity between engram cells as a mechanism for memory storage. a Cellular connectivity in a feedforward excitatory circuit, b synaptic configuration, c dendritic spine density, and d protein synthesis state, shown in a naïve circuit, a circuit during encoding, a circuit after consolidation, or a circuit in an amnesic condition. Engram circuit, cells, and synapses are displayed in green, non-engram in gray. In the naïve state, the circuit displays a variety of synaptic patterns, including strong (thick gray lines) and weak synapses (thin gray lines) as well as silent synapses (dotted lines) exclusively expressing NMDA receptors. During encoding, a network of engram cells is recruited. The preferential connection between engram cells occurs either by potentiation of existing connections (blue dotted circles) or by unsilencing synapses (red dotted circles). A spine density increase supports the synaptic changes. During consolidation, the steady state synthesis of AMPA receptors is shifted to a higher level and the disruption of consolidation with protein synthesis inhibitors (PSI) results in retrograde amnesia. However, during PSI-induced amnesia, memory storage persists within an engram-specific set of weak synaptic connections

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