Synaptic Plasticity: How Brains Change with Learning
Neuroplasticity is the brain's remarkable ability to change its structure and function in response to experience, learning, injury, and development. Long considered a property only of developing brains, we now know that the adult brain retains substantial plasticity — continuously modifying its neural circuits in response to experience throughout life. The cellular mechanism of learning-related plasticity is synaptic plasticity: changes in the strength, number, and structure of synaptic connections between neurons.
Hebbian plasticity, summarized in the phrase 'neurons that fire together wire together,' describes how synchronous activation of connected neurons strengthens their synaptic connection. Psychologist Donald Hebb proposed this mechanism in 1949, before its molecular details were known. Long-Term Potentiation (LTP) — discovered by Tim Bliss and Terje Lømo in 1973 — is the most studied cellular mechanism of learning-related synaptic plasticity. When a presynaptic neuron repeatedly activates a postsynaptic neuron at high frequency, a biochemical cascade is triggered: NMDA receptors (which act as 'coincidence detectors,' requiring simultaneous activity in both pre- and postsynaptic neurons) open, allowing calcium influx that activates kinases, which insert more AMPA receptors at the synapse. The result is a stronger, faster synaptic response — a potentiated synapse that represents a cellular 'memory' of the co-activation.
LTP occurs primarily in the hippocampus and is critical for declarative memory formation. Patients with bilateral hippocampal damage (the most famous being H.M., Henry Molaison, whose hippocampi were removed surgically to treat intractable epilepsy in 1953) are unable to form new declarative memories — they cannot learn new facts or events — while retaining older pre-surgery memories and implicit learning capacity. H.M.'s case established the hippocampus's central role in episodic and semantic memory formation and remains one of the most important case studies in neuroscience.
Beyond synaptic strength changes, neuroplasticity encompasses: growth of new dendritic branches and synapses (synaptogenesis), pruning of unused connections (synaptic pruning — a major process in adolescent brain maturation), myelination of axons (increasing conduction speed — continued until the mid-20s in prefrontal cortex), and adult neurogenesis (the generation of new neurons — demonstrated in the hippocampal dentate gyrus and olfactory bulb, though the functional significance is still debated).