Ascending and descending tracts

Question 1

What sort of signals do descending pathways transport?

Answer 1

Motor signals

Question 2

What are the major functional groups of the descending pathway?

Answer 2

Pyramidal group

Extrapyramidal group

Question 3

Where do pyramidal tracts originate and where do they carry signals?

Answer 3

These tracts originate in the cerebral cortex, carrying motor fibres to the spinal cord and brain stem

Question 4

What movement do pyramidal signals produce?

Answer 4

They are responsible for the voluntary control of the musculature of the body and face

Question 5

Where do extrapyramidal tracts originate and where do they carry signals?

Answer 5

These tracts originate in the brain stem, carrying motor fibres to the spinal cord

Question 6

What movement do extrapyramidal signals produce?

Answer 6

They are responsible for the involuntary and automatic control of all musculature, such as muscle tone, balance, posture and locomotion

Question 7

What tracts compromise the pyramidal tract?

Answer 7

Corticospinal tracts

Corticobulbar tracts

Question 8

What does the corticospinal tract supply?

Answer 8

Musculature of the body

Question 9

What does the corticobulbar tract supply?

Answer 9

Musculature of the head and neck

Question 10

What neuronal areas supply the corticospinal tract?

Answer 10

Primary motor cortex
Premotor cortex
Supplementary motor area

They also receive nerve fibres from the somatosensory area, which play a role in regulating the activity of the ascending tracts.

Question 11

How would you categorise the corticospinal tract?

Answer 11

The fibres within the lateral corticospinal tract decussate. They then descend into the spinal cord, terminating in the ventral horn. From the ventral horn, the lower motor neurones go on to supply the muscles of the body

The anterior corticospinal tract remains ipsilateral, descending into the spinal cord. They then decussate and terminate in the ventral horn of the cervical and upper thoracic segmental levels.

Question 12

Where does the corticobulbar tract arise?

Answer 12

The lateral aspect of the primary motor cortex

The neurones terminate on the motor nuclei of the cranial nerves. Here, they synapse with lower motor neurones, which carry the motor signals to the muscles of the face and neck.

Question 13

Which cranial nerves have contralateral innervation?

Answer 13

Facial nerve
Hypoglossal nerve

Every other cranial nerve is supplied bilaterally

Question 14

How would you categorise the extrapyramidal tracts?

Answer 14

Vestibulospinal Tracts
Reticulospinal Tracts
Rubrospinal Tracts
Tectospinal Tracts

Question 15

Which descending tracts decussate

Answer 15

Lateral corticospinal tract
Corticobulbar
Rubrospinal
Tectospinal

Question 16

Where do the vestibulospinal tracts originate?

Answer 16

vestibular nuclei, which receive input from the organs of balance

Question 17

Are vestibulospinal tracts ipsi or contralateral?

Answer 17

The tracts convey this balance information to the spinal cord, where it remains ipsilateral

Question 18

What motor function do vestibulospinal tracts have?

Answer 18

Fibres in this pathway control balance and posture by innervating the ‘anti-gravity’ muscles (flexors of the arm, and extensors of the leg), via lower motor neurones.

Question 19

What are the two different reticulospinal tracts and where do they originate?

Answer 19

The medial reticulospinal tract arises from the pons. It facilitates voluntary movements, and increases muscle tone.

The lateral reticulospinal tract arises from the medulla. It inhibits voluntary movements, and reduces muscle tone.

Question 20

Where does the rubrospinal tract originate and what is its function?

Answer 20

The rubrospinal tract originates from the red nucleus, a midbrain structure. As the fibres emerge, they decussate (cross over to the other side of the CNS), and descend into the spinal cord. Thus, they have a contralateral innervation.

Its exact function is unclear, but it is thought to play a role in the fine control of hand movements

Question 21

Where does the tectospinal tract originate and what is its function?

Answer 21

This pathway begins at the superior colliculus of the midbrain. The superior colliculus is a structure that receives input from the optic nerves. The neurones then quickly decussate, and enter the spinal cord. They terminate at the cervical levels of the spinal cord.

The tectospinal tract coordinates movements of the head in relation to vision stimuli.

Question 22

What are signs of damage to the corticospinal tract?

