Upper and Lower Motor Neurones and Pathway

Question 1

Where are the cell bodies for upper and lower motor neurones found?

Answer 1

Both cell bodies within CNS

Question 2

What are lower motor neurones? (3)

Answer 2

Final common path - when activated will call muscle contraction

Cell body projects from CNS to PNS

Controlled by upper motor neurones - these descend through brainstem/cord and synapse onto them

Question 3

Where in particular are cell bodies for lower motor neurones found?

Answer 3

Ventral horn or cranial nerve nuclei (eg oculomotor nucleus, optic nucleus)

Question 4

Which neurones are present in spinal reflexes?

Answer 4

Lower motor neurone

Question 5

When are lower motor neurones activated during reflex?

Answer 5

Incoming impulses from sensory neurones communicate with muscle spindles

Question 6

Are lower motor neurones inhibited?

Answer 6

Yes - inhibition dominates LMN from descending UPN synapsing onto inhibitory interneurons

They are also inhibited by sensory neurones synapsing onto inhibitory interneurons in spinal levels below during reflexes eg hamstrings in patella reflex

Question 7

What are ‘primitive’ reflexes?

Answer 7

Present in babies (eg palmar grasp) but disappear as baby grows due to maturation of descending upper motor neurone pathways

Question 8

LMN damage signs and why (5)

Answer 8

Weakness - due to denervation

Areflexia/hyporeflexia - due to denervation

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

Hypotonia - loss of muscle activation

Fasciculation - up regulation of muscle nAchR’s to try and compensate for denervation, become extra sensitive to Ach

Question 9

Where are origin of upper motor neurones found?

Answer 9

Pre-central gyrus (primary motor cortex) in frontal lobe

Question 10

Where do upper motor neurones go and what do they do?

Answer 10

Synapse onto LMNs directly/indirectly in the ventral horn or cranial nerve motor nuclei (if supplying head and neck)

Question 11

What type of neurones are present in basal ganglia and cerebellum?

Answer 11

NOT upper motor neurones - damage to them does not cause an UMN syndrome

Question 12

Overall majority effect of UMNs on LMNs

Answer 12

Inhibition - most synapses are inhibitory for neurone, this explains the signs of UMN lesion

Question 13

Where do the UMNs axons descend from and to - name the structures (9)

Answer 13

Corona radiata - lots of axons are sprayed out (fingers on a hand)
(between
Internal capsule - narrowed here (palm of hand)
Cerebral peduncle in midbrain
Pons
Medullary pyramids
Decussate of the pyramids (in caudal medulla - 85% cross over here)
Lateral corticospinal tract
Ventral horn
Synapse onto LMNs

Question 14

What are the two options for UMN when synapsing?

Answer 14

Can synapse direct onto neurone
Or indirect via inhibitory interneurons onto LMNs

Question 15

Where do the upper motor neurones decussate?

Answer 15

Decussation of pyramids - in pyramids of medulla

Question 16

What do the decussated fibres of the UMNs form vs the others?

Answer 16

85% decussate forming lateral corticospinal tract
15% don’t decussate and form ventral corticospinal tract

Question 17

What is the lateral corticospinal tract involved in?

Answer 17

Fine motor control of the limbs - primarily the distal extremeties

Question 18

What is the route of the UMNs supplying the face via cranial nerve nuclei?

Answer 18

Leave pathway in the brainstem and form the corticobulbar (corticonuclear) tract, which innervate the lower motor neurones in the cranial nerve nuclei

Question 19

What is special about the CN VII the facial nerve nucleus?

Answer 19

Split into two halves - one supplies superior half of face and the other supplies inferior half of face

Superior half of nucleus supplying superior half of face receives contralateral supply from primary motor cortex BUT also ipsilateral supply (both hemispheres)

Inferior half of nucleus supplying inferior face only receives contralateral supply from PMC (just contralateral hemisphere)

Question 20

What does the special fact about facial nerve nucleus mean for pts presenting with facial nerve palsy?

Answer 20

UPN lesions involving the face = forehead sparing as there is supply from the opposite and ipsilateral hemisphere

Eg in stroke on R hemisphere, will get left facial nerve palsy with forehead sparing on left

But in true facial nerve palsy where facial nerve/nucleus is affected, the whole face will be paralysed (eg on L facial nerve lesion, L half of face will be paralysed as fibres have already decussated)

Question 21

What are the signs of upper motor neurone lesion? and why

Answer 21

Weakness - denervation

Hypertonia - loss of descending inhibition (net effect of UMNs on LMNs is inhibition)

Hyperreflexia - overactive reflex arc

Extensor plantar reflexes - loss of descending modulation (toe normally curls under and flexes when foot is stroked, opposite of this happens)

Question 22

What is spinal shock?

Answer 22

Phenomenon that occurs in the days following an UMN lesion

Initially flaccid paralysis with areflexia (like LMN lesions)

But then tone increases (hypertonia) and reflexes become exaggerated (hypereflexia)

Related to neuroplasticity of spinal cord

Question 23

What can be damaged in either LMN or UMN lesion?

Answer 23

Either cell body or axons of the neurones will cause the signs/lesion

Question 24

What signs do you get when you get UMN and LMN lesion?

Answer 24

You get LMN lesion signs as these are needed to transmit UMN signs
LMN signs dominate UMN signs basically