Representational Plasticity

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Cortical sensory and motor neurons are not permanently fixed in the way they subserve sensing and movement. On the contrary, these neurons quickly adapt to changing demands as new sensory and motor associations are experienced. In the adult and developing animal (see Experimental Case Studies 1-3 and 1-4) and in humans (see Chapter 3), the topographical maps of sensory and motor neuronal representations are capable of rapid and long-term physiologic and structural reorganiza-tion.33,218,219 Electrophysiologic and metabolic imaging experiments reveal changes in the cortical maps for sensorimotor,220 visual retino-topic,221 auditory tonotopic,222 and other representations induced by experience. This mutability is a ubiquitous property of adult cortical output and receptive fields.

Table 1-3 summarizes the sequence of important steps that lead to a memory trace for a skill. Consideration of these steps may give clinicians insight into the training and other input conditions that optimize remodeling and, in turn, improve motor control and higher cognitive activities during rehabilitation. How these modifications of functional pathways are modulated and how they may be manipulated to enhance and not to inhibit functional recovery is one of the most impor-

Table 1-3. General Steps for Learning Motor Skills

1. Goal-oriented, skilled movement task

2. Behavior reinforced by a learning paradigm

3. Repetitive practice under varying conditions

4. Sensory feedforward drive of motor cortex

5. Sensorimotor feedback

6. Neuronal representational map for movement expands by unmasking latent synapses

7. Increased synaptic efficacy of connections with related sensorimotor cortices and spinal motor pools

8. Dendritic branching and growth of spines

9. Lasting cortical and subcortical representational map for the skilled movement tant applications of basic neuroscience to neu-rorestoration.

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