Descending Motor Pathways Flashcards
What are motor pathways divided into?
Upper and lower motor neuron regions (UMN and LMN)
Where do UMNs originate? Where do they end?
- Originate in the cerebrum and subcortical structures
- End/synapse in spinal cord or brainstem
- Synapse with cranial nerve nuclei in brainstem which then go on to supply head
- Synapse in spinal cord with nuclei that go on to supply the body
Function of UMNs?
- Influence LMN activity
- Modify local reflex activity
- Superimpose more complex patterns of movement
Where do LMNs originate?
Brainstem or spinal cord (ventral grey horn)
What are motor end plates?
Also called neuromuscular junctions - specialised chemical synapses formed at the sites where the terminal branches of the axon of a motor neuron contact a target muscle cell.
Where does the cell body of a LMN lie?
Within the ventral horn of the spinal cord OR the brainstem motor nuclei of the cranial nerves (within the CNS)
After the axon of a LMN exits the CNS, it forms the somatic motor part of the PNS. Where does it terminate?
On the muscle fibres which it innervates.
N.B. It is important to note that although one LMN will innervate several muscle fibres, a single muscle fibre is innervated by only one LMN.
What is released by LMNs at the neuromuscular junction (NMJ)? What does this cause?
The neurotransmitter acetylcholine, which causes firing of an action potential in the receiving muscle fibre.
What is acetylcholine? Function?
The chief neurotransmitter of the parasympathetic nervous system, the part of the autonomic nervous system (a branch of the peripheral nervous system) that contracts smooth muscles, dilates blood vessels, increases bodily secretions, and slows heart rate.
Cross section through spinal cord. Sensory info enters via dorsal root. How does motor info leave the spinal cord?
Cell bodies of LMNs found in ventral grey horn. Axons (efferent fibres) then exit via the ventral root to go towards muscle
Different types of descending motor pathways:
N.B. the name of the pathway tells you the origin and destination of the pathway
The motor tracts can be functionally divided into two major groups: Pyramidal tracts and Extrapyramidal tracts.
Describe the origin and function of both of these tracts.
- Pyramidal:
- These tracts originate in the cerebral cortex
- Carry motor fibres to the spinal cord and brain stem
- They are responsible for the voluntary control of the musculature of the body and face.
- Extrapyramidal:
- These tracts originate in the brain stem
- Carry motor fibres to the spinal cord
- They are responsible for the involuntary and automatic control of all musculature, such as muscle tone, balance, posture and locomotion
Where do the pyramidal tracts derive their name from?
the medullary pyramids of the medulla oblongata, which they pass through
Functionally, the pyramidal tracts can be subdivided into two. What are these?
- Corticospinal tracts
- Corticobulbar/corticonuclear tracts
What do the corticospinal tracts supply?
Supplies the musculature of the body.
Where do the corticospinal tracts originate/terminate?
- Originate in cerebral cortex
- After originating from the cortex, the neurones converge, and descend through the internal capsule
- Then the neurones pass through the crus cerebri of the midbrain, the pons and into the medulla.
- Terminate in ventral horn of spinal cord
Where do the corticonuclear tracts orignate/terminate?
- Arise from the lateral aspect of the primary motor cortex
- The fibres converge and pass through the internal capsule to the brainstem.
- The neurones terminate on the motor nuclei of the cranial nerves
- Here, they synapse with lower motor neurones, which carry the motor signals to the muscles of the face and neck.
What is the internal capsule?
A white matter pathway, located between the thalamus and the basal ganglia
Why is the internal capsule clinically relevant?
The internal capsule is particularly susceptible to compression from haemorrhagic bleeds, known as a ‘capsular stroke‘. Such an event could cause a lesion of the descending tracts.
What are the 4 extrapyramidal tracts?
- Vestibulospinal Tracts
- Reticulospinal Tracts
- Rubrospinal Tracts
- Tectospinal Tracts
The extrapyramidal tracts originate in the brainstem, carrying motor fibres to the spinal cord.
What are the extrapyramidal tracts responsible for?
They are responsible for the involuntary and automatic control of all musculature, such as muscle tone, balance, posture and locomotion.
Where do the reticulospinal tracts originate/terminate?
Originate in the reticular formation.
Terminates in spinal cord.
Where do the vestibulospinal tracts originate/terminate?
