Muscular Contraction: Cross-Bridge Formation

An error occurred trying to load this video.

Try refreshing the page, or contact customer support.

Coming up next: Excitation-Contraction Coupling & Muscular Contraction Regulation

You're on a roll. Keep up the good work!

Take Quiz Watch Next Lesson
Your next lesson will play in 10 seconds
  • 0:57 Sarcomere Shortening
  • 1:44 Cross-Bridge Cycling
  • 3:42 Lesson Summary
Add to Add to Add to

Want to watch this again later?

Log in or sign up to add this lesson to a Custom Course.

Login or Sign up

Create an account to start this course today
Try it free for 5 days!
Create An Account

Recommended Lessons and Courses for You

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?

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.

Imagine that the guys are myosin, and the rope is actin.
guys pulling rope to contract muscle

While this is clearly not what happens inside of our muscle cells, what you see in the picture above can help you 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.

The cross bridge in a single sarcomere
Muscle Sarcomere Diagram

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.

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.

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

Let's start at the top 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

To unlock this lesson you must be a Study.com Member.
Create your account

Register for a free trial

Are you a student or a teacher?
I am a teacher

Unlock Your Education

See for yourself why 30 million people use Study.com

Become a Study.com member and start learning now.
Become a Member  Back

Earning College Credit

Did you know… We have over 95 college courses that prepare you to earn credit by exam that is accepted by over 2,000 colleges and universities. You can test out of the first two years of college and save thousands off your degree. Anyone can earn credit-by-exam regardless of age or education level.

To learn more, visit our Earning Credit Page

Transferring credit to the school of your choice

Not sure what college you want to attend yet? Study.com has thousands of articles about every imaginable degree, area of study and career path that can help you find the school that's right for you.

Create an account to start this course today
Try it free for 5 days!
Create An Account