The motor system Flashcards
What is the internal capsule?
- A bidirectional white matter pathway
- Connects cerebral hemispheres with the rest of the CNS
What is the internal capsule continuous with?
- Superiorly continuous with corona radiata
- Inferiorly continuous with cerebral peduncle of midbrain
What does the internal capsule contain?
- Primarily contains descending axons of upper motor neurones
- Also contains ascending axons of third order sensory neurones
What are the three anatomical divisions of the internal capsule?
- Anterior limb
- Genu
- Posterior limb
What does the anterior limb of the internal capsule contain?
- Contains axons connecting the motor cortex with the cerebellum
What does the genu of the internal capsule contain?
- The bend in the internal capsule
- Contains axons of upper motor neurones supplying the face
What does the posterior limb of the internal capsule contain?
- Contains axons of upper motor neurones supplying upper limb, trunk and lower limb in that order from anterior to posterior
- Also contain third order sensory axons connecting thalamus to postcentral gyrus
- The third order sensory fibres run in the order face-arm-trunk from anterior to posterior
Which stroke can affect the internal capsule?
- Lacunar strokes
What is the blood supply to the internal capsule?
- Lenticulostriate arteries (branches of the MCA)
Which part of the corticospinal tract is most frequently affected by clinical conditions?
- The lateral cerebral spinal tract
- Supplies distal muscles
Where do the motor axons of the internal capsule decussate?
- 85% of axons decussate at the decussation of the pyramids to form the lateral corticospinal tract
- The remaining 15% remain ipsilateral and descend in the ventral funiculus as the ventral corticospinal tract
- They then decussate at the level of the target
What does the ventral corticospinal tract supply?
- Proximal and trunk muscles
What is the spinal cord like at the cervical enlargement?
- It’s wide
- Correlates with the presence of the brachial plexus
- There are many LMN cell bodies and second order sensory neurone cell bodies at these levels to supply the upper limbs
What is the spinal cord like at the thoracic levels?
- Narrow
- Because there are relatively few LMN cell bodies and second order sensory cell bodies
- Due to relatively small dermatomes and myotomes at these levels
What is the spinal cord like at the lumbosacral enlargement?
- Correlates with the presence of the lumbosacral plexus
- There are many LMN cell bodies and second order sensory neurone cell bodies at these levels to supply the lower limbs
What is the spinal cord like at the lower sacral levels?
- Relatively few muscles that need supplying and a small area of skin
- However S2-S4 contains parasympathetic preganglionic cell bodies and the cell bodies of LMNs
- These distribute to perineum in the pudendal nerve
Outline the trends in white matter at different cord levels
- At C1, there is the maximum amount of white matter
- As we descend the cord, the UMN axons are distributed to LMNs in the cord grey matter
- But as we ascend the cord, sensory axons are gradually added
- The net effect is a gradual increase in white matter as we ascend the cord
Why is the maximum amount of white matter found at C1?
- It contains UMN axons yet to be distributed as well as the convergence of all sensory axons from levels below
What can the dorsal and ventral horns be thought of as?
- Cell columns running the length of the cord
- Within these columns, we have subdivisions containing groups of LMN cell bodies
What sits most laterally in the ventral horn?
- Lower motor neurones supplying distal muscles
- This makes sense as they sit closest to the lateral CST which supplies distal muscles
What sits most medially in the ventral horn?
- Lower motor neurones supplying proximal muscles
- This makes sense as they sit closest to the anterior/ventral CST which supplies proximal muscles
What are lower motor neurones?
- The final common path
- Cause muscle contraction when activated
- Controlled by upper motor neurones
Where do upper motor neurones descend?
- Descend through cord or brainstem and synapse on LMNs
- Their cell bodies are found in the ventral horn and in cranial nerve motor nuclei
Which cranial nerve motor nuclei contain upper motor neurones?
- Oculomotor nucleus
- Trochlear nucleus
- Trigeminal motor nucleus
What reflexes do upper motor neurones participate in?
- Spinal reflexes
- Particularly deep tendon reflexes
What typically activates upper motor neurones?
- Incoming impulses from sensory neurones that communicate with muscle spindles
- This causes the muscle stretch reflex
- But can also be inhibited (inhibition of antagonist muscles)
Why do primitive spinal reflexes disappear as a baby grows?
- Due to maturation of descending upper motor neurones
What are lower motor neurone signs?
- Weakness (due to denervation)
- Areflexia (due to denervation)
- Wasting
- Hypotonia (due to loss of activation)
- Fasciculation
Why does damage to lower motor neurones cause wasting?
- Due to loss of trophic support to the muscle across the neuromuscular junction
Why does damage to lower motor neurones cause fasciculation?
- Up-regulation of muscle nicotinic acetylcholine receptors
- In an attempt to compensate for denervation
Where are upper motor neurones found?
- Primary motor cortex (precentral gyrus)
Where do upper motor neurones synapse?
- They synapse on lower motor neurones in the ventral horn or cranial nerve motor nuclei
Describe motor neurones in the basal ganglia and cerebellum
- NOT upper motor neurones
- Because they do not synapse on lower motor neurones in the ventral horn or cranial nerve motor nuclei
What is the net effect of UMNs on LMNs?
- Inhibition
Outline the descent of UMNs
- Corona radiata
- Internal capsule
- Cerebral peduncle in the midbrain
- Pons
- Medullary pyramids
- Decussation of the pyramids (in caudal medulla)
- Lateral corticospinal tract (in lateral funiculus of cord)
- Ventral horn
- Synapse on LMNs
How do UMNs indirectly synapse on LMNs?
- Via inhibitory interneurones
What is the lateral corticospinal tract involved with?
- Fine motor control in the limbs, primarily the distal extremities
- But all of limb can be affected by an UMN lesion
Outline how UMNs that supply facial structures travel to their targets
- Facial structures are innervated by cranial nerves, not spinal nerves
- They leave the pathway in the brainstem and form the corticobulbar tract
- This innervates LMNs in the cranial nerve motor nuclei
Describe the facial motor nucleus
- It is split into two halves - one supplies the superior face (occipitofrontalis) and the other supplies the inferior face (most of the remaining muscles)
Why do UMN lesions involving the face spare the forehead?
- The part of the facial motor nucleus that supplies the upper half of the face receives UMNs from both hemispheres
- The part that supplies the lower face only receives contralateral input
What makes an upper motor neurone lesion different from a facial nerve palsy?
- These affect all the muscles of facial expression
What are upper motor neurone signs?
- Weakness due to loss of direct excitatory inputs onto LMNs from UMNs
- Hypertonia due to loss of descending inhibition
- Hyperreflexia due to loss of descending inhibition
- Extensor plantar reflexes due to loss of descending modulation of spinal reflexes
What is spinal shock?
- Phenomenon that occurs in the days immediately following a UMN lesion
- Initially there is flaccid paralysis with areflexia
- But then tone increases (becomes hypertonia)
- Reflexes become exaggerated (hyperreflexia)
- This is related to neuroplasticity in the spinal cord
