Lecture 9- The motor system- upper motor neurones and descending tracts Flashcards
Major motor tract which descends through the spinal cord and controls activity of LMN
The corticospinal tract
The corticospinal tract is responsible for
- Voluntary, precise movements
upper motor neurones supplying the upper limb
- UMN originates in lateral aspect of the motor cortex (homunculus)
- Axon of the UMN passes down through the corona radiata and into the internal capsule
- It then descends down through the midbrain via the cerebral peduncle, then through the pons and then to the medulla
- The medulla has medullary pyramids which carry corticospinal axons on their way down into the spinal cord
- Once the corticospinal axons have left the medullary pyramids the axons decussates and descends via the lateral corticospinal tract in the spinal cord where it synapses with a lower motor neurone and supplies muscles in the upper limb
Upper motor neurone supplying the lower limb
- UMN originates in medial aspect of the motor cortex (homunculus)
- Axon of the UMN passes down through the corona radiata and into the internal capsule (discrete white matter pathway running between the thalamus and lentiform nucleus)
- It then descends down through the midbrain via the cerebral peduncle, then through the pons and then to the medulla
- The medulla has medullary pyramids which carry corticospinal axons on their way down into the spinal cord
- Once the corticospinal axons have left the medullary pyramids the axons decussates and descends via the lateral corticospinal tract in the spinal cord where it synapses with a lower motor neurone and supplies muscles in the lower limb
internal capsule
discrete white matter pathway running between the thalamus and lentiform nucleus
somatotopic organisation of the lateral corticospinal tract
- Axons destined for the upper limb are found more medially
- Need upper limbs axons more medially so they can peel off and synapse in the ventral horn earlier on (as to not waste axon and myelin)
- Axons destined for the lower limb are found more laterally
Not all axons cross at the decussation of the pyramids
15% remain ipsilateral and run down through the ventral corticospinal tract and cross over at the level where their LMN is
summary of the corticospinal system
- System originates in the primary motor cortex
- Lower limb= medial
- Upper limb- laterally
- Pathway descends through the corona radiata and condenses further to form the internal capsule
- Fibres for the upper limb are found most anteriorly within the internal capsule
- Fibres for the lower limb are found most posteriorly within the internal capsule
- As we descend further we enter the brainstem via the cerebral peduncle (ears of mini mouse) of the midbrain
- A number of fibres will leave the corticospinal system and synapsing within the brainstem (fibres of the corticonuclear pathway which supply LMN present within the cranial nerve nuclei)
- The pathway then descend further through the ventral pons and then condense to form the medullary pyramids
- Contain fibres of the corticospinal system which have not yet decussated
- At the decussation of the pyramids the corticospinal fibres cross the midline forming the lateral corticospinal tract
- Not all will cross the midline, some will run ipsilateral (15%)
- These ipsilateral corticospinal fibres will form the ventral corticospinal tract
- Only when they get to the level of the lower motor neurone will they synapse will they cross the midline
Fibres for the upper limb are found most …….. within the internal capsule
anteriorly
Fibres for the lower limb are found most…….. within the internal capsule
posteriorly
A number of fibres will leave the corticospinal system, synapsing within the brainstem and become the
corticonuclear pathway which supply LMN present within the cranial nerve nuclei
when do most corticospinal fibres decussate
at the decussation of the pyramis forming the lateral corticopsinal tract
what happens to the fibres which dont decussate after the medullary pyramids
these will stay ipsilateral forming the ventral corticospinal tract
- Only when they get to the level of the lower motor neurone will they synapse will they cross the midline
Cortical spinal system is composed of 2 separate components:
Lateral corticospinal tract and Ventral corticospinal tract
features of the Lateral corticospinal tract
- Much larger
- Function: innervating muscle of the distal parts of the extremities e.g. arm, hand and leg, foot
- E.g. fine movements
features of the Ventral (anterior) corticospinal tract
- Much smaller and crosses at the level of the lower motor neurone
- Function: innervating muscle of the proximal parts of the extremities e.g. limb girdle
Somatotopic organisation of the lateral corticospinal tract
- Upper limbs = most medially
- Lower limbs= most laterally
corticonuclear pathways
- Cranial nerves can contain the axons of lower motor fibres
- Corresponding upper motor neurones descend from the primary motor cortex and synapse upon on cranial nerve motor nuclei (e.g. trigeminal motor nucleus)
corticonuclear pathway of the trigeminal
- Contains the axons of lower motor neurones
- Cell bodies of lower motor neurones are found in the pons in the trigeminal motor nucleus
- These LMN follow the course of the trigeminal nerve to muscles of mastication (mandibular branch)
cranial nerve motor nuclei (cell bodies of cranial nerve LMN) receive ……… innervation from the cerebral cortex
bilteralteral
- Means that if there was a stroke in one hemisphere destroying one UMN, both nuclei would still have some innervation
the trigeminal nerve and bilateral innervation via the corticonuclear pathway
The trigeminal nerve
- Trigeminal motor nuclei (collection of cell bodies of LMN that will distribute along the mandibular branch of the trigeminal nerve and innervate muscles of mastication)
- How upper motor neurones stimulate lower motor neurones running alongside CNs:
- Bilateral innervation
- UMN descends from the primary motor cortex and synapses with LMN in the trigeminal motor nucleus on the ipsilateral side
- There will also be projections from the same UMN that will innervate the contralateral trigeminal motor nuclei
- Also have projections coming from the other cerebral hemisphere (contralateral and ipsilateral innervation
Effect on muscles of mastication if stroke affecting one hemisphere?
