Chapter 17. Diseases Of The Motor Unit

Front 1

Where is the motor neuron?

Back 1

The ventral horn of the spinal cord

Front 2

What is a motor unit?

Back 2

The basic unit of motor function - a motor neuron and the group of muscle fibers it innervates

Front 3

What is electromyography?

Back 3

The technique for recording the action potentials from single motor units in human muscles

Front 4

What are the four functional components of the motor unit?

Back 4

1) The cell body of the motor neuron

2) The axon of the motor neuron that runs in the peripheral nerve

3) The neuromuscular junction

4) The muscle fibers innervated by that neuron

Front 5

Which motor neurons control more muscle fibers?

Back 5

Less fine movements

Front 6

What is the final expression or output of the motor system?

Back 6

Contraction of muscle

Front 7

What do most diseases of the motor unit cause?

Back 7

Weakness and wasting of skeletal muscles

Front 8

What do neurogenic diseases affect?

Back 8

Neuropathies, changes in the nerve cell bodies or peripheral nerves but only minor changes in muscle fibers

Front 9

What are the two types of neurogenic diseases?

Back 9

Motor neuron diseases (affect the nerve cell bodies)

Peripheral neuropathies (affect the peripheral axons)

Front 10

What are myopathic diseases?

Back 10

Diseases of advanged degeneration of muscles, with little their axons

Front 11

What are two important features of neurological disease?

Back 11

1) Disease can be functionally selective (affect only sensory systems, or motor)

2) May be regionally selective, affecting only one part of the neuron

Front 12

What happens when a peripheral nerve is cut?

Back 12

The muscles innervated by that nerve immediately become paralyzed and then waste progressively

Tendon reflexes lost immediately

Sensation is lost (because sensory axons lost as well)

Front 13

What are the terms for degeneration of muscle for neurogenic vs myopathic diseases?

Back 13

Wasting: myopathic

Atrophic: neurogenic

Front 14

What is myotonia?

Back 14

The inability to relax

Front 15

What is myalgia?

Back 15

Pain

Front 16

What are muscular dystrophies?

Back 16

Hereditary myopathies where all symptoms are due to weakness, the weakness becomes progressively more severe, and degeneration and regeneration can be seen histologically

Front 17

Where do neurogenic diseases show weakness?

Back 17

Distal limb

Front 18

Where to myopathic diseases show weakness?

Back 18

Proximal limb

Front 19

What are signs that are only found in neurogenic diseases?

Back 19

Fasciculations and fibrillations

Front 20

What are fasciculations?

Back 20

Visible twitches seen as ripples under the skin

Occur within a single motor unit

Characteristic of progressive diseases of the motor neuron, not peripheral

Front 21

What are fibrillations?

Back 21

Spontaneous activity in single muscle fibers

Only visible electromyographically

Front 22

What is evidence of upper motor neuron disease?

Back 22

Overactive tendon reflexes

Front 23

What is evidence of lower motor neuron disease?

Back 23

Weak, wasted, and twitching muscles

Front 24

What if both signs for upper and lower motor neuron disease are present?

Back 24

Amyotrophic lateral sclerosis (ALS)

Front 25

What lab tests are used when the only symptom is limb weakness?

Back 25

Measurement of serum enzyme activity

Electromyography

Nerve conduction studies

Muscle biopsy

Front 26

What would one expect to see in the serum in myopathic disease?

Back 26

Increases in levels of creatine kinase from the sarcoplasmic reticulum of muscles

Front 27

What three measurements are important in electromyography?

Back 27

1) Spontaneous activity at rest

2) Number of motor units under voluntary control

3) The duration and amplitude of each motor unit potential

Front 28

What does one see in a normal versus neurogenic disease EMG?

Back 28

Normal: potentials overlap in an interference pattern, quiet at rest

Weak: altered amplitudes, spontaneously active at rest, no interference, fewer units under voluntary control, increased amplitude and duration of individual potentials (because surviving units contain more than normal numbers of muscle fibers)

Front 29

What do you see in a myopathic disease EMG?

Back 29

No activity at rest

No change in number of units

Potentials are smaller and of shorter duration

Front 30

Which neuropathies have slowed conduction velocity?

Back 30

Demyelinating neuropathies

Front 31

What are non-demyelinating neuropathies?

Back 31

Axonal neuropathies

Front 32

What are the two types of muscle fibers?

Back 32

Fast-twitch (type I): anaerobic metabolism

Slow-twitch (type II): red, oxidative metabolism

Front 33

Are all muscle fibers in a motor unit of the same type?

Back 33

Yes

Front 34

Do muscle fibers of a motor unit lie side by side?

Back 34

No, they are interspersed among others

Front 35

Do the muscle fibers assume the histochemical type of the motor neuron or vice versa?

Back 35

Former

Front 36

What would you expect to see in histochemical analysis of neurogenic diseases?

Back 36

Clusters rather than alternating of muscle fiber type (fiber-type grouping)

Front 37

What is group atrophy?

Back 37

If, after clustering of muscle fiber type, the disease progresses to that motor unit

Front 38

How are muscle fibers affected in myopathic disease?

Back 38

Randomly

Front 39

What is the best-known disorder of motor neurons?

