Part 1: Image-Mapped Tutorial
Part 2: Matching Self-Test:
33a |
33b |
33c
Part 3: Multiple-Choice Self-Test
The Ventromedial Cortico-Brainstem-Spinal Tract provides an indirect extrapyramidal route from the primary motor cortex to the ventral horn of the spinal cord via a complex network of brain structures. This network includes the tectum, vestibular nuclei, reticular formation, and the cranial nerve motor nuclei. Axons in this tract descending from this network of brainstem structures contain information from both hemispheres and travel downward in the ventromedial portion of the spinal cord. They eventually activate motor interneurons located bilaterally in the ventral horns along several segments of the spinal cord. The target muscles of this tract and its subcomponents are located in the body trunk and proximal region of the limbs (i.e. shoulders and thighs). The movements controlled by this projection include coordinated movements of the trunk and limbs, as needed in locomotion and in maintaining posture.
The four major structures that modulate the activity of neurons in the ventromedial cortico-brainstem-spinal tract are the tectum, vestibular nuclei, reticular formation, and motor nuclei of the cranial nerves controlling the facial musculature. The tectum of midbrain (superior and inferior colliculi) contributes information via the tectospinal tract regarding the spatial location of visual and auditory events. This information is used to generate accurate, orienting movements toward these events (e.g., head and trunk movements are coordinated with eye movements). The vestibular nuclei of the pons contribute information via the vestibulospinal tract regarding balance and body position from the inner ear. This information is used to accurately guide postural movements. The reticular formation nuclei are spread throughout the brainstem and contribute motor programs via the reticulospinal tract for common movements such as jumping, walking, sneezing, coughing, and respiration. Some nuclei of the reticular formation maintain muscle tone and posture by their effects on the gamma motor neurons involved in proprioception. In addition, the mesencehpalic locomotor region of the reticular formation is involved in the locomotion that results from alternating movement of the limbs. Finally, the motor nuclei of cranial nerves are affected via the corticobulbar pathway, modulating for example the movements of the face (CN VII), jaw (CN V), and thetongue (CN IX and X).
| Advanced |
Transection of the brain stem above the level of the vestibular nuclei of the medulla but below the red nucleus of the midbrain will often produce a decerebrate rigidity with an increase in tone of the extensor muscles (Parent, 1996). The vestibulospinal and reticulospinal tracts normally activate the alpha and gamma motor neurons that innervate the extensor muscles. With the loss of inhibition of this activation from higher centers, the stretch reflexes are hyperactive and muscle tone is increased.
| Reference |
Parent, A. (1996). Carpenter's human neuroanatomy (9th ed.). London: Williams & Wilkins.