Name: sarcomere structure in skeletal muscle
Uploaded: 2025-05-22T09:05:34.000Z
Duration: 1 min 16 s
Channel: JoVE

K12 Standards

Florida Department of Education

High School

Life Science

SC.912.L.14

SC.912.L.14.17

Actin and Myosin in Muscle Contraction

Sarcomeres, the smallest muscle contractile units, comprise parallel overlapping troponin-tropomyosin bound actin filaments and bundles of myosin II filaments.
During muscle contraction, calcium ions bind the troponin proteins attached to the tropomyosin. This binding causes a conformational change in the tropomyosin, exposing myosin-binding sites present in the actin filaments.
The myosin head with an ADP and loosely attached phosphates&#160; from the previous cycle of ATP hydrolysis releases the bound phosphate, allowing the globular myosin head to bind the actin filament and form a cross-bridge.
As the ADP dissociates from the two myosin filaments, the energy released pulls the actin filaments towards the center, shortening the sarcomere.
ATP molecules quickly occupy the vacated nucleotide-binding site in the myosin head, stopping the sliding movement and separating the actin and myosin filaments.
The ATPase in the globular head hydrolyzes the ATP to ADP and phosphate for the next round of sliding of actin filaments.

actin and myosin in muscle contraction

Actin and myosin are contractile proteins that form the sarcomere found in skeletal muscle tissues for regulating muscle contraction. Actin, a globular contractile protein, interacts with myosin for muscle contraction. The skeletal tissue appears striped or striated under a microscope due to the repeated arrangement of contractile proteins actin and myosin along the length of myofibrils. Dark A bands and light I bands repeat along myofibrils, and the alignment of myofibrils in the cell causes the entire cell to appear striated or banded. Each I band has a dense line running vertically through the middle called a Z disc or Z line. The Z discs mark the border of units called sarcomeres, the functional skeletal muscle units.
Sarcomere comprises of two main types of filaments: thick filaments and thin filaments; each has different compositions and locations. The tail of a myosin connects with other myosin molecules to form the central region of a thick filament near the M line. In contrast, the heads align on either side of the thick filament where the thin filaments overlap. The thin filaments consist of actin and two other accessory proteins, tropomyosin and troponin. The actin protein has myosin-binding sites for attachment. Strands of tropomyosin block the binding sites and prevent actin-myosin interactions when the muscles are at rest. Troponin consists of three globular subunits. One subunit binds to tropomyosin, one subunit binds to actin, and one subunit binds to Ca2+ ions. The motion of muscle shortening occurs as myosin heads bind to actin and pull the actin inwards. This action requires energy, which ATP provides. Myosin binds to actin at a binding site on the globular actin protein. Myosin has another binding site for ATP at which enzymatic activity hydrolyzes ATP to ADP, releasing an inorganic phosphate molecule and energy.
This text is adapted from <a href="https://openstax.org/books/biology-2e/pages/38-4-muscle-contraction-and-locomotion">Openstax, Biology 2e, Section 38.4: Muscle Contraction and Locomotion</a>

Actin and myosin are contractile proteins that form the sarcomere found in skeletal muscle tissues for regulating muscle contraction. Actin, a globular ...

Education

JoVE Core

Cell Biology

Unit 1

The Cytoskeleton I: Actin and Microfilaments

the-cytoskeleton-i-actin-and-microfilaments

KEY TERMS AND CONCEPTS

sarcomere structure in skeletal muscle

Sarcomere structure in skeletal muscle explains how actin and myosin help muscles contract. These contractile proteins are arranged in repeating units called sarcomeres, which are found in skeletal muscle tissue. Their repeated pattern gives skeletal muscle a striped, or striated, look under a microscope.

The bands in a myofibril create this striped pattern. Dark A bands and light I bands repeat along the length of the myofibrils. When many myofibrils line up in a muscle cell, the whole cell appears banded or striated. Each I band also contains a dense line in the middle called a Z disc, or Z line.

Z discs mark the borders of the sarcomeres, which are the functional units of skeletal muscle. A sarcomere contains two main filament types: thick filaments and thin filaments. The thick filament is built from myosin. Myosin tails connect with other myosin molecules near the M line, while the heads sit on either side where they overlap with the thin filaments.

The thin filaments contain actin and two accessory proteins, tropomyosin and troponin. Actin has myosin-binding sites that allow attachment. At rest, tropomyosin blocks these sites and stops actin and myosin from interacting. Troponin has three globular subunits. One binds tropomyosin, one binds actin, and one binds Ca2+ ions.

Muscle shortening happens when myosin heads bind to actin and pull the actin inward. This process needs energy, and ATP provides it. Myosin binds to a site on globular actin and also has a separate ATP-binding site. At that site, ATP is hydrolyzed to ADP, releasing inorganic phosphate and energy.

