Motor Neurons

Question 1

Name the lobe of the cerebral hemisphere in which the motor homunculus lies on each side of the brain, and name the relevant gyrus (1)

Answer 1

Frontal lobe; Precentral gyrus

Question 2

Describe the clinical signs of LMN dysfunction (3)

Answer 2

Damage to motor neurones at this level will compromise the nerve supply to muscle bellies, resulting in hypotonia or atonia and hyporeflexia or areflexia, atrophy, etc. Flaccid paralysis, as both agonists and antagonists are affected.

Question 3

Describe the reasons behind the differing clinical presentation of a patient with an upper (UMN) versus a lower (LMN) motor neurone lesion (4) [Pic on right]

Answer 3

UMN is inhibitory over LMN. LMN stimulates the effector (e.g. skeletal muscle).

UMN lesion causes loss of inhibition of LMN and therefore spastic paralysis.

LMN lesion results in a lack of stimulation of effector and therefore flaccid paralysis.

Question 4

A child with a neural tube defect has lower motor neurone (LMN) damage at the level of the lesion.

State the exact location of LMNs and explain their function (2)

Answer 4

Lower motor neurones lie in the grey matter of the ventral horn – fibres pass out of ventral root to innervate muscle bellies.

Question 5

Newborn infant in the labour ward. O/E: normal vital signs and appearance with the following exception - a bulging cyst-like structure approximately 4 cm in diameter protruding from his back. Baby has limited movement of the lower extremities and that both feet are plantar flexed and inverted at the ankle.

Explain why the lower limbs of the neonate present in the way described in the case scenario (3)

Answer 5

LMN lesion in lumbosacral area = reduced movement in lower limbs; unopposed plantar flexors if dorsiflexors affected; unopposed flexors of the ankle and foot and invertors if extensors and evertors are affected.

Question 6

Baby was born with a meningomyelocoele in the lumbosacral area of his spine. O/E his limbs showed no sensory or motor deficits of the upper limbs, but the lower limbs had reduced muscle reflexes, were bowed in position, and there were areas of sensory loss on the skin. Preparations are underway for surgery to the lesion on his back.

The baby has lower motor neurone damage. Explain what this is, and how it was deduced from the clinical examination (5)

Answer 6

Lower motor neurones lie in the grey matter of the ventral horn – fibres pass out of ventral root to innervate muscle bellies. Damage to motor neurones at this level will compromise the nerve supply to muscle bellies, resulting in hypotonia and hyporeflexia, atrophy, etc. Flaccid paralysis, of agonist / antagonist, etc.

Question 7

22 m/o girl has developed progressive weakness of the lower limbs and is no longer able to move her legs. Weak cough & chokes when she is being fed. Did not receive all her vaccinations as scheduled. O/E her leg muscles have atrophied, she has depressed reflexes in her lower limbs and he is concerned that she may have poliomyelitis with involvement of her respiratory muscles.

Explain the weakness experienced by the patient, and the relative roles of upper and lower neurones in the motor pathway based on the clinical signs outlined in the case (3) In addition outline the motor pathway. May use a well-labelled diagram (7)

Answer 7

Polyneuropathy - symmetrical, multiple nerves

Weakness = lower motor neurone. UMN = inhibition of LMN, loss of tonic LMN firing.

Diagram showing motor pathway from cortex to muscle, including decussation and synapses