Descending Motor Pathways

Question 1

How are motor pathways divided?

Answer 1

Into upper and lower motor neurone regions

They involve a 2 neurone pathway

Question 2

What are the characteristics of upper motor neurones?

Where do they originate from?

Answer 2

Originate in the cerebrum and subcortical structures

they influence lower motor neurone activity

they modify local reflex activity

they superimpose more complex patterns of movement

Question 3

What are the roles of lower motor neurones?

Where do they originate from?

Answer 3

They originate from the brainstem and the ventral grey horn of the spinal cord

they are peripheral nerves which travel to motor end plates / neuromuscular junctions

they are in direct contact with the muscle

Question 4

Label the diagram showing a typical lower motor neurone

Answer 4

Question 6

What type of information is carried by the lower motor neurones?

Answer 6

Afferent nerve:

• visceral sensory and somatic
• these travel in the dorsal root

Efferent nerve:

• somatic motor
• these travel in the ventral root
• the cell body is in the ventral grey horn

Question 7

What are the 3 main categories of descending motor pathways?

Answer 7

Corticospinal:

• the cell body is in the cortex and it runs to the spinal cord

Corticobulbar / corticonuclear:

• corticobulbar is from the cortex to the brainstem
• corticonuclear is from the cortex to the cranial nerve nuclei in the brainstem

Extrapyramidal:

• these originate in other regions outside of the cerebrum

Question 8

What are examples of extrapyramidal descending motor pathways?

Answer 8

• reticulospinal - from reticular formation
• rubrospinal
• tectospinal
• vestibulospinal

These are mainly involved in modification of the main pathways elicited in the cortex

Question 9

In general, what is the pathway like of the corticospinal / corticonuclear descending motor pathway?

Answer 9

1. cerebral cortex
2. cell bodies in the precentral gyrus
3. UMN descends via the internal capsule
4. it passes between the cerebral peduncles and through the medullary pyramids
5. crosses the midline at the decussation of pyramids

Question 10

What is the difference between corticonuclear and corticospinal pathways?

Answer 10

corticonuclear:

• from the cortex to the brainstem
• lower motor neurone located in the cranial nerve nuclei for a specific function

corticospinal:

• from the cortex to the spine
• involves spinal nerves

Question 12

Label the components involved in the corticospinal / corticonuclear descending motor pathway

Answer 12

Question 13

What areas of the somatotopic organisation of descending fibres are missing?

Answer 13

Question 14

Through which regions of the internal capsule do the corticospinal/nuclear fibres pass through?

Answer 14

the fibres retain somatotopic organisation as they pass through the internal capsule

those travelling to the face travel in the genu

those travelling to the arm, trunk and leg travel in the posterior limb

Question 15

Label the components of the internal capsule

Answer 15

the posterior limb is located between the thalamus and the lentiform nucleus

Question 16

Which

Question 17

What is the difference between the fibres contained within the internal capsule and the crus cerebri (cerebral peduncles)?

Answer 17

the internal capsule also contains ascending sensory fibres that connect to the thalamus

the internal capsule connects to the crus cerebri

the peduncles contain descending fibres only

Question 18

Label the features of the cerebral peduncles

Answer 18

Question 19

What is significant about the descending motor fibres within the crus cerebri?

Answer 19

somatotopic representation is still present

Question 20

Label the location of descending fibres in different parts of the brain

Answer 20

• fibres move from the peduncles, through the basal pons and pyramids
• in the pons, the fibres are interrupted by transpontine fibres
• the fibres recollect to travel in the pyramids of the medulla
• some fibres will cross the midline at the decussation of pyramids

Question 21

Label the components of the corticospinal tract

Do all the fibres cross at the decussation of pyramids?

What kind of innervation does this produce?

Answer 21

85% of fibres cross at the decussation of pyramids and then enter the lateral corticospinal tract

these produce contralateral innervation

15% of UMNs descend the cord ipsilaterally in the anterior corticospinal tract

these produce bilateral innervation

Question 22

where do most UMNs contact the cell bodies of the LMNs?

Where do the 2nd order neurones (LMNs) leave the spinal cord?

Answer 22

UMNs contact cell bodies of LMNs in the contralateral ventral grey horn

the 2nd order neurones then leave the spinal cord as ventral rootlets to form spinal nerves

Question 23

What types of muscles receive bilateral and contralateral innervation?

Answer 23

bilateral innervation:

• comes from the anterior corticospinal tract
• for axial musculature (in the midline)
• at the appropriate SC level, some fibres will cross and some remain ipsilateral

contralateral innervation:

• comes from the lateral corticospinal tract
• for limb musculature
• fibres cross at the decussation of pyramids

Question 24

What is meant by bilateral and contralateral innervation and how is it produced?

Answer 24

contralateral:

• 85% of corticospinal tract fibres decussate at the pyramids and descend in the lateral corticospinal tract
• they contact the LMN in the contralateral ventral grey horn

bilateral:

• 15% of fibres remain ipsilateral in the anterior / ventral corticospinal tract
• they contact the LMN that project to both sides of the respective spinal cord level (ipsilateral and contralateral ventral grey horns)

Question 25

What happens once the corticospinal tract fibres have left the prefrontal gyrus?

Answer 25

they descend through the posterior limb of the internal capsule the fibres descend through the cerebral peduncle of the midbrain, ventral pons and pyramids of the medulla

Question 26

What causes a lower motor neurone lesion?

Answer 26

lesion to ventral grey horn cells of the spinal cord / brainstem or their axons peripheral nerve injury (crush or cut) poliomyelitis - an infection of the cell bodies of LMNs

Question 27

What are the consequences of a lower motor neurone lesion?

