Skeletal Muscles

• Skeletal muscle is striated with fast and slow twitch fibres
• Cardiac muscle, unlike skeletal, is involuntary
• Smooth muscle is non striated and is under involuntary control, often forming a ring to contract
• Myofibrils make up muscle fibres, each contains many sarcomeres making up the striated muscle with light and dark bands
• A sarcomere consists of myosin - the thicker protein filament, and actin - the thinner protein filament
• The Z line marks the end of a sarcomere
• The I band is isotropic and light due to only having actin
• The A band is anisotropic and is dark as it is made of both myosin and actin filaments
• The H zone is the centre of the sarcomere made of only myosin
• There are many ATP in the sarcoplasm of a sarcomere as it is needed to detach myosin heads from the actin filaments
• When the sarcomere contracts, the I band gets smaller and the A band gets larger, the H band stays the same size
• Slow twitch fibres contract more slowly than fast twitch
• Slow twitch muscle fibres have less glycogen stores than fast twitch
• Slow twitch muscle fibres do aerobic respiration and fast twitch do anaerobic respiration due to only being used for short bursts of energy
• Slow twitch fibres are more red due to a better blood supply than fast twitch fibres as they need oxygen for aerobic respiration
• Slow twitch fibres are also redder in colour due to myoglobin, which stores oxygen in the muscles for aerobic respiration
• An action potential arrives at the neuromuscular junction and calcium ions diffuse into the synaptic knob causing vesicles of acetylcholine to fuse with the membrane, releasing it
• After sodium ions diffuse across the sarcolemma, depolarising it, T tubules carry the action potential deep into the sarcomere, calcium ions are released from the sarcoplasmic reticulum
• T tubules are extensions of the sarcolemma allowing action potentials to travel deep into the fibre
• After calcium ions diffuse from the sarcoplasmic reticulum, they move the tropomyosin binding the actin, revealing the myosin head binding site
• Myosin heads bind to the exposed myosin binding site on actin, ADP + Pi are released from the myosin head causing it to bend, pulling the actin toward the centre of the sarcomere
• ATP binding to the myosin head will release it from the myosin binding site on the actin, separating it from the actin, ATP's hydrolysis by ATPase will cause the myosin head to return to its original position, allowing it to bind to the next exposed binding site, moving the actin toward the centre of the sarcomere
• ATP can be supplied to skeletal muscle by anaerobic and aerobic respiration, phosphocreatine is also found in the sarcoplasm, phosphates are stored that can quickly be attached to ADP during intense exercise, creatine is replenished when an organism is at rest
• Antagonistic muscles are how the skeleton moves, muscles work against each other in antagonistic pairs to extend each muscle in turn
• Antagonistic pairs can both be contracted at once to provide rigidity, supporting the posture and skeleton
• Muscles are attached to the skeleton by tendons
• Smaller contractions require fewer fibres so less are stimulated, larger contractions require a larger mass of fibres so more action potentials are sent to more fibres
• ATP becoming ADP + Pi causes the power stroke where myosin heads move toward the centre of the sarcomere
• Calcium ions diffuse from the sarcoplasmic reticulum when an action potential arrives
• Calcium ions activate ATPase for ATP hydrolysis for the power stroke