Skeletal Muscle Tissue

1. Skeletal Muscle Tissue Chapter 10 Part II

2. 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

3. 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

4. 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

5. 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

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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’)