Myofibrils and Sarcomeres: The Sliding Filament Model

1. Introduction and Review

To answer the question “how do muscles contract,” we’re taking a deep look at muscle tissue structure.

In the previous tutorial, we started that process by learning about the image below. Quickly quiz yourself by identifying the name and function of each indicated structure. Clicking on the diagram will bring up a version with labels.

Muscle structure and connective tissues. Click on the diagram to see a labeled version.

In this tutorial, we’re going to go all the way down to the level of the two key proteins involved in muscle contraction: actin and myosin. These are organized into a sarcomere: the functional unit of muscle contraction.

2. Myofibrils and Sarcomeres

Skeletal muscle is also called striped or striated muscle. The striations are caused by a regularly repeating pattern of molecules within skeletal muscle tissue. Using a light microscope set to high magnification, you can see these striations (though not the underlying molecules). Here’s what it looks like.

Light micrograph of skeletal muscle tissue viewed at high magnification. Parallel muscle fibers show a repeating pattern of light and dark bands (striations) running across each fiber, reflecting the organized arrangement of actin and myosin within myofibrils. The striations are visible only at higher magnification, where individual A bands and I bands can be distinguished. — Micrograph of striated or striped muscles viewed at high magnification

To understand these striations, let’s examine the diagram below.

On the top is a skeletal muscle. The first outtake jumps directly to the level of a muscle fiber (a multinucleated cell), where you can see the striated pattern. In the second outtake, which shows a myofibril, the striations are even clearer. The repeating structure, bracketed on the bottom, is called a sarcomere. As we’ll see, sarcomeres are the functional unit of muscle contraction.

The sarcomere is bracketed by a structure called a Z disc. Z discs are made of proteins that anchor actin filaments (also known as thin filaments). Interspersed between the actin filaments, at the center of the sarcomere, are myosin filaments (also known as thick filaments).

The area of the sarcomere that consists of only thin (actin) filaments is called the I band. Between the I bands is an A band. The A band is equal to the length of the myosin (thick fibers). The ends of the thin filaments also protrude into the A band. The M line is a protein that keeps all of the thick filaments in alignment during a muscle contraction.

If you’ve noticed that the actin filaments look smooth and the myosin filaments look bumpy, you’re on to something important.

Three-panel diagram illustrating the sliding filament mechanism at the molecular level. Panel 1 shows a myosin head on a thick filament with ADP bound, positioned near a thin (actin) filament whose myosin-binding sites are blocked. Panel 2 shows calcium ions (Ca²⁺) binding to the regulatory proteins on the thin filament, exposing the myosin-binding sites so the myosin head can attach. Panel 3 shows the myosin head pivoting (power stroke), pulling the thin filament toward the center of the sarcomere while ADP remains bound, illustrating how filament sliding produces muscle contraction. — Myosin-actin interaction that leads to muscle shortening

Myosin filaments have “heads” that act as levers to power muscle contraction When a muscle contracts,

The tip of a myosin molecule (called the “myosin head”) bends forward.
The myosin head attaches to a myosin-binding site on an actin filament.
Like a rower pulling back on the oars of a boat, the myosin head pulls back on the actin filament, which has the effect of shortening the sarcomere.

This is called a “power stroke.” In the same way that rowing requires a lot of energy, so does the power stroke: skeletal muscle (which is what you use for rowing) is the largest consumer of ATP in the body.

3. Sarcomeres and Muscle Contraction

So, what happens during a muscle contraction?

A muscle contracts when its sarcomeres shorten.

In the relaxed state (see the upper image in diagram to your right), the thin (actin) filaments extend partway into the region occupied by the thick (myosin) filaments. The Z discs are farther apart, and the I bands (regions containing only thin filaments) are relatively wide. The H zone, the area that designates where there are thick filaments without overlapping thin filaments, is also relatively wide.

During contraction, myosin heads bind to actin and pull the thin filaments inward, toward the center of the sarcomere (the M line). As this happens:

The Z discs move closer together
The I bands become shorter
The H Zone becomes shorter
The overlap between actin and myosin increases

Importantly, the filaments themselves do not shorten.

The thick filaments stay the same length
The thin filaments stay the same length
The A band remains constant

What changes is their position. The sliding of thin filaments past thick filaments shortens each sarcomere. When millions of sarcomeres shorten at the same time, myofibrils shorten, muscle fibers shorten, and the entire muscle contracts, producing movement.

This mechanism is called the sliding filament model of muscle contraction. Every time you take a step, throw a ball, or pick up a spoon, that’s what’s happening.

4. The Sarcomere: Checking Understanding

[qwiz style=”min-height: 450px !important; width: 700px !important;” qrecord_id=”sciencemusicvideosMeister1961-Sarcomere Labeled Diagrams”]

[h] The Sarcomere: Checking Understanding

[i]

[q labels=”top”]

[l]H zone

[f*] Excellent!

[fx] Sorry, no.

[l] Sarcomere

[f*] Correct!

[fx] Sorry, no.

[l] Myofibril

[f*] Correct!

[fx] No, that’s not correct.

[l]Actin

[fx] No. Please try again.

[f*] Good!

[l]Myosin