Originate in vestibular nuclei in brainstem.
Terminate in spinal cord.
Where do the tectospinal tracts originate/terminate?
Originate in tectum of brainstem.
Terminate in spinal cord.
Where do the rubrospinal tracts originate/terminate?
Originate in red nucleus (a midbrain structure).
Terminate in spinal cord.
Corticospinal and corticonuclear pathways summary:
Cerebral cortex –> Precentral gyrus –> internal capsule –> braimstem/spinal cord
- Corticospinal:
- cortex to spinal cord
- spinal nerves
- cortex to spinal cord
- Corticonuclear:
- cortex to brainstem nuclei
- cranial nerves
- cortex to brainstem nuclei
Somatotopic organisation of descending fibres: motor homunculus
Fibres originate at specific location in precentral gyrus; leg medially, face laterally
The internal capsule consists of three parts and is V-shaped when cut horizontally, in a transverse plane.
What are these 3 parts?
- The genu is the bend in the V
- Anterior limb: the part in front of the genu, between the head of the caudate nucleus and the lentiform nucleus
- Posterior limb: between the thalamus medially and lentiform nucleus laterally
The descending fibres fibres retain somatotopic representation as they pass through internal capsule. Where do the fibres going to the a) face b) arm c) trunk d) leg
a) travel via the genu
Fibres of the corticospinal tract use the posterior limb –> somatotopically organised:
b) fibres going to arm travel more anteriorly in the posterior limb
c) fibres going to trunk travel in the middle in the posterior limb
d) fibres going to leg travel more posteriorly in the posterior limb
What are the descending fibres of the internal capsule continuous with inferiorly?
Crus cerebri (cerebral peduncles)
After the internal capsule, where do the descending neurones pass?
through the crus cerebri of the midbrain, the pons and into the medulla.
Somatic representation is still present as the fibres pass to the crus cerebri. What is the location of a) face b) arm c) trunk d) leg?
- Face fibres: medially
- Leg: laterally
What is the blood supply of the internal capsule?
The internal capsule and basal nuclei are supplied by perforating branches of the anterior cerebral artery (ACA), Heubner’s artery, middle cerebral artery (MCA), internal carotid artery (ICA) and anterior choroidal artery (AChA).
Also known as lenticulostriate arteries
What happens if there is a haemorrhage of the lenticulostriate arteries? Why is the internal capsule particularly vulnerable?
- Vulnerable to vascular damage.
- Many important sensory and motor fibres in small area
- Serious sensory and motor deficits
Lenticulostriate arteries are small end arteries. In which condition are they at risk?
Hypertension
Brainstem path of descending pathways
- Start in cortex
- Pass through genu/posterior limb of internal capsule
- Pass through crus cerebri
- Fibres pass through ventral pons
- In the medulla, they pass through medullary pyramids and pass through midline at decussation
Overview of pathway of corticospinal tracts:
- UMN in pre-central gyrus sends axon via the posterior limb of the internal capsule
- Axon descends and travels through cerebral peduncles
- Axon descends in ventral pons and then medullary pyramids
- At level of caudal medulla, the majority of fibres cross the midline
- Then join the lateral corticospinal tract
- Here fibres are contralateral descending fibres
- At a specific spinal cord level, then synapse with a LMN at the ventral grey horn
- LMN then exits spinal cord
In the most inferior (caudal) part of the medulla, the corticospinal tract divides into two. What are these 2 tracts it divides into?
-
Lateral corticospinal tract (85%):
- These fibres decussate (cross over to the other side of the CNS).
- They then descend into the spinal cord, terminating in the ventral horn (at all segmental levels).
- From the ventral horn, the lower motor neurones go on to supply the muscles of the body.
-
Anterior/ventral corticospinal tract (15%):
- Remains ipsilateral, descending into the spinal cord.
- They then decussate and terminate in the ventral horn of the cervical and upper thoracic segmental levels OR remain ipsilateral
Which category of muscles does the lateral corticospinal tract innervate?
Appendicular limb musculature (contralateral innervation)
Which category of muscles does the anterior corticospinal tract innervate?
Axial musculature (bilateral innervation)
How does the anterior corticospinal tract provide bilateral innervation?
At appropriate cord level some fibres cross, others remain ipsilateral.
