Physiology





(http://health.howstuffworks.com/human-body/systems/musculoskeletal/muscle4.htm)

This pretty much just gives a more detailed description to what's going on in the pictures above.

Step by Step Process on How Skeletal Muscles Work:


 * 1) An electrical signal (**action potential **) travels down a nerve __ [|cell] __, causing it to release a chemical message (**neurotransmitter **) into a small gap between the nerve cell and muscle cell. This gap is called the **synapse **.
 * 2) The neurotransmitter crosses the gap, binds to a protein (**receptor **) on the muscle-cell membrane and causes an action potential in the muscle cell.
 * 3) The action potential rapidly spreads along the muscle cell and enters the cell through the T-tubule.
 * 4) The action potential opens gates in the muscle's calcium store (**sarcoplasmic reticulum **).
 * 5) **Calcium ions **flow into the __ [|cytoplasm] __, which is where the actin and myosin filaments are.
 * 6) Calcium ions bind to **troponin-tropomyosin molecules **located in the grooves of the actin filaments. Normally, the rod-like tropomyosin molecule covers the sites on actin where myosin can form crossbridges.
 * 7) Upon binding calcium ions, **troponin **changes shape and slides tropomyosin out of the groove, exposing the actin-myosin binding sites.
 * 8) **<span style="font-style: inherit; font-weight: bold !important;">Myosin **interacts with actin by cycling crossbridges, as described previously. The muscle thereby creates force, and shortens.
 * 9) <span style="color: black; font-size: 12px; line-height: 16px; list-style-position: outside; list-style-type: decimal !important; margin-bottom: 0px; margin-bottom: 14px; margin-left: 20px; margin-left: 25px; margin-right: 0px; margin-right: 20px; margin-top: 0px; margin-top: 14px; padding-bottom: 0px; padding-bottom: 0px; padding-left: 0px; padding-left: 1.5em; padding-right: 0px; padding-right: 14px; padding-top: 0px; padding-top: 0px;">After the action potential has passed, the **<span style="font-style: inherit; font-weight: bold !important;">calcium gates close **, and calcium pumps located on the sarcoplasmic reticulum **<span style="font-style: inherit; font-weight: bold !important;">remove calcium **from the cytoplasm.
 * 10) <span style="color: black; font-size: 12px; line-height: 16px; list-style-position: outside; list-style-type: decimal !important; margin-bottom: 0px; margin-bottom: 14px; margin-left: 20px; margin-left: 25px; margin-right: 0px; margin-right: 20px; margin-top: 0px; margin-top: 14px; padding-bottom: 0px; padding-bottom: 0px; padding-left: 0px; padding-left: 1.5em; padding-right: 0px; padding-right: 14px; padding-top: 0px; padding-top: 0px;">As the calcium gets pumped back into the sarcoplasmic reticulum, calcium ions come off the troponin.
 * 11) <span style="color: black; font-size: 12px; line-height: 16px; list-style-position: outside; list-style-type: decimal !important; margin-bottom: 0px; margin-bottom: 14px; margin-left: 20px; margin-left: 25px; margin-right: 0px; margin-right: 20px; margin-top: 0px; margin-top: 14px; padding-bottom: 0px; padding-bottom: 0px; padding-left: 0px; padding-left: 1.5em; padding-right: 0px; padding-right: 14px; padding-top: 0px; padding-top: 0px;">The troponin returns to its normal shape and allows tropomyosin to cover the actin-myosin binding sites on the actin filament.
 * 12) <span style="color: black; font-size: 12px; line-height: 16px; list-style-position: outside; list-style-type: decimal !important; margin-bottom: 0px; margin-bottom: 14px; margin-left: 20px; margin-left: 25px; margin-right: 0px; margin-right: 20px; margin-top: 0px; margin-top: 14px; padding-bottom: 0px; padding-bottom: 0px; padding-left: 0px; padding-left: 1.5em; padding-right: 0px; padding-right: 14px; padding-top: 0px; padding-top: 0px;">Because no binding sites are available now, no crossbridges can form, and the muscle **<span style="font-style: inherit; font-weight: bold !important;">relaxes **.

