Ascending and Descending Tract Courses

Question 1

List all ascending and descending tracts.

Answer 1

Ascending:

1 - Spinothalamic.

2 - Dorsal column-medial lemniscal (DCML).

3 - Spinocerebellar.

4 - Trigeminothalamic.

Descending:

1 - Corticospinal.

2 - Corticobulbar.

3 - Rubrospinal.

4 - Tectospinal.

5 - Vestibulospinal.

6 - Reticulospinal.

Question 2

Describe the course of the spinothalamic tract.

Answer 2

1 - The first order (sensory) neurones travel from the periphery and enter the spinal cord.

2 - They then synapse with second order neurones at the tip of the dorsal horn in the substantia gelatinosa:

• Axons carrying pain and temperature synapse and then decussate promptly within one segment of their origin, while those carrying touch and pressure may ascend for several segments before synapsing and crossing.

3 - The second order neurones form two distinct tracts that run alongside each other: the anterior spinothalamic tract (carrying crude touch and firm pressure information) and the lateral spinothalamic tract (carrying pain and temperature information).

4 - These tracts ascend the spinal cord to the brainstem, in which they run through the spinal lemniscus.

5 - Some fibres of the lateral spinothalamic tract carrying information relating to burning pain terminate in the reticular formation of the brainstem for widespread activation of the nervous system. Other fibres report to the limbic system about the nature of the stimulus.

6 - Both anterior and lateral tracts continue to ascend to the thalamus, where they synapse with third order neurones at the ventral posterolateral nucleus.

7 - The third order neurones pass through the internal capsule to synapse with the ipsilateral somatosensory cortex of the cortex.

Question 3

Describe the course of the dorsal column-medial lemniscal pathway.

Answer 3

1 - The first order (sensory) neurones travel from the periphery and enter the spinal cord.

2 - The first order (sensory) neurones then ascend the spinal cord in the dorsal column:

• If the information is from the upper limb (T6 and above), the first order neurones travel in the lateral part of the dorsal column known as the fasciculus cuneatus.
• If the information is from the lower limb (below T6), the first order neurones travel in the medial part of the dorsal column known as the fasciculus gracilis.

3 - First order (sensory) neurones travelling in the fasciculus cuneatus synapse in the nucleus cuneatus of the medulla oblongata. First order (sensory) neurones travelling in the fasciculus gracilis synapse in the nucleus gracilis of the medulla oblongata.

4 - Second order neurones decussate within the medulla oblongata.

5 - Second order neurones then ascend in the contralateral medial lemniscus to reach the thalamus, where they synapse with third order neurones at the ventral posterolateral nucleus.

6 - The third order neurones pass through the internal capsule to synapse with the ipsilateral somatosensory cortex of the cortex.

Question 4

Describe the course of the spinocerebellar tracts.

What is Clarke’s column / nucleus dorsalis?

Answer 4

1 - Primary proprioceptive fibres arise in:

• The lumbar cord (anterior spinocerebellar).
• The thoracic cord (dorsal spinocerebellar).
• The cervical cord (cuneocerebellar and rostral spinocerebellar).

2 - They then synapse with second order neurons in the dorsal horn of the spinal cord in all tracts other than the cuneocerebellar tract:

• Second order neurones of the cuneocerebellar tract arise in the lateral cuneate nucleus in the caudal medulla.
• Second order neurones of the dorsal spinocerebellar tract originate from a group of cells in lamina 7 of cord segments T1-L2, known as clarke’s column / nucleus dorsalis.

2 - The anterior spinocerebellar tract decussates and ascends contralaterally. The rest ascend ipsilaterally.

3 - The tracts ascend to the cerebellum:

• The anterior spinocerebellar tract ascends to the superior cerebellar peduncle in the pons, where it decussates again and exits to terminate in the cerebellum.
• All other tracts ascend to the inferior cerebellar peduncles in the rostral medulla oblongata, where they exit to terminate in the cerebellum.

Question 5

Describe the course of the trigeminothalamic tract.

Answer 5

• First-order afferents of the trigeminothalamic tract arise from the:

1 - Trigeminal nerve (CN V).

2 - Facial nerve (CN VII).

3 - Glossopharyngeal nerve (CN IX).

and

4 - Vagus nerve (CN X).

• These first order afferents terminate in either:

1 - The principal sensory nucleus of the dorsal pons.

or

2 - The spinal trigeminal nucleus of the medulla oblongata.

