Lecture 6 -- Motor Systems 1

Flashcards based on the Motor Systems lecture notes to aid in exam preparation.

What are the two main types of motor neurons?

Upper motor neurons (UMNs) and Lower motor neurons (LMNs)

Where do upper motor neurons reside?

Entirely within the central nervous system (CNS)

What is the function of upper motor neurons (UMNs)?

1. Initiate, regulate, modify, and terminate LMN activity

→ LMN always want to be active → Upper motor neurone help suppress their activity

2. Voluntary movement of skeletal muscle

How can UMNs interact with LMNs?

Information comes down from the brain via upper motor neurones to the brain stem -> Synapse/ via interneuron with lower motor neurones of the nerve e.g. brachial plexus/ lumbosacral plexus -> Supply muscle

What is the dominant influence of most UMNs on LMNs?

Inhibitory

Which funiculus do UMNs travel to FLEXOR muscles?

Lateral funiculus of the spinal cord

Which funiculus do UMNs travel to EXTENSOR muscles?

Ventral funiculus of the spinal cord

What effect might an injury localised to the lateral cervical spinal cord have on the ability to move forelimb muscles?

Cervical spinal cord = Within the CNS = Affect the upper motor neurons

→ As the UMNs are mostly inhibitory, muscles may be more tense that usual

→ No voluntary movement → UMNs are responsible for voluntary movement of limb + Flexion of the limb → Ability to extend the limb (e.g. patella reflex) remains BUT the ability to voluntarily flex the limb may be reduced

Where are the cell bodies of LMNs located?

Within the CNS (brain or spinal cord)

Where do the axons of LMNs run?

Via spinal or cranial nerves to innervate skeletal muscle fibers in the PNS

From where do LMNs for the forelimb originate?

Cervical intumescence (C6-T2)

From where do LMNs for the hindlimb originate?

Lumbar intumescence (L4-S3)

What happens when an action potential occurs in a LMN?

Muscle contraction

LMN can fire without UMN input. What is the action that can be done by LMN without UMN input?

Reflex action e.g. patellar reflex

Why the muscle become faccid and atrophy if LMN is severed?

LMNs have a constant sub-threshold depolarisation → ACh release → Muscles tone and trophic support

→ If sever LMN → Sub-threshold depolarisation of ACh lost, muscle become flaccid/atrophy (smaller, lose bulk)

What clinical signs would you expect in the hindlimbs of an animal which had damage to the spinal cord in the region of the lumbosacral intumescence?

Spinal cord = Lower motor neurone

→ LMN cell bodies damaged → Cannot move the hindlimb → Reduced reflex + Flaccid muscle tone + Muscle atrophy

Define a motor unit

LMN + NMJ + skeletal muscle fibres

How would an injury to the radial nerve affect the function of the muscles supplied by this nerve?

Radial nerve is responsible for triceps, which is responsible for extending the elbow

Damage of radial nerve = Damage of LMN = Cannot contract triceps → Lose reflexs + Flacid muscle tone + Muscle atrophy over a few days

What types of muscles are dominant in maintaining posture against gravity?

Extensor muscles

What inhibits the extensor muscles?

UMNs inhibit the LMNs

What happens if the inhibitory UMNs fail?

LMN become active → Spasticity results

Do reflexes require UMN input?

No

What does RAT stand for, in regards to the motor neurons?

Reflexes, Atrophy, Tone

What would you expect to find in an injury to the LMN

Muscle will lose tone, rapidly lose bulk (atrophy) and have reduced or absent reflexes

What would you expect to find in an injury to the UMN

1. Reflexes will be normal or increased as will tone

2. Atrophy will be relatively mild due to disuse

3. Coordination will be reduced because UMN comes from the brain, deciding what those neurones are going to do

4. Strength will be normal

Which of the following is NOT required for reflex action to occur?

Upper motor neuron

If a dog has a prolapsed intervertebral disc at C7, what effect can we expect to see in the forelimbs?

Within the cervical intumescence, forelimb LMN cell bodies are compressed → Reduced forelimb movement + Absent reflexes + Flaccid muscle tone + Muscle atrophy over a few days

If a dog has a prolapsed intervertebral disc at C7, what effect can we expect to see in the hindlimbs?

UMNs pass through the injury sit also damage → Hindlimb reflex normal or exaggerated + Muscle tone normal or increased + Impaired voluntary muscle movement