1. The document discusses various mechanisms of muscle dysfunction including disorders at the motor neuron, neuromuscular junction, and muscle levels. 2. Electromyography is described as a key test to detect neuromuscular disorders by examining motor unit recruitment patterns and action potentials. 3. Specific muscle and nerve diseases are outlined like myopathies, motor neuropathies, myasthenia gravis, muscular dystrophies, motor neuron disease and spinal muscular atrophy. 4. The roles, features, causes and electromyography patterns of these diseases are explained to characterize different types of muscle dysfunction.

1. Mechanisms of
dysfunction of
muscles
Prof. Vajira Weerasinghe
Professor of Physiology, Faculty of Medicine
and Consultant Neurophysiologist, Teaching Hospital, Peradeniya
Lecture is available at www.slideshare.net/vajira54

2. Objectives
1. Recall the physiology of the motor unit and its
neural control
2. Outline how disorders at different levels in the
control mechanisms affect muscle function

3. Site of lesions
UMN & LMN
Motor nerve
NMJ
Muscle
Motor neuron disease
Diseases
Motor neuropathy
Myasthenia gravis
Myopathy
Site

4. Motor unit
• muscle contraction occurs in
terms of motor units rather
than by single muscle fibres
• a motor unit is defined as
– anterior horn cell
– motor neuron
– muscle fibres supplied by the
neuron

5. Principle of recruitment of motor units
• Increase in the tension of a muscle is due to
progressive recruitment of motor units
• eg.
– Mild contraction – few motor units are recruited – mild
tension
– Moderate contraction – many motor units are recruited –
moderate tension
– Strong contraction – all the motor units are recruited –
maximum tension

6. Electromyography (EMG)
• This is a neurophysiological test done in
order to detect many neuromuscular
disorders
• Recording electrodes are needles (EMG
needles)
• They contain cathode and anode in the
form of a needle
• This is inserted into the muscle
• Motor unit recording pattern is recorded
visually in the screen and sound pattern is
recorded from a loudspeaker
• Motor unit action potentials (MUAP) can
be recorded

7. EMG recording - normal
• At rest
– No activity (screen or
sound)
• Ask the subject to make
a voluntary contraction
– Motor unit action
potentials amplitude and
duration are calculated
Normal resting
Motor unit action potentials (MUAP)

8. EMG recording - normal
• Ask the subject to make graded voluntary
contraction (mild – moderate - strong)
– Recruitment pattern is recorded
Normal full recruitment

9. Muscle dysfunction

10. Muscle fatigue
• Decline in muscle force generated over
sustained periods of activity or due to
pathological issues
• Causes
– Ion imbalance within muscle
– Nervous Fatigue and Loss of Desire
– Metabolic Fatigue
– Exercise and Aging
– Lactic Acid Accumulation

11. Muscle fatigue
Ion Imbalance
– Contraction of a muscle requires Ca ions to interact
with troponin, exposing the actin binding site to
myosin head
– Osmotically active molecules outside the muscles
are lost through sweating
– Difficult for the required Ca ions to be delivered to
muscle fiber

12. Muscle fatigue
• Nervous Fatigue and Loss Of Desire
– Nerves are responsible for controlling the
contraction and force of muscles
• Metabolic Fatigue
– Accumulation of metabolites from Mg ions, induce
fatigue by interfering with release of Ca ions
– Reduction in sensitivity of troponin to Ca ions

13. Muscle fatigue
• Exercise and Aging
– With sufficient training, the metabolic capacity of a
muscle can change delaying the onset of muscle
fatigue
– With aging, levels of ATP, CTP, and myglobin begin
to decline, reducing muscle’s ability to function
– Muscle fiber shrink or lost
• Lactic Acid
– Byproduct of anaerobic respiration which strongly
contributes to muscle fatigue

14. Muscle cramps
• At one time or another most athletes will experience a muscle cramp
• Muscle cramps are involuntary
• Usually a painful contraction of skeletal muscle
• Muscle cramps will usually occur without warning
• Muscle cramps are usually a warning to an athlete as the muscle cramp is
often a symptom of a more serious physical issue that requires attention

15. Muscle cramps
• Some common causes of muscle cramps include
– fatigue
– strenuous exercise
– overuse of particular muscle groups
– a failure to stretch or properly warm up prior to activity
– dehydration and the related problem of sodium deficiency
– low blood sugar (glucose) levels
– magnesium deficiency
– calcium deficiency
– the presence of the hydrogen ion that is a byproduct of lactic acid formation in working
muscles
– thyroid gland irregularity
– kidney dysfunction
– side effects of certain medications
– neuromuscular disorder
• In order for an athlete to prevent a muscle cramp occurring the causes
above should be monitored continuously.

