5.1.1 Lower Motor Neurones

Question 1

Where are LMN cell bodies found?

Answer 1

Ventral horn and in cranial nerve motor nuclei

Axons are both in CNS and PNS, axons synapse with muscles in PNS

Question 2

What controls LMNs?

Answer 2

Controlled by UMNs descending through the cord or brainstem

Final common path- when activated, causes muscle contraction

Question 3

What reflexes do LMNs participate in?

Answer 3

Spinal reflexes
Deep tendon reflexes

Question 4

What activates LMNs?

Answer 4

Incoming impulses from sensory neurones that communicate with muscle spindles

Can also be inhibited (e.g. hamstring inhibition in knee-jerk reflex)

Question 5

What spinal reflexes exist in babies but disappear later and why?

Answer 5

Moro reflex (baby throws head back and stretches out when they feel like they are going to fall)
Palmar (grasp reflex)
Going up plantars reflex (toes go up rather than down when the sole is stroked)

These reflexes disappear due to maturation of descending upper motor neurone pathways inhibiting their action

Question 6

What does it mean if you have a plantar reflex where your toes go up in adults?

Answer 6

Positive babinski sign

Means there is an upper motor neurone lesion as there is not sufficient lmn inhibition

May be due to MS

Question 7

What signs are present in LMN damage?

Answer 7

Weakness- due to denervation

Areflexia/hyporeflexia- due to denervation

Wasting- due to loss of trophic support (growth factors) to the muscle from the LMN across the NMJ

Hypotonia- due to loss of muscle activation

Fasciculation- up-regulation of muscle nAChRs to try compensate for denervation causing hypersensitivity to any ACh