Motor & Sensory Projections
Major Motor & Sensory Projections
Overview
- Consolidated view of the major descending (motor) and ascending (sensory) pathways from the cerebrum through the brainstem into the spinal cord.
Major pathway projections:
Motor
- Corticospinal tract (CST)
- Corticobulbar (aka corticonuclear) tract
Sensory
- Posterior column/medial lemniscus
- Anterolateral system (spinothalamic) tract
- Trigeminothalamic tract
Motor Pathways
Anatomical Structures
Begin with the motor pathways.
- Label the superior/inferior axes.
- Draw a coronal view of the right brain.
- Then, the brainstem.
- The upper cervical spinal cord.
- And the lumbar cord.
Innervation
Motor fibers:
- The corticospinal tract fibers innervate the body via spinal motor neurons.
- The lateral CST innervates distal musculature for fine motor movements
- The anterior CST innervates proximal musculature for gross motor movements.
- The corticobulbar fibers (aka corticonuclear fibers) innervate the face via the CNs.
Projections
- Motor fibers consolidate at the internal capsule, deep within the cerebrum, before entering the ipsilateral cerebral peduncle in the midbrain.
- Each fiber group twists and descends through the subcortical white matter.
- Facial fibers [RED] emerge from the lateral convexity, descend medially and, generally, decussate to synapse on different cranial nerve nuclei throughout their descent.
- The leg fibers [GREEN] emerge paracentrally, descend laterally through the brainstem into the ipsilateral medullary pyramid.
- The arm fibers [BLUE] emerge from the upper convexity, descend in between the facial and leg fibers, medial to the leg fibers through the brainstem into the ipsilateral medullary pyramid.
- They decussate at the cervicomedullary junction.
Key Anatomical Landmarks
- Within the subcortical white matter/internal capsule – here the leg fibers switch medial/lateral position with the arm fibers.
- Within the medullary pyramid – again, here, the leg fibers switch medial/lateral position with the arm fibers.
Decussation
- As the lateral corticospinal tracts decussate and descend through the medullo-cervical junction, they shift posterolaterally to enter the lateral funiculus of the cervical spinal cord.
- The arms remain medial to the legs.
- 75 to 90% percent of the corticospinal fibers undergo the aforementioned decussation as the lateral CST fibers due to their position in the lateral spinal cord.
- The remaining arm fibers travel ipsilaterally through the anteromedial spinal cord as the anterior CST remaining uncrossed until they decussate in the cervical spinal cord.
- The leg fibers do the same within the lumbar cord.
- They descend the spinal cord through the anterior funiculus in axial view, as well.
Sensory Pathways
Anatomical Structures
Now, the sensory pathways.
- Again, draw a coronal view of the right brain.
- Then, the brainstem.
- The upper cervical spinal cord.
- And the lumbar cord.
Innervation
There are three main somatosensory pathways we'll address.
- The posterior column/ medial lemniscus carries input from the sensory fibers from the body, which transmits vibration and joint position sensory information along with select other large fiber modalities, including two-point discrimination.
- The anterolateral system (which, notably comprises the spinothalamic tract), which carries input from the small sensory fibers from the body, which transmit pain, itch, and thermal sensory information.
- The trigeminothalamic tract, which projects sensory information from the face (CN 5).
Projections
- Indicate the ventroposterior thalamus, because all three pathways ascend the brainstem separately and then bundle within the thalamus before they project to the somatosensory cortex.
Spinothalamic Tract
- The position of the spinothalamic tract is largely unchanged throughout its spinal cord and brainstem ascent, so start with it.
- Show leg fibers [GREEN] synapse within the lumbar spinal cord and then decussate and ascend the anterolateral cord and brainstem to the thalamus.
- Then, show arm fibers [BLUE] synapse within the cervical spinal cord, and, also decussate and ascend the anterolateral cord and brainstem to the thalamus.
Posterior Column/Medial Lemniscus
- Next, let's draw the posterior column/medial lemniscus tract as it enters the brainstem.
- Show that the arm fibers synapse laterally, and the leg fibers synapse medially.
- Show that the arm fibers ascend the brainstem medial to the leg fibers as the medial lemniscus to the thalamus.
- Mark where the fibers switch medial/lateral positions.
Decussation
- Now, draw a section of the medulla to highlight this switch, which occurs in the internal arcuate decussation.
- Show that the leg fibers emerge from the gracile nucleus, medially, and pass anteriorly and then laterally.
- Indicate that the arm fibers emerge from the cuneate nucleus, laterally, and pass posterior to the legs and then medially.
- See that the arm fibers ascend medial to the leg fibers.
Trigeminothalamic Tract
- Now, show the trigeminothalamic fibers decussate within the pons and ascend the brainstem, medially, to the thalamus.
- We simplify the originations of the trigeminothalamic tracts for our purposes, here.
- Next, specify that within the thalamus, the facial fibers [RED] project to the ventroposterior medial nucleus (VPM) and the body fibers (arms and legs) project to the ventroposterior lateral nucleus (VPL).
Thalamoccortical Projections
- Finally, draw the twisting thalamocortical sensory projections to the cerebral cortex.
- In their ascent, show that the sensory fibers again reverse their orientation: the leg fibers project medially to terminate in the posterior paracentral gyrus.
- The arm fibers project lateral to the legs and terminate in the upper convexity of the postcentral gyrus.
- The facial fibers project lateral to both of them and terminate in the lower lateral postcentral gyrus.
Kinesthetic Activities
Motor
- We begin in the cerebral cortex with our hand turned palm towards us and our arm across our body.
- To demonstrate the twist and bundling of the motor fibers as they descend through the subcortical white matter and into the internal capsule, lower your hand and turn it over (palm away from you) as you bring your fingers together.
- Your pinky (the arm fibers) should be medial to your thumb (the leg fibers).
- To demonstrate the motor fiber descent through the brainstem, simply continue to drop your hand.
- Now, we need to demonstrate the pyramidal decussation at the medullo-cervical junction and the second twist of the corticospinal fibers.
- This rotation is important because it keeps the leg fibers lateral to the arm fibers when the corticospinal tract crosses midline in the upper cervical cord.
- For this step, bring your hand across midline and turn your palm back towards you so that your thumb is lateral and your little finger is medial when your hand crosses midline.
Sensory
- Place your right hand in front of you, palm down, with your fingers pointing to your right.
- Your thumb represents the leg fibers and your little finger represents the arm fibers.
- To demonstrate the decussation of the internal arcuate fibers, move your hand across midline and bring it in towards you.
- In the process, rotate your hand into anterior–posterior orientation with your little finger behind your thumb: arm fibers posterior to leg fibers.
- Next, to demonstrate the orientation reversal that occurs during the medial lemniscus ascent to the thalamus, rotate your hand back into medial–lateral orientation (the little finger [the arm] is medial and the thumb [the leg] is lateral) and simultaneously raise it.
- Then, to demonstrate the thalamocortical projection to the somatosensory cortex, twist your hand over so that the thumb (the leg) is medial and the little finger (the arm) is lateral.