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Questions and Answers
What is the main role of proprioceptors in the context provided?
Which type of skeletal muscle fibers is particularly adapted for long, slow contractions needed to maintain erect posture?
What feature makes fresh tissue rich in Type I (slow oxidative) muscle fibers dark or red in color?
Which type of skeletal muscle fibers contain large amounts of glycogen and are capable of anaerobic glycolysis?
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What is the main differentiating factor between Type I and Type IIa skeletal muscle fibers?
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Which type of skeletal muscle fiber is predominant in the muscles of high-endurance athletes like marathon runners?
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Which type of skeletal muscle fiber is classified as intermediate and has medium size with high myoglobin content?
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What characteristic makes Type I (slow oxidative) muscle fibers adapted for slow contractions over long periods without fatigue?
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What enables Type IIa (fast oxidative glycolytic) muscle fibers to be capable of anaerobic glycolysis?
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What feature distinguishes Type IIb (fast glycolytic) fibers from Type I (slow oxidative) fibers in terms of energy production?
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Study Notes
Myofibrils and Sarcomeres
- Myofibrils consist of an end-to-end repetitive arrangement of sarcomeres
- The A and I banding pattern in sarcomeres is due to the regular arrangement of thick and thin myofilaments
- The dark A band is bisected by a less dense, or light, region called the H band
- The M line contains a myosin-binding protein myomesin and creatine kinase
Thick Filaments
- Thick filament consists primarily of myosin molecules
- Myosin is a large complex (~500 kDa) with two identical heavy chains and two pairs of light chains
- Thick myosin filaments are 1.6-μm long and 15-nm wide and occupy the A band at the middle region of the sarcomere
Thin Filaments
- Thin filament primarily consists of polymerized actin molecules coupled with regulatory proteins and other thin filament–associated proteins
- G-actin is a small, 42 kDa molecule that polymerizes to form a double-stranded helix, the F-actin filament
Skeletal Muscle
- Skeletal muscle consists of muscle fibers, which are long, cylindrical multinucleated cells with diameters of 10-100 μm
- Elongated nuclei are found peripherally just under the sarcolemma
- A small population of reserve progenitor cells called muscle satellite cells remains adjacent to most fibers of differentiated skeletal muscle
Organization of a Skeletal Muscle
- Thin layers of connective tissue surround and organize the contractile fibers in skeletal muscle
- The epimysium, an external sheath of dense irregular connective tissue, surrounds the entire muscle
Sarcoplasmic Reticulum and Transverse Tubule System
- The membranous smooth ER, called here sarcoplasmic reticulum, contains pumps and other proteins for Ca2+ sequestration and surrounds the myofibrils
- Calcium release from cisternae of the sarcoplasmic reticulum through voltage-gated Ca2+ channels is triggered by membrane depolarization produced by a motor nerve
Mechanism of Contraction
- Contraction occurs when thin actin filaments slide over thick myosin filaments
- Actin filaments enter the A band, the H band narrows, and the sarcomere becomes shorter
- The lengths of actin and myosin do not change, but their positions change, causing the I bands to narrow and the length of the A band to remain unchanged
Motor Innervation
- Skeletal muscle fibers are richly innervated by motor neurons that originate in the spinal cord or brainstem
- Motor nerves branch out within the perimysium, where each nerve gives rise to several unmyelinated terminal twigs that pass through endomysium and form synapses with individual muscle fibers
Sensory Innervation
- Striated muscles contain sensory receptors acting as proprioceptors providing the central nervous system (CNS) with data from the musculoskeletal system
- Proprioceptors detect increases in tension and help regulate the amount of effort required to perform movements that call for variable amounts of muscular force
Skeletal Muscle Fiber Types
- There are three types of skeletal muscle fibers: Type I (slow oxidative), Type IIa (fast oxidative glycolytic), and Type Iıb (fast glycolytic) fibers
- Type I fibers are adapted for slow contractions over long periods without fatigue, having many mitochondria, many surrounding capillaries, and much myoglobin
- Type IIa fibers are intermediate fibers seen in fresh tissue, with many mitochondria and high myoglobin content
- Type Iıb fibers are capable of anaerobic glycolysis
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Description
Learn about Duchenne Muscular Dystrophy as an X-linked recessive trait affecting primarily boys, and the anatomy of muscle fibers with a focus on sarcoplasmic reticulum and transverse tubule system. Understand the role of calcium in muscle contraction and the process of calcium release triggered by membrane depolarization.
