Muscular Contraction: Cross-Bridge Formation

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  • 0:57 Sarcomere Shortening
  • 1:44 Cross-Bridge Cycling
  • 3:42 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 muscles contract as a result of cross-bridge formation between actin and myosin? This lesson describes the stages of cross-bridge cycling and how this results in sarcomere shortening and muscular contraction.

How Do Muscles Contract?

The cross bridge in a single sarcomere
Muscle Sarcomere Diagram

We all know that muscles contract, but what we might not know is how they contract. More specifically, what happens inside our muscle cells to cause contraction? Let's get out our trusty magnifying glass and find out.

While this is clearly not what happens inside of our muscle cells, what you see on the screen can help us understand how they contract. This is how the contractile proteins that we call actin and myosin interact with each other to cause contraction. So you can think of the guys as the myosin and the rope as actin as we move into discussing how the contractile proteins interact with one another during cross-bridge cycling.

Sarcomere Shortening

The sarcomere is the functional unit of striated muscle. Let's look at the cross-bridge within the context of a single sarcomere to understand how contraction occurs.

As you can see, actin makes up the thin filaments, and they're attached to the Z lines. Myosin makes up the thick filaments, which overlap the thin filaments in the middle of a sarcomere. Perhaps you can imagine myosin forming a cross-bridge with actin much like a person would grab a rope and pull on it. Myosin pulls the thin filaments towards the middle on each side, thus shortening the sarcomere and causing contraction.

At the high-energy state, myosin contains potential energy waiting to be released.
Myosin Loaded with Potential Energy

Cross-Bridge Cycling

In the context of muscular contraction, a cross-bridge refers to the attachment of myosin with actin within the muscle cell. All muscle types - whether we're talking about skeletal, cardiac, or smooth - contract by cross-bridge cycling - that is, repeated attachment of actin and myosin within the cell. Let's get out that trusty magnifying glass again and focus now on a single cross-bridge within a sarcomere.

Let's start at the top of the animation with what we call the high-energy or attached state of the cross-bridge. In this stage of the cycle, myosin is loaded with potential energy and attached to actin, just as a mouse trap is loaded with potential energy when we set it to hopefully catch a mouse.

Much like a mouse trap tripping, myosin binding releases the stored energy and the myosin head changes its shape, pulling the thin filament towards the middle of the sarcomere. This is referred to as the working stroke of the cross-bridge cycle, as work requires movement, and now movement is being done.

The four stages of cross bridge cycling
Cross Bridge Cycling Flow Chart

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