FIGURE 33a - Dorsolateral and Ventromedial Pathways

Intro

Figure 33a:	Fibers from Cerebral Cortex | Upper Level of the Medulla | Lower Level of the Medulla | Spinal Cord
Figure 33b:	Cerebral Hemisphere | Midbrain | Medulla and Cerebellum | Spinal Cord
Figure 33c:	Ventromedial Corticospinal Tract | Ventromedial Cortico-Brainstem-Spinal Tract

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Part 3: Multiple-Choice Self-Test

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4 - Spinal Cord

Axons of the upper motor neurons are conveyed in the corticospinal tract (dorsolateral pathway) of the pyramidal system. They descend to their destination within the lateral white matter of the spinal cord, where they branch off to synapse primarily with interneurons located within the ventral horn of the spinal segments where the lower motor neurons for the appropriate muscles are located. Overall, the neurons of the dorsolateral pathway control movements of peripheral parts of the body on the side opposite to the cerebral hemisphere of origin. Some dorsolateral neurons (upper motor neurons) synapse directly on the motor neurons (lower, alpha motor neurons) that control the independent movement of the fingers.

The axons of the ventromedial neurons from the cortex descend in the ventromedial areas of the spinal cord white matter. These axons branch diffusely to activate the interneurons located bilaterally along several segments of the spinal cord. Overall the neurons of the ventromedial pathway control bilateral movements of the body along the midline, such as bending, turning, standing up, sitting down, walking, and swimming.

Although each tract exerts a unique influence over movement, they must work in cooperation to execute most actions smoothly.

Lesions within the spinal cord or of the motor neurons themselves may result in the lower motor neuron syndrome (Parent, 1996). Polio is an example of a viral disease resulting in this syndrome. The lower motor neuron syndrome typically affects a small group of muscles and is characterized by muscular fasciculation (small involuntary contractions of a number of muscle fibers due to spontaneous discharge of motor neurons) and atrophy (muscle loss or wasting). Hypotonia or decreased muscle tone is also associated with the lower motor neuron syndrome and is probably due to dysfunction of the gamma motor neurons.

Parent, A. (1996). Carpenter's human neuroanatomy (9th ed.). London: Williams & Wilkins.