Although one UMN would be destroyed, each nuclei would still have some innervation i.e. from the contralateral UMN–> therefore muscles of mastication can still be used–> survival measure
the facial nerve and bilateral innervation
- Facial motor nucleus (collection of cell bodies of LMN that will distribute along the facial nerve and innervate muscles of facial expression)
-
Split into two halves
- Upper half- upper half of the face
- Bilateral innervation
- Think – stroke = forehead sparing
- Bilateral innervation
- Lower half- lower half of the face
- Only contralateral innervation
- Stroke pts will have much worse set of signs affecting lower half than upper half of face
- Only contralateral innervation
- Upper half- upper half of the face
-
Split into two halves
vagus nerve and bilateral innervation
- Nucleus ambiguous (collection of cell bodies of LMN that will distribute along the vagus nerve and innervates muscles of the vocal cords and pharynx)
- Bilateral innervation
- Required because nucleus ambiguous controls swallowing necessary for survival
The internal capsule
- White matter tract (not really a capsule) between the thalamus (medially) and the lentiform nucleus (laterally -globus pallidus and putamen) and caudate nucleus (laterally)
- Basically the condensation of the corona radiata
types of pathways which run through the internal capsule
Bidirectional pathway
- Descending motor pathways (UMN)
- Ascending sensory pathways (third order sensory neurones)
Structure of the internal capsule
- anterior limb
- posterior limb
- Genu (convergence of the anterior and posterior limb)
posterior limb contains
- Contains descending motor fibres (UMN axons)- Corticospinal tract
-
From anterior to posterior
- Arms
- Trunk
- Lower limbs
-
From anterior to posterior
Genu (knee) contains
UMN supplying facial muscles are found at the genu- contains the corticobulbar (corticonuclear) tract–> CN 7
Within the internal capsule is a discrete region called the thalamic radiation (both anterior and posterior) (light brown)
-
Posterior thalamic radiation
- Contains 3rd order ascending sensory fibres form part of the somatosensory system
-
Anterior thalamic radiation
- Contains projections going from the thalamus to the frontal lobes (not directly related to sensation)
Outside regions of the internal capsule (dark brown) called corticofugal fibres
- Go from the cortex away from the brain
- Don’t worry to much about these
midbrain mickey mouse anatomical orientation
midbrain can be roated 180 degrees to make it more memorable
- label
mickey mouse
Mickey mouse ears = Cerebral peduncles
- Connect the cerebral hemispheres to the midbrain
- Contain descending motor fibres which will enter the corticospinal tract
- Important motor structures
- Eyes = red nucleus
- In fresh brain appears red
- Involved in motor control
- If damaged= corse tremor
- Important landmark
- Eyebrows= Substantia nigra
- Appears dark normally
- produces dopamine for nigrostriatal pathway
- Tears = medial lemniscus
- (part of dorsal column / spinothalamic system)
- Parts of the sensory system
- Ascending sensory fibres on their way to the thalamus
- Mouth= cerebral aqueduct
Channel connecting the third and fourth ventricles
- Lips= periaqueductal grey matter
- Pain regulation
- Micturition
- Nose= oculomotor nucleus
- Cell bodies of LMN
- Edwinger Westphal nucleus (cell bodies of parasympathetic neurones)
- Fibres coming off the nose= fibres of the oculomotor
- Double chin= superior colliculus
Reflex actions related to the visual system
summarise the motor neurone route
- Cell body within the primary motor cortex
- Sends axon down via the corticospinal tract/ Corticonuclear tract where it synapses on the cell body of a lower motor neurone
- LMN then send sits axon out through the peripheral nervous system until it reaches its target muscle
causes of lesions to UMN
- Stroke
- Spinal cord injury
- Motor neurone disease
causes of lesions to LMN
- Peripheral neuropathy
- Motor neurone disease
- Spinal cord injury
- Disc prolapses
clinical features of UMN and LMN lesions
why is power, tone and reflexes decreased in LMN lesions
muscle is deprived of nervous supply- No ACH release therefore wont move
–> atrophy ‘dont use it lose it’
why is power decreased in UMN lesion
because LMN deprived of input
