Descending Pathways

Question 1

What structures, other than the cortex, are capable of generating and controlling movement?

Answer 1

• basal ganglia & cerebellum
• reticular formation
• tectum & red nucleus
• vestibular system
• these structures are able to send descending pathways to stimulate muscles at the level of the body

Question 2

What are the 3 descending motor pathways?

Answer 2

• corticospinal
• corticobulbar
• extrapyramidal

Question 3

Where does the corticospinal tract travel to and from?

What movement is it associated with?

Answer 3

• it travels from the cortex to the spinal cord
• also called the “pyramidal tract” as part of this tract crosses the midline at the level of the pyramids in the medulla
• associated with fine movement and flexor function

Question 4

Where do the corticobulbar tracts start and end?

What are their roles?

Answer 4

• this refers to any pathway that starts in the cortex and runs to the brainstem
• this includes the corticonuclear tracts that innervate cranial nerve nuclei
• they are associated with control over cranial nerves** and **modulation of descending pathways

Question 5

What is meant by an extrapyramidal tract?

What are the 4 main extrapyramidal tracts and what are they involved with?

Answer 5

• an extrapyramidal tract does NOT start in the cortex and does NOT pass through the pyramids

pathways arising in the midbrain:

• tectospinal - involved with head movements
• rubrospinal - involved with fine movement & flexor function

pathways arising in pons / medulla:

• vestibulospinal
• reticulospinal
• both involved with maintaining tone - antigravity, balance and posture

Question 7

Which funiculi are the descending motor tracts found in?

Answer 7

Lateral funiculus:

• lateral corticospinal tract
• rubrospinal tract (runs parallel)

Ventral funiculus:

• tectospinal tract
• vestibulospinal tract
• some of the reticulospinal tract
• MLF
• ventral corticospinal tract

Question 8

What are the 2 neurones involved in a typical motor pathway?

Answer 8

Upper motor neurone (UMN):

• originates in cerebrum and subcortical structures
• descends to either the brainstem or the spinal cord to stimulate a LMN

Lower motor neurone (LMN):

• originates from the brainstem (cranial nerve nuclei) and spinal cord (ventral grey horm)
• gives rise to peripheral nerves (cranial or spinal nerves) that travel to motor end plates / neuromuscular junctions

Question 9

What are the typical symptoms and causes of UMN and LMN lesions?

Answer 9

UMN lesion:

• leads to hyperreflexia** and **hypertonia
• pathological reflexes may be present - e.g. positive Babinski sign
• typical causes include:

1. stroke at the level of the cortex / internal capsule
2. tumours compressing the pyramids or pons

LMN lesion:

• leads to muscle weakness, hyporeflexia and hypotonia
• typical causes include:

1. trauma to the peripheral nerve
2. tumours of the ventral grey horn
3. demyelination
4. lack of blood supply

Question 10

Describe the main pathway of the corticospinal tract

Answer 10

• precentral gyrus sends an UMN which descends via the posterior limb of the internal capsule
• UMN descends through the cerebral peduncles, ventral pons and into the medulla
• within the medulla, the UMNs collect together to form the pyramids
• at the cervicomedullary junction, 90% of fibres cross the midline as the decussation of the pyramids
• these fibres will enter the CONTRALATERAL lateral corticospinal tract
• the lateral corticospinal tract descends to reach a specific level of the spinal cord and then sends its axons to the ventral grey horn
• LMN travels from the ventral grey horn to innervate a specific muscle of the body

Question 11

What happens to the fibres that do not cross the midline at the decussation of the pyramids?

Answer 11

• 10% of descending fibres do not cross the midline at the decussation of the pyramids in the caudal medulla
• these fibres enter the IPSILATERAL anterior corticospinal tract
• some fibres will travel to the contralateral ventral grey horn and some will travel to the ipsilateral ventral grey horn
• this provides bilateral innervation

Question 12

What is significant about the corticospinal tracts being able to provide contralateral and bilateral innervation?

