3. Motor Cortex
Part 2 of 5
James Knierim, Ph.D.
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Cortical Afferents and Efferents
The motor cortex exerts its influence over muscles by a variety of descending routes (Figure 3.4). Some of the descending pathways reviewed in the last chapter can be influenced by motor cortex output. Thus, in addition to the direct cortical innervation of alpha motor neurons via the corticospinal tract, the following cortical efferent pathways influence the remaining descending tracts:
- the corticorubral tract allows cortex to modulate the rubrospinal tract
- the corticotectal tract allows cortex to modulate the tectospinal tract
- the corticoreticular tract allows cortex to modulate the reticulospinal tracts
Figure 3.4 |
Parallel pathways from the motor cortex allow the cortical motor areas to influence the processing of all descending motor tracts and side loops of the motor system. Mouse over the pathways for more information. |
The cortex can also influence the processing of the side loops of the motor hierarchy. The corticostriate tract innervates the caudate nucleus and putamen of the basal ganglia. The corticopontine tract and corticoolivary tract innervate important inputs to the cerebellum. Finally, cortical areas can influence other cortical areas, directly via corticocortical pathways and indirectly via the corticothalamic pathways (Figure 3.5). Most of these pathways are bi-directional. Thus, motor cortex receives input from other cortical areas, directly and indirectly through the thalamus, and it receives input from the cerebellum and basal ganglia, always through the thalamus.
Figure 3.5 |
Major connections of motor cortex. The cross-section on the left is a schematic version of an idealized brain section that contains the major structures of the motor system hierarchy for illustrative purposes; no actual brain section would contain all of these structures. Move the cursor over each box on the right to highlight the inputs (blue) and outputs (red) of each region. |
Motor Cortex Cytoarchitecture
Like all parts of the neocortex, the primary motor cortex is made of six layers (Figure 3.6). Unlike primary sensory areas, primary motor cortex is agranular cortex; that is, it does not have a cell-packed granular layer (layer 4). Instead, the most distinctive layer of primary motor cortex is its descending output layer (Layer 5), which contains the giant Betz cells. These pyramidal cells and other projection neurons of the primary motor cortex make up ~30% of the fibers in the corticospinal tract. The rest of the fibers come from the premotor cortex and the supplementary motor area (~30%), the somatosensory cortex (~30%), and the posterior parietal cortex (~10%).
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Pyramidal and non-pyramidal neurons in motor cortex. The cerebral cortex is organized into six layers. These layers contain different proportions of the two main classes of cortical neurons, pyramidal and non-pyramidal cells. Pyramidal cells send long axons down the spinal cord and are the major output neurons. They are abundant in layer 5. Non-pyramidal cells have axons which terminate locally. |
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Contact the author(s) at nba_course@uth.tmc.edu
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