Major Tracts
Major Tracts of the Spinal Cord
Overview
- Here, we will draw the major tracts of the spinal cord, specifically the posterior column pathway, the anterolateral system (which includes, most notably, the spinothalamic tract), and the lateral corticospinal tract.
- First, draw an axial cross-section of the ovoid outline through the spinal cord.
- Then, draw the gray matter horns.
- Divide the posterior column in half and then include the central canal.
- Next, label the anatomic right and anatomic left sides of the page.
- Now, on the right, draw a mixed spinal nerve.
- Then, on the left, label the primary motor cortex and label the primary sensory cortex.
- Include a table so we can keep track of key elements of the major tracts.
Posterior column pathway
Posterior column pathway Synapses
Posterior funiculus
- Shade the posterior column in the posterior funiculus of the spinal cord.
- To begin, add a dorsal root ganglion to the mixed spinal nerve and indicate that proximal to the dorsal root ganglion, the posterior division of the nerve becomes the posterior nerve root — we address nerve root anatomy elsewhere.
1st order neuron: Dorsal root ganglion
- Show that the first neuron we come to along the path of the posterior column pathway (the first-order neuron), which we abbreviate as 1 PCP, lies in the dorsal root ganglion.
2nd order neuron: Medulla - gracile and cuneate nuclei
- Next, show that the second-order sensory neuron lies in the gracile and cuneate nuclei in the medulla.
3rd order neuron: Thalamus (contralateral)
- Then, show that the third-order sensory neuron lies in the contralateral thalamus (in the ventroposterior lateral nucleus).
Posterior column pathway Pathway
Spinal cord projection (posterior column)
- Indicate that the central process enters and ascends the posterior column without forming a synapse in the spinal cord and that it instead first synapses in the gracile and cuneate nuclei in the medulla.
Brainstem projection (internal arcuate fasciculus and medial lemniscus)
- Next, in midline, label the internal arcuate fasciculus.
- Then, show that the gracile and cuneate nuclei send decussating fibers across the medulla via the internal arcuate fasciculus to synapse in the thalamus.
- These fibers ascend the brainstem via the medial lemniscus to complete the brainstem course of the posterior column pathway.
Thalamic projection
- Finally, indicate that the thalamus projects to the sensory cortex.
Ipsilateral
Ipsilateral to side of origin
- Note in our table that the posterior column pathway ascends the spinal cord ipsilateral to its side of origin.
Anterolateral System (spinothalamic tract)
Anterolateral System Synapses
Anterior funciculus
- Shade the anterolateral system in the anterior funiculus of the spinal cord.
1st order neuron: Dorsal root ganglion
- Indicate that its first order neuron lies within the dorsal root ganglion.
2nd order neuron: Dorsal horn of spinal cord
- Its second-order neuron lies within the dorsal horn of the spinal cord.
3rd order neuron: Thalamus (contralateral)
- Its third order neuron lies within the contralateral thalamus (in the ventroposterior lateral nucleus).
Anterolateral System Pathway
Lissauer's tract
- Next, draw Lissauer's tract in between the dorsal pial surface and the dorsal horns because the anterolateral system fibers ascend or descend a variable number of spinal levels within Lissauer's tract prior to synpasing in the dorsal horn.
Spinal cord entry: dorsal horn
- Then, show that the anterolateral system projects centrally from the dorsal root ganglion into the dorsal horn.
Decussation: ventral commissure
- Now, draw the ventral commissure between the anterior gray matter horns.
- Show that at or near their level of entry into the spinal cord, the anterolateral system fibers decussate via the ventral commissure and bundle in the anterolateral spinal cord, where they ascend the spinal cord and brainstem to synapse in the thalamus.
Thalamic projections
- Finally, show that the thalamus projects to the sensory cortex.
Contralateral
**Contralateral to side of origin
- Note in our table that the anterolateral system ascends the spinal cord contralateral to its side of origin.
Corticospinal Tract
Corticospinal Tract synapses
Lateral funiculus
- Shade the lateral corticospinal tract in the lateral funiculus of the spinal cord.
1st order neuron: Motor cortex (cerebrum)
- Indicate that the prototypical first-order neuron lies in the motor cortex (precentral gyrus).
- 1st order corticospinal tract neurons do lie in other cortical areas, as well, such as in the premotor and sensory cortices.
2nd order neuron: anterior gray mattern horn
- Next, draw a spinal motor neuron within the contralateral anterior gray matter horn of the spinal cord label it as the second-order neuron.
- Now, label the cervicomedullary junction.
Corticospinal Tract Pathway
Brainstem projection
- Show that lateral corticospinal fibers descend from the motor cortex through the ipsilateral brainstem
Cervicomedullary junction
- Decussate within the medullary pyramids at the cervicomedullary junction.
Spinal cord
- And then descend through the spinal cord in the lateral corticospinal tract to synapse in spinal motor neurons.
Anterior nerve root
- Then, show that the spinal motor neuron projects motor fibers via the anterior nerve root to form the motor division of the peripheral nerve.
Contralateral
**Contralateral to side of origin
- Note in our table that the lateral corticospinal tract system descends the spinal cord contralateral to its side of origin.