5a & 5b.) Motor System

Question 1

Summarise where:

• Cell bodies of upper motor neurones (UMNs)
• Axons of UMNs
• Cell bodies of lower motor neurones (LMNs)
• Axons of LMNs

… are found

Answer 1

UMNs

• Cell bodies in primary motor cortex (pre-central gyrus)
• Axons in CNS (synapse onto LMNs)

LMNs

• Cell bodies in ventral horn or in cranial nerve motor nuclei in brainstem
• Axons partly in CNS and partly in PNS

Question 2

What do lower motor neurones synapse onto?

Answer 2

Skeletal muscle (when activated they will cause skeletal muscle contraction)

Question 3

How are LMNs controlled?

Answer 3

Controlled by UMNs which descend through cord or brainstem and synapse onto LMNs

Question 4

Explain how the cranial nerve motor nuclei ‘developed’ from the ventral horn of the spinal column and hence explain why cell bodies of LMNs are found in both ventral horn and cranial nerve motor nuclei

Answer 4

Idea that the extension of the grey and white matter of neural tube into brainstem and brain got interuppted. In the brainstem these bits of grey matter, which are no longer continuous tubes like the dorsal and ventral horns are, become the cranial nerve nuclei.

• If ventral horn component persists: cranial nerve will have motor nuclei
• If dorsal horn component persists: cranial nerve will have sensory nuclei
• If lateral horn component persists: cranial nerve will have autonomic nuclei

… therefore CNs can be thought of as specialised spinal nerves

Question 5

LMNs are typically activated by impulses from sensory muscles that communicate with what structures in a muscle?

*HINT: these structures detect stretch

Answer 5

LMNs are typically activated by impulses from sensory muscles that communicate with muscle spindles (which detect stretch in a muscle)

Question 6

Lower and uppper motor neurones are involved in spinal reflexes; true or false?

Answer 6

FALSE- only lower motor neurones involved in spinal reflexes

Question 7

Describe some primitive reflexes in babies

Answer 7

• Up going plantars: stimulus brough across ball of foot and the great toe should extend and the other toes fan out (NOTE: this is the normal babinski reflex for babies but not for adults)
• Moro reflex: when baby startled they will stretch out arms and unclench fists and curl up their upper limbs
• Palmar grasp: babies will grasp/clench fist if they have stimulus in their palm

Question 8

Why do reflexes in baby, such as Moro reflex, disappear a baby grows?

Answer 8

Due to maturation of descending upper motor neurone pathways

Question 9

State, and explain why each occurs, 5 signs of LMN damage

Answer 9

• Weakness: due to denervation (NOTE: if one cord segement damaged will get weakness but if multiple damaged you may get complete paralysis as areas are supplied by multiple segments)
• Areflexia: efferent arm of spinal reflex damaged
• Muscle wasting: due to loss of trophic support from LMN across neuromuscular junction (LMNs provide trophic support e.g. growth hormones to muscles)
• Hypotonia: due to loss of muscle activation
• Fasciculation: due to up-regulation of muscle nAChRs to try and compensate for denervation

Question 10

What is meant by fasiculation in LMN injury?

Answer 10

A brief spontaneous contraction affecting a small number of muscle fibres, often causing a flicker of movement under the skin.

Question 11

State the 5 signs of damage to LMN

Answer 11

• Weakness
• Areflexia
• Wasting/atrophy
• Hypotonia
• Fasciculations

Question 12

Whate state, excitation or inhibition, are LMNs held in most of the time?

Answer 12

Inhibition due to inhbitory interneurones

Question 13

Remind yoursel of the knee jerk reflex

Answer 13

Tapping patella stretches quadriceps

• Muscle spindles detect stretch
• Causes action potentials to be fired by afferent fibres
• Afferent fibres synapse in spinal cord (L3&L4) with alpha motoneurones
• Efferent lower motor neurones innverate extrafusal fibres causing agonist to contract
• The afferent fibres also synapse with segment controlling hanstrings (L4.L5) and activate inhibitory interneurones; the hamstrings (antagonist muscle) then relax

Question 14

Which motor tract are we concerned with/which is clinically important?

Answer 14

Lateral corticospinal tract (found between dorsal and ventral horn)

Question 15

UMNs synapse directly or indirectly onto LMNs in ventral horn or cranial nerve motor nuclei; true or false?

Answer 15

True

Question 16

The net effect of UMN on LMNs is….?

