Week 6 Lecture 13: Descending pathways Flashcards
(43 cards)
What does an ascending pathway do? give 2 examples
pathways that bring information to the brain e.g DCML, spinothalamic tract
What are the lateral corticospinal and ventral corticospinal tracts responsible for?
carrying our descending motor information controlling voluntary movement
What are the features of upper motor neurones?
- originate in the cerebrum and subcortical structures e.g basal ganglia
- influence LMN activity
- motor local reflex activity
- superimpose more complex patterns of movement
How are UMNs that supply the head and neck different?
they don’t travel through the spinal cord - they travel a short distance until they reach the cranial nerve nuclei and from there they synapse and contact with LMN which sends its axon via a cranial nerve
Where do LMNs originate?
- from the brainstem if they are going to innervate structures in the head and neck
- ventral grey horn of spinal cord if they send their axons peripherally to innervate body muscles
How do LMNs leave the spinal cord to go to specific muscle?
via the ventral root
What are the 3 main types of descending motor pathways?
- corticospinal
- corticonuclear/ corticobulbar
- extrapyramidal
How are the extrapyramidal pathways different to corticonuclear and corticospinal pathways?
- they don’t initiate movement, they fine tune movement
- don’t take origin in the cortex
- do not pass through the medullary pyramids (which is why their called extrapyramidal?
Where does the corticospinal tract travel from and to?
from cortex to spinal cord
Where does the corticonuclear tract travel from and to?
from the cortex to the cranial nerve nuclei
What structures does the corticospinal/corticonuclear tract pass through in the brain before reaching the brainstem/spinal cord?
- cerebral cortex
- pre-central gyrus
- internal capsule
- brainstem/ spinal cord
Explain the pathway of fibres in the corticonuclear tract
- UMNs originate in pre-central gyrus in cerebral cortex
- travel through IC
- stops at specific nucleus of a cranial nerve
- make contact with LMN and the axon of LMN leaves the brainstem via a specific cranial nerve to innervate muscle
Explain the pathway of fibres in the corticospinal tract
- UMNs originate in pre-central gyrus in cerebral cortex
- travel through IC
- makes its way through the medullary pyramids and cross the midline at the decccusation of pyramids
- fibres enter the ventral grey horn and make contact with LMNs
- LMNs use specific spinal nerves to innervate skeletal muscle
Where would you find UMNs that innervate the face, arms, trunk and leg in the pre-central gyrus?
face is most lateral
arm
trunk
leg is most medial
(somatotopic organisation)
How do descending fibres retain somatotopic representation as they pass through the internal capsule?
anterior limb–>
genu –> face
posterior limb –> arm, trunk, leg
from lateral to medial is anterior to posterior on a horizontal section
Fibres of the corticospinal tract use which part of the internal capsule?
posterior limb
What is the crus cerebri?
- anterior portion of the cerebral peduncle
- internal capsule connects to crus cerebri
- there is continuity between the fibres of the internal capsule and cerebral peduncles
How is somatotopic representation still present at the level of the cerebral peduncle?
fibres that innervate the face are most medial and the fibres that innervate the legs are most lateral
Explain the corticospinal pathway (8 steps)
- UMN in the pre central gyrus sends its axon via the posterior limb of the internal capsule
- Fibres continue descending - continuous with cerebral peduncles
- Fibres then make their way through the ventral pons
- In the caudal medulla they connect with the pyramids then cross the midline at the decussation of pyramids
- These fibres then join the lateral corticospinal tract: this is contralateral descending motor fibres (started in the opposite hemisphere)
- At a specific spinal cord level, the UMN makes contact with a LMN in the ventral grey horn
- Exit of the lower motor neuron
- OR some fibres do not cross at the pyramids and remain ipsilateral and join the anterior corticospinal tract (15%) and innervates LMNs on both sides of the cord, providing bilateral innervation
How can fibres provide contralateral OR bilateral innervation?
- For limb musculature: 85% of fibres cross at decussation of pyramids, then enter the lateral corticospinal tract –> contralateral innervation
so 85% of UMNs contact cell bodies of LMNs in contralateral ventral grey horns
- For axial musculature: 15% of UMNs descend cord ipsilaterally (entering anterior corticospinal tract) –> bilateral innervation
Which corticospinal tract is responsible for contralateral innervation?
lateral corticospinal tract
Which corticospinal tract is responsible for bilateral innervation?
anterior corticospinal tract
What mechanisms of injury could cause LMN lesions and where would the lesion be?
mechanisms: peripheral nerve injury (crush or cut), infection (poliomyelitis), IV disc herniation
occurs due to lesion to ventral gray horn cells of the spinal cord/brainstem or their axons
What are the clinical consequences of LMN lesions?
- flaccid (soft/weak) paralysis of muscles involved
- dimished (hypoeflexia) or absent (areflexia) tendon reflexes at the level of lesion
- muscle wasting
- muscle weakness (paresis)/ reduced power
- hypotonia - decreased muscle tone
- fasciculation (spontaneous muscle twitches)/ fibrillation (rapid spontaneous muscle contractions)