Answer 12

Contralateral innervation:

• provided by the lateral corticospinal tract
• mainly for the upper and lower limbs
  • CST is particularly important for the flexor functions of the upper limb

Bilateral innervation:

• provided by the anterior corticospinal tract
• mainly for axial and proximal girdle muscles

Question 14

Where can the corticospinal / corticobulbar tracts arise from?

Where do they pass after this?

Answer 14

• they mainly arise from the precentral gyrus
• they can also arise from the sensorimotor strip of the postcentral gyrus (BA 3) and the premotor & supplementary motor areas (BA 6)
• fibres pass from the cortex via the corona radiata and enter the posterior limb of the internal capsule

Question 15

How is somatotopic organisation maintained at the level of the internal capsule?

Answer 15

• descending corticospinal / corticobulbar fibres retain somatotopic representation as they pass through the internal capsule
• motor neurones passing to the face are located more anterior and travel through the genu and anterior part of the posterior limb
• motor neurones passing to the legs are located most posterior in the posterior limb of the IC

Question 16

How is somatotopic organisation maintained as fibres pass from the internal capsule to the next structure?

Answer 16

• the internal capsule connects to the crus cerebri (cerebral peduncles)
• neurones travelling to the face are located more medially
• neurones travelling to the legs are located more laterally

Question 17

How do corticospinal fibres appear as they travel through the pons?

Answer 17

• as they pass through the ventral pons, the corticospinal tracts appear as scattered fascicles
• this is due to the presence of transverse pontine fibres travelling between the pons and cerebellum
• the fibres recollect as the pyramids at the level of the medulla

Question 18

What fibres are found within the lateral corticospinal tract?

Where do they meet the LMN and what does this innervate?

Answer 18

• the lateral CST contains the 90% of fibres that crossed the midline at the pyramidal decussation of the medulla
• they synapse with the LMN within the CONTRALATERAL ventral grey horn
• they provide contralateral innervation to the limb musculature

Question 19

What fibres are found within the anterior corticospinal tract?

Where do these meet the LMN and what does this innervate?

Answer 19

• the anterior CST contains the 10% of fibres that did NOT cross the midline at the pyramidal decussation
• at the appropriate spinal cord level, some fibres will cross the midline via the ventral white commissure and some will remain ipsilateral
• the UMN synapses with the LMN in either the ipsilateral or contralateral ventral grey horn
• this provides bilateral innervation to the axial musculature

Question 21

What are the 4 main corticobulbar tracts?

Answer 21

• corticobulbar tracts pass from the cortex to the brainstem

1. corticonuclear (travel to cranial nerve nuclei)
2. corticorubral (to red nucleus)
3. corticoreticular (to reticular formation)
4. corticocollicular (to superior colliculus)

Question 22

What is the purpose of the corticoreticular pathway?

Answer 22

• it passes to the reticular formation of the pons and medulla
• the pons gives rise to the medial reticulospinal tract and the medulla to the lateral reticulospinal tract
• the corticoreticular pathway provides a route for cortical control over the reticulospinal descending pathways via inhibitory interneurones

Question 23

What is the corticorubral pathway and how is it organised?

Answer 23

• this runs from the cortex to the red nucleus
• it allows for cortical control over the rubrospinal pathways
  
  • ​these are diminished in humans with most fibres only going to cervical regions
• it is somatotopically organised such that upper limb fibres travel in the dorsal part and lower limb fibres travel in the ventral part

Question 24

What is the purpose of the corticocollicular pathway?

Answer 24

• this allows for cortical input to the superior colliculus and horizontal gaze centres
• this allows for cortical coordination of voluntary and involuntary eye movements

Question 25

Where does the vestibulospinal tract pass to and from? How is it divided?

Answer 25

* the vestibulospinal tract passes from the **labyrinth** to the **spinal cord** * there are both **_medial_** and **_lateral_ vestibulospinal tracts**

Question 26

Describe the pathway of the medial vestibulospinal tract What is the role of this tract?

