Skeletal Muscle Tissue

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Skeletal Muscle Tissue

• Chapter 10
• Part II

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Types of Skeletal Muscle Fibers

• Skeletal muscle fibers are categorized according
  to
• How they manufacture energy
• - oxidative fibers produce ATP aerobically using
  oxygen
• - glycolytic fibers produce ATP anaerobically
  (without oxygen)
• How quickly they contract divided into 3
  classes
• - Slow oxidative fibers (Type I) red slow
  twitch
• - Fast glycolytic fibers (Type IIx) white
  fast-twitch
• - Fast oxidative fibers (Type IIa)
  intermediate fibers

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Slow Oxidative Fibers (Type I)

• Red color due to abundant myoglobin
• Obtain energy from aerobic metabolic reactions
• Contain a large number of mitochondria
• Richly supplied with capillaries
• Contract slowly and resistant to fatigue
• Fibers are small in diameter
• Many of these fibers are in the postural muscles
  of the
• lower back that must contract continuously to
  keep the
• spine straight and maintain posture

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Fast Glycolytic Fibers (Type IIx)

• Contain little myoglobin and few mitochondria
• About twice the diameter of slow-oxidative fibers
• Contain more myofilaments and generate more power
• Depend on anaerobic pathways
• Contract rapidly and tire quickly
• Common in muscles of the upper limbs, which often
  
• lift heavy objects for brief periods

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Fast Oxidative Fibers (Type IIa)

• Have an intermediate diameter
• Contract quickly like fast glycolytic fibers
• Are oxygen-dependent
• Have a high myoglobin content and rich supply of
  capillaries
• Somewhat fatigue-resistant
• More powerful than slow oxidative fibers
• Abundant in the muscles of the lower limbs, which
  
• must move the body for plong periods during
• locomotion

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Disorders of Muscle Tissure

• Muscle tissues experience few disorders except
  heart muscle
• Skeletal muscle is remarkably resistant to
  infection
• Smooth muscle problems stem from external
  irritants
• Noninfections skeletal muscle disorders include
• muscular dystrophy, myofascial pain syndrome,
• and fibromyalgia

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Muscular Dystrophy

• A group of inherited muscle destroying disease
• - affected muscles enlarge with fat and CT
  causing muscles to degenerate
• Types of muscular dystrophy
• Duchenne muscular dystrophy lack dystrophin
• - most common and most serious form
• - inherited as a sex-linked recessive disease
• Myotonic dystrophy
• - slow-progressing disease can present anytime
  between birth and age 60

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Myofacial Pain Syndrome Fibromyalgia

• MPS pain is caused by tightened bands of muscle
  fibers that twitch when the skin over them is
  touched
• - sensitive areas of skin are called trigger
  points
• - pain is felt some distance from the trigger
  point at reference zones
• Fibromyalgia (algia pain)
• - a mysterious chronic-pain syndrome
• - affects mostly women
• - symptoms include fatigue, slepp
  abnormalities, severe musculoskeletal pain, and
  headache

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Muscle Tissue Throughout Life

• Muscle tissue develops from embryonic mesoderm
• - myoblast cells fuse to form skeletal muscle
  fibers
• - skeletal muscles contract by the 7th week of
  development
• - nerves grow into the muscle masses from the
  spinal cord
• - skeletal muscle fibers never undergo mitosis
  after they are formed
• - during childhood and adolescence, cells
  lengthen and thicken to keep up with the growing
  body
• - during youth, satellite cells fuse into
  existing muscle fibers to help them grow
• - after an injury, satellite cells proliferate
  in the damaged muscle tissue but regenerative
  capacity is not complete, and damaged tissue is
  replaced by scar tissue

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Cardiac Muscle

• Pumps blood 3 weeks after fertilization
• Satellite cells surround skeletal muscle fibers
• - resemble undifferentiated myoblasts
• - fuse into existing muscle fibers to help them
  grow

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Muscle Tissue Throughout Life

• With increased age the amount of CT increases in
  muscles while the number of muscle fibers
  decrease
• Loss of muscle mass with aging leads to a
  decrease in muscular strength
• - usually by 50 by the age of 80
• - this is called sarcopenia (flesh wasting)