Skeletal Muscle Contraction: Histology: Sarcolemma: *The plasma membrane of a muscle fiber -T- tubules - Terminal Cisternae - Sarcoplasmic Reticulum *Transverse tubules- tube like invaginations or inward extensions found along the sarcolemma *conducts the action potention(nerve impulse) towards the center of the muscle fiber *Enlarged area of the sarcoplasmic reticulum- forms the triad system with T- tublues *highly organized smooth endoplasmic reticulum associated with T-tubules *functions in calcium ion (Ca 2+ ) storage Sarcoplasm: *Muscle cell cytoplasm- contains organelles such as mitochondria and other cellular materials - Myofibril *bundles of protein filaments found in the sarcoplasm *1-3 micrometers in diameter *runs the length of the muscle Actin: *myofilament found in the myofibril *thin filament *8 nm in diameter and 1000 nm in length Myosin: *myofilament found in the myofibril *thick filament *12nm in diameter and 1800 nm in length - Sarcomere *fundamental unit of skeletal muscle fiber *highly organized unit formed from the myosin and actin myofilaments *gives skeletal muscle its striated or striped appearence
Actin and Myosin: Actin: *composed of F-actin(fibrous actin), tropomyosin and troponin. *this complex regulates the cross bridge formations that occur between the active sites of actin and the myosin protein -F actin: *protein strands that form a double helix that extends the length of the actin filament *composed of 200 hundred units called G- actin (globular actin) -G actin: *globular protein units that contain and active site, or bonding site for the myosin myofilament - Tropomyosin: *elongated protein that winds along the length of the actin filament *covers or blocks the active site of the G actin *each tropomyosin convers seven (7) active sites - Troponin: *trimer- protein with three sub units *TnT- subunit that binds to tropomyosin removing it from the active sites *TnC- subunit that binds to Calcium allowing TnT to bind to tropomyosin *TnI- subunit that gets phosphorylated and binds tog- Actin Myosin: Properties of myosin: *each myosin myofilament contains two, golf club like filaments (heavy chains) wound together and arranged parallel to the actin filament and two globular heads composed of four myosin (light) chains. *each myosin myofilament contains about 300 myosin molecules with 150 globular heads projecting towards each end 1. globular heads bind to active sites of the actin filament (cross bridge formation) 2. globular heads are attached to rod portion of myofilament by a hinged region 3. globular heads are ATPase enzymes that break down ATP which provides energy for the hinge region to bend causing muscle contraction
Sarcomere: Sarcomere: Z disk: *fundamental unit of skeletal muscle *smallest portion of muscle capable of contracting *Z- disk to Z-disk *filamentous network of protein forming a disk-like structure for the attachment of actin myofilaments I band: A band: H zone: M line: Titin: *isotropic band- light band- allows light to pass *extends from Z- disk to the end of the myosin filament *consists of actin filaments only *anisotropic band- dark band- light cannot pass *extends the length of the myosin myofilament within a sarcomere *consists of actin and myosin myofilaments (overlapping portions) *smaller band in center of sarcomere where actin and myosin do not overlap *consists of myosin myofilaments only *a dark line in the middle of the H zone *consists of filaments that attach to the center of the myosin myofilament *one of the largest known proteins 27, 000 amino acids *attaches Z disks to the myosin myofilaments *helps hold them in position.
Steps to muscle contraction and relaxation: A. Contraction: The Slide- Filament Theory 1. Excitation: 2. Release of Calcium 3. Charging of globular heads 4. Calcium binds to troponin 5. Troponin binds to tropomyosin 6. Cross bridge formation 7. Power stroke 8. Release of ADP B. Relaxation: 1. Signal stops 2. Phosphorilation of TnI 3. Tropomyosin covers active sites 4. Calcium is released 5. Calcium is pumped back to Sarcoplasmic reticulum
Structure of a Sarcomere: