18.5: Microfilaments - Structure and Role in Muscle Contraction

At 7 nm in diameter, microfilaments (actin filaments) are the thinnest cytoskeletal component. Globular actin (G-actin) monomers polymerize to form linear F-actin polymers. Two polymers then combine to form a twin-helical actin microfilament. As with microtubules, microfilaments have a +end, to which new actin monomers are added to assemble F-actin, and a −end, at which they disassemble when they are in a dynamic state, such as when a cell is changing shape. When one end of a microfilament is anchored to a cellular structure, (e.g., to plaques in the cell membrane, motor proteins like myosin can use ATP to generate a force that deforms the plasma membrane and, thus, the shape of the cell. One of the best-studied examples of myosin/actin interaction is in skeletal muscle, where the sliding of highly organized thick myosin rods and thin actin microfilaments results in muscle contraction.

18.5.1 The Thin (Micro) and Thick Filaments of Skeletal Muscle

Bundles of parallel muscle cells make up a skeletal muscle. Thin sections of skeletal muscle cells, called myocytes, appear striated in the light microscope (Figure 18.13).

Figure 18.13: Light micrograph of skeletal muscle, stained to show characteristic striations.

During development, myoblast cells (muscle cell precursors) fuse to form a myoctes (myofibers). The dark structures surrounding the striations are the multiple nuclei that end up in the mature myocyte. Multinucleate cells resulting from such fusions are called syncytia (singular: syncytium). Each syncytial myocyte also contains many mitochondria to provide ATP to fuel contraction. Skeletal muscle is made up of aligned, bundled myocytes (the myofibers), which are in turn, further organized into fascicles, which are finally bundled to form a muscle.

Figure 18.14 shows the anatomical organization and fine structure of a muscle.

Figure 18.14: Skeletal muscle organization and the anatomy of a muscle cell sarcomere.

Light passing through the more or less ordered regions of the sarcomere will bend to different degrees because each region has a different refractive index. Polarizing light microscopy detects these differences, enhancing the contrast between the regions of the sarcomere and defining them as either isotropic (i.e., with a low refractive index) or anisotropic (with a high refractive index). High-resolution electron microscopy from the 1940s revealed the fine structure of skeletal muscle (right panel). Paired, dark, vertical Z-lines define a sarcomere in the electron micrograph. In the illustration of the myofiber (lower left) the Z-lines (shown in gray) are aligned in register in bundled myofibers as well as in the fascicles (upper left); these are what appear as the striations characteristic of skeletal muscle seen in Figure 18.13. Based on light and electron microscopy, we can define regions within a sarcomere:

• The A band (anisotropic band) of overlapping, aligned actin and myosin filaments, which runs down the middle of the sarcomere, is more ordered than the I-band, so it has a higher refractive index.
• The I-band (isotropic band) has a low refractive index compared to the A band. It is largely made up of thin (actin) microfilaments.
• The paired Z-lines demarcate the sarcomere (Z for zwischen, German for between).
• The H zone is a region where myosin does not overlap actin filaments.
• An M-line lies at the center of the H zone.

18.5.2 The Sliding Filament Model of Skeletal Muscle Contraction

Electron microscopy of relaxed and contracted muscle is consistent with the sliding of thick and thin filaments during contraction (Figure 18.15).

Figure 18.15: The transmission-electron micrograph and its corresponding illustration show a sarcomere shortening, consistent with the sliding of filaments during skeletal muscle contraction.

Additional key structures of the sarcomere can be seen in the drawing at the right. Note that, in the sarcomeres of a contracted muscle cell, the H zone has almost disappeared. While the width of the A band has not changed after contraction, the width of the I-bands has decreased and the Z-lines are closer. The best explanation here was the Sliding Filament Hypothesis (or model) of skeletal muscle contraction.

330-2 The Sliding Filament Model of Skeleton-Muscle Contraction

18.5.3 The Contraction Paradox: Contraction and Relaxation Require ATP

The role of ATP in fueling the movement of sliding filaments during skeletal muscle contraction was based in part on experiments with glycerinated fibers. These are muscle fibers that were soaked in glycerin to permeabilize the plasma membrane.

The soluble cytoplasmic components that are normally contained in myocytes leak out of these fibers in this experiment, but they leave sarcomere structures intact, as seen in electron micrographs. Investigators found that if ATP and calcium were added back to glycerinated fibers, the ATP was hydrolyzed, and the fibers could still contract—and even lift a weight! Contraction of a glycerinated muscle fiber in the presence of ATP is illustrated below in Figure 18.16.

Figure 18.16: The skeletal muscle contraction Paradox: When given ATP, glycerinated muscle fibers contract and pull a weight. When all the ATP is hydrolyzed, the fiber can’t stretch unless more ATP is added.

Assays showed that when all of the added ATP had been hydrolyzed, the muscle remained contracted. It would not relax, even with the weight it had lifted still attached. Attempts to manually force the muscle back into its relaxed position didn’t work. But the fiber could be stretched when fresh ATP was added to the preparation. Moreover, if the experimenter let go immediately after stretching the fiber, it would again contract and lift the weight. A cycle of forced stretching and contraction could be repeated until all of the added ATP was hydrolyzed. At that point, the fiber would again no longer contract—or if contracted, it could no longer be stretched.

The contraction paradox, then, was this: ATP hydrolysis is required for muscle contraction as well as for relaxation (stretching). The paradox was resolved when the functions of the molecular actors in contraction were finally understood. Here we review some of the classic experiments that led to this understanding.

331 The Contraction Paradox