Ascending tracts

Question 1

What are the ascending tracts?

Answer 1

The ascending tracts refer to the neural pathways by which sensory information from the peripheral nerves is transmitted to the cerebral cortex.

Question 2

How are the ascending tracts divided functionally?

Answer 2

Conscious tracts - DCML pathway and anterolateral system - pain, temperature and touch

Unconscious tracts - spinocerebellar tracts - proprioception

Question 3

What is essential for conscious tracts/sensaitons?

Answer 3

The destination for ascending tracts for conscious sensations is in the post-central gyro (primary sensory or somatosensory cortex) in the parietal lobe.

Question 4

What sensory modalities are carried in each of the ascending tracts?

Answer 4

Dorsal column medial lemniscal pathway - fine touch (tactile sensation), vibration and conscious proprioception

Anterior spinothalamic tract - crude touch and pressure

Lateral spinothalamic tract - pain and temperature

Anterior spinocerebellar pathway - unconscious proprioception in the lower limb (from golgi tendon organs)

Posterior spinocerebellar tract - unconscious proprioception in the lower limb and trunk (muscle spindle and tendon afferents

Cuneocerebellar tract - unconscious proprioception from the upper limb

Question 5

What sensory modalities does the dorsal medial lemniscal pathway carry?

Answer 5

Fine touch (tactile sensation)
Vibration
Conscious proprioception

Question 6

What is the structure of the DCML pathway?

Answer 6

The pathway travels via the dorsal columns in the spinal cord and through the medial lemniscus in the brainstem.

The fascicule gracilis exists at all levels of the spinal cords, containing long fibres from the lower limbs and is located medially in the dorsal column.

The fascicule cuneatus exists above the T6 level and carries fibres from the upper limb. It is in the lateral part of the dorsal column.

Question 7

Describe the dorsal column medial lemniscal pathway.

Answer 7

First order neurones:

Signals from the upper limb (T6 and above) – travel in the fasciculus cuneatus (the lateral part of the dorsal column). They then synapse in the nucleus cuneatus of the medulla oblongata.

Signals from the lower limb (below T6) – travel in the fasciculus gracilis (the medial part of the dorsal column). They then synapse in the nucleus gracilis of the medulla oblongata.

The second order neurones begin in the cuneate nucleus or gracilis. The fibres receive the information from the preceding neurones, and delivers it to the third order neurones in the thalamus.

Within the medulla oblongata, these fibres decussate (cross to the other side of the CNS). They then travel in the contralateral medial lemniscus to reach the thalamus.

Lastly, the third order neurones transmit the sensory signals from the thalamus to the ipsilateral primary sensory cortex of the brain. They ascend from the ventral posterolateral nucleus of the thalamus, travel through the internal capsule and terminate at the sensory cortex.

Question 8

Overview of DCML

Answer 8

1ON LL –> fasciculus gracilis (medial)
UL –> fasciclus cuneatus ( lateral)
Synapse in Medualla oblongata (nucleus gracilis/cuneatus)
2ON decssate in medulla oblongata
Through medial lemniscus
Synapse in Thalamus
3ON from thalamus to medial aspect of somatosensory cortex.

Question 9

What does damage of the DCML result in?

Answer 9

Ipsilateral symptoms below the level of the spinal cord region damaged. Loss of tactile sense, proprioception and inability to identify object placed in hand.

Question 10

What does the anterior spinothalamic tract carry

Answer 10

Crude touch and pressure

Question 11

What does the lateral spinothalamic tract carry?

Answer 11

Pain and temperature

Question 12

Describe the path of the spinothalamic tracts.

Answer 12

The first order neurones arise from the sensory receptors in the periphery. They enter the spinal cord, ascend 1-2 vertebral levels, and synapse at the tip of the dorsal horn – an area known as the substantia gelatinosa.

The second order neurones carry the sensory information from the substantia gelatinosa to the thalamus. After synapsing with the first order neurones, these fibres decussate within the spinal cord, and then form two distinct tracts:

Crude touch and pressure fibres – enter the anterior spinothalamic tract.
Pain and temperature fibres – enter the lateral spinothalamic tract.
Although they are functionally distinct, these tracts run alongside each other, and they can be considered as a single pathway. They travel superiorly within the spinal cord, synapsing in the thalamus.
o From medial to lateral, fibres ascend cervical, thoracic, lumbar and sacral.

The third order neurones carry the sensory signals from the thalamus to the ipsilateral primary sensory cortex of the brain. They ascend from the ventral posterolateral nucleus of the thalamus, travel through the internal capsule and terminate at the sensory cortex.

Question 13

Overview of anterolateral spinothalamic tracts.

