- Synaptic plasticity
- The ability of synapses to strengthen or weaken over time in response to activity. The foundational mechanism of learning and memory at the cellular level. Hebbian plasticity (neurons that fire together wire together, proposed by Donald Hebb in 1949) captures the key principle: repeated co-activation of pre- and postsynaptic neurons leads to lasting strengthening of the synapse. Synaptic plasticity occurs through changes in neurotransmitter release, receptor number, receptor sensitivity, and spine morphology.
- Long-term potentiation (LTP)
- A persistent increase in synaptic strength following high-frequency stimulation of a neural pathway. First demonstrated in the rabbit hippocampus by Bliss and Lømo in 1973. LTP depends on NMDA receptors (which act as "coincidence detectors," requiring both presynaptic glutamate release and postsynaptic depolarisation) and subsequent AMPA receptor insertion into the postsynaptic membrane. LTP is considered the primary cellular mechanism underlying Hebbian plasticity and memory formation. Its counterpart, long-term depression (LTD), weakens synaptic connections and is equally important for learning.
- Long-term depression (LTD)
- A persistent decrease in synaptic strength following low-frequency stimulation or asynchronous pre- and postsynaptic activity. LTD is mediated by NMDA receptors and mGluR receptors and leads to AMPA receptor internalisation. LTD is not simply the "forgetting" mechanism — it actively prunes weak or incorrect synaptic connections and is essential for motor learning (in the cerebellum), spatial memory, and the refinement of neural circuits during development.
- Critical periods
- Developmental windows during which the brain is especially sensitive to specific types of environmental input, and during which experience is necessary for normal neural and behavioural development. The classic example is the visual critical period: monocular deprivation in kittens during the first weeks of life (Hubel & Wiesel, 1963) causes permanent loss of binocular vision and cortical columns for the deprived eye. In humans, the critical period for language acquisition extends from infancy to early adolescence. Critical periods are defined by elevated synaptic plasticity and are regulated by the balance of excitation and inhibition (parvalbumin interneurons and perineuronal nets play key roles).
- Cortical remapping
- The reorganisation of topographic cortical maps in response to altered sensory input or motor demands. Following amputation, the somatosensory cortex representation of the missing limb can be taken over by adjacent body part representations — a potential mechanism of phantom limb pain. In profoundly deaf individuals, auditory cortex is recruited for visual processing. In blind Braille readers, visual cortex is activated during tactile reading. Remapping can be rapid (within hours after nerve block) or gradual (years of practice), suggesting multiple mechanisms.
- Adult neurogenesis
- The generation of new neurons in the adult brain. In mammals, neurogenesis occurs in two regions with high certainty: the subgranular zone of the hippocampal dentate gyrus (producing granule cells integrated into hippocampal circuits) and the subventricular zone (producing neurons that migrate to the olfactory bulb). Hippocampal neurogenesis is increased by physical exercise, environmental enrichment, and antidepressants; decreased by stress, alcohol, and aging. Its precise functional role is debated, but it is implicated in pattern separation, forgetting (clearance of old memories to prevent interference), and mood regulation.
- Hebbian plasticity ("fire together, wire together")
- The principle, proposed by Donald Hebb (1949), that "when an axon of cell A is near enough to excite cell B, and repeatedly and persistently takes part in firing it, some growth process or metabolic change takes place in one or both cells such that A's efficiency, as one of the cells firing B, is increased." This has become the foundational principle of computational neuroscience and models of memory: synapses between co-active neurons strengthen. The NMDA receptor implements this rule biologically by requiring simultaneous presynaptic glutamate release and postsynaptic depolarisation.
- Use-dependent plasticity
- Changes in neural circuitry driven by repeated use of specific circuits — the neural basis of skill learning. In London taxi drivers, who spend years learning the detailed street map of the city ("The Knowledge"), the posterior hippocampus is enlarged relative to controls (Maguire et al., 2000). In musicians, the motor cortex representation of the playing hand is expanded. In people who become blind, visual cortex is recruited for auditory and tactile processing. These findings establish that the brain's architecture responds to the demands placed on it across the lifespan.