Answer 16

Inhibition

Question 17

Describe the descent of the axons of UMNs

Answer 17

• Cell body in primary motor cortex
• Axon descend through coronoa radiata, internal capsule, crus cerebri of midbrain and pons

UMNs synapsing in brainstem

• UMNs that are synapsing with LMNs in brainstem (in cranial nerve motor nuclei) will leave the pathway in the brainstem and form the corticobulbar (also known as corticonuclear) tract

UMNs synapsing in ventral horn

• 85 % of axons that are going to ventral horn then decussate in medullayry pyramids (in caudal medulla) and then descend in lateral corticospinal tract (in lateral funiculus of cord)
• Other 15% dont’ decussate in medullary pyramids and descend in ventral corticospinal tract
• Synapse in Ventral hrn

Question 18

What is the lateral corticospinal tract involved in (in terms of motor control)?

Answer 18

Involved with fine motor control in imbs, primarily the distal extremeties (but all of limb can be affected by an UMN lesion)

Question 19

What structures is the internal capsule between?

Answer 19

• Thalamus
• Lentiform nucleus

Question 20

What is the ventral corticospinal tract responsible for?

Answer 20

Drives proximal postural muscles

Question 21

Describe the structure of the facial motor nuclues

Answer 21

• Split into a superior and inferior half
  
  • Superior half: supplies superior face (above supra-orbital ridge- hence mainly supplies occipitofrontalis)
  • Inferior half: supplies inferior face (below supra-orbital ridge so most of remaining muscles of face)

Question 22

Describe the UMN supply to each half of the facial nerve motor nucleus

Answer 22

• Superior half (supplying superior face) receives UMNs from both hemispheres
• Inferior half (supplying inferior face) receives UMNs from the contralateral hemisphere

Question 23

Explain why an UMN lesion involving the face will spare the forehead

Answer 23

• Forehead (part of superior face) supplied by superior half of CNVII motor nuclei
• Superior half of CNVII motor nucleus receives input from both cerebral hemispheres hence if lesion was on right, the left could compensate etc…
• Inferior half of CNVII motor nucleus only receives input from contralateral side hence lesion on right would cause left sided weakness in lower half of face

NOTE: if you had a facial nerve palsy (damage to facial nerve) would have paralysis of all face on that side

Question 24

State 4 signs of UMN damage

Answer 24

• Weakness
• Hypertonia
• Hyperreflexia
• Extensr plantar reflexes

Question 25

State, and explain the reasons behind, the 4 signs of UMN damage

Answer 25

• Weakness: due to loss of excitatory inputs to LMNs from UMNs
• Hypertonia: due to loss of descending inhibition (remember net effect of UMNs on LMNs is inhibition)
• Hyperreflexia: over reactive reflex arc due to loss of descednign inhbition
• Extensor plantar reflexes: reverion to the situation in a baby due to loss of descending modulation of spinal reflexes

Question 26

Describe what spinal shock is and state the mechanism

Answer 26

Phenomenon that occurs in days immediately following a UMN lesion. Initially there is flaccid paralysis with areflexia (like in a LMN lesion) but then tone increases leading to hypertonia and reflexes become exagerrated leading to hyperreflexia.

Mechanism is unclear but it is releated to neuroplasticitiy of spinal cord

Question 27

Where, medial or lateral, does the upper body map to in the lateral corticospinal tract?

Answer 27

• Upper body maps medially
• Lower body maps laterally

Question 28

What injury, UMN or LMN, is clasp knife rididity seen in?

Answer 28

Upper motor neuron

Question 29

Explain clasp-knife rididity

Answer 29

• UMN leading to hypertonia (which often presents as spasticity since all muscles experiecing equal lack of inhbition but in arm flexors are stronger and in leg extensors are stronger)
• Pull the limb from its spastic position- it will initially be very hard to this and then eventually it will just suddenly become easy to move
• This is the high pressure/tension has activated golgi tendon organs (they have a high tension threshold)
• Golgi tendon organs synpase with inhbitory interneurons which inhbit LMN and prevent muscle contraction

Question 30

The internal capsule can be thought of as a condensation of the…?

Answer 30

Corona radiata

Question 31

Describe the shape of the internal capule in a transverse section and label each section

Answer 31

V shape with:

• Anterior limb
• Genu (the bend)
• Posterior limb

Question 32

Which limb of the internal capsule contains descending UMN axons that are going to the ventral honr?

Answer 32

Posterior limb (as the corticospinal tract is found in the posterior limb)

Question 33

Which region of internal capsule contains UMN axons that are going to the brainstem to supply muscles of the face?

Answer 33

Genu (as the corticobulbar tract is found in genu region)

Question 34

What do the thalamic regions in the internal capsule contain?

Answer 34

Thalamic regions contain third order sensory fibres, in particular:

• In posterior limb: third order sensory fibres of somatosensory system (hence fibres are going to post-central gyrus)
• In anterior limb: third order sensory fibres going from thalamus to other parts of brain

Question 35

What do the corticofugal fibres in internal capsule contain?

Answer 35

Fibres from cerebral cortex downwards… don’t need to worry about