Neuronal plasticity is the grand idea that the brain is not a fixed object, but is capable of change and is constantly changing. Students of medical history would know that this idea was only developed in earnest in the last century, and represented a massive paradigm shift in the fields of neurology and phrenology (the study of the shape of the skull to determine character traits and intelligence—now debunked as pseudoscience).
But neuronal plasticity represents far more than the sum total of the research conducted on this topic; it represents hope for millions of sufferers of various brain and psychological injuries around the world. If the brain can change, truly change, and be induced to change for the better, that would have tremendous implications for several life-altering diseases of the central nervous system, as well as in the field of psychology.
Is the brain a fatalistic, unyielding box of a machine, or does it grow and repair and improve with the right stimuli? Can the central nervous system be nursed back into full health? This is the subject of intense research and speculation, especially in the case of multiple sclerosis (MS). Researchers recently reviewed a few key studies regarding the possibility of neuromodulation and rehabilitation exercises for MS symptomatic improvement and published their findings in the Annals of Neurology. In this article, we will discuss some of their findings and what they mean for MS research.
Promising Studies in Animal Models
MS is a major disease of the central nervous system and can cause numerous neurological pathologies. In the central nervous system, myelin is formed through the differentiation and maturation from oligodendrocyte precursor cells (OPCs) into oligodendrocytes (OLs), a process that persists into adulthood. It has been suggested that the ability to produce new myelin is crucial in preserving neuronal plasticity. Broadly speaking, researchers are investigating 2 possible pathways that affect plasticity outcomes: learning-induced and experience-induced.
Learning-induced neuronal plasticity is the theory that continuous learning triggers OPC proliferation and the production of new myelin. This has been demonstrated in abundance in mouse models. For example, a study shows that mice who were trained in a forelimb reaching task displayed a doubled increase in oligodendrogenesis in the motor cortex. In addition, pre-existing myelin sheaths demonstrated greater rates of dynamic length changes, which is all the more remarkable considering that they are usually inert in baseline conditions.
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In another study, mice were trained to run on a complex wheel and demonstrated increased proliferation of OPCs and differentiated OLs. Mice that received Morris water maze training displayed similar neuronal plasticity. In another study, mice were given fear-based, aversive shock stimuli and also displayed proliferation of OPCs and the long-term increase in mature OLs.
Experience-induced neuronal plasticity is the hypothesis that brain changes can occur in response to one’s environment. To put this hypothesis to the test, a study placed a group of mice in an enriched environment for 20 days. In this study, an “enriched environment” meant creating a multisensory environment in which the mice enjoyed an expanded cohort of social companions. The results found that they had increased proliferation and differentiation of OPCs in the sensorimotor cortex compared to mice who were placed in standard cages. It was also discovered that mice placed in an enriched environment experienced a 5-fold increase in the successful integration of newly differentiated OLs.
Conversely, in a study that placed mice in social isolation for two weeks, reduced myelin thickness was observed. This shouldn’t come as a surprise, as existing literature shows that decreasing OL differentiation and myelination occurs in deprived sensory environments, with the opposite effect observed in an enriched environment. This suggests that physicians and physiotherapists should consider carefully the environmental stimuli that patients are exposed to when recovering from neurological injury.
White Matter Plasticity in Humans
In human studies, white matter adaptation and putative changes in myelination have been observed when performing complex motor or cognitive tasks. In a fascinating study looking into concert pianists, researchers discovered that they displayed increased fractional anisotropy (FA) in the internal capsule when compared to age-matched nonmusicians. Of course, achieving a concert pianist-level of musical expertise requires a lifetime of learning, so what about short-term tasks? Are they likewise able to induce observations of neuronal plasticity?
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Research suggests that this is the case. A study in which subjects were given a mere 2 hours of training in a car racing game was enough to induce white matter plasticity in the fornix. Another study had participants complete a texture discrimination task (3 daily sessions); researchers found that they displayed increased FA in the white matter underlying visual-associated cortex post-training. A study in which patients were given 6 weeks of juggling training displayed increased density in the parietal gray matter and increased FA in the posterior intraparietal sulcus a few weeks post-training.
The Next Chapter
The findings of this study reinforce the theory that not only can the brain change, but it is constantly changing, responding to various environmental stimuli and tasks. This has tremendous clinical implications because so many diseases either stem from the central nervous system directly (such as multiple sclerosis) or affect it in some way. If researchers are able to discover methods that can greatly accelerate remyelination with quantifiable clinical results, we may yet be entering into a whole new chapter of neurology altogether.
Pan S, Chan JR. Clinical applications of myelin plasticity for remyelinating therapies in multiple sclerosis. Ann Neurol. 2021;90(4):558-567. doi:10.1002/ana.26196
Filippi M, Riccitelli G, Mattioli F, et al. Multiple sclerosis: effects of cognitive rehabilitation on structural and functional MR imaging measures–an explorative study. Radiology. 2012;262(3):932-40. doi:10.1148/radiol.11111299