Descending Pathways Flashcards
1
Q
What structures, other than the cortex, are capable of generating and controlling movement?
A
- 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
2
Q
What are the 3 descending motor pathways?
A
- corticospinal
- corticobulbar
- extrapyramidal
3
Q
Where does the corticospinal tract travel to and from?
What movement is it associated with?
A
- 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
4
Q
Where do the corticobulbar tracts start and end?
What are their roles?
A
- 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
5
Q
What is meant by an extrapyramidal tract?
What are the 4 main extrapyramidal tracts and what are they involved with?
A
- 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
6
Q
A
7
Q
Which funiculi are the descending motor tracts found in?
A
Lateral funiculus:
- lateral corticospinal tract
- rubrospinal tract (runs parallel)
Ventral funiculus:
- tectospinal tract
- vestibulospinal tract
- some of the reticulospinal tract
- MLF
- ventral corticospinal tract
8
Q
What are the 2 neurones involved in a typical motor pathway?
A
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
9
Q
What are the typical symptoms and causes of UMN and LMN lesions?
A
UMN lesion:
- leads to hyperreflexia** and **hypertonia
- pathological reflexes may be present - e.g. positive Babinski sign
- typical causes include:
- stroke at the level of the cortex / internal capsule
- tumours compressing the pyramids or pons
LMN lesion:
- leads to muscle weakness, hyporeflexia and hypotonia
- typical causes include:
- trauma to the peripheral nerve
- tumours of the ventral grey horn
- demyelination
- lack of blood supply
10
Q
Describe the main pathway of the corticospinal tract
A
- 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
11
Q
What happens to the fibres that do not cross the midline at the decussation of the pyramids?
A
- 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
12
Q
What is significant about the corticospinal tracts being able to provide contralateral and bilateral innervation?
A
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
13
Q
A
14
Q
Where can the corticospinal / corticobulbar tracts arise from?
Where do they pass after this?
A
- 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
15
Q
How is somatotopic organisation maintained at the level of the internal capsule?
A
- 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
16
Q
How is somatotopic organisation maintained as fibres pass from the internal capsule to the next structure?
A
- 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
17
Q
How do corticospinal fibres appear as they travel through the pons?
A
- 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
18
Q
What fibres are found within the lateral corticospinal tract?
Where do they meet the LMN and what does this innervate?
A
- 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
19
Q
What fibres are found within the anterior corticospinal tract?
Where do these meet the LMN and what does this innervate?
A
- 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
20
Q
A
21
Q
What are the 4 main corticobulbar tracts?
A
- corticobulbar tracts pass from the cortex to the brainstem
- corticonuclear (travel to cranial nerve nuclei)
- corticorubral (to red nucleus)
- corticoreticular (to reticular formation)
- corticocollicular (to superior colliculus)
22
Q
What is the purpose of the corticoreticular pathway?
A
- 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
23
Q
What is the corticorubral pathway and how is it organised?
A
- 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
24
Q
What is the purpose of the corticocollicular pathway?
A
- this allows for cortical input to the superior colliculus and horizontal gaze centres
- this allows for cortical coordination of voluntary and involuntary eye movements
25
Where does the vestibulospinal tract pass to and from?
How is it divided?
* the vestibulospinal tract passes from the **labyrinth** to the **spinal cord**
* there are both **_medial_** and **_lateral_ vestibulospinal tracts**
26
Describe the pathway of the medial vestibulospinal tract
What is the role of this tract?
* 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_**
27
Describe the pathway of the lateral vestibulospinal tract
What is its function?
* 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)
28
How can the medial longitudinal fasciculus (MLF) be divided into 2 components?
**_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
29
Describe the pathway of the tectospinal tract
What is the purpose of this pathway?
* 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*
30
Describe the pathways and actions of the medial and lateral reticulospinal tracts
* 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_***
31
Describe the pathway of the rubrospinal tract
What is its function?
* 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
32
What structure does the rubrospinal tract run parallel to and why is this significant?
* 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*
33
What happens if there is an injury above and below the level of the red nucleus in a comatose patient?
**_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_**
34
Label the descending tracts
