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Action Potential:

a) RESTING MEMBRANE POTENTIAL (RMP): RMP at -70mV. Na+ on outside and K+ on inside of cell

As depolarization reaches THRESHOLD of -55mV, the action potential is triggered (opening the voltage regulate Na+ gates) and Na+ rushes into cell. Membrane potential reaches +30mV on action potential

c) PROPAGATION: Propagation (which means spreading or moving) of the action potential at 100 m/sec (which is 225 mph). Membrane potential at +30mV

d) REPOLARIZATION: Repolarization occurs with K+ exiting the cell to return to -70mV (i.e., the cell restores itself to RMP)

e) NA+K+ PUMP: Return of ions (Na+ and K+) to their extracellular (Na+) and intracellular (K+) sites by the sodium potassium (Na+K+) pump

NOTE: An Action Potential is a substantial depolarization. At homeostasis, the cell is continuosly alternating from positive (depoloarization) and negative (hyperpolarization) charges. When the cell attains thresshold (-55mV) the action potential occurs.
Force Production Short Story by Jenny Thomas, B.S., Now Exercise Science M.S.
She wanted to scratch her nose. A simple task, one might suggest. Yet this seemingly elementary chore is, in reality, little more than the most intricate and complex events in the history of the world. “But how is this so?” you might ask. Well, sit back, and I will impart the tale upon your ears...

It all started with the activation of the central nervous system (CNS), the king of all the land. The CNS was a recluse and only communicated with the outside world via his only companion, the peripheral nervous system (PNS). “I must scratch my itch!” the CNS said. “Please release me from this agony!” “Right away!” the PNS replied as he shot an action potential down its nerves, causing Na+ to rush into the cell. The action potential of the PNS finally met the motor nerve fiber. “Motor nerve fiber! Please help!” the PNS said. “The CNS must scratch his itch! You are our only hope!” “I will not let you down! On my honor, I will not disappoint the CNS!” the motor nerve fiber replied.

With great haste, the motor nerve fiber relayed the impulse message of the CNS until it met the treacherous axon terminal. As the impulse arrived, the motor nerve fiber enlisted the help of his friends, the Ca++ ions to rush into the terminal through the voltage-gated calcium channels to react with the synaptic vesicles and push their forces back to the cell membrane of the axon terminal. Success! The synaptic vesicles were defeated, and they were forced to fuse with the cell membrane of the axon terminal and release their prisoners and precious cargo, Acetylcholine (ACh), into the synaptic cleft.

The ACh was free, and as soon as he was released, he saw, across the vast expanse of the synaptic cleft, his old friend, the post-synaptic ACh receptor (AChR). “AChR!” ACh cried as he diffused across the gap. “It has been too long,” AChR replied as he embraced (and bound with) his old friend. As soon as the embrace occurred, there was a glorious influx of Na+ through the opening of the chemically-gated cation channels. This spark of love and friendship (and the influx of Na+ into the cell) caused a depolarization of the surrounding sarcolemma. Sadly, some of the ACh was not lucky enough to meet (bind with) their old loved ones (AChR). These not-so-lucky ACh's were either defused away or hydrolyzed by the evil acetylcholinesterase (AChE). But I digress.
The depolarization brought about by our friends ACh and AChR had a great journey ahead of him. The depolarization was brave and strong, and he traveled along the vast sarcolemma and down the t-tubules. There, his mere presence caused the release of Ca2+ from the sarcoplasmic reticulum into the cytoplasm of the muscle fiber like a crowd of teen girls squealing for Justin Bieber.

The sudden convergence of the Ca2+ aggravated (activated) the C subunit of the troponin molecule. Like Cerberus, the troponin molecule was a three-headed monster, so it quickly aggravated the other subunits (I and T). The T subunit, in a moment of weakness, was suddenly so annoyed that he completely dissociated the tropomyosin away from the binding sites of the actin molecules revealing the binding sites he normally was so careful to conceal.
The myosin head saw his golden opportunity and took his chance. “Here I come!” the myosin head cried as he bound with the binding site of the actin filament, his long lost love. “Myosin head!” the binding site cried. “Oh, how I've missed you!” “Let's run away together,” the myosin head pleaded. “I never want to be parted from you again!” “I would love nothing more,” the binding site replied. However, their fervent joy was cut short when the binding site's father, the actin filament, overheard their plans to depart. “I will not have you steal my daughter away!” the actin filament cried. With that, a battle was waged between the myosin head and the actin filament. The myosin head bent back, pulling with the actin filament. This caused a sliding of the actin over the myosin, bringing the z-discs closer together, eliminating the H-zone, and shortening the muscle. This violent behavior resulted in the dissociation of the ADP molecule and phosphate group from the myosin head.

Just when the myosin head thought he was about to be victorious, a new ATP molecule appeared. “I have friends everywhere,” the actin filament said deviously. “NOOOOOOO!!!” the actin binding site cried as she saw the new ATP molecule. The binding site and the myosin head both knew the ATP molecule would react with the myosin head and cause a reaction that would lead him to lose the strength to continue clinging to his one-and-only love, the actin binding site. “I will never let go,” he whispered to her as his strength gave out. “Until we meet again,” she called out in response.

In his agony from leaving his actin binding site, the myosin head bent back which split the new ATP molecule into ADP+P. These two molecules would not dissociate from him and would be his constant companions until he could meet his beloved binding site again; however, these companions encouraged, motivated, and energized the myosin head to be ready to react with his binding site again when the time came. These cycles of unity and aching separation between the myosin head and the actin binding site continued until the brave and strong depolarization left (due to lack of ACh), and the pesky Ca2+ left the sarcoplasm (the cisternal membrane calcium pumps will pump Ca2+ back into the cisternae). At this time the troponin subunit T could gain his composure and protect the binding site yet again. And the actin slides back to its original relaxation position.

Little did the myosin head know, his aching and swiveling set forth motion that he could not imagine. His pain was not in vain. His epic battle with the actin filament is what created force production. The force was imparted upon the arm and hand bones as the arm lifted, and the hand reached out for the nose, and the nails traversed the skin. And thus, the fulfillment of the CNS's wishes, the nose was scratched. “Ahhh....” sighed the CNS in relief.
The end