Polymer-Gel Phase-Transition as the Mechanism of Muscle Contraction
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ABSTRACT The thesis offered here is that the muscle-contraction mechanism is similar to the mechanism of contraction in many artificial muscles. Artificial muscles typically contract by a phasetransition. Muscle is thought to contract by a sliding-filament mechanism in which one set of filaments is driven past another by the action of cyclically rotating cross-bridges-much like the mechanism of rowing. However, the evidence is equally consistent with a mechanism in which the filaments themselves contract, much like the condensation of polymers during a phasetransition. Muscle contains three principal polymer types organized neatly within a framework. All three can shorten. The contributions of each filament may be designed to confer maximum strength, speed and versatility on this biological machine. The principles of natural contraction may be useful in establishing optimal design principles for artificial muscles.
FUNCTIONAL ELEMENTS The essence of the textbook structure of muscle is shown in Fig. 1. The fundamental molecular unit is the sarcomere, bounded on either end by the Z-line. Muscle shortening is the summated shortenings of the many sarcomeres in series and in parallel. Sarcomeres in series build velocity, whereas sarcomeres in parallel build force. thin filament (actin)
cross-bridge Figure 1. Textbook view of muscle-unit structure. The sarcomere is symmetrical about the middle (M) region. It consists of three longitudinally Zconnecing filament (titin) orientedfilaments:thick thin half-sarcomere and connecting. Cross bridges arrayedalongthe thickfilament are thought to interactwith the thinfilament to mediate contraction.
M
thick filament (myosii n)
The sarcomere is built of three principal proteins-or polymers. The central polymer is the thick filament, which is built of multiple repeats of the protein myosin. The thick filament connects at either end to the connecting filament, which in turn connects to the Z-line. In vertebrate muscle the connecting filament is built largely of the protein titin. Titin is built of tandem repeats of discrete domains, the principal one being the immunoglobulin-like domain, which repeats many times between thick filament and Z-line. The thin filament is also a polymer. Along with regulatory proteins of various kind, it consists of multiple repeats of the 237 Mat. Res. Soc. Symp. Proc. Vol. 600 ©2000 Materials Research Society
protein actin. Thus, all three longitudinally oriented structures of the sarcomere are biological polymers. These three polymers are cross-linked along their length: actin and connecting filaments are crosslinked through the Z-line; thick filaments cross-linked to parallel thick filaments through M-lines; and while cross-bridges (Fig 1) are thought to make transient cross-links with thin filaments, structural evidence obtained by freeze-substitution and thin sectioning (Baatsen et al., 1988) indicates that the cross-bridges form permanent cross-links between thick filaments, much like the M-line connections. There is also evidence that thin an
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