Lecture 33: Diseases affecting motor units

Question 1

What are the common motor symptoms of neurological disorders:

Answer 1

1. Paralysis: ~plegia: limb loss of motor function.
2. Paresis: weakness
3. Abnormalities in muscle tone: eg. hypertonia (spasticity in stroke), rigidity
4. Ataxia: incoordination of movements. Decomposition (jerky) or Dysmetria (loss of proprioception leads to missing the target
5. Involuntary movements : intention, resting and essential tremor, dyskinesia: excessive movement

Question 2

What is likely to be the cause of an acute focal, chronic focal, acute diffuse and chronic diffuse “lesion”

Answer 2

A.F: trauma, vascular disease
C.F: brain tumour
A.D: Toxin or infection
C.D: Neurodegeneration

Question 3

Where are motoneurons found and what are the 2 types

Answer 3

Motoneurons are LMN. Found in anterior grey horn of spinal cord or in cranial motor nuclei in the brainstem.

1. Alpha motoneuron: innervating extrafusal fibres: generation of force by muscles
2. Gamma motoneurons: innervate intrafusal fibres. found near Alpha, control excitability of stretch receptors in muscle spindles

Question 4

What is the motor unit

Answer 4

Nerve + all the muscles it innervates, varying in the innervation ratio but also the properties of the unit.
FF: fast twitch , fatiguable and S; slow twitch

Question 5

What are the 3 inputs for Alpha motoneurons and their motor units

Answer 5

1. Descending tracts converging on alpha motoneurons: eg. corticospinal, rubrospinal, vestibule spinal, tectospinal, reticulospinal
2. Local spinal/brainstem interneurons
3. Ia afferents from muscle spindles

Question 6

What are the places along the motor unit that can be affected by disease and give an example

Answer 6

1. Only the muscle itself - eg. Myopathies
2. Neuromuscular junction- eg. Myasthenia Gravis
3. Presynaptic neuron - eg. Botulism
4. Axon -eg Axotomy
5. Peripheral neuropathy - eg. Guillan barre syndrome

Question 7

Describe Myotonic muscular dystrophy: action of disease, epidemiology, and symptoms

Answer 7

• dominant inherited disease, in early adult life
• progressive wasting and weakness of striated muscle due to mutation in myotonin protein.
• muscle stiffness, hypertonia and delayed relaxation of muscle immediately after contraction

Question 8

Describe Myasthenia gravis : action of disease, symptoms and diagnosis

Answer 8

• Autoimmune disease against Ach receptors in the post synaptic membrane: lead to fewer Ach binding sites, smaller End plate P - reduced transmission of AP.
• Muscle weakness without wasting (atrophy). Reduced junctional folds.
• Occular form Diagnosed via open eye facial appearance after injection of tensilon an ACh esterase blocker
• Otherwise antibodies detected in serum (in 85% of patients)

Question 9

Describe Botulism : action of disease and symptoms

Answer 9

• Botulinum toxins produced under anaerobic conditions impair ACh release at all presynaptic synapses by entering pre syn. neurons outside the BBB and causing proteolysis of proteins involved in neurotransmitter release.
• Leads to weakness/paralysis of striated and smooth muscle (somatic and autonomic dysfunction- dry mouth, postural hypotension). Death in ug.
• Infant botulism: resp arrest, difficulty feeding, flaccid paralysis.

Question 10

Describe the process of Wallerian degeneration following Axotomy

(Compression, piercing of the axon leading to discontinuation of the axon.)

Answer 10

1. At first
   - Loss of synaptic transmission within 24hrs in the distal segment. Will degenerate in days due to loss of axon survival factor from cell body.

• Proximal part survives (in nonCNS). The cell body will undergo chromatolysis.

2. Next few days
   - Regrowth of axon (1-2mm/day) due to Schwann cells that enhance signalling
3. Re innervation of muscles
4. Re myelination of axons (partial functional recovery sometimes)