<span style="font-family: arial,helvetica,sans-serif; font-size: 13px; line-height: 19px;">Step by Step Process on How Cardiac and Smooth Muscles Work: <span style="font-family: arial,helvetica,sans-serif; font-size: 13px; line-height: 19px;"><span style="font-family: arial,helvetica,clean,sans-serif; font-size: 12px; line-height: 14px;">
 * 1) <span style="color: black; font-size: 12px; line-height: 16px; list-style-position: outside; list-style-type: decimal !important; margin-bottom: 0px; margin-bottom: 14px; margin-left: 20px; margin-left: 25px; margin-right: 0px; margin-right: 20px; margin-top: 0px; margin-top: 14px; padding-bottom: 0px; padding-bottom: 0px; padding-left: 0px; padding-left: 1.5em; padding-right: 0px; padding-right: 14px; padding-top: 0px; padding-top: 0px;">**<span style="font-style: inherit; font-weight: bold !important;">Calcium ions ** come from outside of the cell.
 * 2) <span style="color: black; font-size: 12px; line-height: 16px; list-style-position: outside; list-style-type: decimal !important; margin-bottom: 0px; margin-bottom: 14px; margin-left: 20px; margin-left: 25px; margin-right: 0px; margin-right: 20px; margin-top: 0px; margin-top: 14px; padding-bottom: 0px; padding-bottom: 0px; padding-left: 0px; padding-left: 1.5em; padding-right: 0px; padding-right: 14px; padding-top: 0px; padding-top: 0px;">Calcium ions bind to an enzyme complex on myosin, called **<span style="font-style: inherit; font-weight: bold !important;">calmodulin-myosin light chain kinase **.
 * 3) <span style="color: black; font-size: 12px; line-height: 16px; list-style-position: outside; list-style-type: decimal !important; margin-bottom: 0px; margin-bottom: 14px; margin-left: 20px; margin-left: 25px; margin-right: 0px; margin-right: 20px; margin-top: 0px; margin-top: 14px; padding-bottom: 0px; padding-bottom: 0px; padding-left: 0px; padding-left: 1.5em; padding-right: 0px; padding-right: 14px; padding-top: 0px; padding-top: 0px;">The enzyme complex breaks up ATP into ADP and transfers the P i directly to myosin.
 * 4) <span style="color: black; font-size: 12px; line-height: 16px; list-style-position: outside; list-style-type: decimal !important; margin-bottom: 0px; margin-bottom: 14px; margin-left: 20px; margin-left: 25px; margin-right: 0px; margin-right: 20px; margin-top: 0px; margin-top: 14px; padding-bottom: 0px; padding-bottom: 0px; padding-left: 0px; padding-left: 1.5em; padding-right: 0px; padding-right: 14px; padding-top: 0px; padding-top: 0px;">This P i transfer activates myosin.
 * 5) <span style="color: black; font-size: 12px; line-height: 16px; list-style-position: outside; list-style-type: decimal !important; margin-bottom: 0px; margin-bottom: 14px; margin-left: 20px; margin-left: 25px; margin-right: 0px; margin-right: 20px; margin-top: 0px; margin-top: 14px; padding-bottom: 0px; padding-bottom: 0px; padding-left: 0px; padding-left: 1.5em; padding-right: 0px; padding-right: 14px; padding-top: 0px; padding-top: 0px;">Myosin forms crossbridges with actin (as occurs in skeletal muscle).
 * 6) <span style="color: black; font-size: 12px; line-height: 16px; list-style-position: outside; list-style-type: decimal !important; margin-bottom: 0px; margin-bottom: 14px; margin-left: 20px; margin-left: 25px; margin-right: 0px; margin-right: 20px; margin-top: 0px; margin-top: 14px; padding-bottom: 0px; padding-bottom: 0px; padding-left: 0px; padding-left: 1.5em; padding-right: 0px; padding-right: 14px; padding-top: 0px; padding-top: 0px;">When calcium is pumped out of the cell, the P i gets removed from myosin by another enzyme.
 * 7) <span style="color: black; font-size: 12px; line-height: 16px; list-style-position: outside; list-style-type: decimal !important; margin-bottom: 0px; margin-bottom: 14px; margin-left: 20px; margin-left: 25px; margin-right: 0px; margin-right: 20px; margin-top: 0px; margin-top: 14px; padding-bottom: 0px; padding-bottom: 0px; padding-left: 0px; padding-left: 1.5em; padding-right: 0px; padding-right: 14px; padding-top: 0px; padding-top: 0px;">The myosin becomes inactive, and the muscle relaxes.