Sarcolemma – Membrane around individual muscle fibers
Neuromuscular Junction – Junction between motor neuron (nerve cell) and muscle fibers it innervates
Motor Unit – Motor neuron and all the fibers it innervates
All the fibers of a motor unit will contract when the motor neuron stimulates it (all or none)
Sarcoplasm – Cytoplasm of muscle which contains contractile componenets such as proteins, mitochondria, glycogen, fat particles, and enzymes
Myofibrils – Contract the muscle cell and contain myosin and actin
Myofilament – Actin and myosin
Myosin filaments– Thick filaments
Actin filaments – Thin filaments
Cross-bridge – Heads of myosin molecules that bind with actin
Sarcomere – Smallest contractile unit of a skeletal muscle formed by myosin and actin filaments (from z-line to z-line)
M-bridge – Where other myosin filaments are anchored to in the middle of the sarcomere
Z-Line – At each end of the sarcomere where actin is anchored
Myosin is surrounded by 6 actin filaments
Actin is surrounded by 3 myosin filaments
I-band – Contains the Z-line and only actin filaments
H-zone – Contains the m line and only myosin filaments
H-zone and I-band both decrease during contraction as actin slides over myosin and Z-lines are pulled toward the center of the sarcomere
Sarcoplasmic Reticulum – System of tubules surrounding each myofibril which contain and pump calcium
T-tubules – Tranverse tubules that run perpendicular to the sarcoplasmic reticulium and transmit action potentials (electrical nerve impulses)
Sliding-filament theory – Actin filiments are puled over myosin filiments bringing the z-lines closer together and shortening the sarcomere which shortens the muscle fiber.
Length Tension Curve
The strongest force in a muscle can be produced when the muscle is partially flexed meaning many myosin heads are posed above the actin filaments, so all of these heads can bind and cause the muscle to contract. When not flexed only a few myosin heads can bind, and when maxmially contracted the actin filaments have overlaped meaning there is some actin that cannot be bound with so there are less cross bridges formed.
Resting Phase
In the resting phase there is little calcium in the myofibril it is in the sarcoplasmic reticulium, so very few cross-bridges are bound to actin and there is no muscle tension.
Excitation-Contraction Coupling Phase
The sarcoplasmic reticulum is stimulated to release calcium ions, calcium binds to troponin which causes tropomyosin to move exposing the myosin binding site on actin. The cross-bridge binds to actin allowing cross-bridge flexion to occur.
At any time the amount of cross-bridges bound to actin dictates how much force the muscle can produce.
Contraction Phase
ATP attaches to the crossbridge and is broken down to ADP which causes flexion. Then another ATP molecule must replace the ADP by binding to the crossbridge allowing it to detach from the actin site and recock.
Calcium and ATP are required for cross-bridge cycling.
Relaxation Phase
Calcium is pumped back into the SR which prevents the link between actin and myosin.
Neuromuscular System
Activation of Muscles
Muscles that perform precise movements, such as the eye, have as few as one muscle fiber per motor unit. On the other-hand
