7 The Motor System

Question 1

Where can the cell bodies for the Upper and Lower motor neurones be found?

Answer 1

• LMNs: ventral horn and cranial nerve motor nuclei (III, IV, V etc)
• UMNs: NOT found in cerebellum or basal ganglia
  
  • Synapse onto LMNs directly or indirectly

Question 2

Give some examples of primitive spinal relfexes that exist in babies.

Answer 2

Upgoing plantars

Moro reflex

Palmar grasp

Question 3

How are lower motor neurons typically activated? Give an example of how they can be inhibited (think patellar reflex activation)

Answer 3

• Impulses- sensory neurones
  
  • Communicate with muscle spindles

LMNs particpate in spinal relflexes - esp deep tendon reflexes (highly clinically relevant)

Question 4

What are the lower motor neurone signs? (ie what signs might the patient have if LMNs are damaged?) (5)

Answer 4

1. Weakness
2. Hypotonia (loss of muscle activation)
3. Fasciculation (upregulation of nAChRs to compensate for loss of denervation)
4. Areflexia
5. Wasting (loss of trophic support to muscle from LMN across neuromuscular junction)

Question 5

What is the net effect of UMNs on LMNs? (neuro)

Answer 5

Net effect of UMNs on LMNs= inhibition

Question 6

What are the upper motor neurone signs? (ie what signs might the patient have if UMNs are damaged?) (4)

Answer 6

1. Hypertonia (loss of descending inhibition)
2. Weakness (loss of excitatory inputs onto LMNs)
3. Hyperreflexia (loss of descending inhibition, overactive reflex arc)
4. Extensor plantar reflexes (loss of descending modulation of spinal reflexes)

Question 7

What structures do the upper motor neurones pass through before they synapse (usually indirectly via inhibitory interneurones) with the lower motor neurone?

Answer 7

1. Corona radiata
2. Internal capsule
3. Cerebral peduncles of midbrain
   
   1. Where descending motor fibres enter the midbrain
4. Pons
5. Medullary pyramids- decussate
6. Lateral corticospinal tract (lateral funinculus of cord)
7. Ventral horn
8. Synapse on LMNs (usually indirectly via inhibitory interneurones)

Question 8

What type of motor control is the lateral corticospinal tract involved in?

Answer 8

Fine motor control of limbs (esp distal extremities)

Question 9

Upper motor neurones that supply the facial structures (ie cranial nerves) leave the brainstem and form what tract which innervates LMNs in the cranial nerve motor nuclei?

Answer 9

Leave brainstem:

Form corticobulbar/corticonuclear tract

Question 10

Explain why upper motor neurone lesions involving the face will spare the forehead.

Answer 10

• Facial nerve nucleus= split into 2 halves
  
  1. Supplies superior face (occipitofrontalis)
  2. Supplies inferior face (remaining muscles)
• Superior part- receives UMNs from BOTH hemispheres
• Inferior part- receives UMNs from CONTRALATERAL hemisphere

Question 11

What is ‘Spinal shock’? How long does it last?

(UMN damage= only CNS; LMN damage= can be PNS or CNS)

Answer 11

Occurs in days following UMN lesion

Initially:

• Flaccid paralysis
• Areflexia (like LMN lesions)

Then

• Hypertonia (tone increases)
• Hyperreflexia

4 to 12 weeks

Question 12

What part of the internal capsule do the upper motor neurones descend through?

Answer 12

Posterior limb- remember Face Arm leg

Remember genu in middle