Covers the structure of skeletal muscle tissue and its contraction. It describes the fibres (rabdomyofibres), their triads and cross-striations, and the sarcomere with its thin filaments of actin, tropomyosin and troponin and thick myosin filaments. A ten-step sequence of the contraction mechanism follows, from the motor neuron signal to calcium return to the sarcoplasmic reticulum. It ends with the connective tissue layers (epimysium, perimysium, endomysium) that organise the muscle.
Topic 39 of 59 in the MU Pleven syllabus for Cytology (Medicine, Year 1). The free AceMed note covers it in 5 sections, about 638 words (4 min of reading), with 28 flashcards.
Skeletal muscle fibres (rabdomyofibres) are characterised with cylindrical shape, numerous peripherally located nuclei and cytoplasm rich in mitochondria, SER and glycogen
During embryonic development, several hundred myoblasts line up end to end, fusing with one another to form long, multinucleated cells called rabdomyofibres
Rabdomyofibres have myofilaments and triads
Triads are combinations of cisternae of the SER associated with the invaginations of the plasmalemma
Skeletal muscles have cross-striations of alternating light and dark bands
Dark band – A band (anisotropic) ; light band – I band (isotropic)
Each I band is bisected by a dark transverse line, the Z line
A sarcomere extends from one Z line to the next Z line and is 2.5 μm long
A sarcomere has two fibres, thick and thin, which lie parallel to the long axis of the myofibres in a symmetrical pattern
The A and I banding pattern in sarcomeres is due mainly to the regular arrangement of thick and thin filaments composed of myosin and actin
Thin filaments are actin, tropomyosin and troponin
The thin filaments run between and parallel to the thick filaments and have one end attached to the Z line
Actin is present as long filament (F-actin) polymers consisting of two strands of globular (G-actin) monomers, twisted around each other in a double helical formation. Actin filaments are anchored to the Z line by α-actinin and desmin
Tropomyosin is a long, thin molecule that runs between the two twisted actin strands
Troponin is a complex of three subunits – TnT, TnC, TnI. The troponin is attached to the tropomyosin molecules
Thick filament – Myosin, is composed of two heavy chains and two light chains. Small globular projections at one end of the heavy chain form heads which have ATP binding sites as well as the enzymatic capacity to hydrolyse ATP and the ability to bind actin. Myosin filaments occupy the A band at the middle of the sarcomere
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