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| Massimo Matelli |
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Istituto di Fisiologia Umana, Via Volturno 39, 43100 Parma, Italy
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Presentation: |
| 2002-10-10, 11:30-12:00 |
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| Motor cortex. |
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The agranular frontal cortex is constituted of a mosaic of at least seven distinct areas (F1-F7). The definition of the various motor areas is mainly based on a combination of cytoarchitectonic histochemical, immunohistochemical, and neurochemical criteria, but is also strongly corroborated by hodological and physiological data:
Tract tracing experiments showed that each frontal motor area has a specific pattern of anatomical connections. However, if the general pattern of connectivity is considered, it becomes obvious that the various frontal motor areas can be grouped into two major classes. The caudal areas (F1, F2, F3, F4, and F5) form the first class. Their unifying feature is that these areas receive a primary cortical input from the parietal lobe and, therefore, this class was referred to as parieto-dependent motor areas. The rostral areas F6 and F7 form the second class. Their primary cortical input originates from the prefrontal cortex and, therefore, this class was referred to as prefronto-dependent motor areas.
The organization of the corticospinal projection is in agreement with the subdivision of the motor areas into two classes. The origin of the corticospinal tract, when compared with the subdivision of the agranular frontal cortex, show that corticospinal projections originate from all the caudal motor areas (F1, F2, F3, F4 and F5). These same areas send connections also to F1. These data indicate that, although F1 is the only motor area provided with a direct access to the motor neuron pools, all the caudal areas can be involved in movement execution, either directly, through their independent access to the spinal cord, or indirectly via F1. In contrast, the prefronto-dependent motor areas (F6 and F7) do not project to the spinal cord and are not connected with F1. Therefore, prefronto-dependent motor areas cannot control movement directly. They may control it indirectly through their subcortical relays or their connections with the parieto-dependent motor areas rostral to F1.
Recent studies on the organization of the parietofrontal connections indicate that motor and parietal areas are connected with each other in a very specific way. Within the general framework of parietofrontal connections it is possible to identify a series of largely segregated anatomical circuits formed by parietal and motor areas linked by predominant connections. The functional correlate of this anatomical organization is that each of these circuits appears to be dedicated to a particular aspect of sensory-motor transformation. It has been proposed that these parietofrontal circuits, and not the frontal motor areas in isolation, should be considered the functional units of the cortical motor system.
From these anatomic considerations, clearly emerges that parieto- and prefronto-dependent motor areas have a quite different functional role in motor control. Parieto-dependent areas, together with parietal areas, are involved in a parallel elaboration of sensory information for the transformation of it into potential motor actions. Prefronto-dependent areas appear to play a high hierarchical role in the control of body or eye movements. These areas convey inputs from the prefrontal and cingulate cortex to the parieto-dependent motor areas with an emphasis on motivation, long term plans, and memories of past actions. On the basis of this information, the potential motor actions could be either implemented, or could remain as they are, that is that may be either cancelled or implemented later, when other contingencies allow it. |
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