Back 39

ALS

Front 40

What does lateral sclerosis refer to?

Back 40

The hardness felt when the spinal cord is examined at autopsy

Front 41

What causes lateral sclerosis?

Back 41

Proliferation of astrocytes and scarring of the lateral columns of the spinal cord

Front 42

What causes the scarring of the lateral columns in ALS?

Back 42

Disease of the corticospinal tracts

Front 43

Where are upper motor neurons?

Back 43

Cortex

Front 44

Where are lower motor neurons?

Back 44

Brain stem and spinal cord

Front 45

What is motor neuron disease when only cranial symptoms occur?

Back 45

Progressive bulbar palsy

Front 46

What is the motor disease when only lower motor neurons are involved?

Back 46

Spinal muscular atrophy

Front 47

Does ALS affect sensory or autonomic neurons?

Back 47

No

Front 48

What are viruses that attack the nervous system called?

Back 48

Neurotropic viruses

Front 49

What is the EMG counterpart to a fasciculation?

Back 49

Compound motor unit potential

Front 50

What does the fact that fasciculations can be stopped by d-tubocurarine mean?

Back 50

ACh is involved

Front 51

Do motor neuron diseases or neuropathies affect both sensory and motor function?

Back 51

Neuropathies

Front 52

What sensory information is altered due to impaired small myelinated fibers?

Back 52

Pain and temperature

Front 53

Are sensory symptoms in neuropathies proximal or distal?

Back 53

Distal

Front 54

Where is the protein content increased in neuropathies?

Back 54

CSF

Front 55

What is the best-known acute neuropathy?

Back 55

Guillain-Barre syndrome

Front 56

Describe Guillain-Barre syndrome.

Back 56

Autoimmune attack on peripheral nerves by circulating antibodies

Treated with plasmapheresis

Front 57

What can cause chronic neuropathies?

Back 57

Genetics, metabolics, intoxications, nutrition, carcinomas, and immunology

Front 58

Which neuropathies (demyelinating or axonal) are more common?

Back 58

Demyelinating

Front 59

What are positive symptoms of peripheral neuropathies?

Back 59

Paresthesias

Front 60

What is ephaptic transmission?

Back 60

Electrical interaction between abnormal axons

Front 61

What are three negative symptoms of demyelinating neuropathies?

Back 61

1) Conduction block

2) Slowed conduction

3) Impaired ability to conduct impulses at higher frequencies

Front 62

What is conduction block?

Back 62

Stimulation of an injured peripheral nerve below the site of injury evoked muscular response, whereas stimulation above does not

Front 63

What is a toxin that causes demyelination and conduction block?

Back 63

Diphtheria

Front 64

Why is conduction velocity faster in myelinated axons?

Back 64

1) Axons of myelinated fibers tend to be larger in diameter

2) Action potential propagates discontinuously from one node of Ranvier to the next

Front 65

What does demyelination mess up?

Back 65

Velocity and synchrony

Front 66

What are the two different types of myopathies?

Back 66

Inherited and acquired

Front 67

What are the best known inherited diseases?

Back 67

Muscular dystrophies

Front 68

What are the four types of muscular dystrophies?

Back 68

Two types weakness alone: Duchenne and facioscapulohumeral

Myotonia in addition to weakness: Myotonic

None of the above: Limb-girdle distrophy

Front 69

Describe Duchenne type muscular dystrophy.

Back 69

Starts in legs

Males only

Progresses rapidly

Front 70

Describe the facioscapulohumeral type.

Back 70

Autosomal dominant

Both sexes

Milder

 

Front 71

Describe myotonic muscular dystrophy.

Back 71

Delayed relaxation of muscle after vigorous voluntary contraction or stimulation

Caused by repetitive firing of muscle APs

Limb weakness distal rather than proximal

Front 72

What is dermatomyositis?

Back 72

An acquired myopathy

Two clinical features: rash and myopathy

Proximal muscle weakness

Autoimmune

Front 73

What happens after wasting of muscle fibers?

Back 73

Regeneration of new fibers that cannot keep pace

Front 74

How could you identify the genes responsible for a disease?

Back 74

Techniques for mapping DNA polymorphisms called restriction fragment length polymorphisms

Front 75

What are genetic polymorphisms?

Back 75

Markers that are expressed in people that express the disease

Front 76

What are markers based on?

Back 76

Restriction fragment lenth polymorphisms, cutting sites for restriction enzymes

Front 77

What is the process of identifying polymorphisms?

Back 77

Restriction enzymes produce DNA fragments of different lengths from the two alleles; the fragments that differ in length separated by electrophoresis and distinguished by specific DNA probes (Southern blot hybridization)

Front 78

What is a haplotype?

Back 78

A consistant pattern that can be used to identify carriers of the gene

Front 79

What is wrong in Duchenne disease?

Back 79

Dystrophin is absent and cannot be part of the cytoskeleton due to gene product being absent

Front 80

What is wrong in Becker dystrophy?

Back 80

Reading frame shift in the same gene for dystrophin that results in a smaller gene product giving an abnormal protein

Front 81

How could you cure Duchenne disease?

Back 81

Injection of normal myoblasts (muscle cell precursors) into the dystrophic muscle or injection of a plasmid that contains the gene