Descending Spinal Cord Pathways
Descending Spinal Cord Pathways
Essentials
General Diagram
Here, we will draw the descending spinal cord pathways in axial cross-section.
- Draw one half of an axial cross-section of the spinal cord.
Key Terms
- Tract
- Origin
- Termination
- Function
Descending Pathways
- Draw the lateral corticospinal tract in the lateral funiculus.
- The anterior corticospinal tract in the antero-medial spinal cord.
Lateral corticospinal tract
- Indicate that the lateral corticospinal tract fibers originate in the motor cortices (most notably), terminate in contralateral spinal motor neurons
- They innervate distal musculature for fine motor movements.
- Label the somatotopic organization of the lateral corticospinal tract.
- Indicate that the medial aspect of the lateral corticospinal tract carries the arm fibers.
- Indicate that the lateral aspect carries the leg fibers.
Anterior corticospinal tract
- Next, indicate that the anterior corticospinal tract fibers originate in the motor cortices (most notably), terminate in contralateral spinal motor neurons.
- They innervate proximal musculature for gross motor movements.
- As discussed elsewhere, although the anterior corticospinal tract fibers fibers travel ipsilaterally through the anteromedial spinal cord as the anterior corticospinal tract and remain uncrossed as they leave the brainstem, when they reach the level of their target neuron in the spinal cord, they cross within the anterior commissure to the opposite side.
Hypothalamospinal tract
- Then, label the hypothalamospinal tract alongside the lateral corticospinal tract in the lateral funiculus.
- Indicate that this tract originates in the hypothalamus, descends ipsilaterally, and carries hypothalamic control of autonomic function.
Somatotopy of the anterior gray matter horns
Next, in regards to the somatotopy of the anterior gray matter horns, show that:
- Posterior nuclei innervate flexor muscles (eg, biceps brachii, which flexes the arm),
- Anterior nuclei innervate extensor muscles (eg the triceps, which extends the arm),
- Lateral nuclei innervate distal muscles (eg, the dorsal interossei which produce finger spread)
- Medial nuclei innervate proximal muscles (eg, the deltoids, which raise the shoulder).
Note that the somatotopic organization of the gray matter parallels the positions of the functionally-related white matter pathways.
- The lateral corticospinal tract lies laterally and innervates gray matter spinal motor neurons that lie laterally and innervate distal muscles.
- The anterior corticospinal tract lies medially and innervates gray matter spinal motor neurons that lie medially and innervate proximal muscles.
rubrospinal, reticulo- and vestibulospinal tracts
- Now let's address the functional role of the rubrospinal, reticulo- and vestibulospinal tracts; we address their anatomy in the advanced section of this tutorial.
Indicate in the bottom of our diagram that in general:
- The rubrospinal tract originates from the midbrain and innervates upper extremity flexors, whereas
- The reticulospinal and vestibulospinal tracts emerge from below the rubrospinal tract in the pons and medulla and innervate extensor musculature.
Frontal Release Signs/Decorticate & Decerebrate Posturing
Now, section off the right-hand side of the page.
Normal Innervation
- First, show a scenario wherein the cerebrum inhibits the rubrospinal tract and also the reticulo-/vestibulospinal tracts.
- Indicate that this is the normal scenario.
Frontal Release Signs
- In global cerebral injury (or in neurodegenerative conditions), there are frontal release signs.
Decorticate Posturing
- Next, indicate that when this cerebral inhibition is lost, these three tracts are left unchecked.
- The rubropsinal tract produces upper extremity flexion.
- The reticulo- and vestibulospinal tracts produce lower extremity extension.
- In the appropriate clinical setting, this response is referred to as decorticate posturing.
Decerebrate Posturing
- Now, indicate that when there is a lesion at or below the midbrain, that cuts off the cerebral suppression and interrupts the rubrospinal tract, the reticulo- and vestibulospinal tracts are left unchecked.
- The reticulo- and vestibulospinal tracts produce neck and limb extension.
- In the appropriate clinical setting, this response is referred to as decerebrate posturing.
END OF ESSENTIAL/START OF ADVANCED
Rubrospinal Tract
- Now, label the rubrospinal tract near to the lateral corticospinal tract.
- Indicate that it originates from the red nucleus in the midbrain, crosses in the ventral tegmental area of the brainstem, and innervates the upper cervical spinal cord to produce arm flexion.
Tectospinal Tract
- Next, label the tectospinal tract in the anterior-medial spinal cord.
- Indicate that it originates in the superior colliculus of the midbrain, decussates in the dorsal midbrain tegmentum, and innervates the upper cervical spinal segments to activate neck movements and produce contralateral head turn.
Reticulospinal/Vestibulospinal tracts
Now, let's address the reticulospinal tracts and vestibulospinal tracts, both of which maintain posture through the activation of antigravity muscles.
- In our axial cross-section, adjacent to the anterior corticospinal tract, draw the medial reticulospinal tract and adjacent to the tectospinal tract, draw the medial vestibulospinal tract.
- Then, farther laterally, in the lateral funiculus, draw the lateral reticulospinal tract and the lateral vestibulospinal tract.
- Next, indicate that the medial (aka rostral) reticulospinal tract originates in the medial zone of the pontine reticular formation and courses predominantly ipsilaterally to activate the axial and proximal limb extensors.
- Then, indicate that the lateral (aka caudal) reticulospinal tract originates in the medial zone of the medullary reticular formation and courses bilaterally to inhibit the axial and proximal limb extensors (and to a lesser degree it also excites axial and proximal limb flexors).
Note that intertextual discrepancy exists regarding the laterality of the course of the lateral reticulospinal tract.
- Next, indicate that the medial vestibulospinal tract arises from the medial and spinal vestibular nuclei (driven by the semicircular canals) and acts on the bilateral cervical spinal cord to excite neck extensor musculature.
- Now, indicate that the lateral vestibulospinal tract arises from the lateral vestibular nucleus (driven by the otolith organ) to activate ipsilateral paravertebral and proximal limb extensor muscles.