17. Muscle strain
• A muscle strain is damage caused by
overstretching the muscle

18. Myopathies
• Primary muscle disorders are called
myopathies
• eg.
– Proximal myopathy
– Myositis
– Congenital myopathy
– Muscular dystrophy

19. Proximal myopathy
• Primary muscle disorders
• Muscle fibers are degenerated
• Clinically proximal muscle weakness & muscle
wasting
• High CPK levels in blood
• EMG specfic pattern
• Muscle biopsy useful

20. Myositis
• Inflammatory muscle disease
• EMG specific pattern

21. Congenital myopathy
(floppy baby)
Congenital myopathy produces floppy baby

22. Muscular dystrophy
Primary muscle disorder produces gross muscle wasting

23. Dystrophin
• A rod-shaped cytoplasmic protein
• Vital part of a protein complex
• Connects the cytoskeleton of a muscle fiber
to the surrounding extracellular matrix
• Through the cell membrane
• It provides an anchoring function to the
muscle proteins

24. Duchenne Muscular dystrophy
• A lethal degenerative disease
of muscles
• Protein dystrophin is absent
• Muscles are more susceptible
to stretch-induced muscle
damage
• Gower’s sign is seen
• Incidence 1 to 5000 male live
births

25. Becker’s muscular dystrophy
• Develop in older children (5 to 15 yrs)
• Slowly progressive
• Muscle weakness of the lower body, including the legs and pelvis area, slowly gets
worse, causing:
• Difficulty walking that gets worse over time; by age 25-30 the person is usually
unable to walk
• Frequent falls
• Difficulty getting up from the floor and climbing stairs
• Difficulty with running, hopping, and jumping
• Loss of muscle mass
• Toe walking
• Incidence 1 in 18450 male live births

26. EMG recording – myopathic pattern
• At rest: fibrillations (myositis)
• Motor unit action potentials amplitude and
duration reduced
• Recruitment pattern is early with complete
recruitment
Myopathic EMG pattern

27. Myotonia
• Two types
– Myotonia dystrophica
– Myotonia congenita
• Contraction is normal, relaxation is slow
– Depolarisation is normal
– Repolarisation will not take place normally
• Due to derangement of electrical activity in the muscle
membrane
– K+ Cl- channel derangements
• Channelopathy

29. EMG recording – myotonia
• Rest:
– “Dive bomber sound”
– Prolonged continuous activity
– Triggered by needle position or percussion

30. Other disorders affecting
muscle dysfunction

31. MND (Motor neuron disease)
or ALS (amyotrophic lateral sclerosis)
• Affect adults (after 40 years)
• Features include weakness and wasting of limb muscles, tongue fasciculations,
dysarthria, dysphagia
• Slowly progressive and poor prognosis
• Both upper motor neuron and lower motor neuron are affected
• No cure or treatment
• Incidence 2-6 per 100,000

32. EMG recording – MND pattern
• At rest: fibrillations and fasciculations
• Motor unit action potentials amplitude
and duration increased, giant motor
units
• Recruitment pattern is reduced
Giant
motor units

33. SMA (Spinal muscular atrophy)
• Anterior horn cell disease
• Affect infants and older children
• Poor prognosis
• Muscle weakness and wasting
• Caused by an abnormal or missing gene responsible for the production of a
protein essential to motor neurons
• Incidence 1 in 10,000 live births
• Several types are present SMA type I, II and III
• Type I (Werdnig-Hoffman disease or infantile-onset SMA)
– Evident at birth or within the first few months
– Symptoms include floppy limbs and trunk, feeble movements of the arms and legs,
swallowing and feeding difficulties, and impaired breathing
• Type II (the intermediate form)
– Usually begins 6 and 18 months of age. Legs tend to be more impaired than arms
– Children with Type II may able to sit and some may be able to stand or walk with help
• Type III (Kugelberg-Welander disease)
– Appear between 2 and 17 years of age and include difficulty running, climbing steps, or rising
from a chair

34. DSMA (Distal spinal muscular atrophy)
• Anterior horn cell disease
• Affect adolesecents
• Main feature is a wasting of small muscles of the hand
• Non-progressive and benign
DSMA (distal spinal muscular atrophy)

35. Neuromuscular junction disorders
• eg. myasthenia gravis
• Muscle fatiguability
• Ptosis
• Rarely congenital myasthenia

36. Single fibre EMG
• This is a specialised EMG
technique
• This is useful to diagnose
myasthenia gravis

37. Neuromuscular diseases
Site Neonate Older child Adolescent Adult
Muscle DMD Becker’s Myopathies,
myositis
Anterior horn
cell
SMA SMA DSMA MND (ALS)
NMJ Congenital
myasthenia
Myasthenia
gravis

38. Clinical use of needle EMG
• Diagnosis of muscle disorders
– Myopathy
– Myositis
– Muscular dystrophy
– Myotonia dystrophica
• Diagnosis of UMN, LMN disorders, anterior horn cell diseases
– MND (ALS)
– SMA
– DSMA
• Diagnosis of NMJ disorders (Single fibre EMG)
– Myasthenia gravis