Skeletal Muscle Structure and Function

Skeletal Muscle Structure

• Muscle fibers can be categorized into type I and type II fibers
• Type I fibers are known as "red" or "slow twitch" fibers and are responsible for endurance activities
• Type II fibers are known as "white" or "fast twitch" fibers and are responsible for short bursts of activity
• The pectoral muscles of chickens and birds are rich in type II fibers, while long-distance flying birds have pectoral muscles rich in type I fibers
• The distribution of fibers can affect the presence of certain diseases

Skeletal Muscle Function

• Involved in locomotion, posture, and fine motor movements
• Supports joint stability and plays a crucial role in proprioception
• Protects the body by absorbing impacts
• Contributes to body homeostasis by generating heat
• Stimulates blood vessels, assisting in blood pumping against gravity

Skeletal Muscle Examination

• Visual examination includes assessing color, volume, texture, and appearance
• Compare the affected muscle to the contralateral side if possible
• Assess muscle response to stimuli
• Microscopic examination through histopathology is essential to determine the underlying pathology

Skeletal Muscle Disease

• Intertwined with the peripheral nervous system as muscle function is driven by motor neuron innervation
• Diseases fall into two categories: neuropathic or myopathic
• Neuropathic disease involves a disruption in nerve signal transmission
• Myopathic disease affects muscle cell metabolism and function

Clinical Signs of Skeletal Muscle Disease

• Atrophy: decrease in muscle size
• Hypertrophy: increase in muscle size
• Swelling
• Weakness
• Pain
• Muscle spasms
• Abnormal gait

Disease Portal Entries for Skeletal Muscle Disease

• Injury: wounds, intramuscular injections, bone fractures
• External pressure
• Infection: blood pathogens, toxins, autoantibodies
• Inflammation: inflammatory cells

Skeletal Muscle Disease: Specific Conditions

• Developmental abnormalities
  • Arthrogryposis: Congenital condition where fetuses and stillborn have small limbs with rigid joints
  • Myofibers Hypoplasia: Hind limbs are affected, with a lower than expected number and diameter of myofibers

Degenerative Alterations

• Atrophy:

  • Denervation atrophy: Loss of muscle mass due to lack of nerve stimulation
  • Disuse atrophy: Reduction in muscle mass due to inactivity.
  • Malnutrition atrophy: Decrease in muscle mass due to lack of nutrients

• Hypertrophy: Increase in muscle size due to an increase in workload or genetic selection

• Degeneration and Necrosis: Myofiber injury can be reversible to a point of no return.

• Calcification: Deposition of calcium in the muscle tissue, common in degeneration and necrosis.

• Regeneration: Muscle repair involves satellite cells migrating to the center to form myoblasts and facilitate myofiber regeneration

Myopathies and Myositis

• Myopathies: Diseases of muscle tissue
• Myositis: Inflammation of muscle tissue
• Types of myositis:
  • Bacterial: Black leg (caused by Clostridium chauvoei), Gas gangrene (caused by Clostridium septicum, perfringens, chauvoei...)
  • Viral: Foot and mouth disease
  • Parasitic: Trichinosis, Cysticercosis, Sarcocystosis
  • Idiopathic - immune mediated: Masticatory muscle myositis, Myasthenia gravis
  • Neoplastic: Rhabdomyoma and rhabdomyosarcoma

### Black leg (Symtomatic Anthrax, Emphysematous Gangrene)

• Caused by Clostridium chauvoei
• A fatal disease in cattle, sheep, and goats
• Infection:
  • Spores ingested from soil or the environment
  • Spores germinate in the digestive tract, liver, or muscle
• Characterized by:
  • Muscle injury
  • Inflammation of the affected muscles
  • Toxin production
  • Edema
  • Muscle necrosis
  • Gas production (emphysema)
  • Toxemia
  • Death
• Pathology:
  • Large masses of muscle (pectoral, pelvic, crural, scapular)
  • Muscles are black with gas (emphysema)
  • Subcutaneous edema
• Histopathology:
  • Degeneration
  • Edema
  • Necrosis
  • Emphysema
  • Bacteria difficult to see

Gas gangrene (Malignant edema)

• Caused by Clostridium septicum, perfringens, chauvoei...
• Typically affects ruminants, horses, and pigs
• Cysts containing trillions of parasites are present
• Often no inflammatory response in the muscle tissue
• Important clinical signs include abortion

Idiopathic – immune mediated myositis (dogs and bovines)

• Masticatory muscle myositis: Bilateral swollen and painful jaws, often with atrophy
• Myasthenia gravis: Rare, sporadic disease in dogs and cats. Causes severe muscle weakness and fatigue after mild exercise.
  • Types:
    • Hereditary: Reduced acetylcholine (ACh) receptors
    • Acquired: Antibodies against ACh receptors
  • Treated with Neostigmine, an anticholinesterase drug

Neoplasia

• Primary:
  • Rhabdomyoma/Rhabdomyosarcoma
  • Fibroma/Fibrosarcoma
  • Lipoma/Liposarcoma
  • Hemangioma/Hemangiosarcoma
  • Lymphoma/Lymphosarcoma
• Secondary:
  • Metastasis of sarcomas and carcinomas

Explore the complexities of skeletal muscle fibers and their functions. This quiz covers the differences between type I and type II fibers, their roles in various activities, and the importance of muscle in overall body function. Test your knowledge on muscle examination techniques and physiological contributions.