Neuroplasticity Overview PDF

Neuroplas*city: An Overview History, Mechanisms, Applica*ons, Rehabilita*on Tushar Issar Ins*tute of Neurological Sciences, Prince of Wales Hospital [email protected] The Brain • Composed of neurons (~85 billion) and glia (~85 billion) • Average neuron makes ~10,000 synapses – Synapses can be excitatory (glutamate) or inhibitory (GABA) • Neurons are specialised for controlling motor output, sensory processing, or connec*ng regions • Neurons form networks that are responsible for speciﬁc func*ons Associate Professor Janet Taylor “Localisa*onism” Pearson Educa8on History of Neuroplas*city A few of the pivotal studies before neuroplas4city became mainstream • 1793 – anatomist Michele Malacarne discovered animals that received motor training had larger cerebellums than untrained animals • 1890 – psychologist William James proposed the brain and its func*on are not ﬁxed • 1945 – neuroscien*st Justo Gonzalo observed dynamic and adap*ve proper*es aXer brain injuries • 1964 – Marian Diamond produced ﬁrst scien*ﬁc evidence of anatomical brain plas*city. Rats in an enriched environment had thicker cor*ces compared to rats in basic environment. History of Neuroplas*city • 1964 – David Hubel and Torsten Wiesel inves*gated the eﬀect of long-‐term closure of one eye on brain ac*vity in corresponding visual region for that eye. Found that same brain region started processing informa*on from the open eye instead of shut eye. • 1984 – Michael Merzenich mapped ﬁve dis*nct areas in sensory cortex corresponding to digits 1,2,3,4,5. Digit 3 was removed. 2 months later, s*mula*on of digit 2 or 4 evoked ac*vity in cor*cal region where digit 3 mapped (adjacent digits invaded unused region) • 1997 – Eleanor Maguire documented changes in hippocampal structure in associa*on with acquiring knowledge of London’s road layout in taxi drivers. Compared to controls, size of hippocampus was same but posterior hippocampus was bigger Deﬁni*on of Neuroplas*city The ability of the nervous system to respond to intrinsic and extrinsic s*muli by reorganising its structure, func*on, and connec*ons. • Can occur at molecular, cellular, system, behavioural level • Can occur during development, in response to the environment, during disease, or aXer therapy – Developing brain exhibits a higher degree of plas*city. Kral et al. (2010) Mechanisms of Neuroplas*city Change in one neuron (molecular changes) For example, -‐ Change in receptor conforma*on -‐ Altered gene expression Associate Professor Janet Taylor Mechanisms of Neuroplas*city Change between two neurons (cellular changes) For example, -‐ Altered synapses -‐ Larger dendri*c tree Associate Professor Janet Taylor Mechanisms of Neuroplas*city Change in a network of neurons (physiological, anatomical, behavioural changes) For example, -‐ Diﬀerent cor*cal map -‐ Improved motor performance Associate Professor Janet Taylor Mechanisms of Neuroplas*city Neuroplas4city is ac4vity-‐ dependent “Neurons that ﬁre together, wire together” Long-‐term poten8a8on -‐ -‐ -‐ Long-‐term depression Strong depolarisa*on in post-‐synap*c neuron results In high inﬂux of Ca2+ Triggers inser*on of AMPA receptors Stronger synapse -‐ -‐ -‐ Weak depolarisa*on Low inﬂux of Ca2+ results removal of AMPA receptors Synapse gets weaker Associate Professor Janet Taylor Mechanisms of Neuroplas*city Neuroplas4city in development Most of brain growth aXer birth is in cerebral cortex Growth is mainly from prolifera*on and elabora*on of axons and processes • Cri*cal period -‐ In early years, we develop more neurons than we need. Synapses are gradually eliminated in ac*vity-‐dependent manner. • Neurogenesis s*ll occurs in hippocampus, olfactory bulb, and cerebellum in adults. Unknown if and how new neurons integrate into established circuits. Degree of neuroplas*city in adults is unknown. • • S8les & Jernigan (2010) Applica*ons of Neuroplas*city Example 1 – Cerebellar agenesis Applica*ons of Neuroplas*city Example 2 – Neurotrophins and Stroke BDNF is a neurotrophic factor involved in neuronal prolifera*on, survival, synap*c plas*city, learning and memory. • BDNF is increased in cortex during motor learning • In this study, BDNF was blocked aXer a focal ischaemia. Received rehab. • BDNF likely has a role in motor map reorganisa*on, learning, and memory aXer stroke. • Applica*ons of Neuroplas*city Example 3 – Hemispherectomy In some cases of severe epilepsy, one hemisphere of the brain may be removed or disconnected. • Extensive reorganisa*on may occur, such that the remaining side is responsible for motor and sensory func*on for both sides of the body. • Greatest poten*al is seen for children under age of 6. • Applica*ons of Neuroplas*city Example 4 Blinded subject Sighted subject Applica*ons of Neuroplas*city Maladap4ve/Adverse Examples • Chronic pain following limb amputa*on (phantom limb) – 80% of amputees report painful feelings from areas where limbs are no longer present – Previous theory was that nerve endings were inﬂamed, causing pain (disproved). – In 1990s, Vilayanur Ramachandran hypothesised that phantom limbs was due to reorganisa*on of the somatosensory cortex. • Stroking face resulted in percep*ons of phantom limb being touched – Other theories also exist. No consensus. cnx.org Applica*ons of Neuroplas*city Maladap4ve/Adverse Examples • Onset of epilepsy aXer cerebral trauma – Arises months or years aXer insult – Delayed onset suggests progressive changes in the brain • Axonal sprou*ng and new connec*ons that alter signalling and induce seizures • Drug Addic*on – Transi*on from casual -‐> compulsive -‐> relapse is thought to be caused by long-‐las*ng neuroadapata*ons in reward pathways – Reward circuits are involuntary ac*vated and execu*ve func*on circuits are hijacked to support drug-‐seeking behavior. Therapies to Promote Neuroplas*city • Brain s*mula*on (non-‐invasive-‐ or invasive) – Non-‐invasive – transcranial magne*c s*mula*on and transcranial direct current s*mula*on – Invasive – deep brain s*mula*on www.pyschscenehub.com • Physical training and exercise www.medium.com • Cogni*ve training • Neuropharmalogical interven*ons Therapies to Promote Neuroplas*city Transcranial Magne4c S4mula4on Nowak et al. 2009 TMS is used to ac4vate lesion side and suppress inhibi4on from unaﬀected side Therapies to Promote Neuroplas*city Transcranial Direct Current S4mula4on www.neuromtl.com Therapies to Promote Neuroplas*city Deep Brain S4mula4on van Hartevelt et al. (2014) Long-‐term DBS can aﬀect structural and func4onal connec4vity Therapies to Promote Neuroplas*city Physical Training and Exercise Sawaki et al. (2008) Constraint therapy for upper limb has been associated with enlarged motor cortex map Therapies to Promote Neuroplas*city Neuropharmalogical interven4ons Asadollahi et al. (2018) Compared with placebo groups, pa4ents on citalopram or ﬂuoxe4ne had signiﬁcant increases in Fugl-‐Meyer Motor Scale Future Direc*ons and Clinical Ques*ons • Improved means to assess neuroplas*city in humans • Op*mal therapy parameters and pa*ent popula*on • Biomarkers for predic*ng and monitoring response to treatment • Combina*on therapies – E.g. Brain s*mula*on with peripheral nerve s*mula*on (neurons that ﬁre together, wire together) – E.g. physical training + stem cell therapy • Comprehensive understanding of all levels of plas*city and bener animal disease models • Bener understanding of how age and cri*cal periods inﬂuence circuit development Neuroplas8city Can be facilitated in disease via therapy • TMS • TDCS • Exercise • Pharmalogically Occurs within and between neurons Has wide-‐scale applica8ons in disease and healthy states Depends on -‐ Age (youth) -‐ Environment -‐ Ac8vity Will occur diﬀerently for each individual -‐ Speed -‐ Possibili8es of change Is always occurring www.quantamagazine.org Is not always a good thing Is a developing ﬁeld

Neuroplasticity Overview PDF

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