How Muscles Move the Skeleton

As you read this blog post, you might be tapping your foot as you sit at your desk or lifting your phone to get a better look at your screen. Even if you’re just sitting there, motionless—you’re using muscles to move (or stabilize) your skeleton! 

In this blog post, we’ll take a look at how your muscles move your bones. We’ll examine the anatomy and physiology behind skeletal muscles, muscle attachments, muscle actions, and your synovial joints. 

The Structure of Skeletal Muscle

The body contains three types of muscle: skeletal, cardiac, and smooth. Skeletal muscles are probably what you think of when you hear the word “muscle.” They’re muscles that connect to bones, creating and stopping movement in response to signals from the nervous system.

Let’s take a deep dive into the structure of skeletal muscle, zooming in with the skeletal muscle microanatomy model in Visible Body Suite. 

Skeletal muscle microanatomy model in VB Suite. 

We will start from the outside and work our way in. 

The muscle is covered by a layer of connective tissue called the epimysium. The epimysium protects the muscle and keeps it from losing its shape. Deep to the epimysium, you will find the perimysium, which surrounds each fascicle. 

A fascicle is a bundle of muscle fibers, and these fibers are long and cylindrical in shape. In skeletal muscle, fibers are large cells formed by the fusion of many myocytes (muscle cells).

These muscle fibers contain:

• Sarcoplasm: the muscle fiber’s cytoplasm. 
• Myofibrils: fibers which cause muscle contraction. 
• Sarcolemma: the cell membrane.
• Nuclei from the fused myocytes.
• Mitochondria: which generate ATP.

Each muscle fiber is surrounded by the perimysium, which allows for passage of the blood vessels that provide the muscle with nutrients, gases, and waste removal. 

Image from VB Suite. 

Now, let’s take an even closer look at myofibrils, the fibers that cause muscle contraction. Sarcomeres are the basic functional unit of the myofibril, and they are made up of long chains of protein filaments.

There are two types of protein filaments: 

• Thin protein filaments, which are mostly composed of actin.
• Thick protein filaments, which are mostly composed of myosin.

Skeletal muscle microanatomy model in VB Suite. 

Signals from the nervous system produce a neurotransmitter (acetylcholine), which kicks off a chemical reaction in the muscle fibers. 

To understand how muscle contraction occurs, let’s look at a few more structures.

Formed by the folding of the sarcolemma, transverse tubules (t-tubules) act as a network to carry membrane action potentials throughout the muscle fiber. Action potentials travel through the sarcolemma (including the t-tubules, which are extensions of the sarcolemma) to the sarcoplasmic reticulum. 

The sarcoplasmic reticulum surrounds each myofibril and stores calcium ions. These ions are released by an action potential, and they allow the actin and myosin filaments to interact. The fibers slide across each other, shortening the muscle.

GIF from the "Physiology of Muscle Contraction" animation in VB Suite. 

In addition to forming the t-tubules, the sarcolemma fuses with the tendon to help with muscle contraction. 

Tendons connect most skeletal muscles to bones. They are made up of dense connective tissue, and when skeletal muscles contract, the tendons transmit that force to the bones.

Muscle Attachments

Most skeletal muscles attach to bones in two or more places. There are two types of attachments: origin and insertion. A muscle’s origin is where it attaches to an immobile bone, and an insertion is where the muscle attaches to the bone that moves during the muscle action.

For example, let’s look at the rectus femoris. The rectus femoris has two origins, one at the pubis and one at the ilium—two bones that (along with the ischium) make up the hip. The rectus femoris’s insertion is at the patella, or the kneecap.

Side-by-side images of the rectus femoris's origins (left) and insertion (right). Images from VB Suite. 

VB Suite pro tip: After you’ve selected a muscle, you can click on the red pin in the info box to instantly access the muscle’s attachments, blood supply, and innervation.

Look at this hip flexion muscle action. See how the rectus femoris’s origins are immobile whereas the insertion at the patella moves. 

Interactive hip flexion animation in VB Suite. 

Movers, Synergists, Stabilizers, and Antagonists 

When it comes to movement, it takes a village.

Muscle actions, like the hip flexion example we just saw, require multiple muscles. The muscles at play in any given action can be divided into four categories: agonists, antagonists, synergists, and stabilizers. 

• An agonist is the primary force that drives the action. The agonist is also called the prime mover.
• An antagonist provides resistance and/or reverses the movement. These muscles help maintain the position of the body or limb during muscle actions, and they also keep muscle movements controlled. 
• Synergists help the agonist. 
• Stabilizers help keep bones immobile.
  
  

Synovial Joints

Last but not least, let’s take a look at synovial joints! Synovial joints make muscle actions possible. 

A joint is simply a point where two bones meet, and a synovial joint is a fluid-filled joint within a fibrous capsule. Of all the joint types, synovial joints allow for the most movement. 

A close look at a synovial joint in VB Suite. 

The fibrous outer capsule protects the joint and prevents it from extreme movement. Synovial fluid helps reduce friction between bones as they move.

Additionally, some synovial joints include:

• Intracapsular ligaments, which limit movements
• Menisci (sg. meniscus), which absorb impact and help space out the joint
• Labrums, which stabilize ball-and-socket joints
• Bursae (sg. bursa), fluid-lined sacs that reduce friction between tendons and other structures

A synovial joint is just one type of joint, and not all joints are movable—think about all the joints that connect the cranial bones.

Conclusion 

To sum things up, skeletal muscle is made up of fascicles, which are groups of muscle fibers. Those fibers contain microfibrils, which are made up of thick or thin protein filaments. 

Signals from the nervous system cause the thick and thin filaments to slide across each other, shortening the muscle and causing it to contract.

Tendons help connect skeletal muscles to bones. A muscle's origin is its attachment to an immobile bone, and its insertion is its attachment to a mobile bone.

Agonists, antagonists, synergists, and stabilizers work together to generate any given muscle action. This movement is made possible through joints, the places where bones connect to each other. Synovial joints like the shoulder and knee are typical examples that show how muscles move bones.

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