Motor Pathways - Introduction

Tutorial 33: Motor Pathways

Part 1: Image-Mapped Tutorial
Part 2: Matching Self-Test: 33a | 33b | 33c
Part 3: Multiple-Choice Self-Test

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As described in Tutorial 32, the motor system executes control over the skeletal muscles of the body via several major tracts. Tutorial 33 will focus on the particulars of several of these tracts, discussing both the unique functions of each and the structures involved in these functions.

The pyramidal and extrapyramidal motor systems utilize separate tracts. The pyramidal system conveys information via the corticospinal tract of the dorsolateral pathway. This tract conveys signals from the primary motor cortex to the alpha motor neurons found in the medulla and the spinal cord. The pyramidal, corticospinal tract crosses in the "pyramids" of the medulla, to control discrete voluntary movement on the contralateral side of the body. The extrapyramidal system includes all pathways connecting the motor structures not included in the pyramidal system, including the basal ganglia, red nucleus, reticular formation, and the vestibular nuclei. The descending dorsolateral and ventromedial pathways include both pyramidal and extrapyramidal elements and are each composed of two major tracts, one which descends directly to the spinal cord and another that synapses for processing in the brain stem prior to acting on the spinal cord motor neurons or interneurons. The ventromedial pathway differs from the dorsolateral pathway, as it is much more diffuse in both its origin and distribution, affecting interneurons on both sides of the spinal cord along several segments. In addition, the ventromedial pathway affects the proximal muscles of the trunk and shoulder, while the dorsolateral pathway affects the distal muscles of the hand and fingers. The dorsolateral, corticospinal tract is synonymous with the pyramidal tract or system, and is the only pathway of control for fine finger movements. The dorsolateral, corticorubrospinal tract originates in motor cortex, but synapses in the red nucleus of the extrapyramidal system prior to controlling movement of the forearms, hands and feet. Both tracts of the dorsolateral pathway are involved in the control of reaching movements.

Figure 33a illustrates the two main descending pathways of the motor system, the dorsolateral pathway (including the pyramidal, corticospinal tract and the extrapyramidal, corticorubrospinal tract) and the ventromedial pathway. Figure 33b illustrates the multiple extrapyramidal pathways underlying the intention, initiation, and coordination of movement. Finally, Figure 33c illustrates the two divisions of the ventromedial pathway (the ventromedial corticospinal tract and the ventromedial cortico-brainstem-spinal tract). The first descends directly to the ventral horn of the spinal cord, whereas the second descends indirectly to the motor nuclei of the cranial nerves and to the ventral horns of the spinal cord via the superior colliculus, reticular formation, and the vestibular nuclei. The ventromedial tracts control movement of the body trunk, maintain trunk posture, as well as whole-body movements such as walking and climbing.

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Tutorial 33: Motor Pathways

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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