7 - The Motor System

Question 1

How many neurones is the motor system made up of and where do these neurones originate from?

Answer 1

2 unlike 3 in the sensory

• Upper motor neurone from primary motor cortex in CNS
• Lower motor neurone from ventral horn of spinal cord or nuclei in brainstem, both in CNS
• LMN projects onto skeletal muscle

Question 2

Where are upper motor neurones NOT found?

Answer 2

• Cerebellum
• Basal ganglia

Question 3

Where in the spinal cord do motor neurones run?

Answer 3

• Corticospinal tract

(85% lateral to distal mucles for fine motor control, 15% anterior to proximal postural)

Question 4

Where are lower motor neurones found and what is their function?

Answer 4

- Ventral horn of spinal cord or motor-nuclei in brain stem

• Cause muscle contraction
• Involved in spinal reflexes, sensory neurones from muscle spindles synapse on LMN in ventral horn of spinal cord

Question 5

How does the patella reflex work?

Answer 5

- Spindle muscle fibres detect stretch so sensory neurone activated and synapses on LMN in L3

• Sensory neurone activates L3 LMN so contraction of quadriceps
• Sensory neurone also branches off and synapses with inhibitory interneurone at L5 to cause hamstring relaxation

Question 6

How can lower motor neurones be damaged?

Answer 6

CNS and PNS lesions as body in CNS but axons in PNS!!!

Question 7

What are signs of a lower motor neurone lesion?

Answer 7

- Hypotonia: due to loss of muscle activation

- Areflexia: no LMN to complete reflex arc

- Dennervation Muscle atropy: LMN supplies trophic factors to muscle so lost

- Fasiculations:due to upregulation of muscle nAChRs to try to compensate for denervation

- Weakness/Flaccid paralysis

- Fibrillations

- Weakness: dennervation

Question 8

What is the connection like between most upper and lower motor neurones?

Answer 8

Via inhibitory interneurones mostly net inhibition until spindles send excitatory sensory neurones to overcome the inhibitory, e.g wanting to stand up

Question 9

What is the internal capsule and what are the different parts of it?

Answer 9

- Genu: UMNs that supply face

- Anterior Limb

- Posterior limb

White matter structure situated in the inferomedial part of each cerebral hemisphere of the brain carrying motor and sensory information. Encapsulates the thalamus

Carpal tunnel of hand

Question 10

What is a peduncle?

Answer 10

White matter connecting a hemisphere

Question 11

What is the pathway of upper motor neurones from the motor cortex to the lower motor neurones for spinal nerves?

Answer 11

1. Corona radiata
2. Internal capsule
3. Cerebral peduncle in the midbrain
4. Pons
5. Medullary pyramids
6. Decussation of the pyramids (in the caudal medulla)
7. Lateral corticospinal tract (in the lateral funiculus of the cord)
8. Ventral horn
9. Synapse (directly but usual indirectly via inhibitory interneurones) on LMNs

Question 12

Where are upper motor neurones found and what is their function?

Answer 12

• Cell body and axon in primary motor cortex (pre-central gyrus) and synapse onto LMN at nuclei or ventral horn
• Excite LMNs by direct synapses or inhibit LMNs by projecting onto inhibitory interneurones
• Net effect is inhibitory so lesion of UMN leads to hyperactivity of muscles

Question 13

How do upper motor neurones travel when they are travelling to spinal nerves as their target?

Answer 13

• Cell bodies lateral/medial in motor cortex depending on which spinal nerve/homunculus
• Axons descend through corona radiata then through the internal capsule (space between lentiform nucleus and thalamus)
• Radiations that create the corona radiata come very close and descend through the cerebral peduncle in the brainstem, the pons and medullary pyramids
• At the pyramids there is decussation
• Axons descend in lateral corticospinal tract to ventral horn of required spinal level where they will synapse with LMN

Question 14

How do upper motor neurones travel when they are travelling to cranial nerves (using the facial nerve as an example) as their target?

Answer 14

• UMN cell body is lateral in motor cortex as going to facial nerve nucleus where they will synapse with LMN
• UMNs supplying the upper face (top eyelid above) prject contralterally and ipsilaterally
• UMNs supplying the lower face project contralaterally only

Question 15

If there was a stroke affecting the left upper motor neurone in the facial area of the homunculus, what would be the motor loss to the muscles of facial expression and why?

Answer 15

• Loss of muscles on lower half of right face due to decussation and the upper right face being supplied by right UMN too
• FOREHEAD SPARING of occipitofrontalis
• Whereas if you had Bell’s palsy this is a lesion to the facial nerve so complete paralysis

Question 16

How could you tell if ipsilateral facial muscle weakness was due to an MCA stroke or Bell’s palsy?

