Experimental Case Studies

Motor Map Plasticity

Motor maps also show functional reorganization. Merzenich and colleagues mapped the motor cortical zones that represent the digital, hand, wrist, elbow, and shoulder movements of monkeys before and after 11 hours of behavioral training.383 Training involved having the primates retrieve food pellets from small food wells without using their thumbs. The Ml territories evoked by digital movements required for the task increased significantly (Fig. 1-9). New movement relationships emerged in the map between digit and wrist extensor activity that were inherent to success at the task. The cortical surface over which this increased coupling of neurons evolved corresponded to a network as great or greater than the spread of the axonal arbors of intracortical pyramidal cells.

Other experiments by this group support long held notions about the plasticity of the movement maps of the primary motor cortex.384 The results also extend the work of others who found that the cortex of primates is composed of multiple representations of distal forelimb movements. For example, Merzenich and colleagues studied each hand's representation for movement in Ml as squirrel monkeys performed an unpracticed task that required skilled use of the arm and digits. In the hemisphere contralateral to the preferred hand, they found representations, especially for digit flexion, wrist extension, and forearm supination, to be greater in number, cover a larger area, and show greater spatial complexity compared to the nondominant hand's cortical representations for the same task.385 The investigators concluded that the number of rerepresentations and the type of movements that overlap are quite variable between individuals and between the hemispheres of an individual. These differences derive in part from ontogenetic development and from experience, such as coactivating muscles or using them in a particular sequence in everyday activities.

Several neurotransmitters may participate in the changes in representational maps with experience and activity. A reduction in intracortical inhibitory pathways mediated by GABA permits the expression of new receptive fields.282 Cholinergic and N-methyl-D-aspartate (NMDA) receptor modulation is also required.386 These messengers apparently unmask preexisting synapses that had been functionally ineffective. For example, following deafferentation, tonic inhibition was diminished in rat and cat so-matosensory cortex and the responsiveness of neurons to acetylcholine increased. This uncovered new receptive fields and strengthened existing ones.387 Drugs used in clinical practice may alter these transmitters and receptors and affect plasticity.

Figure 1-9. Movement maps derived by recording from a grid placed over Brodmann area 4 of the macaque before and after the animal was trained in a food-retrieval task. The upper cortex shows territorial gains for the neuronal assemblies of the digits (black) and new evocations by combined digit and wrist movements (parallel lines) that were needed to learn the motor task. Source: Merzenich et al., 1990,395 with permission.

Figure 1-9. Movement maps derived by recording from a grid placed over Brodmann area 4 of the macaque before and after the animal was trained in a food-retrieval task. The upper cortex shows territorial gains for the neuronal assemblies of the digits (black) and new evocations by combined digit and wrist movements (parallel lines) that were needed to learn the motor task. Source: Merzenich et al., 1990,395 with permission.

unconscious knowledge of the task. When they achieved explicit or verbalizable knowledge of how to do the task, the map of cortical output returned to baseline. These examples of rapid motor learning with enlarging maps suggest that synaptic connections are rapidly unmasked and the motoneurons form new movement associations. Once the procedure is overlearned and automatic, the same level of synaptic excitation for learning is no longer needed. Indeed, the internal knowledge about automatic performance may be stored elsewhere, perhaps in the basal ganglia and cerebellar networks.203

Cortical and subcortical networks, of course, also reveal adaptations with motor learning. For example, subjects moved a robotic manip-ulandum against a changing force field to targets on a screen. Within 6 hours of completion of practice on this novel task, in which performance skills were unchanged, a PET study revealed that the brain had come to engage new areas to consolidate learning the skill.233 The internal model for the new skill shifted from an increase in activation of the visuomo-tor association cortex in BA 46, prior to practice, to an increase in rCBF in the contralateral posterior parietal and ipsilateral anterior cerebellar cortices. A reduction in activation was found in the bilateral middle frontal gyri in BA 46. These results and other studies reveal that the prefrontal cortex temporarily maintains arbitrary sensorimotor information, followed by long-term storage of an internal model for the skill.

The properties of neurons and their synapses are continuously shaped by use-dependent experience as a motor skill is learned. Bizzi and colleagues recorded from Ml neurons in monkeys as they performed the same reaching movements with a manipulandum in a force field to assess for plasticity during learning.234 Loads on the arm strongly modulated the activity of Ml neurons. Both muscle and movement representations were found in Ml. The investigators showed that both motor performance during the task and motor learning involved the same neuronal population. Apparently, single neurons change their activity as a new internal model for motor learning evolves. In parallel, the entire neuronal population reorganizes in relation to the direction of movements and to activation of muscles to transmit signals appropriate for the behavioral goal and motor performance. Thus, plasticity induced by practice supports both motor performance and motor learning. This association is critical to theories about how to enhance motor skills after a CNS or PNS injury.

Was this article helpful?

0 0

Post a comment