Skeletal muscle

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Skeletal muscle
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Reading material
1. Review the normal structure and function of
skeletal muscle (in your notes or general
textbooks) 2. McGavin M.D. 1995 Muscle Chapter
9 in Thomsons Special Veterinary Pathology
Carlton W.W. et al pp 461 - 498 NB - Put
special emphasis on the topics covered during the
lectures.
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Objectives for Muscle dz

• Describe mechanisms of
• Muscular regeneration
• Muscular repair

• Define following conditions and give examples
• Denervation atrophy
• Disuse atrophy
• Malnutrition atrophy
• Muscular hypertrophy
• Ischemic myopathy
• - Occlusion of the vascular system
• - External pressure on a muscle
• Swelling of a muscle in a non-expandable
• compartment

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Objectives for Muscle dz
You should be able to define describe
pathogenesis list lesions and know how to
diagnose the following conditions/diseases

• Nutritional myopathy (White muscle disease)
• Toxic myopathy (Monensin toxicity)
• Exertional myopathy
• Gas gangrene and malignant edema
• Blackleg
• Botulism
• Tetanus
• Myasthenia gravis
• Masticatory muscle myositis

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Skeletal muscle
Unique features

• Complex biochemistry and physiology
• Simple pathology
• Very long multinucleated cells
• Segmental necrosis
• Unique regeneration

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Muscle (epimysium)
Muscle bundle (perimysium)
Myocyte (endomysium)
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Myocyte
Endomysium
Basal lamina
Satellite cell
Plasmalemma
Nucleus
Mitochondria
Myofibril
Myosin Actin
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• Type I fibers (red)
• Postural
• Oxidative
• Slow contracting
• Slow fatiguing

• Type II fibers (white)
• Fast movement
• Glycolytic
• Fast contracting
• Fast fatiguing

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Review
Contractile mechanism (Histology physiology
notes)
Rigor mortis (Thomsons Special Veterinary
Pathology p. 396)
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Response to injury - Atrophy
Response to injury

• Atrophy
• Hypertrophy
• Degeneration / Necrosis
• Inflammation

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Response to injury - Atrophy
Atrophy

• Denervation

Suprascapular nerve paralysis (sweeney) Radial
nerve paralysis in dogs

• Disuse

Fracture Recumbency

• Malnutrition

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Response to injury - Hypertrophy
Hypertrophy
Work hypertrophy Compensatory hypertrophy
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Response to injury
Necrosis
Myocyte necrosis Cell membrane
disintegration Leakage of cytosol substances
(creatine kinase (CK) myoglobin) Indicators
of muscle damage.
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Regeneration and repair
Regeneration
Repair
Both function structure are re-established
COMPLETELY
Function structure are re-established ONLY
PARTIALLY (due to fibrosis)
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Regeneration and repair
Prerequisites for regeneration

• Debridement
• Tissue scaffold
• Germinal cells
• Blood supply

Repair (scarring)
If any of these is missing
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Regeneration and repair
Basal lamina
SCAFFOLD
Satellite cell
GERMINAL CELLS
BLOOD SUPPLY
DEBRIDEMENT
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Regeneration and repair - regeneration
Regeneration
See regeneration described in Thomsons Spec.
Path on page 467 and 468 Figure 9-5.
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Regeneration and repair - Regeneration
Regeneration
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Regeneration and repair - Regeneration
Regeneration
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Regeneration and repair - Regeneration
Regeneration
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Regeneration and repair - Repair
Repair
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Regeneration and repair - Repair
Repair
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Regeneration and repair - Repair
Repair
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Response to injury - Necrosis
Ischemia
Vascular occlusion
Saddle thromboembolus (Fe Eq) Equine Purpura
Hemorrhagica
External pressure on muscle
Prolonged recumbency Too-tight plaster casts
Muscle swelling in non-expandable compartment
Pressure in muscle exceeds perfusion pressure in
capillaries
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Response to injury - Necrosis
(White muscle dz)
Nutritional Myopathy

• Vit E and/or Se deficiency
• Economically important muscle dzin Bo Ov Cap
  Po and Eq

• Lesions opaque white muscle (mineralization)

Ruminants skeletal muscles and heart Pigs liver
(hepatosis diatetica) heart (mulberry heart
disease)
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Response to injury - Necrosis
Pathogenesis
Vit E and/or Se deficiency
Loss of protection against free radicals
Peroxidation of membrane lipids
Disturbance of membrane permeability
Influx of calcium into cytosol
Accumulation of calcium in mitochondria
Damaged mitochondria
Energy exhaustion
Necrosis
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Response to injury - Necrosis
Diagnosis
Histology Se and Vit E levels in the liver feed
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Response to injury - Necrosis
Monensin toxicity

• Feed additive overdose
• Lesions necrosis (ill-defined pale streaks) in
  myocardium and skeletal muscles
• Pathogenesis

Interferes with ion transmembrane
transport Calcium influx and mitochondiral
damage Energy exhaustion Necrosis
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Response to injury - Necrosis
Diagnosis
Histology Monensin levels in feed and stomach
content
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Response to injury - Necrosis
Exertional myopathy
Excessive muscle activity Sudden production of
heat and lactic acid Coagulation of
contractile proteins Muscle swelling and decrease
blood perfusion Ischemia and necrosis
Often with myoglobinuria
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Response to injury - Necrosis
Equine Rhabdomyolysis (muscle necrosis) Azoturia
(Monday morning disease Tying-up (Transient
exertional rhabdomyolysis)
Capture myopathy (wildlife)
Lesions necrosis of the major muscles
Diagnosis History Myoglobinuria Muscle
necrosis
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Response to injury - Inflammation
Clostridial myositis
Lesions Dry red-black porous necrotic
muscles Serohemorrhagic exudation
Diagnosis Demonstration of Clostridial
agents Presence of the characteristic lesions
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Response to injury - Inflammation
Gas gangrene and malignant edema
Bo Eq Ov Cap Po Penetrating wounds infected
by C. septicum C. perfringens C. novyi and
C. chauvoei
Blackleg (Bo Cap Ov)
Activation of spores of C. chauvoei disseminated
from the GI tract to muscles
Clostridial requirements for low O2 Death is
due to toxemia and septicemia.
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Response to injury - Inflammation
Other infections

• Systemic or local infections
• Bacterial
• Viral
• Parasitic

Dz associated with clostridial toxins Tetanus Bot
ulism
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Response to injury - Inflammation
Immune-mediated myositides
Masticatory Muscle Myositis
Eosinophilic myositis - acute form Atrophic
myositis - chronic form

• Autoantibodies against type IIM fibers

• Lesions
• Acute - extremely swollen hard painful
  muscles
• Chronic - marked atrophy

• Diagnosis
• Muscle biopsy
• Serology for autoantibodies

Polymyositis
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Congenital hereditary disorders
Myasthenia Gravis
Muscle weakness and fatigue exacerbated by
exercise and resolves with rest (in dogs and
occ. in cats and goats

• Acquired

Autoimmune dz - Ab against acetylcholine
receptors Adult dogs with megaesophagus
aspiration pneumonia

• Congenital

Inherited autosomal recessive deficiency in AchRs

• Diagnosis

Cholinesterase inhibitors Demonstration of Ab
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Neoplasia
Primary
Rhabdomyosarcoma
Rhabdomyoma
Secondary
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