Structure of Muscle III: Molecular Structure

Understand and name all the parts of the thick and thin filaments.
Identify the structures and control proteins found within a sarcomere (the basic unit of muscle contraction).
Recognize the proteins that connect the muscle's working parts (myofilaments) to its outer covering (sarcolemma).
Focus on the main building blocks of the sarcomere; know that many other supporting proteins also work here.

Parts we'll cover: We'll talk about thick filaments, thin filaments, and the proteins that hold everything together.
Thick filaments: These contain tiny motors called myosin. They are held in the center by a very large protein called titin.

Made mainly of myosin molecules which are arranged together to form a thick, rope-like structure.
- How it's built: Each thick filament has many myosin molecules grouped together.
- Central Bare Zone: This is a part in the middle of the thick filament where there are no myosin heads sticking out. It's a key feature of this structure.
- M Line: This is the very center line of the sarcomere where the myosin heads meet and connect.

Myosin Structure

Myosin Heavy Chains: These are two long protein strands that wrap around each other like a twisted rope. They form the long tail of the myosin molecule that overlaps with other myosin tails in the central bare zone.
- Myosin Heads: These are the 'motor' parts of the myosin. They are responsible for creating movement by pulling on the thin filaments.
- How it works: The myosin heads grab onto the thin filaments and 'ratchet' (like a tiny pulling motion), pulling the thin filaments closer to the middle M line.

Myosin Head Components

Hinge Region: This is like a flexible joint that connects the long tail of the myosin molecule to the head, allowing the head to move.
Light Chains: These are smaller proteins located near the myosin heads. They are important for the myosin motor to work correctly and are often highlighted in diagrams as blue.
ATPase Activity: Myosin can break down a high-energy molecule called ATP (adenosine triphosphate) into ADP (adenosine diphosphate) and an inorganic phosphate (PI). ATP \to ADP + PI. This breakdown releases energy that powers the movement of the myosin head.
Regulatory Elements: These parts help control when and how the muscle contracts, especially important in smooth muscles.

Titin

What it is: The largest protein in the human body, extremely long (over 3,000 kilodaltons). It acts like a giant spring or rubber band, keeping the thick filaments in place by connecting them to the Z disc (the boundary of a sarcomere).
What it does: It stretches all the way from the Z line to the center of the sarcomere and then folds back on itself. This gives the muscle its elasticity (ability to stretch and recoil) and makes sure the filaments stay perfectly lined up.

Involves several proteins: actin, tropomyosin, and troponin.
Actin: Starts as small, round protein units called globular actin (G-actin). These G-actin units then link together to form long, twisted strands called filamentous actin (F-actin), creating a helical (spiral) structure.
- Active Sites: On the actin strands, there are specific spots where the myosin heads can attach. These spots are normally exposed once the actin filaments are formed.

Tropomyosin

What it is: A long, thread-like protein that wraps around the actin filament.
What it does: In a relaxed muscle, tropomyosin lies over and blocks the active sites on actin. This prevents myosin heads from binding and causing contraction.

Troponin Complex

Made up of three smaller proteins: Troponin C, Troponin I, and Troponin T.
What it does: This complex acts like a 'molecular handle'. When calcium ions are present, they bind to Troponin C. This causes the entire troponin complex to shift, pulling tropomyosin away from the active sites on actin. This 'uncovers' the active sites, allowing myosin to bind and muscle contraction to begin.

Additional Proteins of the Thin Filament

Nebulin: Acts like a molecular 'ruler' that helps determine the exact length and direction of the thin filaments. It's often shown in blue in diagrams.
Cap Z: This protein is found at the end of the thin filament and firmly attaches it to the Z disc, ensuring it's properly anchored.
Tropomodulin: Located at the other end of the thin filament (the end away from the Z disc), it helps regulate and maintain the correct length of the actin strands.
Alpha-Actinin: Another protein that helps anchor the actin filaments to the Z disc, assisting in the structural organization.

Dystrophin-Associated Glycoprotein Complex: This is a group of proteins that forms a bridge between the internal structure of the muscle cell (the actin framework, connected by dystrophin) and the material outside the cell (extracellular matrix) through special sugar-coated proteins.
Dystrophin: A very important protein that provides structural support. It helps maintain the strength and integrity of the sarcolemma (the muscle cell membrane) during the stresses of muscle contraction.
Desmin: This protein works with alpha-actinin to create a structural 'scaffolding'. It anchors the Z discs (the ends of the sarcomeres) to the sarcolemma and helps keep the overall organization of muscle fibers stable.

Dystrophin and desmin are crucial because they ensure that when things contract inside the cell, it leads to smooth, coordinated movement between the inside of the cell and its surroundings. This prevents damage to the muscle during activity.
They make sure that all the muscle cells contract together as one unified unit, much like how connective tissues help organize entire muscle bundles.

Thick Filaments: Made of myosin, which has both long 'heavy' chains and shorter 'light' chains. Myosin is responsible for generating the force during muscle contraction.
Thin Filaments: Composed of actin, tropomyosin, and troponin. Tropomyosin and troponin have important roles in regulating (turning on and off) the activity of myosin.
Structural Proteins: These include titin, nebulin, Cap Z, tropomodulin, and alpha-actinin. They are vital for controlling the length of the filaments and maintaining the overall structure of the muscle cell.
Dystrophin and Desmin: These are critical for connecting the sarcomeres to the sarcolemma (muscle cell membrane). They ensure that the muscle fiber contracts in a coordinated way with its surrounding environment.