E.g. postural muscles tend to work in pairs not individually
Somatotopic representation is retained through the cord and through the lateral corticospinal tract.
- Fibres to arm travel more medially in tract
- Fibres to trunk in middle
- Fibres to leg more laterally
What is the Babinski reflex? Who is this seen in?
- Sole of foot is firmly stroked
-
A positive Babinski sign: happens when the big toe bends up and back to the top of the foot and the other toes fan out
- ONLY normal in children under 2 years
- In people above 2 years, this is a sign of neurological condition (UMN lesion)
- Normal plantar reflex: downward flexion of toes towards source of stimulus
-
A positive Babinski sign: happens when the big toe bends up and back to the top of the foot and the other toes fan out
Where do the corticonuclear neurons originate?
Arise in the head region (lateral aspect) of primary motor cortex/precentral gyrus
Overview of corticonuclear pathway
- Cell body in precentral gyrus (lateral aspect)
- Send axons via the genu of the internal capsule
- Axons then contact cell bodies of LMNs on BOTH SIDES (bilateral)
- This occurs for most LMNs
- LMN then sends fibres via cranial nerves to innervate head and neck
The innervation of the corticonuclear tract is largely bilateral. What are the exceptions to this?
- Lower facial nucleus
- E.g. right side of precentral gyrus innervates the left lower facial nucleus
- Hypoglossal nucleus
- Extrinsic tongue muscles innervates contralaterally
These are under contralateral innervation.
Ventral view of emergence of facial nerve from brainstem
Cerebellopontine angle
View of motor terminal branches of facial nerve
What is the facial colliculus?
An elevated area located on the pontine tegmentum (dorsal pons) in the floor of the fourth ventricle. It is formed by fibres from the facial motor nucleus of the facial nerve as they loop over the abducens nucleus (NOT formed by the facial nucleus).
Cross section through caudal pons: view of facial colliculus
Corticonuclear input to facial motor nuclei:
- At the level of the upper facial region of the precentral gyrus:
- UMN sends axon via genu of internal capsule
- Sends bilateral projections to both sides of upper facial nucleus
- Synapses with LMNs which go on to innervates the upper face regions
- At the level of the lower facial region of the precentral gyrus:
- UMN sends axon via genu of internal capsulse
- Axon crosses midline to supply the contralateral lower facial nucleus
- Synapses with LMNs which go on to innervates the lower face contralateral regions
All of these LMNs travel together via the facial nerve.
- Which case presents a transient ischaemic attack at the level of the internal capsule?
- Which case presents an infection within the facial canal (Bell’s palsy)?
Which side is the lesion? At what level (UMN or LMN)?
- Weak lower face on right side
- LEFT corticonuclear damage
- Upper face spared as fibres coming from left precentral gyrus innervate right upper facial nucleus (bilateral)
- Transient ischaemic attack at the level of the internal capsule
- Weak upper and lower face on right side
- RIGHT facial nerve or nucleus damage
- Fibres from upper and lower right facial nuclei lost
What does a supranuclear lesion to the corticonuclear/corticobulbar tracts result in?
- 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
Diagram of right corticobulbar damage (UMN) and left facial nerve damage (LMN)
describe the corticonuclear route regarding CN XII
- UMN fibres originate from (e.g. left) precentral gyrus
- Pass through genu of internal capsule
- Reach contralateral hypoglossal nucleus where they synapse with LMN
- LMNs then travel via hypoglossal nerve to extrinsic muscles of tongue
Where do you think the tongue will deviate if we lesion the left hypoglossal nerve?
The tongue will deviate to the left (ipsilateral to the lesion) - fibres have already decussated
Where do you think the tongue will deviate if we lesion the fibres coming from the right side of the cortex ?
The tongue will deviate to the left (contralateral to the lesion)
What is the reticulospinal pathway responsible for?
- Reticular formation (pons & medulla) to spinal cord
- Voluntary movement/ breathing/consciousness
What is the vestibulospinal pathway responsible for?
- Vestibular nuclei (pons and rostral medulla) to spinal cord
- Controls posture
What is the rubrospinal pathway responsible for?
- Red nucleus (midbrain) to spinal cord
- Controls muscle tone
Summary of descending tract location:
Major ascending and descending fibre tracts: Heavily shaded tracts are sensory or motor to the right side of the body