Sarcomere Structure in Skeletal Muscle

introduction to the cytoskeleton

Cytoskeleton Structure and Cell Movement

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Cytoskeletal filaments help cells change shape, divide, and move. Their flexibility lets them stay stable for hours during interphase or reorganize quickly during cell division. Strong regulation controls how these filaments assemble, fall apart, and form networks. Microfilaments and microtubules are dynamic, while intermediate filaments are considered more static. Microfilaments are spread throughout the cell body and can shift into different structures. Many proteins in the cell, including...

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Accessory proteins help the cytoskeleton work as a connected system. The cytoskeleton is a key cell structure with both structural and functional roles. Its filaments do not work well on their own, so accessory proteins assist with filament formation, stability, and function. These proteins also support communication between cytoskeletal filaments. They are found in both prokaryotes and eukaryotes, although bacterial accessory proteins differ a lot from one species to another. The evolutionary...

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How Viruses and Bacteria Ride the Cytoskeleton

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The cytoskeleton can be studied with microscopy and biochemical methods. Electron microscopy helped reveal its architecture in the 1960s. It gives very high-resolution structural detail, but sample preparation can limit what is seen in biological samples. One electron microscopy method is rotary shadowing, also called metal replica. In this protocol, cytoskeletal filaments are fixed with chemicals such as glutaraldehyde or formaldehyde. The sample is then dehydrated with alcohol to a critical...

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Actin Structure, Isoforms, and Roles

Actin is a major cytoskeletal protein in eukaryotic cells. In muscle cells, it can make up about 10% of the total cellular protein. In non-muscle cells, it is lower and usually makes up around 1 to 5 percent of total cell protein. Actin is also highly conserved, which means it has changed very little over time. Actin in unicellular amoebae and in complex multicellular animals is about 80% similar. Actin genes are also conserved within a species and across different species. For example, actin...

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Actin Filament Growth Stages

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Actin Filament Growth and Loss

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generation of straight or branched actin filaments

generation-of-straight-or-branched-actin-filaments

Actin Filament Branching and Straight Growth

Actin filaments can grow as either straight strands or branched networks. Two nucleating proteins control this shape choice: formins and the Arp2/3 complex. Formins build straight actin filaments, while the Arp2/3 complex builds branched ones. The Arp2/3 complex is made of seven subunits. Two of them, Arp2 and Arp3, are actin-like proteins, and five other subunits help keep them inactive until they are needed. Activation happens with help from other proteins called nucleation-promoting...

actin filament depolymerization

Actin Filament Breakdown at Cell Ends

Actin filaments are built from actin subunits, and they can break down from either end. This process is called depolymerization. It happens faster at the minus-end, also called the pointed end, where the subunits are usually in the ADP-actin form. Several proteins help speed up actin filament breakdown. The actin-depolymerizing factor, or ADF, and cofilin proteins bind to ADP-actin in a one-to-one ratio. When they bind, the filament twists, which creates stress and makes it brittle. That...

formation of higher-order actin filaments

formation-of-higher-order-actin-filaments

Actin Bundles and Cell Shape

Actin bundles and cross-links help shape cells by organizing F-actin into larger networks. G-actin monomers first polymerize into filamentous F-actin, and those filaments then assemble into higher-order structures that support cell function. These structures can help build lamellipodia and filopodia at the leading edge of a migrating cell. They also help form microvilli on the brush border of epithelial cells. Higher-order actin networks can form in two main ways. Straight F-actins may bundle...

overview of myosin structure and function

overview-of-myosin-structure-and-function

Myosin Types in Muscle and Cell Transport

Myosins are molecular motor proteins that help cells move materials and generate force. They were first identified in skeletal muscle, where they drive muscle contraction. They also help move molecules and vesicles inside cells. Myosin proteins are divided into 24 classes based on their domain sequence and organization. Six classes are especially well characterized: Myosin I, Myosin II, Myosin V, Myosin VI, Myosin VII, and Myosin X. These proteins also take part in contractile ring formation...

Sarcomere structure in skeletal muscle explains how actin and myosin help muscles contract. These contractile proteins are arranged in repeating units called sarcomeres, which are found in skeletal muscle tissue. Their repeated pattern gives skeletal muscle a striped, or striated, look under a microscope. The bands in a myofibril create this striped pattern. Dark A bands and light I bands repeat along the length of the myofibrils. When many myofibrils line up in a muscle cell, the whole cell...

the role of actin and myosin in non-muscle cells

the-role-of-actin-and-myosin-in-non-muscle-cells

How Actomyosin Bundles Shape Cell Movement

Actin and myosin form contractile bundles in non-muscle cells. These actomyosin structures help cells change shape, attach to surfaces, and divide. They were first observed in Dictyostelium and Acanthamoeba. Two common bundles are stress fibers and actomyosin adherence belts. These bundles are smaller and less organized than the ones found in muscle cells. Accessory proteins such as fascin, filamin, and fimbrin help hold them together, depending on where they are in the cell. Their formation...

key terms and concepts

Learn how sarcomeres, actin, myosin, troponin, and tropomyosin create the striated look of skeletal muscle and drive contraction.

Sarcomere Structure in Skeletal Muscle | JoVE K12

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Sarcomere Structure in Skeletal Muscle

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