Highly reliable in a healthy person, the human nervous system has inherent modifiability, though in adulthood this attribute cannot approach that in invertebrates (moths and snails) or certain other vertebrates (teleosts and amphibians). In mammalian development, neural plasticity is striking. In continues postnatally. Abnormal visual experience at certain sensitive periods profoundly affects ocular dominance and orientation columns in the visual cortex. If an eye is closed at birth, ocular dominance columns for the other eye enlarge at the expense of adjacent blind eye columns, with thalamic fibers arriving in the cortex expanding terminal fields into them. If, shortly after birth, visual stimuli are restricted for a few weeks or even days to stripes of one orientation, cortical cells develop a response preference to lines of that orientation.
In humans, PET imaging studies of cortical blood flow show that tasks requiring tactile discrimination activate visual cortex in people blind at birth or having lost sight in childhood. This suggests that cortical connections reorganize after blindness: that afferent fibers to nearby cortical areas serving polymodal sensory integration usurp the bereft visual cortex. Such plasticity may explain the well-known tactile acuity of the blind.
In later development, neural plasticity operates on many levels, as in fine-tuning circuits to changing body dimensions. Depth perception is recalibrated as the skull enlarges and interpupillary distance increases. Even in adulthood, plasticity persists. Vilayanur Ramachandran has shown that a stroke with a cottonswab on the cheek of a young man who had accidentally lost his left arm led him to feel touch on his missing left hand. Later, the whole hand could be mapped on his face. The findings suggest that the deprived somatosensory cortical region for the hand becomes innervated by fibers from the adjacent face areas and that secondary input to a cortical neuron's broad receptive field becomes functional when primary input is lost.
After injury to the CNS, intact neurons form new terminals, by axon sprouting, to replace those of other neurons lost to trauma and thus reoccupy vacated synapses. Such reactive synaptogenesis, the clinically proven effectiveness of long-range regrowth of PNS axons, and the evident potential for axon regeneration in the CNS (as in teleosts and amphibia) hold promise for circuit reestablishment. But in mammals, these factors are thwarted by myelin debris, glial scarring, usurpation of sprouts, unresponsive injured neurons, and complex central connections. Developmental neuroscience now focuses on the cerebral cortex. The human nervous system appears to learn very rapidly by using preconstructed circuits and by locking neurons into specific types and functions after cell origin."
About that last paragraph suggesting that deliberate neurogenesis is difficult in mammals, check out this new blogpost Growing new neurons by Kevin McHenry at painonline.com:
"Wernig et al in Proc Natl Acad Sci U S A May (2008) have achieved a real breakthrough. They have been able to convert fibroblasts to neurons. These converted cells form into neurons, glia, and even dopaminergic cells. There has always been concern that converted cells might form tumors, but these scientists painstakingly separated the cells turned into neurons from pluripotential cells with fluorescent stains."
Seems like ordinary cells can be turned into neurons if they can be recoded, using appropriate transcription factors, "Oct4, Sox2, Klf4, and c-Myc"
Also, work by Peter Eriksson and Fred Gage showed that neurogenesis is intrinsic to the human brain, even in elderly people on the brink of death (see this history module, The Growth of New Neurons in the Adult Human Brain).
Neuroplasticity has been a favorite topic on this blog. It's starting to dawn on a few of us PTs that this is what "improved outcomes", be they pain reduction or increased function, strength etc, have always been all about. Here are some old posts with extensive links:
1. Neuroplasticity Dec 11/07
2. Learning Dec 12/07
3. History of neuroplasticity Dec 12/07
4. About mirror therapy Dec 16/07
5. The devil is in the details Dec 18/07
6. A few types of learning Dec 18/07
7. Cart ruts: More about UN-doing something Dec 29/07
8. It's all about movement Dec 30/07
9. And it's about brain parts: like Hippocampus Dec 30/07
10. Function only Jan 15/08
11. Smart Prosthetics, smart nerves, smart brains Feb 10/08