why does tone increase in UMN lesion
most influence of UMN is inhibitory
therefore if lesion, inhibition will be lost, therefore amount of activity of LMN increases
why do reflexes increase in UMN lesions
- intitial phase of decreased reflex activity (spinal shock)
- however after some time reflexes increase due to loss of descending inhibition
- therefore reflexes more reactive
Fasciculations and fibrillations
*
Both features seen in LMN lesion (pathophysiology not fully understood)
- Fasciculations
- Visible
- May be caused by aberrant activity in the distal portion of the LMN axon
- Fibrillations
- Only detectable using electromyography
- May be caused by increased sensitivity of muscles fibres to ACh (changes in localisation of nicotinic receptors)
why does atrophy occur in all LMN and some UMN lesions
- Growth factors (as well as AP) are delivered to the muscle via the LMN0
- If you damage the LMN nucleus or transect the axon you decrease the amount of GF getting to the muscle–> atrophy
- Spasticity= due to
hypertonicity
Monosynaptic reflex arc can help us understand pahtophysiology of hypertonia and spaticity
e.g. in a healthy person
- Sensory neurone receptor within spindle brings info into cord via the dorsal root ganglion
- Enters ventral horn and synapses with lower motor neurone
- Lower motor neurone also synapses with excitatory corticospinal tract neurone
- Important for voluntary movement in this particular muscle i.e. you decide to move a muscle and it cause that motor neurone to cause muscle movement
- Also other tracts will descend down the cord and terminate on inhibitory interneurons which inhibits the lower motor neurone
- Most of the time it is in a state of inhibition
- Controlled by the cortex
- Lower motor neurone also synapses with excitatory corticospinal tract neurone
- Motor neurone sends axons via the ventral root to the muscle of interest
- Muscle twitch
Monosynaptic reflex arc can help us understand pahtophysiology of hypertonia and spaticity
e.g. in someone who has had a stroke
- Cortical lesion e.g. stroke destroys excitatory corticospinal tracts and the descending inhibitory inputs
-
Initially lower motor neurone will go into a state of ‘spinal shock’
- Frequency of AP running along LMN will decrease significantly
- Therefore tone of muscle supplied by this LMN will increase
-
As time goes on the LMN starts to wake up
- Realises it still has the excitatory input of the muscle spindles (sensory afferents)
- However, loss of CST- therefore no voluntary movements
- Lost descending inhibitory influences
- Therefore breaks have been taken off LMN, therefore it will fire off more frequent AP, leading to more frequent muscle contracts = increased muscle tone
- Loss of net inhibition therefore increase excitation
why if someone has a strpke affecting the upper limb, will the arm and hand be held in a flexed position
- The flexor and extender muscles will be affected equally, but in the upper limb the flexors are more powerful
- Flexed arm, wrist and fingers (think cerebral palsy)
Following the diffusion of water pathway along the corticospinal tract
- can see that motor cortex axon found mostly in the precentral gyrus
- yellow fibres= trunk
- blue fibres= lower limb
- red fibres= face
why do lacunar infarcts affect the brain, upper and lower limb equally
- This is due to fibres from the corticospinal tract showing less somatotopic organisation in the internal capsulee.g. face and lower limb fibres mixes
Visualising the internal capsule
Basal ganglia model
- Key components
- Thalamus
- Internal capsule
-
Lentiform nucleus
- Putamen- superficial
- Putamen and caudate nucleus (C shaped) closely related
- Connected to each other- same ancestral nucleus together referred to as the striatum
- The striatum receives input from the cortex
- Globus pallidus- further subdivided
- External
- Internal
- Sends output back to the cortex via the thalamus
- Putamen- superficial
-
Substantia nigra (part of the midbrain)
- Dopaminergic neurones which sends axons up to striatum (putamen)via the Nigro striatal pathways
somatotopic organisation in the ventral horn of the spinal cord
- ventral (anterior) cotricospinal tract supplying the axial musculature and limb gurdles
- lateral corticospinal tract supplying the intrinsic limb muscles and distal portion of the limbs
