Ascending Tracts

Question 1

Describe the organisation of the white matter of the spinal cord.

Answer 1

Split into three funiculi

Dorsal funiculus is found between the midline and medial edge of the dorsal root

Lateral funiculus found on the lateral edge of the grey matter of the spinal cord

Ventral funiculus is found between the midline and medial edge of the ventral root

Question 2

How is the grey matter of the spinal cord organised?

Answer 2

Its cell bodies are arranged into laminae

Rexed Laminae

There are X (10) rexed laminae

Each laminae is equivalent to a neuronal nucleus

Some have discreet names.

Question 3

How is somatosensory information conveyed from the body top the brain?

Answer 3

Spinal sensory system:
Sensation arising from the top of the neck down to the toes
Sensory signals are carried by sensory division of spinal (or segmental) nerves
There are 31 pairs of segmental nerves carrying sensory signals from the body surface to the brain

Cranial sensory system:
Sensations arising from cranial structures
Sensory signals are carried by sensory roots of cranial nerves
(The trigeminal nerve carries the majority of somatosensory signals)

Question 4

What are ascending tracts?

Answer 4

They are fibres by which sensory information is conveyed to the brain

Divisible by two broad categories

Question 5

What are the two categories of sensation do the sensory signals of the body give rise to?

Answer 5

Conscious sensation (We are directly aware of this information)

• Pain, temp., crude touch
• Conscious proprioception, fine touch

Non-conscious sensation (We are not directly aware of this information)
- Tactile strength, muscle length, muscle tension, joint
position, joint angle

Question 6

What are the routes the ascending tracts can take?

And what is the difference between them?

Answer 6

Spinothalamic tract
(conscious sensation - Pain, temp and crude touch)
Dorsal column
(Conscious proprioception and fine touch)
Spinocerebellar tract
(Unconscious sensation - proprioception etc….)

Question 7

Describe the route of the primary afferent neurone of the spinothalamic nerves.

Answer 7

They enter the spinal cord via the dorsal root

It enters the spinal cord and divides, sending branches to adjacent spinal segments

Unbranched main axon enters the grey matter of its respective dorsal horn

It then terminates in the dorsal horn

(The laminae it terminates in determines its modality, it terminates in laminae I to IV)
(laminae III & IV are known as Nucleus Proprius)

Question 8

What does the primary afferent conscious neurone do just after entering spinal cord?

Answer 8

They enter the spinal cord and then divide, sending branches to adjacent spinal segments

The branches will go 3 to 4 spinal segments in both directions, rostrally and caudally

Axonal branches travelling to adjacent spinal segment rostral and caudal to the neural level of entry travel in the posterolateral tract of Lissauer
(Specific for pain & temp. ascending tracts)

Question 9

Describe the route of the secondary neurone of the spinothalamic tract.

Answer 9

It sends its axon towards the central canal, then ducks under and crosses the midline

It then joins the lateral funiculus of the white matter

And then joins the fibres of the lateral spinothalamic tract

These fibres then travel to the thalamus

To the ventral posterolateral (VPL) nucleus of the thalamus

There they terminate onto 3rd order sensory neurones.

Question 10

What is meant by spinal decussation?

Answer 10

When spinal neurones cross the midline

Question 11

What are the spinal decussation fibres of the spinothalamic tract known as?

Answer 11

Anterior commissures

Spinal Arcuate fibres

Question 12

What are the consequences of spinal decussation in the spinothalamic tract?

Answer 12

Sensory signals arising from the right side of the body are carried by secondary sensory fibres of the spinothalamic fibres of the left side of the body

And vice versa

Question 13

Describe the route of third order sensory neurones of the spinothalamic tract.

Answer 13

They ascend from the VPL nucleus to terminate in the post-central gyrus of the cerebral cortex.

They travel via the internal capsule

Question 14

What are the different spinothalamic tracts?

Answer 14

Anterior spinothalamic tract
(Crude touch pressure)

Lateral spinothalamic tract
(Pain, Temp)

Question 15

What is syringomyelia?

