The Motor System

Question 1

Differentiate between upper and lower motor neurones

Answer 1

• Upper motor neurone
  
  • Cell body found in primary motor cortex
    
    • Synapse with lower motor neurone within CNS
  • Can be damaged by only CNS lesions
• Lower motor neurone
  
  • Cell bodies in ventral horn or brainstem motor nuclei
    
    • Synapse with target (skeletal muscle)
    • Can be damaged by CNS or PNS lesions

Question 2

Outline the patellar reflex

Answer 2

• Hitting the patellar tendon sends signal through to the ventral horn, which synapses with stimulatory fibres causing contraction of quadriceps
• Reciprocal inhibition of hamstring occurs
  
  • L3 fibres synapses with inhibitory interneurones at L5 level which reduces signals to hamstring

Question 3

Describe how the patellar reflex can be enhanced

Answer 3

• The Jendrassik manoeuvre can be done to enhance the patellar reflex response
• Patient asked to clench teeth, flex both sets of fingers into a hooked form and interlock those sets of fingers together
• Takes the mind off of moving the leg

Question 4

Describe the signs of lower motor neurone damage

Answer 4

• Weakness - interruption of information from nerve to muscle
• Areflexia - absence of muscle reflexes
• Wasting - normally LMN provides trophic factors to muscle
• Fasciculation - uncoordinated muscle contractions
• Hypotonia - decreased muscle tone

Question 5

Describe the effect of UMN on LMN

Answer 5

• UMN can directly excite LMN
• UMN can also excite inhibitory interneurones to LMN
• Net effect of UMN on LMN is inhibitory
  - Lesion in UMN will lead to excitatory effects of LMN

Question 6

Describe the motor pathway from UMN to LMN

Answer 6

• Upper motor neurone cell bodies reside in primary motor cortex and sends axon through white matter
  
  • Has millions of radiating fibres forming white matter called corona radiata
  • Radiating fibres become condensed between thalamus and lentiform nucleus
    
    • Known as internal capsule
    • UMNs towards most of the body found in posterior limb of internal capsule
    • UMNs destined for the face in the genu
• Cerebral peduncle - part of midbrain where descending fibres run
• Decussation takes place in medulla through medullary pyramids
  
  • Pyramids are swelling of the ventral horn
• Fibres descend as lateral corticospinal tract and synapse with LMN to allow complex movement in hands and lower limb
  
  • Not all tracts decussate - anterior cortical spinal tract crosses at level of target muscle

Question 7

Outline the motor pathway from UMN to LMN for facial motor nuclei

Answer 7

• UMN controlling the face originate from lateral homunculus and travel through corticobulbar/corticonuclear tract to the brainstem motor nuclei
• 1/2 of nuclei supplies the upper face and other 1/2 supplies the lower face
• UMN from one half of brain supplies both upper halves of facial motor nuclei but only the contralateral lower half

Question 8

Describe how a UMN lesion synapsing to the facial motor nuclei will present

Answer 8

• A lesion of UMN towards the origin maintains innervation of upper face due to contralateral innervation (UMN lesion of forehead does not affect upper face)
  
  • Leads to contralateral palsy of lower face
  • Not facial nerve palsy - only damages UMN to facial nerve and not LMN
• A lesion causing problems with the upper face shows greater problems as not only UMN is damaged, but LMN as well

Question 9

Describe the signs of upper motor neurone lesions

Answer 9

• Weakness - loss of excitatory projections of LMN
• Hypertonia - increased muscle tone as descending inhibition to LMN taken away
  
  • Can’t move someone’s limb
• Hyperreflexia - reflexes enhanced as don’t need much stimulation to get reflex as easily excited
• Extensor plantar reflex

Question 10

Describe the concept of spinal shock

Answer 10

• Spinal shock - short term consequence of UMN lesions
  
  • Weakness, hypotonia, reduced signally
  • Flaccid paralysis
  • Early UMN lesion signs similar to LMN lesion
• Later signs of UMN lesions are hypertonia, hyperreflexia, extensor plantar reflex

Question 11

Describe the medial longitudinal fasciculus and vestibular nuclei

Answer 11

• Many fibres which connect the brainstem motor nuclei responsible for coordinated eye movements (CN III, IV, VI)
• Vestibular nuclei - sensory nuclei from the vestibular apparatus which convey position of head
  
  • Connected to the medial longitudinal fasciculus through fibres
  • Eg. When walking up hill, allow eyes to compensate so that body is in vertical position
    
    • Also sends signals to neck and trunk to move into vertical position

Question 12

Describe the effect of damage to the medial longitudinal fasciculus

Answer 12

• A lesion or plaque within the MLF will cause uncoordinated eye movements leading to internuclear ophthalmoplegia
  
  • Connection between CN III and CN VI crucial to allow conjugate eye movements when looking between left and right
    
    • Requires quick white matter connection
  • Leads to slower movement of one eye compared to the other
  • When eyes converge to middle, eyes should move at same speed
• Vulnerable to multiple sclerosis - slows down conduction through MLF

Question 13

Name the structures running through the midbrain

Answer 13

• Ears - cerebral peduncle - descending tracts
• Eyebrows - substantia nigra
• Eyes - red nucleus - descending tracts
• Tears - medial lemniscus - 2˚ sensory neurones to the thalamus
• Mouth - cerebral aqueduct
• Nose - CN III and Edinger Westphal nuclei