Plastic changes in the adult brain can occur following injury. Although new axon sprouting is observed in the damaged central nervous system (CNS), it does not necessarily restore normal function, and some neural pathways do not produce sprouts. To improve medical therapy for CNS diseases and neural trauma, it is crucial to gain further knowledge about neural plasticity in the adult CNS. Current research aims to explore the potential of human embryonic stem cells in restoring CNS function [Fig. 10.10].
Neural plasticity in adults is exemplified by phantom limb sensation. When a limb is amputated, patients often experience sensations on the missing limb when other areas of the body are stimulated. Functional imaging studies indicate that this phenomenon occurs due to the spread of connections from the surrounding cortical territories into the cortical region that previously served the amputated limb.
Traditionally, corrective surgery for congenital cataracts in children was delayed until they were older and better able to cope with the stress of surgery. However, if correction is deferred beyond the "critical period," full recovery of function becomes unlikely. Similarly, children born with amblyopia, a condition characterized by strabismus (cross-eye) due to weakness in one of the extraocular muscles, tend to prefer using the unaffected eye. Early surgery is now commonly practiced in both cases to ensure correct sculpting of cortical circuitry through balanced input from both eyes.
Remapping can also occur after surgical amputation of the second and third digits of the hand. Prior to surgery, each digit was represented in distinct and somatotopically organized areas of the postcentral gyrus (SI cortex). Following surgery, the area representing the amputated digits is now mapped with an enlarged representation of the adjacent digits [Fig. 10.11]. Conversely, individuals born with syndactyly (fusion of two or more digits of the hand) have a single or mostly overlapping representation of these digits in the SI cortex. After corrective surgery, the individual digits acquire distinct representations. Monkeys trained on a sensory discrimination task requiring daily use of their fingertips exhibited cortical differences after training. Their SI cortical territories of the fingertips expanded, and the number of cortically recorded receptive fields on the fingertips increased.
Plastic changes can also occur after injury to the brain in adults. Sprouting of new axons does occur in the damaged CNS; however, the sprouts do not necessarily restore normal function, and many neural pathways do not appear to produce sprouts. Additional knowledge concerning neural plasticity in the adult CNS is vital if medical therapy is to be improved for many diseases of the CNS and after neural trauma. Research is currently being conducted to explore •Fig. 10.10 Plasticity in the Visual Pathway as a Result of Sensory Deprivation During Development. Theoculardominancecolumnsaredemonstratedbyautoradiographyafterinjectionofaradioactivetracerintooneeye.Thetraceristransportedtothelateralgeniculatenucleusandthentransneurallytransportedtothestriatecortex.Thecortexislabeledinbandsthatalternatewithunlabeledbandswhoseinputisfromtheuninjectedeye.A, Normalpattern.B, Changedpatterninananimalraisedwithmonocularvisualdeprivation.Theinjectionwasmadeintothenondeprivedeye,andtheoculardominancecolumnsforthiseyewereclearlyexpanded.Otherexperimentsshowedthattheoculardominancecolumnsforthedeprivedeyecontracted.(A, FromHubelDH,WieselTN.Proc R Soc Lond B. 1977;198:1.B, FromLeVayS,etal.J Comp Neurol. 1980;191:1.) the potential of human embryonic stem cells for restoring CNS function.
Phantom limb sensation is an example of neural plasticity in adults. A patient whose limb has been amputated often perceives sensations on the missing limb when stimulated elsewhere on the body. Functional imaging studies suggest that this is a result of the spread of connections from the surrounding cortical territories into the cortical region that had served the amputated limb.
Itwastraditionalpolicytodelaycorrectivesurgeryforachildbornwithacongenitalcataractuntilthechildwasolderandmoreabletocopewiththestressofsurgery.However,ifthecorrectionisdeferreduntilafterthe“criticalperiod,”fullrecoveryoffunctionisunlikely.Similarly,childrenbornwithamblyopia, aconditioncharacterizedbystrabismus(crosseye)becauseofrelativeweaknessofoneoftheextraocularmuscles,tendtousetheunaffectedeyeinpreference.Inbothcases,earlysurgeryisnowcommonpracticesothatthecorticalcircuitrycanbecorrectlysculptedbybalancedinputfromthetwoeyes.
Such remapping can also occur after surgical amputation of the second and third digits of the hand. Before surgery, each of the digits was represented in discrete and somatotopically organized areas of the postcentral gyrus (SI cortex). After surgery, the area that represented the amputated digits is now mapped with an enlarged representation of the adjacent digits (
Fig. 10.11 ). Conversely, individuals born with syndactyly (fusion of two or more digits of the hand) have a single or mostly overlapping representation of these digits in the SI cortex. After corrective surgery, the independent digits come to have distinctive representations. Even more remarkable is that monkeys that were trained on a sensory discrimination task requiring repeated daily use of their fingertips showed cortical differences after training. Not only were the SI cortical territories of their fingertips larger than before training but also the number of cortically recorded receptive fields on the fingertips was likewise increased.