Answer 15

A chronic progressive disease in which longitudinal cavities form in the cervical region of the spinal cord. This characteristically results in wasting of the muscles in the hands and a loss of sensation

Due to damage at the central canal of the spinal cord, which results in scar tissue, this leads to central cavitation, and if the cyst expands ventrally the patient can suffer anaesthesia of upper limbs bilaterally

Question 16

What sensory information is carried by the dorsal column?

Answer 16

Conscious proprioception and fine touch

Question 17

Describe the route of the primary afferent neurone of the dorsal column.

Answer 17

Comes into the spinal cord via the dorsal root ganglion

Stays on same side

Immediately ascends in the dorsal funiculus still as a primary sensory neurones (without terminating)

It ascends in the dorsal columns

In either:

• Fasciculus cuneatus (lateral) - Signal from above T6
• Fasciculus gracilis (medial) - Signal from below T6

They then terminate in the medulla on the same side they arose from

Fasciculus cuneatus terminates in the nucleus cuneatus of the medulla

Fasciculus gracilis terminates in the nucleus gracilis of the medulla

Question 18

Describe the route of the secondary neurones of the dorsal column.

Answer 18

The secondary sensory neurone in the nucleus gracilis or nucleus cuneatus sends its axon cranially

They travel ventromedially as internal Arcuate fibres
And cross the midline to become the medial lemniscus

The crossed fibres then ascend the medulla, pons and midbrain towards thalamus

All fibres terminate in the ventral posterolateral (VPL) nucleus of the thalamus

Question 19

Describe the route of the 3rd order sensory neurones of the dorsal column.

Answer 19

From the VPL they ascend further

Travelling through the internal capsule

To finally terminate on the sensori-motor cerebral cortex

Question 20

What is Brown-Sequard Syndrome?

Answer 20

damage to hemi-section of the spinal cord will lead to a sensory dissociation syndrome known as:
Brown-Sequard Syndrome

For example if the right half of the spinal cord was damaged at the level of T10
Then the right half below T10 would have no proprioception and fine touch,
Whereas the left side would have no pain and temperature sense.

Question 21

How does the cerebellum receive proprioceptive information?

Answer 21

Non-conscious proprioceptive sensory information from muscle receptors and joint receptors

It receives the information through:

• Dorsal Spinocerebellar Tract (DSCT)
• Ventral Spinocerebellar Tract (VSCT)
• Cuneo-cerebellar Tract (CCT)

Question 22

What happens when a proprioceptive sensory neurone enters the spinal cord?

Answer 22

It divides into two branches:
1st branch - hardly enters the dorsal horn before joining
the dorsal column
2nd branch - Terminates in laminae VII of the dorsal horn
also known as nucleus of Clarke

Question 23

Tell me about Clarke’s column.

Answer 23

Successive nuclei of Clarke from all neural levels form a continuous column of cells in the rostro-caudal axis

This is Clarke’s column

Clarke’s column neurones act locally as interneurons and also send there axons to the cerebellum

Question 24

Describe the route of the dorsal Spinocerebellar tract.

Answer 24

Derived from neurones of Clarke’s column

Axonal processes of neurones in Clarke’s column leave the grey matter to enter the white matter in the lateral funiculus

Here they join the fibres of the dorsal spinocerebellar tract

DSCT fibres ascend ipsilaterally without decussating

They pass through the medulla to terminate in the Vermis & anterior lobe of the Cerebellum

Question 25

Briefly describe the route of the ventral spinocerebellar tract.

Answer 25

Parallel to DSCT

Primary afferent neurone has cell body in dorsal root ganglion

It enters the dorsal horn of the grey matter and synapses with a second neurone

This neurone then crosses the midline via the anterior white commissure and joins the VSCT in the lateral funiculus

It then travels to the cerebellum where it crosses back over to the same side it originated on.

Question 26

What is the clinical significance of the cerebellar projections?

Answer 26

A patient with unilateral cerebellar damage will tend to sway, falling on the side in which the lesion is located

(If the Vermis of the cerebellum is damaged then the patient will fall backwards.)