Synaptic plasticity · Lobeworks/17

Synaptic plasticity is the change, driven by activity and experience, in the strength of a synapse or in whether the synapse exists at all, and it is the physical form that learning and memory take in a nervous system.

Synaptic plasticity. Synaptic plasticity is the change, driven by activity and experience, in the strength of a synapse or in whether the synapse exists at all, and it is the physical form that learning and memory take in a nervous system.

A synapse can grow stronger or weaker by releasing more or less transmitter per spike, by gaining or losing receptors on the receiving side, or by being built or removed outright. The changes run on very different clocks. Short-term plasticity lasts milliseconds to seconds and depends on the recent history of firing; long-term potentiation and its opposite last hours to a lifetime, and often follow the rule that cells which fire together strengthen their link (Hebbian learning); structural plasticity adds and removes the contacts themselves.

It is distinct from neuromodulation. Modulation changes how a circuit responds while it lasts and rewires nothing; plasticity changes the circuit and stays after its cause is gone, although modulators such as dopamine often decide whether it happens.

A connectome records at most its residue. A fixed map shows how many synapses join two cells, which may reflect past learning, and shows nothing of the strength of each synapse or of the changes under way when the tissue was fixed.

Whole-brain models of the fly leave it out. The leaky integrate-and-fire model of the fly brain has fixed weights, so the simulated fly cannot learn.

Plasticity writes the program, and modulation sets how it runs.

A map made at one instant captures neither the writing nor the setting, only the program as it stood.

Questions: Does a connectome record what an animal learned? It records only a residue of it. Learning changes the strength of synapses, and sometimes their number; a connectome counts synapses but cannot measure how strong each one was, so a change in strength without a change in count is invisible. In flies, where most wiring is laid down by genes and repeats from animal to animal, that matters less than in a human cortex, where far more of the wiring reflects experience. What is the difference between neuromodulation and plasticity? Neuromodulation changes how a circuit responds for as long as the modulator is present, by changing excitability, release or gain, and rewires nothing. Plasticity changes the circuit itself, the strength of synapses or their existence, and the change outlasts its cause. Modulators often decide whether plasticity happens, as dopamine does for reward learning. A fixed connectome contains neither, and the fly whole-brain models include neither. Why would a human connectome describe one person rather than the species? Because the human cortex is far less stereotyped than an insect brain. Much more of its wiring is shaped after birth by experience, through synaptic plasticity, so two people differ in ways two flies do not. A generic human map would miss most of what makes any one cortex work, while the map of one fly describes most flies.