Cortical Plasticity

In the past 10 years, many observations have indicated that the cerebral cortex in the adult is not as hardwired as previously thought. Although the concept of plasticity was associated with the developing brain, it was not thought to be possible in the mature central nervous system. It has been demonstrated that a major change in sensory input can result in a reorganization of the cerebral cortex. Either an increase in afferent activation or a deficit in sensory input can cause substantial changes in cortical representation. For example, in blind subjects who rely extensively on Braille for reading, it has been shown that cortical representation for the digits that are utilized can be substantially increased. Deafferentation due to amputation of a limb or digit or severing of afferent nerves leads to an invasion of the dennervated cortical region by an adjacent intact region. For example, amputation of a digit results in encroachment of the cortical region previously representating the digit by the areas repre-sentating adjacent digits. In addition, the smaller region that represents the amputated digit has a greater degree of representation by the remaining stump and a much reduced area in the region previously representing the distal skin of the digit. The extent of the invasion can be quite substantial. Deafferentation of an upper limb by severing the dorsal roots results in an invasion of the region representing the face by as much as 15-20 mm.

The immediate response in the cortex is an expansion of representation from an adjacent region into the region previously serving the deafferented limb or digit. At a later stage, the topography in the region is reorganized based on the extent of use or level of afferent input that is being received. The mechanisms responsible for this cortical plasticity are thought to be the same for both the expansion of a cortical region in response to an increase in peripheral input and the invasion of a cortical area following the deafferenta-tion of a peripheral input. In the case of the deaf-ferentation of a digit or limb, there are two distinct phases. For several weeks, the region of the cortex for representation of the deafferented limb or digit becomes totally silent. At a later stage, 6 months or longer, the invasion by the adjacent region takes place.

Currently, there are two proposed mechanisms for how this large amount of cortical plasticity can occur. First, it is known that connections from adjacent regions already exist. These residual connections are present but are not responsive to afferent input. When the cortical region loses its primary input these silent connections begin to respond to afferent input. This causes changes in neurotransmitters levels and the numbers of neurotransmitter receptors, which in turn results in an increase in the synaptic representation of the alternative region. This increase in synaptic representation may even lead to sprouting of nerve terminals resulting in an even greater degree of spreading of the cortical reorganization. A mechanism restricted to the cortex is supported by evidence that local microstimulation in the cortex is sufficient to produce an increase in cortical representation similar to the effect of an enhancement of peripheral afferent input. However, this may not account for the complete extent of cortical reorganization that occurs. There is good evidence that a great deal of reorganization takes place at subcortical levels. It is postulated that small amounts of reorganization at thalamic and brain stem levels will be amplified at the level of the cerebral cortex.

Conquering Fear In The 21th Century

Conquering Fear In The 21th Century

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