Neuroplasticity is the ability of neural networks in the brain to change through growth and reorganization.
These changes range from individual neuron pathways making new connections, to systematic adjustments like cortical remapping. Examples of neuroplasticity include circuit and network changes that result from learning a new ability, environmental influences, practice, and psychological stress.
Activity-dependent plasticity can have significant implications for healthy development, learning, memory, and recovery from brain damage.
Christopher Shaw and Jill McEachern in “Toward a theory of Neuroplasticity”, state that there is no all-inclusive theory that overarches different frameworks and systems in the study of neuroplasticity. However, researchers often describe neuroplasticity as “the ability to make adaptive changes related to the structure and function of the nervous system.”
In 1793, Italian anatomist Michele Vicenzo Malacarne described experiments in which he paired animals, trained one of the pair extensively for years, and then dissected both. He discovered that the cerebellums of the trained animals were substantially larger.
But these findings were eventually forgotten. The idea that the brain and its function are not fixed throughout adulthood was proposed in 1890 by William James in The Principles of Psychology, though the idea was largely neglected. Until around the 1970s, neuroscientists believed that the brain’s structure and function was essentially fixed throughout adulthood.
While the brain was commonly understood as a nonrenewable organ in the early 1900s, Santiago Ramón y Cajal, father of neuroscience, used the term neuronal plasticity to describe nonpathological changes in the structure of adult brains. Based on his renowned Neuron doctrine, Cajal first described the neuron as the fundamental unit of the nervous system that later served as an essential foundation to develop the concept of neural plasticity.
Correspondingly, two types of neuroplasticity are often discussed:
Structural neuroplasticity- is often understood as the brain’s ability to change its neuronal connections. New neurons are constantly produced and integrated into the central nervous system throughout the life span based on this type of neuroplasticity. Researchers nowadays use multiple cross-sectional imaging methods (i.e. magnetic resonance imaging (MRI), computerized tomography (CT)) to study the structural alterations of the human brains.
This type of neuroplasticity often studies the effect of various internal or external stimuli on the brain’s anatomical reorganization. The changes of grey matter proportion or the synaptic strength in the brain are considered as examples of structural neuroplasticity.
Functional neuroplasticity- refers to brain’s ability to alter and adapt the functional properties of neurons. The changes can occur in response to previous activity (activity-dependent plasticity) to acquire memory or in response to malfunction or damage of neurons (reactive plasticity) to compensate a pathological event. In the latter case the functions from one part of the brain transfer to another part of the brain based on the demand to produce recovery of behavioral or physiological processes.
Regarding physiological forms of activity-dependent plasticity, those involving synapses are referred to as synaptic plasticity. The strengthening or weakening of synapses that results in an increase or decrease of firing rate of the neurons are called long-term potentiation (LTP) and long-term depression (LTD), respectively, and they are considered as examples of synaptic plasticity that are associated with memory.