Review of Musculoskeletal System

1. Review of Musculoskeletal System Chapter 18

2. Muscle • Skeletal muscle • > 600 muscles in body • Fascia • Epimysium – forms tendons at ends • Perimysium – divides into fascicles • Endomysium – surrounds individual fibers

4. Motor Unit • One motor neuron and all the muscle fibers it innervates • recruitment

6. Muscle cell structure • Sarcolemma motor end plate transverse ( t- ) tubules • Sarcoplasm • Sarcoplasmic Reticulum – Stores Ca++

9. Proteins: • Thick filaments – myosin • Thin filaments – actin • Troponin • Tropomyosin • Sliding Filament Model

14. Muscular Dystrophy • Group of rare diseases characterized by a genetic etiology and progressive degeneration of skeletal muscle. • X-linked recessive defect • Most common of the muscular dystrophies • 1 in 3,000 live births • Affects males • Gene located on the short arm of the X chromosome.

15. 30% of cases arise as a new mutation • Can be diagnosed immediately after birth by high serum creatine kinase • Muscle weakness and delayed motor skills can be detected early – obvious by age 5 • Age 10 – require leg bracing • Age 12 – wheelchair • Age 15 completely bedridden • Death by 20 – 30 of cardiac arrest or respiratory failure.

16. Fibrosis → contracture distorts skeletal development • Lordosis • Scoliosis • Compromised respiration • Respiratory insufficiency • Respiratory infection • Cardiac muscle • Dysrythmias • Congestive heart failure • Mental sluggishness

17. Dystrophin is lacking • Membrane damage • Replaced by fibrous connective tissue and fatty deposits • Therapy • Sustain mobility • Sustain respiratory function

18. Myesthenia gravis • Autoimmune disease in which antibodies (IgG) bind with acetylcholine receptors on muscle cells. • Reduces the number of acetylcholine receptors at the neuromuscular junction • Characterized by muscle weakness and fatigability • Also associated with other autoimmune disorders, such as SLE, rheumatoid arthritis, and thyrotoxicosis

19. In 10-25% of people with MG thymic tumors are found • More common in males than females • 70 – 80 % have pathologic changes in the thymus

20. Classification of myasthenia • Neonatal myasthenia • Transitory condition in which 10-15 % of infants born to mothers with MG show symptoms of the disease • Congenital myasthenia • Juvenile myasthenia – onset us.about 10 years • Ocular myasthenia • More common in males • Weakness of eye muscles and eyelids, may also include swallowing difficulties and slurred speech

21. Generalized autoimmune myasthenia • Involves proximal musculature throughout the body, and has several courses: • A course with periodic remissions • Slowly progressive course • Rapidly progressive course • Fulminating course

22. Pathophysiology • Defect in the nerve impulse transmission at the NMJ • Postsynaptic acetylcholine receptors are no longer recognized as “self” and antibodies are produced against them. • IgG blocks the binding of ACh • Eventually destroys the receptor • Causes diminished transmission of nerve impulse across the NMJ and lack of muscle depolarization • Cause is unknown.

23. Clinical manifestations • Onset typically insidious • May first appear during pregnancy, postpartum or with the administration of certain anesthetic agents • Complaints are fatigue and progressive muscle weakness • Fatigue after exercise • Recent history of recurrent upper respiratory infections

24. Clinical manifestations • Muscles of the eyes, face, mouth, throat and neck are usually affected first • Levator and extraocular muscles affected most -Diplopia, ptosis, and ocular palsies • Muscles of facial expression, mastication, swallowing and speech are the next most involved • Facial droop, expressionless face; difficulties in chewing and swallowing, drooling, episodes of choking and aspiration • Nasal, low volume, high-pitched monotonous speech pattern

25. Less frequently involved are the muscles of the neck, shoulder girdle and hip flexors • Fatigue requires periods of rest • Weakness of arms and legs • Difficulty maintaining head position • Respiratory muscles of chest wall and diaphragm become weak • In advanced stage all muscles are weak

26. Myasthenic crisis • Severe weakness causes quadriparesis or quadriplegia, respiratory insufficiency and extreme difficulty in swallowing

27. Cholinergic crisis • Anticholinesterase drug toxicity • Intestinal motility increases • Fasciculation • Bradycardia • Pupillary constriction • Increased salivation • Increased sweating

28. Evaluation • Improvement with edrophonium chloride (Telison) for several minutes • EMG – amplitude of action potentials declines • Antiacetylcholine receptor antibody titers • Antistriated muscle antibody titers • MRI to rule out thymoma

29. Progression • Varies • Appears first as a mild case that spontaneously remits with a series of relapses and symptom free intervals • Over time can progress leading to death • Ocular myasthenia has a good prognosis

30. Treatment • Anticholinesterase drugs • Steroids • Immunosuppressant drugs • Cyclophosphamide • Plasmapheresis during myasthenic crisis • Thymectomy is treatment of choice for individuals with thymoma