| Popis: |
The configuration of the map of the body surface in the primary somatosensory cortex (SI) has been shown to be dependent on the configuration of the sensory peripheral sheet. Not only does the periphery have an important role in the ontogenetic establishment of its representation in SI, but it also plays a crucial role in its maintenance during adulthood (for references see Van der Loos et al., this volume). This latter aspect has been first shown for the hand representation of the adult monkey (Merzenich et al., 1983). Since then, the peripheral somatosensory system has been modified in adults of a variety of species. The chosen paradigm in most of these cases has been either surgical removal of a part of the sensory sheet, or a partial denervation through division of a sensory nerve. The common observation in these experiments was that, after a brief period in which the zone of the cortex that corresponds with the removed or denervated part of the periphery was silent, neuronal responses could be evoked, in that same zone, from skin areas neighboring the damaged periphery. The first sign of plasticity1 in the functionally defined maps may occur within only a few hours after the division of a nerve (Calford & Tweedale, 1988). This very short time-lapse makes it unlikely that the cortical adaptations are due to axonal sprouting, peripheral or central. Therefore, it has been proposed that these adaptations may be due to alterations in the chemical environment of cortical neurons. The inhibitory neurotransmi-tter GABA (γ -aminobutyric acid) seemed to be a potential candidate since Dykes et al. (1984) have shown that the iontophoretic application of the GABA-antagonist bicuculline methiodide in the environment of neurons in the primary somatosensory cortex of the cat led to an increase of the receptive field of these neurons. Therefore, GABA does seem to be a good candidate to play a role in the expression of adult plasticity in SI. |
