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In more advanced forms of animal life, reflexive motion is based on the transmission of impulses from a receptor through an afferent neuron and ganglion cell to motor neurons and muscles. This arrangement is found in the reflex arc of higher animals, including humans, in whom the spinal cord has further developed into a central regulating mechanism. Superimposed on these reflex circuits, the brain is concerned with the initiation and control of movement and the integration of complex motions.
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Control of Movement in Humans
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The motor system in humans controls a complex neuromuscular network. Commands must be sent to many muscles, and multiple ipsilateral and contralateral joints must be stabilized. The motor system includes cortical and subcortical areas of gray matter; the corticobulbar, corticospinal, corticopontine, rubrospinal, reticulospinal, vestibulospinal, and tectospinal descending tracts; gray matter of the spinal cord; efferent nerves; and the cerebellum and basal ganglia (Figs 13–1 and 13–2). Feedback from sensory systems and cerebellar afferents further influences the motor system.
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Movement is organized in increasingly complex and hierarchical levels.
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Reflexes are controlled at the spinal or higher levels.
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Stereotypic repetitious movements, such as walking or swimming, are governed by neural networks that include the spinal cord, brain stem, and cerebellum. Walking movements can be elicited in experimental animals after transection of the upper brain stem, probably as a result of the presence of central pattern generators, or local circuits of neurons that can trigger simple repetitive motor activities, in the lower brain stem or spinal cord.
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Specific, goal-directed movements are initiated at the level of the cerebral cortex.
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Corticospinal and Corticobulbar Tracts
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A. Origin and Composition
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The fibers of the corticospinal and corticobulbar tracts arise from the sensorimotor cortex around the central sulcus (see Fig 13–1); about 55% originate in the frontal lobe (areas 4 and 6), and about 35% arise from areas 3, 1, and 2 in the postcentral gyrus of the parietal lobe (see Fig 10–11). About 10% of the fibers originate in other frontal or parietal areas. The axons arising from the large pyramidal cells in layer V (Betz's cells) of area 4 contribute only about 5% of the fibers of the corticospinal tract and its pyramidal portion.
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