Answer 13

1ON enter spinal cord, ascend 1/2 vertebral levels
Synapse at substantia gelatinosa (tip of dorsal horn)

2ON decussate in spinal cord –> form anterior and lateral tracts (cervical to sacral medial to lateral)

Synapse in thalamus with 3ON that ascend to sensory cortex (homunculus)

Question 14

What does damage to the lateral spinothalamic tract result in?

Answer 14

Complete loss of pain and temperature sensation on the contralateral side of the body, at and below the damage.

Pain loss occurs at 1/2 levels below lesion as 1OIN ascend when they enter the spinal cord.

Question 15

What is sacral sparing?

Answer 15

Due to the sacral and lumbar fibres lying dorsolateral to the thoracic and cervical fibres, any expanding tumour or lesion in the grey matter will affect the thoracic and cervical fibres first, resulting in the sacral and lumbar fibres having intact pain and temperature still present initially, known as sacral sparing.

Question 16

What do the spinocerebellar tracts carry?

Answer 16

Fibres of unconscious proprioception

Question 17

Describe the posterior spinocerebellar pathway

Answer 17

1ON enter spinal cord c(cell body in dorsal root ganglion)
Synapse in spinal grey matter in nucleus dorsals of Clarke
2ON ascend on ipsilateral side to medulla.
Continue up to cerebellar hemispheres.

Don’t decussate!

Question 18

What does damage to the posterior spinocerebellar tract lead to

Answer 18

Loss of unconscious proprioception and co-ordinated movement of ipsilateral lower limb and trunk

Question 19

Describe the anterior spinocerebellar pathway.

Answer 19

1ON enter spinal cord
Synapse in spinal grey matter (dorsal horn)
2ON decussate in spinal cord and ascend through medulla to pons
Decussate again in pons before terminating in ipsilateral cerebellum.

Decussates twice!

Question 20

What is the function of anterior spinocerebellar pathway and what does damage lead to?

Answer 20

• This tract acts to convey information about whole limb movement and postural adjustments to the cerebellum.
• Damage to this tract results in loss of proprioception and coordination in the lower limb.

Question 21

Describe the cuneocerebellar tract.

Answer 21

1ON enters spinal cord (cell body in dorsal root ganglion)
Ascends to the accessory cuneate nucleus in medulla
Synapses with 2ON which projects to the cerebellar hemisphere

Doesn’t decussate!

Question 22

What is the function of the cuneocerebellar tract?

Answer 22

Unconscious proprioception of upper limb to ipsilateral cerebellum

Question 23

Draw and label cross section of spinal cord with motor and sensory tracts.

Answer 23

-

Question 24

What happens in DCML lesion? What if the lesion is in the spinal cord? When are DCML lesions seen?

Answer 24

A lesion of the DCML pathway causes a loss of proprioception and fine touch. However, a small number of tactile fibres travel within the anterolateral system, and so the patient is still able to perform tasks requiring tactile information processing.

If the lesion occurs in the spinal cord (which is most common), the sensory loss will be ipsilateral – decussation occurs in the medulla oblongata. DCML lesions can be seen in vitamin B12 deficiency and tabes dorsalis (a complication of syphilis).

Question 25

What happens in injury to the anterolateral system?

Answer 25

Injury to the anterolateral system will produce an impairment of pain and temperature sensation. In contrast to DCML lesions, this sensory loss will be contralateral (the spinothalamic tracts decussate within the spinal cord).

Question 26

What is Brown-Sequard syndrome? What happens?

Answer 26

hemisection (one sided lesion) of the spinal cord. This is most often due to traumatic injury, and involves both the anterolateral system and the DCML pathway:

DCML pathway – ipsilateral loss of touch, vibration and proprioception below level.

Anterolateral system – contralateral loss of pain and temperature, crude touch and pressure a few segments below the level of damage.

It will also involve the descending motor tracts, causing an ipsilateral hemiparesis.

Question 27

What happens in lesions of the spinocerebellar tracts?

Answer 27

Lesions of the spinocerebellar tracts present with an ipsilateral loss of muscle co-ordination.

However, the spinocerebellar pathways are unlikely to be damaged in isolation – there is likely to be additional injury to the descending motor tracts. This will cause muscle weakness or paralysis, and usually masks the loss of muscle co-ordination.

Question 28

What is romberg’s signs and what does it test?

Answer 28

A test for the function of the posterior (DCML) pathway is demonstrated as Romberg’s sign– where the patient stands with feet together, and closes their eyes- swaying will be observed, as they are unable to do this.

Without sensory input relating to the proprioception, the orientation of the body, awareness of limbs, execution of movement can be severely impaired. Such is the ‘stamp and stick’ gait of sensory ataxia.