Upper and Lower Motorneuron lesions

Question 1

What is a lower motor neuron

Answer 1

A neuron that projects from the anterior horn of the spinal cord to the muscle

Question 2

What is an upper motor neuron?

Answer 2

A neuron running from the cortex down to the anterior horn of the spinal cord.

Question 3

Describe some of the signs of a lower motor neuron lesion

Answer 3

• Atrophy as the muscle will waste
• Fasciculations as the nerve fibres try to regenerate
• Tone may be reduced, definitely not increased.
• Weakness
• Loss of reflexes as the reflex arc is not intact

Question 4

Describe some of the signs of an upper motor neuron lesion

Answer 4

• No muscle wasting as the reflex arc is exaggerated so the muscles are under greater electrical stimulation.
• This also means tone is increased
• May be clonus which is repeated muscle contractions when the muscle is under tension (most commonly seen at the ankle in an UMNL)
• There will be weakness
• Reflexes are increased (may be positive babinski response)

Question 5

What is the upper motor neuron pattern of weakness?

Answer 5

Where arm extensors are weaker than flexors and when leg flexors are weaker than extensors

Question 6

What is spasticity?

Answer 6

A condition where the muscles are continuously contracted

Question 7

What occurs if a muscle looses part of its nerve supply?

Answer 7

• Muscle will have reduced bulk
• Individual muscle fibres can produce spontaneous action potentials (produce fibrillations) which can be seen on a myogram.
  Fasciculations seen.
• When patients asked to contract muscle the deflections on the myogram will be less frequent but have a larger amplitude because there are fewer motorneurons