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Excitation-Contraction Coupling & Muscular Contraction Regulation

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  • 0:05 Contraction Regulation
  • 1:04 Cross-Bridge
  • 1:52 Excitation-Contraction…
  • 3:16 Stopping Contraction
  • 4:22 Lesson Summary
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Lesson Transcript
Instructor: John Simmons

John has taught college science courses face-to-face and online since 1994 and has a doctorate in physiology.

Did you know that calcium couples neural stimulation with contraction of skeletal muscle? This lesson describes the role of calcium, troponin, tropomyosin, and ATP in the regulation of muscular contraction. Rigor mortis is explained to help you understand the regulation of contraction as well.

Contraction Regulation

Just as much as muscular contraction is needed, muscular relaxation is needed as well. In other words, we need to regulate contraction. For example, our diaphragm is a muscle, and it contracts and relaxes every time we breathe. If we lose the ability to regulate contraction, then, well, we will die.

While the contractile proteins actin and myosin are responsible for muscular contraction, regulatory proteins regulate interaction between actin and myosin, and thus regulate contraction. Troponin and tropomyosin are the regulatory proteins in striated muscle, while calmodulin replaces troponin in smooth muscle. Let's take a quick look at how a cross-bridge is formed between the contractile proteins actin and myosin during muscular contraction before we discuss how that's regulated.

Cross-Bridge

In the context of muscular contraction, a cross-bridge simply refers to the attachment of myosin with actin within a muscle cell. All muscle types - skeletal, cardiac, and smooth - contract by what we call cross-bridge cycling; that is, the repeated attachment of actin and myosin within the cell.

When myosin binds to actin, it changes shape and moves the actin.
Myosin Changes Shape Binding

The image below illustrates a single cycle in which myosin binds to actin, forming the cross-bridge. After binding, myosin changes its shape, thus pulling actin towards the middle of the sarcomere, kind of like a person would pull on a rope. ATP then binds to myosin, thus detaching the cross-bridge and also providing energy for yet another cycle to occur.

Cross bridge cycle
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Excitation-Contraction Coupling

I've got a question for you. Have you ever wished you could simply switch off the light and not have to get up or get out of bed? I have. No matter how hard I try, ultimately, I've got to get out of bed and flip the switch. Regulation of muscular contraction kind of works this way. Once the muscle is excited by a neuron, calcium is released from the sarcoplasmic reticulum, and the calcium binds to troponin, thus coupling the excitation of the muscle with the contraction of the muscle itself.

Calcium binds to troponin after being released from the sarcoplasmic reticulum.
Calcium Binds to Troponin

Similarly, after I decide to turn off the light, I have to get up and flip the switch, thus coupling my thought or desire with an appropriate action. In relaxed muscle, tropomyosin covers the myosin binding sites that are on actin. That prevents the cross-bridge from forming.

Once released from the sarcoplasmic reticulum into the cytoplasm, calcium then binds to troponin, and it changes its shape - or what we call conformation. As troponin is attached to tropomyosin, the conformational, or shape, change in troponin moves tropomyosin away from the myosin binding sites. So, as long as calcium is available, the cross-bridge can form, and the muscle contracts.

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