Do ensembles form by excitability, not just synapses?
Coactivating mouse L2/3 pyramids raised correlated firing with only small biphasic synaptic changes but large, persistent increases in intrinsic excitability.
Source
Intrinsic excitability mechanisms of neuronal ensemble formation
What they did
Replicated an in vivo optogenetic imprinting protocol in mouse visual-cortex slices, coactivating L2/3 pyramids and measuring pair correlations, EPSPs, membrane resistance, and spike threshold with whole-cell and perforated patch recordings.
What they found
Coactivated cells increased correlated activity. Presynaptic plasticity was small and biphasic (depression then potentiation). Spontaneous EPSP frequency/amplitude rose even after single-cell stimulation. Strong lasting excitability gains (higher Rm, lower threshold) appeared and reversed when membrane-resistance changes were blocked pharmacologically—supporting an ‘iceberg’ model where subthreshold connections become suprathreshold.
The limits
What it doesn't show
Slice coactivation may not fully recapitulate in vivo ensemble learning; human cortical ensembles and long-term behavior were not tested.
Key terms
- Neuronal ensemble
- Coactive group of cortical neurons linked to perception/behavior.
- Intrinsic excitability
- How readily a neuron spikes given its membrane properties.
- Iceberg model
- Raised excitability lifts existing subthreshold synapses into functional drive.
- Biphasic plasticity
- Initial synaptic depression then later potentiation after recovery.
- Perforated patch
- Recording mode that better preserves intracellular milieu than whole-cell dialysis.
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Common questions
What cells were stimulated?
Layer 2/3 pyramids in mouse visual cortex (both sexes).
Ensemble hallmark observed?
Increased correlated activity after coactivation.
Main intrinsic changes?
Higher membrane resistance and lower spike threshold.
What is the iceberg model?
Excitability makes weak existing connections effectively suprathreshold.