Imagine your brain as a living house that constantly remodels itself. Its rooms, or parts of the brain with different responsibilities, constantly evolve; instead of remaining in a fixed shape, your brain is designed to change constantly in response to your lived experiences. This flexibility is called neuroplasticity, or the brain’s ability to reorganize and rewire its neural connections, enabling it to adapt and function in ways that differ from its prior state. Much like plastic, the brain is highly malleable in response to learning new skills, experiencing environmental changes, recovering from injuries, and adapting to sensory and cognitive deficits [1]. This dynamic process occurs on a cellular level through synaptic plasticity, the strengthening or weakening of connections between neurons from damage or recovery, and neurogenesis, the process by which new neurons are formed in the brain for regulation [2,3]. Neuroplasticity is often categorized into two types: structural and functional.
Image: Johns Hopkins Medicine (Under BP2)
Structural plasticity physically changes the brain structure, occurring when more neurons and synaptic connections are formed through learning, memory, and experience [4]. New neurons developed from neurogenesis enable the adult brain to rewire itself, especially in the hippocampus. As the brain’s processing center, the hippocampus controls memory, learning, navigation, perception, and emotions [5]. It experiences rapid growth in neurons and synaptic connections specifically during infancy and childhood as a result of heightened learning, experience, and memory development. This is an example of structural plasticity, as the brain’s anatomy is physically altered in size and complexity through growth and development [4].
Structural plasticity can be imagined as pathways that extend the brain’s map; when a new skill is learned, a strengthening in connection between neurons from their synaptic plasticity triggers them to sprout new physical neural pathways called dendrites, leading to tangible changes in brain structure. In other words, synaptic plasticity involves rapid, temporary functional changes in connection strength, whereas structural plasticity represents long-term, physical alterations from the formation of dendrites. While synaptic adjustments allow for immediate learning, structural involves processes like dendritic spine remodeling, sprouting, and pruning to solidify memories and skills [1,6]. An example of this is learning how to walk as a baby. As infants learn how to take their first steps, the brain creates new neural pathways that extend its informational map in which the new adaptation is stored. Through this learning process, it is the structural flexibility of synaptic connections that rewires the brain’s physical structure.
Functional plasticity is the synaptic remodeling following brain dysfunction or injury, where already existing neurons propagate and form new synaptic connections [4]. This remodeling takes place when the brain adapts parts of its functions to compensate for malfunction or damage of neurons in another part of the brain. This takes place in the cerebral cortex and amygdala [4]. The amygdala, which acts alongside the cerebral cortex to heavily regulate emotional responses, is especially active during childhood, similar to the hippocampus region [4,7]. Early experiences of stress and trauma are supported to increase the rate of cognitive decline because the dendrites of neurons in the prefrontal cortex and hippocampus shrink in response to these stress events. And those who are in a chronically stressful environment or state have reduced amounts of gray matter volume in emotional regulation areas like the amygdala. As gray matter volume is inversely related with emotional reactivity, individuals in the states described above tend to have higher emotional reactivity. Conversely, those with higher gray matter volume experience more stable emotional control [8, 9].
Negative experiences like these at an early age cause the brain to have tendencies of “protecting” itself by altering neural connections, therefore altering emotional responses in future experiences. Since functional plasticity is reorganizable, however, it can also be used to heal trauma. By intentionally retraining neural responses, the brain can reshape established patterns and connections when faced with familiar situations or triggers [4].
Meanwhile, the cerebral cortex is responsible for the higher-level processes of the human brain: conscious thought, memory, language, reasoning, decision-making, emotions, intelligence, and personality. Functional plasticity allows the cerebral cortex to reorganize its structure and function from physical damage in the brain [10]. In the same way structural plasticity builds new pathways, the brain can “re-route” its map after an injury. This is from the undamaged areas sprouting new dendritic branches to create stronger synaptic connections, allowing the undamaged tissues of the brain to quickly adapt and take over the functions of the damaged parts. For example, cases show that stroke victims could temporarily or even permanently lose their basic motor or speech skills [10]. However, functional plasticity can help the victim regain these skills, where the healthy brain tissues around the damaged area will try to aid recovery by reorganizing neural circuits and compensating for the damaged areas often through therapy and repetitive practice [11]. Beyond just reorganizing existing circuits, the brain initiates a regenerative response from neurogenesis. Following a stroke, neural stem cells in specialized regions are triggered to multiply and produce new neuroblasts [2]. These newborn cells migrate towards the damaged area to replace potentially lost neurons and release growth factors to keep surviving cells alive. Through this cellular process, functional plasticity gives the brain the flexibility that allows individuals to potentially regain lost functions by finding new ways of performing tasks [12].
Both structural and functional plasticity are highly active and rapidly developing during childhood, explaining why young brains master new skills quickly while remaining deeply sensitive to early life experiences in both positive and negative lights [4]. While neuroplasticity naturally declines with age, the brain remains a life-long “work in progress” as it continually adapts, learns, and grows in response to new experiences, environments, and emotions [9].