Answer 26

* originates in the **_medial vestibular nucleus_** (rostral medulla) * this sends **_bilateral projections_** to the **_cervical spinal cord_** via the **descending part of the _MLF_** * at the correct level of the cervical spinal cord, the UMN synapses with a LMN that innervates **muscles of the neck and shoulder** * *most of these are innervated by the accessory nerve* * this allows us to **_stabilise our head_** in response to forces that **_change our balance or posture_**

Question 27

Describe the pathway of the lateral vestibulospinal tract What is its function?

Answer 27

* originates from the **_lateral vestibular nucleus_** (level of pons) * descends **_ipsilaterally_** to **_all levels_** of the spinal cord * synapses with LMN that will innervate the **_extensor muscles of the lower limb_** (anti-gravity muscles) that allow us to **_keep an upright posture_** (e.g. when a bus brakes suddenly)

Question 28

How can the medial longitudinal fasciculus (MLF) be divided into 2 components?

Answer 28

**_Ascending component:_** * has a role in **connecting the abducens nucleus** with the **oculomotor nucleus** **_Descending component:_** * originates in the **medial vestibular nucleus** * fibres of the **_medial vestibulospinal trac_****_t_ descend within the MLF** * fibres of the **lateral** vestibulospinal tract **DO NOT** descend via the MLF

Question 29

Describe the pathway of the tectospinal tract What is the purpose of this pathway?

Answer 29

* arises from the **_superior colliculus_** in the midbrain * fibres **rapidly _cross the midline_** via the **_dorsal tegmental decussation_** * fibres descend **_contralaterally_** to **_all levels_ of the _cervical_ spinal cord**, where they connect with a LMN that is capable of moving the **muscles of the neck** * the role of this pathway is to allow for **_postural changes in response to a visual stimulus_** * it is under **corticocollicular modulation** * *it can send a LMN without cortical involvement, but the stimulus can be modified by the cortex via corticocollicular fibres*

Question 30

Describe the pathways and actions of the medial and lateral reticulospinal tracts

Answer 30

* these pathways mainly act on axial musculature and are under corticoreticular modulation (inhibition) **_Medial reticulospinal tract (pons)_****_:_** * originates from the **reticular formation in the pons** and descends to **_all levels_** of the spinal cord * it ***_facilitates voluntary movement_*** and ***_increases muscle tone_*** **_Lateral reticulospinal tract (medulla):_** * originates from the **reticular formation in the medulla** and descends to **_all levels_** of the spinal cord * it ***_inhibits voluntary movement_*** and ***_decreases muscle tone_***

Question 31

Describe the pathway of the rubrospinal tract What is its function?

Answer 31

* originates from the **_red nucleus_** of the midbrain * **rapidly _crosses the midline_** via the **_ventral tegmental decussation_** * descends to **_all levels_** of the **_cervical_ spinal cord** * facilitates motor neurones that innervate **_flexor muscles_** that are involved in **precision movements** * under corticorubral modulation

Question 32

What structure does the rubrospinal tract run parallel to and why is this significant?

Answer 32

* it runs **parallel to the _lateral corticospinal tract_** and is somatotopically organised * it can take over some of the functions of the lateral corticospinal tract if it is injured * *as the cortex is not involved, these actions will not be as precise or accurate*

Question 33

What happens if there is an injury above and below the level of the red nucleus in a comatose patient?

Answer 33

**_Injury above the level of the red nucleus:_** * the **_rubrospinal tract_** (associated with fine movement and flexor function) is **_still intact_** * if the patient receives a pain or auditory stimulus, their **_arms will flex_** * this is **_decorticate posturing_** **_Injury below the level of the red nucleus:_** * the **vestibulospinal tract** and **reticulospinal tract** are involved in **extensor functions** and are present at the level of the pons and medulla * injury affecting **_all of the descending tracts_** leads to **_extension of the limbs_** upon a pain / auditory stimulus * this is **_decerebrate posturing_**

Question 34

Label the descending tracts

Answer 34