Introduction The primary contractile unit of skeletal muscle is the sarcomere Skeletal and cardiac muscle contraction is explained by the sliding filament theory with four key steps attachment power stroke release cocking Requirements for contraction stimulatory impulse (action potential) from a motor neuron high calcium concentration within muscle cells ATP for energy Definitions motor unit is defined as the individual motor neuron and the muscle fibers it stimulates motor end plate (neuromuscular junction) is defined as the junction between the motor neuron and its associated muscle fibers Sarcomere Structure One sarcomere is defined as the segment between two Z-lines Important defining structures Z-line anchoring point for actin filaments (thin filaments) distance between Z-lines shortens with contraction I-band zone of thin filaments not superimposed by thick filaments decreases in size with contraction A-band entire length of one thick filament stays constant in size with contraction H-zone zone of thick filaments not superimposed by thin filaments decreases in size or disappears entirely with contraction M-line midline of the sarcomere does not change with contraction Important proteins actin thin filament anchored to the Z-line extend from the Z-line into the A-line myosin thick filament extends across the A-band linked at the center by the M-line tropomyosin actin-binding protein at rest, is bound tightly to actin to prevent cross-bridge formation with myosin during contraction, calcium binding to troponin triggers a conformational change that releases tropomyosin from actin, allowing cross-bridge formation to occur troponin complex of three proteins (C, I, and T) troponin C is a calcium-binding protein that regulates the conformational state of tropomyosin titin links the Z-line to the thick filaments Sliding Filament Theory An action potential triggers calcium release from the sarcoplasmic reticulum Calcium ions bind to troponin, inducing a conformational change in the troponin-tropomyosin complex tropomyosin is released from actin, exposing actin binding sites allows cross-bridge formation to occur between myosin heads and actin binding sites Contraction attachment myosin head is "cocked" and bound to actin, forming a cross-bridge ADP and Pi are bound to myosin power stroke myosin head pivots centrally, pulling the actin toward the M-line ADP and Pi are released release myosin head is "uncocked" and not bound to nucleotide ATP binds to myosin, triggering myosin detachment from actin rigor mortis, also known as postmortem rigidity, is caused by ATP deficiency secondary to loss of oxygen and glucose in death without ATP, myosin can not detach from actin, leading to muscle rigidity cocking myosin head hydrolyzes ATP, using the energy from hydrolysis to undergo a conformational change from uncocked (low energy) to cocked (high energy) myosin head is now ready to bind actin again and repeat the contraction cycle ADP and Pi are bound to myosin