Answer 27

* **flaccid paralysis** of the muscles involved * diminished (**hyporeflexia**) or absent (**areflexia**) tendon reflexes at the level of the lesion * muscle wasting * muscle weakness / reduced power * hypotonia * fasciculation / fibrillation (spontaneous abnormal contractions)

Question 28

What causes damage to the corticospinal tract (UMN) only? What are the initial symptoms?

Answer 28

this is due to lesion to cerebral hemisphere or as they descend to lateral white column of the spinal cord initial symptoms: * **flaccid paralysis** of opposite limbs * loss of tendon reflexes

Question 29

What are the longer term consequences of an upper motor neurone lesion?

Answer 29

after several days to a week, motor function recovers but there is **hypertonia** (increased muscle tone) **long term:** * increased, brisk (**hyperreflexia**) spinal reflex BELOW lesion * spastic paralysis of the involved muscles * loss of fine motor control and permanent inability to carry out fine movements of hands and feet

Question 30

Why do the symptoms of an upper motor neurone lesion change over time?

Answer 30

other pathways appear to take over most "corticospinal" functions these pathways do not originate in the cortex, so fine movements cannot be controlled

Question 32

Why are axial muscle groups not affected in an upper motor neurone lesion?

Answer 32

the symptoms are contralateral to the location of the lesion axial muscles still have supply form the other side due to bilateral control

Question 33

What test can be used to determine whether there is an upper motor neurone lesion?

Answer 33

**Babinski test** a sharp object is run along the surface of the foot and the toes should become flexed In UMN lesion, the big toe will be extended and the other toes will fan out

Question 34

Where do the fibres of the corticonuclear pathway originate and terminate? What is the innervation of LMNs like?

Answer 34

fibres originate laterally within the **precentral gyrus** they influence LMNs in the **cranial nerve motor nuclei** innervation of LMNs is mostly **bilateral** (if there is a lesion in one side, the face is not affected as the other side can take over)

Question 35

What is the exception to the bilateral innervation of the head and neck muscles?

Answer 35

the lower facial and extrinsic tongue muscles are under contralateral control

Question 36

Label the branches of the facial nerve

Answer 36

Question 37

What injury would be associated with the following facial symptoms?

Answer 37

**lower motor neurone lesion:** * Bell's palsy * painless unilateral weakness of the facial muscles * weakness is present on the upper and lower face of the ipsilateral side **upper motor neurone lesion:** * weakness of the lower face only * lesion is contralateral to lower facial muscles

Question 38

What is meant by a supranuclear lesion? What are the side effects?

Answer 38

it is unilateral damage to the corticobulbar fibres this deprives the **lower half** of the opposite facial motor nucleus of corticobulbar input results in paralysis of the **lower half** of the face on the opposite side to the lesion

Question 40

What sign would indicate damage to the facial nerve itself?

Answer 40

paralysis of the whole of one side of the face indicates damage to the facial nerve itself

Question 41

What is the difference in the sides of the face affected by upper and lower motor neurone lesions?

Answer 41

**upper motor neurone lesion:** * affects the contralateral side of the face * this is the opposite side to the lesion **lower motor neurone lesion:** * affects the ipsilateral side of the face * this is the same side as the lesion

Question 42

what are the facial signs of an upper motor neurone lesion?

Answer 42

contralateral **lower quadrant** weakness this affects the angle of the mouth

Question 43

What are the facial signs of a lower motor neurone lesion?

Answer 43

it affects the ipsilateral **half** of the face this affects the **orbicularis oculi** muscle and facial muscles leading to: 1. inability to close the eyes 2. weakness of the angle of the mouth 3. inability to elevate the eyebrows

Question 44

What is the innervation to the extrinsic muscles of the tongue like?

Answer 44

innervation of the extrinsic muscles of the tongue is **contralateral** these change the shape and direction of the tongue

Question 45

If there is a lesion of the **left hypoglossal nerve**, where will the tongue deviate to?

Answer 45

the tongue will deviate to the left this is ipsilateral to the lesion

Question 46

If there is a lesion of fibres coming from the **right side of the cortex**, where will the tongue deviate to?

Answer 46

the tongue will deviate to the **left** this is **contralateral** to the lesion

Question 47

What is the difference in UMN and LMN lesions affecting hypoglossal innervation to the extrinsic muscles of the tongue?

Answer 47

**upper motor neurone lesion:** * deviation is contralateral to the lesion **lower motor neurone lesion:** * the peripheral hypoglossal nerve itself is paralysed * deviation is ipsilateral to the lesion

Question 48

What structures, other than the cerebral cortex, can be involved in control of muscles?

Answer 48

* basal ganglia * tectum and red nucleus * reticular formation * vestibular system

Question 49

What are some of the roles of the **reticulospinal pathway**?

Answer 49

this runs from the **reticular formation** (in pons and medulla) to the spinal cord it is invovled in voluntary movement, breathing and consciousness

Question 50

What are some of the roles of the **vestibulospinal pathway**?

Answer 50

this runs from the **vestibular nuclei** (in pons and rostral medulla) to the spinal cord it is invovled in controlling posture

Question 51

What are some of the roles of the **rubrospinal pathway**?

Answer 51

this runs from the **red nucleus** (in the midbrain) to the spinal cord it is invovled in controlling muscle tone

Question 52

Label the descending tracts

Answer 52

Question 53

Label the locations of the major ascending and descending tracts What sensory modalities are they concerned with?

Answer 53

**dorsal column:** * fine touch * pressure * vibration * joint position sense **spinothalamic tract:** * crude touch * pain * temperature **lateral & ventral corticospinal tract:** * voluntary movement