Myofilaments

Let's return to the myofibrils just mentioned—the long protein cords that fill most of the muscle cell—and look at their structure at a finer, molecular level. It is here that the key to muscle contraction lies. Each myofibril is a bundle of parallel protein microfilaments called myofilaments. There are three kinds of myofilaments:

  1. Thick filaments (fig. 11.3 a, b) are about 15 nm in diameter. Each is made of several hundred molecules of a protein called myosin. A myosin molecule is shaped like a golf club, with two polypeptides intertwined to form a shaftlike tail and a double globular head, or cross-bridge, projecting from it at an angle. A thick filament may be likened to a bundle of 200 to 500 such "golf clubs," with their heads directed outward in a spiral array around the bundle. The heads on one half of the thick filament angle to the left, and the heads on the other half angle to the right; in the middle is a bare zone with no heads.
  2. Thin filaments (fig. 11.3 c, d), 7 nm in diameter, are composed primarily of two intertwined strands of a protein called fibrous (F) actin. Each F actin is like

Saladin: Anatomy & Physiology: The Unity of Form and Function, Third Edition

410 Part Two Support and Movement

410 Part Two Support and Movement

Structure Skeletal Muscle

Figure 11.2 Structure of a Skeletal Muscle Fiber. This is a single cell containing 11 myofibrils (9 shown at the left end and 2 cut off at midfiber).

-Myofibrils

Figure 11.2 Structure of a Skeletal Muscle Fiber. This is a single cell containing 11 myofibrils (9 shown at the left end and 2 cut off at midfiber).

a bead necklace—a string of subunits called globular (G) actin. Each G actin has an active site that can bind to the head of a myosin molecule.

A thin filament also has 40 to 60 molecules of yet another protein called tropomyosin. When a muscle fiber is relaxed, tropomyosin blocks the active sites of six or seven G actins, and prevents myosin cross-bridges from binding to them. Each tropomyosin molecule, in turn, has a smaller calcium-binding protein called troponin bound to it.

3. Elastic filaments (fig. 11.4b, c), 1 nm in diameter, are made of a huge springy protein called titin3

(connectin). They run through the core of a thick filament, emerge from the end of it, and connect it to a structure called the Z disc, explained shortly. They help to keep thick and thin filaments aligned with each other, resist overstretching of a muscle, and help the cell recoil to resting length after it is stretched.

Myosin and actin are called the contractile proteins of muscle because they do the work of shortening the muscle fiber. Tropomyosin and troponin are called the regulatory proteins because they act like a switch to determine when it can contract and when it cannot. Several clues as to how they do this may be apparent from what has already been said—calcium ions are released into the sarcoplasm to activate contraction; calcium binds to troponin; troponin is

Saladin: Anatomy & I 11. Muscular Tissue I Text I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

Chapter 11 Muscular Tissue 411

Heads

Myosin moiecuie

Myosin head

Myosin moiecuie

Myosin head

Thick filament

Thick filament

Tropomyosin Troponin complex

G actin

Thin filament

Tropomyosin Troponin complex

G actin

Thick filament Thin filament

Thin filament

Thick filament Thin filament

Myofilament Structure

Figure 11.3 Molecular Structure of Thick and Thin Filaments. (a) A single myosin molecule consists of two intertwined polypeptides forming a filamentous tail and a double globular head. (b) A thick filament consists of 200 to 500 myosin molecules bundled together with the heads projecting outward in a spiral array. (c) A thin filament consists of two intertwined chains of G actin molecules, smaller filamentous tropomyosin molecules, and a three-part protein called troponin associated with the tropomyosin. (d) A region of overlap between the thick and thin filaments.

Figure 11.3 Molecular Structure of Thick and Thin Filaments. (a) A single myosin molecule consists of two intertwined polypeptides forming a filamentous tail and a double globular head. (b) A thick filament consists of 200 to 500 myosin molecules bundled together with the heads projecting outward in a spiral array. (c) A thin filament consists of two intertwined chains of G actin molecules, smaller filamentous tropomyosin molecules, and a three-part protein called troponin associated with the tropomyosin. (d) A region of overlap between the thick and thin filaments.

also bound to tropomyosin; and tropomyosin blocks the active sites of actin, so that myosin cannot bind to it when the muscle is not stimulated. Perhaps you are already forming some idea of the contraction mechanism to be explained shortly.

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