Answer 16

• Bell’s palsy is lesion of facial nerve so all muscles affected, e.g cannot raise eyebrow. LMN lesion
• Stroke may be forehead sparing so can still raise eyebrow. UMN lesion

Question 17

What is the corticobulbar/corticonuclear tract?

Answer 17

Two-neuron white matter motor pathway connecting motor cortex to medullary pyramids, and are primarily involved in carrying the motor function of the non-oculomotor cranial nerves

Question 18

What is the ventral corticospinal tract?

Answer 18

• 15% of UMN do not decussate in the inferior medullary pyramids, they continue ipsilaterally until the level of the LMN and then they decussate
• For proximal postural muscles
• Other 85% of UMNs decussate at pyramids and travel in lateral corticospinal tract for distal musculature

Question 19

What is the golgi tendon reflex?

Answer 19

• Proprioceptive sensoryreceptor organ that senses changes in muscle tension and activated in high tension to protect the bone and muscles
• Inhibits the lower motor neuron in a reflex to protect bone and muscle (biceps curl example) and excites the antagonising muscle

Question 20

What are signs of an upper motor neurone lesion?

Answer 20

- Hypertonia: loss of descending inhibition by inhibitory interneurones

- Spasticity: LMN still in tact but no inhibition

- Hyperreflexia: still LMN

- Clasp Knife rigidity: golgi tendon reflex

- Weakness

- Disue atropy not due to loss of trophic factors

- Babinski’s reflex (Extensor plantar response)

- Acute flaccid paralysis: spinal shock

Question 21

What would the upper and lower limbs look like in an upper motor neurone lesion?

Answer 21

Upper: spastic flexed

Lower: spastic extended

STILL HAVE REFLEXES

Question 22

What is spinal shock?

Answer 22

• In the acute phase (days) of UMN lesion

- Flaccid paralysis with areflexia (like in LMN lesions) but then after a few weeks tone increases (becoming hypertonia) and reflexes become exaggerated (hyperreflexia)

• Related to neuroplasticity in the spinal cord and LMN shutting down following UMN lesion

Question 23

What is Babinski’s sign?

Answer 23

Present in babies and UMN lesions

Question 24

In the lateral corticospinal tract what is the topography of the trunk, arm and legs?

Answer 24

Arm most medially then trunk then leg laterally

Question 25

Where is the internal caspule on this cross section?

Answer 25

Question 26

Where does the corona radiata arise from on this cross section?

Answer 26

Question 27

What are some examples of primitive reflexes that disappear as we mature into adults, and why do they disappear?

Answer 27

**- Babinski:** stimulation of sole of the foot causes plantar flexion in adults but dorsiflexion in babies **- Moro** **-Palmar** Lost due to maturation of descending inhibitory pathways in the corticospinal system

Question 28

What is the best answer?

Answer 28

**E** - Lesion is in the genu which affects the face, may track back into the anterior posterior limb which affects the upper limbs too. Left sided lesion so right sided issue

Question 29

Answer 29

a. Upper limbs as brachial plexus will have LMN signs and sensory loss as loss of grey and white matter in a spinal cord injury so loss of dorsal columns too b. Hypotonia, Hyporeflexia, Weakness, Fasiculations, Atrophy c. Anything below T2 will have upper motor neurone signs as loss of anything coming through the lesion d. Hypertonia, Hyperreflexia, Weakness, Spasticity

Question 30

What part of the brain has to be affected to get Parkinsonian symptoms?

Answer 30

Basal ganglia

Question 31

What is the effect of UMNs on LMNs?

Answer 31

Net inhibitory effect

Question 32

If someone presents with mixed lower and motor neurone signs and they have had no trauma and they are fairly young, what diagnosis should you be thinking about?

Answer 32

Motor Neurone Disease

Question 33

If you lose both your UMN and LMN what signs will you show?

Answer 33

LMN signs

Question 34

Where is the lateral geniculate nucleus located and what is its blood supply?

Answer 34

- Thalamus - PCA - MCA supplies the optic radiations

Question 35

Where does noradrenaline and acetylcholine get released from in the CNS?

Answer 35

Acetylcholine mainly from basal forebrain nuclei and is involved in arousal, learning, memory and motor control

Question 36

What are the different pathways that dopamine is released from?

Answer 36

**Nigrostriatal:** motor control **Mesolimbic and Mesocortical:** mood, arousal and reward

Question 37

Why may a stroke affecting the lateral aspect of the motor cortex compromise swallowing?

Answer 37

Denervation of cranial nerve nuclei which distribute lower motor neurones in the vagus nerve

Question 38

What connects the midbrain to the cerebellum?

Answer 38

Crus Cerebri

Question 39

What structure in the midbrain is important in motor control?

Answer 39

Red Nucleus

Question 40

Which lobe of the brain is the hippocampus found?

Answer 40

Temporal