Definition: a somatotopically organized fiber bundle, relatively small in humans, arising from the red nucleus, immediately crossing in the ventral tegmental decussation, and descending near the lateral surface of the brainstem into the lateral funiculus of the spinal cord at the ventral border of the lateral pyramidal tract. It terminates in the zona intermedia of the spinal cord, where its distribution coincides with that of the lateral pyramidal tract; in contrast to the latter, it appears not to have direct connections with spinal motor neurons. Impulses conveyed by this tract indirectly increase flexor muscle tone.
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Examples: glitazone, GI cocktail, etc.