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.
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.
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.
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
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.
In the contracted sarcomere, the I band shortens while the A band remains the same. More specifically, the length of the thick and thin filaments do not change, but rather they slide over each other, bringing the thin filaments closer together in the middle and the thick filaments closer to the Z lines on the sides of the sarcomere.
Each sarcomere shortens by about 1/3 of its resting length. If an individual sarcomere is 3 micrometers at rest and 2 micrometers when contracted, it shortens by only 1 micrometer. One micrometer is not very far. But remember, a myofibril running the length of the muscle fiber contains some 10,000 sarcomeres. In this case, the muscle fiber would shorten by 10,000 micrometers or 10 millimeters. Now that's noticeable.
In summary, striated muscle fibers are composed of myofibrils, which are packs of myofilaments packed together in a parallel arrangement. These myofilaments include thick filaments that are made up of myosin and thin filaments composed of actin, troponin and tropomyosin. Actin and myosin are the contractile proteins, and they interact with each other, causing contraction, while troponin and tropomyosin are regulatory proteins that regulate the interaction between actin and myosin.
Myofilaments are packed in functional units of striated muscle referred to as sarcomeres. Sarcomeres are connected end to end by Z lines along the length of each myofibril. Thin filaments attach to the Z lines and make up the I zone of the sarcomere. Thick filaments overlap the thin filaments in the middle of the sarcomere, making up the A zone. The thick and thin filaments slide over each other, thus shortening the sarcomere and causing muscular contraction.
At the end of this lesson, you will be able to:
- Describe the structure and function of a sarcomere
- Explain how sarcomeres cause muscles to contract
- List the myofilaments and contractile proteins involved in muscle contraction
Sarcomere and Sliding Filaments Crossword Puzzle
This activity will assess what you've learned about the definition and structures of the sarcomere and sliding filaments.
Complete the crossword by filling in a word that fits each of the given clues. To do this, you can just right-click the crossword to save the image and print it. With a pencil and an eraser, neatly write your answers in the boxes provided.
3. Any of a family of muscle proteins that regulate the interaction of actin and myosin.
4. Initially, the thick filaments reside at the _____ part of the sarcomere.
5. The shortening of the _____ results in the contraction of the muscle.
6. Any of the cylindrical organelles found within muscle cells that are the contractile unit of muscles.
7. Thousands of myofibrils are tightly packed together in a _____ arrangement to form the muscle fiber.
8. A _____ muscle is a muscle tissue that features repeating functional units called sarcomeres.
1. These are the filaments that are brought closer together at the middle of the sarcomere as it shortens.
2. A filament within a myofibril, constructed from proteins.
3. The lengths of the thick and thin filaments do not change since they _____ past each other.
4. The thin filaments are attached to both _____ of the sarcomere.