Descending Motor Pathways Flashcards
How are motor pathways divided?
Into upper and lower motor neurone regions
They involve a 2 neurone pathway
What are the characteristics of upper motor neurones?
Where do they originate from?
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
What are the roles of lower motor neurones?
Where do they originate from?
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
Label the diagram showing a typical lower motor neurone
What type of information is carried by the lower motor neurones?
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
What are the 3 main categories of descending motor pathways?
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
What are examples of extrapyramidal descending motor pathways?
- reticulospinal - from reticular formation
- rubrospinal
- tectospinal
- vestibulospinal
These are mainly involved in modification of the main pathways elicited in the cortex
In general, what is the pathway like of the corticospinal / corticonuclear descending motor pathway?
- cerebral cortex
- cell bodies in the precentral gyrus
- UMN descends via the internal capsule
- it passes between the cerebral peduncles and through the medullary pyramids
- crosses the midline at the decussation of pyramids
What is the difference between corticonuclear and corticospinal pathways?
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
Label the components involved in the corticospinal / corticonuclear descending motor pathway
What areas of the somatotopic organisation of descending fibres are missing?
Through which regions of the internal capsule do the corticospinal/nuclear fibres pass through?
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
Label the components of the internal capsule
the posterior limb is located between the thalamus and the lentiform nucleus
Which
What is the difference between the fibres contained within the internal capsule and the crus cerebri (cerebral peduncles)?
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
Label the features of the cerebral peduncles
What is significant about the descending motor fibres within the crus cerebri?
somatotopic representation is still present
Label the location of descending fibres in different parts of the brain
- 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
Label the components of the corticospinal tract
Do all the fibres cross at the decussation of pyramids?
What kind of innervation does this produce?
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
where do most UMNs contact the cell bodies of the LMNs?
Where do the 2nd order neurones (LMNs) leave the spinal cord?
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
What types of muscles receive bilateral and contralateral innervation?
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
What is meant by bilateral and contralateral innervation and how is it produced?
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)
What happens once the corticospinal tract fibres have left the prefrontal gyrus?
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
What causes a lower motor neurone lesion?
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
What are the consequences of a lower motor neurone lesion?
- 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)
What causes damage to the corticospinal tract (UMN) only?
What are the initial symptoms?
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
What are the longer term consequences of an upper motor neurone lesion?
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
Why do the symptoms of an upper motor neurone lesion change over time?
other pathways appear to take over most “corticospinal” functions
these pathways do not originate in the cortex, so fine movements cannot be controlled
Why are axial muscle groups not affected in an upper motor neurone lesion?
the symptoms are contralateral to the location of the lesion
axial muscles still have supply form the other side due to bilateral control
What test can be used to determine whether there is an upper motor neurone lesion?
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
Where do the fibres of the corticonuclear pathway originate and terminate?
What is the innervation of LMNs like?
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)
What is the exception to the bilateral innervation of the head and neck muscles?
the lower facial and extrinsic tongue muscles are under contralateral control
Label the branches of the facial nerve
What injury would be associated with the following facial symptoms?
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
What is meant by a supranuclear lesion?
What are the side effects?
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
What sign would indicate damage to the facial nerve itself?
paralysis of the whole of one side of the face indicates damage to the facial nerve itself
What is the difference in the sides of the face affected by upper and lower motor neurone lesions?
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
what are the facial signs of an upper motor neurone lesion?
contralateral lower quadrant weakness
this affects the angle of the mouth
What are the facial signs of a lower motor neurone lesion?
it affects the ipsilateral half of the face
this affects the orbicularis oculi muscle and facial muscles leading to:
- inability to close the eyes
- weakness of the angle of the mouth
- inability to elevate the eyebrows
What is the innervation to the extrinsic muscles of the tongue like?
innervation of the extrinsic muscles of the tongue is contralateral
these change the shape and direction of the tongue
If there is a lesion of the left hypoglossal nerve, where will the tongue deviate to?
the tongue will deviate to the left
this is ipsilateral to the lesion
If there is a lesion of fibres coming from the right side of the cortex, where will the tongue deviate to?
the tongue will deviate to the left
this is contralateral to the lesion
What is the difference in UMN and LMN lesions affecting hypoglossal innervation to the extrinsic muscles of the tongue?
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
What structures, other than the cerebral cortex, can be involved in control of muscles?
- basal ganglia
- tectum and red nucleus
- reticular formation
- vestibular system
What are some of the roles of the reticulospinal pathway?
this runs from the reticular formation (in pons and medulla) to the spinal cord
it is invovled in voluntary movement, breathing and consciousness
What are some of the roles of the vestibulospinal pathway?
this runs from the vestibular nuclei (in pons and rostral medulla) to the spinal cord
it is invovled in controlling posture
What are some of the roles of the rubrospinal pathway?
this runs from the red nucleus (in the midbrain) to the spinal cord
it is invovled in controlling muscle tone
Label the descending tracts
Label the locations of the major ascending and descending tracts
What sensory modalities are they concerned with?
dorsal column:
- fine touch
- pressure
- vibration
- joint position sense
spinothalamic tract:
- crude touch
- pain
- temperature
lateral & ventral corticospinal tract:
- voluntary movement
