disorders of the motor neurone

Question 1

what is weakness/paresis?

Answer 1

impaired ability to

move a body part in response to will

Question 2

what is paralysis?

Answer 2

ability to move a body part in

response to will is completely lost.

Question 3

what is ataxia/incoordination?

Answer 3

willed

movements are clumsy, ill-directioned or uncontrolled

Question 4

what are involuntary movements?

Answer 4

spontaneous

movement of a body part, independently of will

Question 5

what is apraxia?

Answer 5

disorder of consciously organised
patterns of movement or impaired ability to recall
acquired motor skills ( loss of the ability to execute or carry out learned purposeful movements - eg holding a hairbrush the wrongway).

Question 6

how is movement brought about?

Answer 6

1. association areas of the motor cortex have the idea of movement
2. upper motor neurones are activated in the precentral gyrus
3. impulses travel to the lower motor neurones and their motor units via the corticospinal (pyramidal) tracts
4. the cerebellum and basal ganglia modulate this activity
5. sensory feedback also modulates movement

Question 7

where are LMN located?

Answer 7

in the anterior horns of the spinal cord and in the cranial nerve nuclei of the brainstem

Question 8

what is a motor unit?

Answer 8

a lower motor neurone and the skeletal muscle fibres it innervates

Question 9

what type of motor neurones are muscle spindles innervated by?

Answer 9

gamma motor neurones

Question 10

what type of motor neurones innervate extrafusal muscle fibres?

Answer 10

alpha motor neurones

Question 11

how do spasticity and rigidity alter muscle tone?

Answer 11

via the stretch reflex carried out by muscle spindles

Question 12

what are the potential site of damage along the LMN pathway?

Answer 12

1. Motor nuclei of cranial nerves
2. Motor neurones in the spinal cord
3. Spinal ventral roots
4. Peripheral nerves
5. Neuromuscular junction
6. Muscle

Question 13

What are the clinical features of LMN lesions?

Answer 13

Muscle tone normal or reduced (flaccid)
Muscle wasting
Fasciculation - visible spontaneous
contraction of motor units
Reflexes depressed or absent

Question 14

what conditions affect the motor neurones of the brainstem or spinal cord?

Answer 14

– Motor neurone disease
– Spinal muscular atrophy
– Poliomyelitis
–Syringomyelia/syringobulbia
– Spinal cord/ brainstem compression
– Vascular disease

Question 15

what conditions affect the motor spinal roots

Answer 15

– Prolapsed intervertebral disc
– Cervical or lumbar spondylosis
– Tumours eg neurofibroma / ependymoma
– Malignant infiltration

Question 16

what conditions affect the peripheral nerves?

Answer 16

– Axonal degeneration or demyelinating
– Symmetrical polyneuropathy – often distal weakness in limbs
and glove and stocking pattern sensory loss with decreased/ absent
reflexes
– Mononeuritis multiplex pattern of weakness and sensory loss
conforms to distribution of particular peripheral nerves.

Question 17

what are the conditions that affect the NMJ?

Answer 17

– Myasthenia gravis
– Lambert Eaton myasthenic syndrome
– Congenital disorders

Question 18

how does the pattern of weakness help you to localise the lesion?

Answer 18

proximal limb and axial weakness in muscle disease

distal weakness in peripheral neuropathy

Question 19

how are LMN problems investigated?

Answer 19

Neurophysiology nerve conduction
studies/electromyography
Neuro-imaging MRI scan head/spine
Blood tests eg muscle enzymes, peripheral
neuropathy screen, auto-antibodies etc
Lumbar puncture

Question 20

in the corticospinal tract, what percentage of fibres decussate at the pyramids?

Answer 20

85% decussate and become the lateral corticospinal tract
15% of fibres remain uncrossed and for the anterior corticospinal tract (decussate later and terminate in the ventral horn of the cervical and upper thoracic segmental levels).

Question 21

what are the clinical features of UMN pathology?

Answer 21

Muscle tone is increased (spasticity)
Tendon reflexes / jaw jerk are brisk
Plantar responses extensor (+ Babinski sign)
Characteristic pattern of limb muscle weakness (pyramidal
pattern).
a) Upper limbs extensor muscles weaker than flexors
b) Lower limbs flexors weaker than extensors
c) Finer more skilful movements most severely impaired.
emotional lability

Question 22

What does pyramidal drift show?

Answer 22

Upper motor neuron lesion on contralateral side

Question 23

what are the features of cortical lesions and why?

Answer 23

Contralateral often circumscribed weakness

– Dispersion of cortical representation over a wide area

Question 24

what are the features of internal capsule lesions and why?

Answer 24

– Complete contralateral hemiparesis

– Descending fibres grouped closely together in a very small area

Question 25

what are the features of brainstem lesions and why?

Answer 25

– Often bilateral weakness due to involvement of both descending
corticospinal tracts
– Involvement of cranial nerve nuclei (Bulbar involvement - CN 9,10,11,12)

Question 26

what are the features of spinal cord lesions and why?

Answer 26

• Often bilateral corticospinal tract lesions
  – Cervical –will involve upper and lower limbs –tetraparesis
  – Below cervical – weakness of both legs – paraparesis
  – Sensory level may help in localising the site of the spinal lesion.

Question 27

what are the causes of UMN lesions?

Answer 27

stroke
MS
compression - tumour of brain/spinal cord 
spondylosis 
MND
hereditary spastic paraplegia

Question 28

How does hereditary spastic paraplegia present?

Answer 28

young pt 
progressive difficulty walking 
stiff legs
balance impaired 
falls 
urgency of micturition 
strong FH 
pes cavus 
spastic gait 
increased tone in the legs 
lower limb reflexes brisk 
Baninski
abnormal neutological signs only in the lower limbs

Question 29

how do we investigate UMN lesions?

Answer 29

MRI head and spine
blood tests for metabolic disorders
CSF examination for oligoclonal antibodies