Introduction to Muscle Contraction

Muscle is a contractile tissue that is essential to human life. Every second of every day muscles contract all over your body, pumping blood through your veins, repositioning you to compensate for the crushing force of gravity and allowing you to physically interact with your environment. But the contraction of muscle on a molecular level consists of an incredible series of events that relies on a specific order of biochemical processes. Muscle contraction is a vastly complex progression, and so it is necessary to break the explination into sections to understand the individual parts and processes before grasping the process as a whole. The physiology cannot be understood without the chemistry, and vice versa. As such, this article will first explain the chemistry behind each aspect of muscle contraction and then explain the chemical and physiological process.

Sarcomere.jpg

The Structure

Muscles, like most body tissues, are made up of proteins. Proteins are enourmous molecules, huge chains of linked molecules called amino acids. A muscle cell is typically called a muscle fiber. Muscle fibers are packed full of contractile fibers call myofibrils, and the myofybrils are divided into distinct units called sarcomeres. Sarcomeres in myofibrils are lined up like train cars, one after another. The sarcomere is made up of two main proteins, actin and myosin. Myocin has a long two-chain "tail" and a polar "head" and links together in bundles. Actin links together in long, thin chains. Wrapped around the actin and blocking its bonding sites are complexes made of the proteins troponin and tropomyosin. The actin appears as a much lighter band and the myosin appears as a darker band. The myosin and actin strands overlap, with the myosin strands in the center and the actin strands coming in from the outside. The picture on the left displays the basic sarcomere, with the myocin heads clearly visible.[1]
external image muscle1fiber.jpg

ATP Hydrolysis
Energy to break the bond between the myosin head and the actin is obtained through ATP hydrolysis. In this reaction, one molecule of water reacts with one ATP with a catalyst such as Hexokinase to yield one ADP, an inorganic phosphate and around 7kcal/mole of energy. In muscle reactions, only 40% of that energy goes to performing additional reactions; the rest is lost as heat.[2] [3] [4]
Hydrolysis.gif + 7kcal/mole energy


The Steps

The Muscle Contraction! [5] [6]
1) Muscle contraction is stimulated by a motor neuron, or nerve cell. This neuron receives an electrical signal that causes it to release acetylcholine into the sarcolemma, the membrane that surrounds the muscle fiber.

2) The acetylcholine binds at special sites on the muscle fiber.

3) The acetylcholine makes the sarcolemma temporarily permeable to sodium ions.

4) Sodium floods into the sarcolemma, disrupting the electrical conditions of the sarcolemma and causing an electrical signal to pass down the muscle fiber.

5) This electrical signal activates organelles called the sarcoplasmic reticula in the muscle cell.

6) These sarcoplasmic reticula respond by flooding the muscle cell with Ca2+ ions.

7) 4 Ca2+ ions bond with the troponin-tropomyosin complex, changing the folding of the tropomyosin and revealing highly polar binding sites on the actin strand.

8) The myosin heads, also highly polar, cross the space between the myosin and the actin and attach to the highly polar binding sites on the actin. The attraction between the myosin head and the actin is so strong and the myosin head has so much potential energy that once the tropomyosin moves out of the way the myosin head will spontaneously bond with the actin, without any activation energy to initiate the reaction. The myosin head springs from its place like a mousetrap and bond with the actin.
Sarcomere_contracted.png
This picture illustrates a contracted sarcomere and a relaxed sarcomere

9) The spring like motion of the myosin continues, pulling the actin fiber along relative to the myosin fiber.

10) 1 ATP binds with the attached myosin head at the myosin head’s active site.

11) This 1 ATP reacts with water through ATP hydrolysis, causing the ATP to become ADP and releasing the energy necessary to break the actin-myosin attraction.

12) The myosin head then attaches with the actin at the next active site on the actin and pulls along again.

13) The continuous pulling of actin towards the center of the individual sarcomeres along the entire length of the muscle fiber and throughout the entire muscle causes it to
shorten and exerts a force.

14) When the muscle relaxes, the nerve ending ceases to supply the sarcolemma with acetylcholine.

15) The remaining acetylcholine is then broken down by the enzyme Acetylcholinesterase and the electric signal shuts down.[7]

16) The calcium is reabsorbed into the sarcoplasmic reticula and the troponin moves back into position, restoring the shape of the tropomyosin and blocking the actin active sites from the myosin heads, which return to their own position.

The whole contraction process ends in an instant and the muscle is ready to contract again at a moment’s notice.

external image Muscle-774348.jpg

Video!

This video starts halfway through the contraction, but even so it is an excellent visualization of this complex process.

  1. ^
    Marieb, Elaine Nicpon. "The Muscular System." Essentials of Human Anatomy & Physiology. San Francisco, Calif.: Benjamin Cummings, 2003. 166-73. Print.
  2. ^
    Chasin, L. "Lecture 8." Columbia University in the City of New York. 2000. Web. 05 June 2010. <http://www.columbia.edu/itc/biology/chasin/lecture8/>.
  3. ^ Okimoto, N. "Theoretical Studies of the ATP Hydrolysis Mechanism of Myosin." Publmed. U.S. National Library of Medicine, Nov. 2001. Web. 04 June 2010.
  4. ^ Diwan, Joyce. "Bioenergetics." Rensselaer Polytechnic Institute. Web. 05 June 2010. <http://www.rpi.edu/dept/bcbp/molbiochem/MBWeb/mb1/part2/bioener.htm>.
  5. ^ "Muscle Contraction." Meat Science at Texas A&M. Web. 05 June 2010. <http://meat.tamu.edu/muscontract.html>.
  6. ^ Marieb, Elaine Nicpon. "The Muscular System." Essentials of Human Anatomy & Physiology. San Francisco, Calif.: Benjamin Cummings, 2003. 166-73. Print.
  7. ^ Chasin, L. "Lecture 8." Columbia University in the City of New York. 2000. Web. 05 June 2010. <http://www.columbia.edu/itc/biology/chasin/lecture8/>.