• The second-order afferents from the spinal trigeminal nucleus and the principal sensory nucleus decussate and join in the pons, completing the trigeminal lemniscus.
• The trigeminal lemniscus terminates in the ventral posteromedial nucleus of the thalamus.
• From the thalamus, third-order afferents project to the area of facial representation in the lower half of the somatosensory cortex in the postcentral gyrus.

Question 6

Describe the course of the corticospinal tract.

Answer 6

First part is common between corticospinal and corticobulbar tracts:

• Upper motoneurones originate from sensorimotor areas around the precentral gyrus, and are known as pyramidal (Betz) cells.
• Upper motoneurones leave the cerebrum through the corona radiata and internal capsule.
• They then enter the crus cerebri of the cerebral peduncles of the midbrain, where they begin to descend the brainstem.

The following parts are exclusive to the corticospinal tract:

• They then pass through the ventral portion of the pons to reach the medulla oblongata, where they form two prominent columns known as pyramids.
• In the medulla, the majority of fibres decussate and enter the contralateral lateral corticospinal tract, and the rest remain ipsilateral and enter the ventral corticospinal tract.
• The upper motoneurones of the lateral corticospinal tract descend and synapse with lower motoneurones innervating muscles of the limbs.
• The upper motoneurones of the ventral corticospinal tract descend, decussate at the level of synapse, and synapse with lower motoneurones innervating muscles of the trunk.

Question 7

Describe the course of the corticobulbar tract.

Answer 7

First part is common between corticospinal and corticobulbar tracts:

• Upper motoneurones originate from sensorimotor areas around the precentral gyrus, and are known as pyramidal (Betz) cells.
• Upper motoneurones leave the cerebrum through the corona radiata and internal capsule.
• They then enter the crus cerebri of the cerebral peduncles of the midbrain, where they begin to descend the brainstem.

The following parts are exclusive to the corticospinal tract:

• The corticobulbar axons leave the tract as it descends in the brainstem and terminate in the motor nuclei of the various cranial nerves.
• Most motoneurones of the corticobulbar tract end bilaterally.
• However, fibres of the hypoglossal nucleus and of the portion of the facial nucleus that supplies muscles of the lower face decussate and therefore end contralaterally.

Question 8

Describe the course of the rubrospinal tract.

Answer 8

• Upper motoneurones originate in the red nucleus of the tegmentum of the midbrain, and receive afferent input from the cerebellum and motor cortex.
• The upper motoneurones course ventromedially and decussate in the central tegmental decussation of the midbrain.
• They then descend to the spinal cord, where they synapse with lower motoneurones in a position slightly ventral to the lateral corticospinal tract.

Question 9

Describe the course of the tectospinal tract.

Answer 9

• Upper motoneurones originate in the superior colliculus of the midbrain, which receives visual afferents.
• The upper motoneurones course ventromedially around the periaqueductal grey area and decussate at the dorsal tegmental decussation of the midbrain.
• They then descend in the ventral funiculus of the spinal cord.
• They then terminate near the ventral median fissure in the upper cervical spinal cord.
• Here, they synapse with lower motoneurones innervating muscles of the head and neck.

Question 10

Describe the course of the vestibulospinal tract.

Answer 10

• Its origin is the medial vestibular nucleus in the medulla oblongata and and lateral vestibular nucleus in the pons.
• The vestibular nuclei receive input from the vestibular nerve and the fastigial nucleus of the cerebellum.

Lateral vestibulospinal tract:

• Axons from the cells of the lateral vestibular nucleus descend ipsilaterally as the lateral vestibulospinal tract in the ventral funiculus of the spinal cord.
• Fibres of the lateral vestibulospinal tract synapse along the length of the spinal cord with extensor motoneurones, exerting excitatory effects.
  Medial vestibulospinal tract:
• Axons from the cells of the medial vestibular nucleus descend ipsilaterally as the medial vestibulospinal tract in the medial longitudinal fasciculus of the spinal cord.
• The medial vestibulospinal tract descends as far as the cervical spine, where they synapse with motoneurones innervating the extraocular muscles.

Question 11

Describe the course of the reticulospinal tract.

Answer 11

• First-order neurones of the reticulospinal tract arise in the pons (pontine division) and the medulla (medullary division).
• The pontine division descends ipsilaterally in the ventral funiculus.
• The medullary division partially decussates and descends bilaterally in the ventral funiculus.
• The first-order neurones synapse with alpha and gamma motoneurones in the ventral grey horn.