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Muscle Contractions

Human Anatomy, Physiology, and Medicine. Anything human!

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Muscle Contractions

Postby raym1 » Fri Feb 18, 2011 8:33 pm

Hi, this is my first post here. I'm looking for a simple effective explanation of how muscles contract. For example, nerve impulses traveling down a motor neuron axon then releasing acetylcholine etc.

Does anyone know of some thorough tutorials, links, or generally helpful pages on these topics. I need to understand this process better for my Physiology class. I'm having a hard time understanding how actin and myosin react during a muscle contraction. The whole A band, I band, Z line thing is confusing me a bit.

Thanks for your time.
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Re: Muscle Contractions

Postby Jpoley » Tue Mar 01, 2011 2:42 am

Hey,

You have the general idea with the nerve pulse and Ach release into the neuromuscular junction... Once the Ach binds to its nicotinic receptor, it opens ligand gated channels for Na+ diffusion into muscular cell. This gives it enough "voltage" to reach its threshold which initiates an action potential on the muscle cell membrane (sarcolemma) which stimulates Ca2+ release from the sarcoplasmic reticulum to stimulate contraction (second messenger).

The myosin fibers (on actin) are blocked by tropomyosin which is anchored by troponin. Ca2+ binds to troponin causing a conformational change thus moving tropomyosin allowing the myosin binding site to become open. These binding sites form a cross bridge with actin with the help of ADP/Pi. The release of this ADP and phosphate powers a stroke between the myosin and actin filaments causing a pass of the long to shortened sacromere. ADP is released and a new ATP binds to myosin, detacing (relaxing) myosin for next stroke. When ATP is converted to ADP/Pi, another stroke occurs and the cycle repeats until Ca2+ or ATP "runs out".

The Ca2+ is then pumped back into SR by Ca2+ ATPase (SERCA) and muscle relaxes.

Hope it helps
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