The Sarcomere and Sliding Filaments in Muscular Contraction: Definition and Structures

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  • 0:05 The Sarcomere Is the…
  • 1:09 Striated Muscle Arrangement
  • 1:41 Sarcomere Arrangement
  • 3:27 Contraction of Sarcomere
  • 4:58 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 a sarcomere is the fundamental functional unit of striated muscle? This lesson describes the thick and thin filaments of a sarcomere and how they interact with each other to cause sarcomere shortening and muscular contraction.

The Sarcomere is the Fundamental Unit

What does it mean to say that muscles contract? The short answer is - no pun intended - muscles shorten when they contract. This begs the question of how muscles shorten and, thus, contract. To answer this question, we must first examine the components of the sarcomere, which is the fundamental functional unit of striated muscle - that is, skeletal and cardiac muscle.

To say that the sarcomere is the functional unit means that all the components needed for contraction are contained within each sarcomere. In fact, muscle is composed of millions of tiny sarcomeres, and each sarcomere shortens, thus resulting in muscular contraction. It's important to note that smooth muscles do not contain sarcomeres. Rather, each smooth muscle is like one giant sarcomere. In this lesson, we will describe the basic components of a sarcomere and how they interact to contract our striated muscles.

Striated Muscle Arrangement

Before we examine the contents of an individual sarcomere, let's take a look at how sarcomeres are arranged within the context of a striated muscle cell - that is, a muscle fiber. Muscle fibers are composed of hundreds to thousands of contractile organelles called myofibrils. The myofibrils are packed tightly together in a parallel arrangement, much like Vienna sausages are packed in a tin can. If you take out a single sausage, it would represent a single myofibril.

Sarcomere Arrangement

Now, let's take a look at an individual myofibril within the muscle cell. At this level, we can see the sarcomeres are butted up end to end, running the length of each myofibril.

Sarcomeres are arranged end to end.
sarcomere is end to end

A given myofibril contains approximately 10,000 sarcomeres, each of which is about 3 micrometers in length. While each sarcomere is small, several sarcomeres added together span the length of the muscle fiber. Each sarcomere consists of thick and thin bundles of proteins referred to as myofilaments.

If we magnify a portion of the myofilaments, we can identify the molecules that compose them. Thick filaments contain myosin, while thin filaments contain actin. Actin and myosin collectively are referred to as the contractile proteins, which cause muscle shortening when they interact with each other. Additionally, thin filaments contain the regulatory proteins troponin and tropomyosin, which regulate interaction between the contractile proteins.

These contractile proteins cause muscles to shorten when they interact.
Contractile Proteins Image

Let's now take a closer look at an individual sarcomere. The bottom image is a drawing of the actual micrograph that you see above.

Diagram and micrograph of a sarcomere
Individual sarcomere

The I band is that part of the sarcomere that contains thin filaments, while the A band contains an area of overlap between the thin and the thick filaments. As you can see, a single I band spans two neighboring sarcomeres. A Z line attaches those neighboring sarcomeres. The thin filaments are attached to the Z lines on each end of the sarcomere, while the thick filaments reside in the middle of the sarcomere.

Contraction of Sarcomere

Now that we've identified the sarcomere as the functional unit of contraction, let's consider how sarcomeres shorten to cause contraction. Sarcomeres within a myofibril can be thought of as a bunch of people holding hands in a line. If everyone in the line pulls their arms to their side, the line will shorten. Likewise, whole muscles contract as a result of millions of tiny sarcomeres shortening in length. The image below illustrates a contracted sarcomere below a relaxed sarcomere. What's the difference between these images?

On top, a relaxed sarcomere; on bottom, a contracted sarcomere.
Contraction Thick Thin Filaments

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