Answer 22

If there is only a unilateral lesion of the left or right corticospinal tract, symptoms will appear on the contralateral side of the body. The cardinal signs of an upper motor neurone lesion are:

Hypertonia – an increased muscle tone
Hyperreflexia – increased muscle reflexes
Clonus – involuntary, rhythmic muscle contractions
Babinski sign – extension of the hallux in response to blunt stimulation of the sole of the foot
Muscle weakness

Question 23

What are signs of damage to the corticobulbar tract?

Answer 23

Due to the bilateral nature of the majority of the corticobulbar tracts, a unilateral lesion usually results in mild muscle weakness

Hypoglossal nerve – a lesion to the upper motor neurones for CN XII will result in spastic paralysis of the contralateral genioglossus. This will result in the deviation of the tongue to the contralateral side.

Facial nerve – a lesion to the upper motor neurones for CN VII will result in spastic paralysis of the muscles in the contralateral lower quadrant of the face.

Question 24

What are signs of damage to the extrapyramidal tracts?

Answer 24

Extrapyramidal tract lesions are commonly seen in degenerative diseases, encephalitis and tumours. They result in various types of dyskinesias or disorders of involuntary movement.

Question 25

What is the internal capsule and why is it relevant?

Answer 25

A white matter pathway, located between the thalamus and the basal ganglia

This is clinically important, as the internal capsule is particularly susceptible to compression from haemorrhagic bleeds, known as a ‘capsular stroke‘. Such an event could cause a lesion of the descending tracts

Question 26

What is the functional division of the ascending tracts?

Answer 26

Conscious tracts - comprised of the dorsal column-medial lemniscal pathway and the anterolateral system

Unconscious tracts - comprised of the spinocerebellar tracts

Question 27

What information does the dorsal column-medial lemniscal pathway carry?

Answer 27

sensory modalities of fine touch (tactile sensation), vibration and proprioception

Question 28

How does the dorsal column-medial lemniscal pathway obtain its name?

Answer 28

Its name arises from the two major structures that comprise the DCML. In the spinal cord, information travels via the dorsal (posterior) columns. In the brainstem, it is transmitted through the medial lemniscus.

There are three groups of neurones involved in this pathway – first, second and third order neurones.

Question 29

Where do the first order neurons synapse onto second order neurons in the DCML pathway?

Answer 29

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.

Question 30

Where do the second order neurons synapse onto third order neurons in the DCML pathway?

Answer 30

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

Question 31

What tracts comprise the anterolateral system?

Answer 31

Anterior spinothalamic tract – carries the sensory modalities of crude touch and pressure.

Lateral spinothalamic tract – carries the sensory modalities of pain and temperature.

Question 32

Where do the first order neurons synapse onto second order neurons in the anterolateral pathway?

Answer 32

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.

Question 33

Where do the second order neurons synapse onto third order neurons in the anterolateral pathway?

Answer 33

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:

1. Crude touch and pressure fibres – enter the anterior spinothalamic tract.
2. 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

Question 34

What is the collective name for the tracts in the unconscious proprioceptive pathway?

Answer 34

Spinocerebellar tracts

Question 35

What are the four individual pathways of the spinocerebellar tracts?

Answer 35

Posterior spinocerebellar tract – Carries proprioceptive information from the lower limbs to the ipsilateral cerebellum.

Cuneocerebellar tract – Carries proprioceptive information from the upper limbs to the ipsilateral cerebellum.

Anterior spinocerebellar tract – Carries proprioceptive information from the lower limbs. The fibres decussate twice – and so terminate in the ipsilateral cerebellum.

Rostral spinocerebellar tract – Carries proprioceptive information from the upper limbs to the ipsilateral cerebellum.

Question 36

What do injuries to the DCML pathway lead to?

Answer 36

A lesion of the DCML pathway causes a loss of proprioception and fine touch.

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 37

What do injuries to the anterolateral pathway lead to?

Answer 37

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

Question 38

What is Brown-Séquard syndrome?

Answer 38

Brown-Séquard syndrome refers to a 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.
Anterolateral system – contralateral loss of pain and temperature sensation.

Question 39

What do injuries to the spinocerebellar pathway lead to?

Answer 39

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.