Senior

Structure of skeletal muscle.
Muscle fibers have the same organelles present in other cells however, skeletal muscle fibers are multinucleated.
Striated muscle appears striated due to dark A bands and light I bands.

Motor Units.
Synapse between the nerve and muscle fiber. The motor end plate is a specialized region of the sarcolemma of a muscle fiber surrounding the terminal end of theaxon.
Each axon may produce many branches to innervate an equal number of muscle fibers.
* Each somatic motor neuron and the muscle fibers it innervates is known at the motor unit.
* When a motor neuron is stimulated, all of the muscle fibers it innervates are stimulated to contract. In order for contractions to be smooth and sustained, different motor units must be activated by rapid,asynchronous stimulation.
* Fine neural control – optimal when there are many small motor units.
* Fine control innervation ratio – one neuron per 23 muscle fibers.
* Powerful contractions – optimal when there is one neuron innervating many muscle fibers.
* Powerful control innervation ratio – one neuron per 1,000 muscle fibers.
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* Contraction.
* Muscle cells are madeof myofibrils (1 micrometer in diameter). Tightly packed so other organelles restricted to narrow cytoplasmic spaces between them.
* Thick filaments – contained in the A band, myosin.
* Thin filaments – contained in the I band, actin.
Thin filaments do not end at the edge of the I bands. Extends partway into the A band. Edges of the A band are darker in appearance.
Central lighterregions are the H bands. Only contain thick filaments that are not overlapped.
The thin, dark Z line is in the center of each I band. Z line to Z line is a sarcomere.
M line are produced by protein filaments located at the center of the thick filaments in the sarcomere. They anchor the thick filaments, helping them to stay together during contraction.
Titin – elastic protein that runs throughthick filaments from the M lines to the Z discs. Contributes to elastic recoil of muscles that returns them to resting length.

Sliding filaments in contraction.
A myofiber, together with all its myofibrils, shortens by movement of the insertion toward the origin of the muscle.
Shortening of the myofibrils is caused by a shortening of the sarcomeres – distance between the Z lines is reduced.Shortening of the sarcomeres is accomplished by sliding of the myofilaments. The length of each filament remains the same during contraction.
Sliding of the filaments is produced by asynchronous power strokes of myosin cross bridges, which pull the think filaments (actin) over the thick filaments (myosin).
The A bands remain the same length during contraction but are pulled toward the origin ofthe muscle.
Adjacent A bands are pulled closer together as the I bands between them shorten.
The H bands shorten during contraction as the thin filaments on the sides of the sarcomeres are pulled toward the middle.

Cross bridges.
Cross bridges extend out from the myosin toward the actin.
Part of the myosin proteins, arms and heads.
Orientation of myosin heads on one side of the sarcomere areopposite on the other side. Pull actin from each side toward the center.
Myosin heads are not attached to actin at rest, each myosin head of a cross bridge contains an ATP binding site closely associated with the actin binding site.
Globular heads function as myosin ATPase enzymes that split ATP into ADP and Pi.
When ATP is hydrolyzed to ADP and Pi, the myosin head cocks. Position of myosinhead has changed and it has the potential energy to contract because it has stored energy. One myosin binds to actin forming a cross bridge, the bound Pi is released. Myosin goes through a conformational change to produce a power stroke.
Thin filaments are pulled toward the center of the A band.
Myosin head is in the flexed position after the powerstroke. ADP unbinds and ATP binds. This...