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Terms in this set (45)

Each muscle fiber is a long cylindrical , multinucleate cell. The multiple nuclei are located just beneath the sarcolemma or muscle cell membrane. The nuclei appear to be pushed to one side. The fibers are 10 to 100 micrometers long. (Think about the length of some of your leg muscles.) The cytoplasm of the cell is termed sarcoplasm and contains glycogen which serves as an energy storage compound, and myoglobin which binds oxygen. Fibers contain the usual organelles, plus myofibrils, a sarcoplasmic reticulum and T-tubules.

-The interior of the cell is filled with myofibrils which are rod like contractile elements. They make up most of the muscle volume and push the nuclei to the edges of the cell. The arrangement of myofibrils within a muscle fiber is such that a perfectly aligned repeating series of dark A bands and light I bands is evident.

-As we saw, the plasma membrane of a skeletal muscle fiber is called the sarcolemma. Invaginations of the sarcolemma are called T-tubules, or transverse tubules. Na+/ K+ pumps along sarcolemma and T-tubules create concentration gradients for Na+ and K+. Three Na+ are pumped out while two K+ are pumped in. The resting membrane potential is maintained by pumps. The inside of the cell is relatively negative in comparison to outside. This is responsible for excitability of skeletal muscle fibers. Voltage-gated Na+ channels and voltage-gated K+ channels are also present. These are necessary for propagation of the electrical change along the sarcolemma.
The sarcoplasmic reticulum is an internal membrane complex similar to the smooth endoplasmic reticulum. It surrounds bundles of contractile proteins. Terminal cisternae are blind sacs of sarcoplasmic reticulum which serve as reservoirs for calcium ions. They combine in twos with central T-tubule to form triads.
The Second physiological event is Excitation-contraction coupling which links skeletal muscle stimulation to the events of contraction. It consists of three events:

1. Development of end-plate potential at motor end plate. Binding of ACh to ACh receptors on motor end plate causes receptors to open. This allows Na+ to rapidly diffuse into muscle fiber and allows K+ to slowly diffuse out. There is a net gain of positive charge inside fiber which reverses the electrical charge difference at motor end plate. The reverse is termed an end plate potential (EPP)

2. Initiation and propagation of action potential along sarcolemma. Action potential triggered by EPP. First, the inside of sarcolemma becomes relatively positive due to influx of Na+ from voltage-gated channels. This is termed depolarization. Then, the inside of sarcolemma returns to the resting potential due to an outflux of K+ from voltage-gated channels. This is termed repolarization. The action potential is propagated along sarcolemma and T-tubules. Inflow of Na+ at the initial portion of sarcolemma causes adjacent regions to experience electrical changes initiating voltage-gated Na+ channels in this region to open. The action potential is propagated down the sarcolemma and t-tubules. The refractory period is the time between depolarization and repolarization. During this period the muscle is unable to be restimulated.

3. Release of Ca2+ from sarcoplasmic reticulum Opening of voltage-gated Ca2+ channels found in terminal cisternae of sarcoplasmic reticulum is triggered by action potential. Ca2+ diffuses out of cisternae and into sarcoplasm. It now interacts with thick and thin filaments.