Contraction

Contraction is the step in which the muscle fiber develops tension and may shorten. (Muscles often "contract," or develop tension, without shortening, as we see later.) How a muscle fiber shortens remained a mystery until sophisticated techniques in electron microscopy enabled cytolo-gists to see the molecular organization of muscle fibers. In 1954, two researchers at the Massachusetts Institute of Technology, Jean Hanson and Hugh Huxley, found evidence for a model now called the sliding filament theory. This theory holds that the thin filaments slide over the thick ones and pull the Z discs behind them, causing the cell as a whole to shorten. The individual steps in this mechanism are shown in figure 11.10.

  1. The myosin head must have an ATP molecule bound to it to initiate the contraction process. Myosin ATPase, an enzyme in the head, hydrolyzes this ATP. The energy released by this process activates the head, which "cocks" into an extended, high-energy position. The head temporarily keeps the ADP and phosphate group bound to it.
  2. The cocked myosin binds to an active site on the thin filament.
  3. Myosin releases the ADP and phosphate and flexes into a bent, low-energy position, tugging the thin filament along with it. This is called the power stroke. The head remains bound to actin until it binds a new ATP.
  4. Upon binding more ATP, myosin releases the actin. It is now prepared to repeat the whole process—it will hydrolyze the ATP, recock (the recovery stroke), attach to a new active site farther down the thin filament, and produce another power stroke.

It might seem as if releasing the thin filament at step 13 would simply allow it to slide back to its previous position, so that nothing would have been accomplished. Think of the sliding filament mechanism, however, as

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11. Muscular Tissue

Text

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  • Motor nerve fiber n->2+
  • Motor nerve fiber n->2+
Muscle Action Potential Creation
2. Acetylcholine (ACh) release

Sarcolemma

Sarcolemma

Sarcolemma Action Potential
  1. Binding of ACh to receptors
  2. Opening of ligand-gated ion channel; creation of end-plate potential
  3. Binding of ACh to receptors
  4. Opening of ligand-gated ion channel; creation of end-plate potential

Motor

Motor

Steps Muscle Contraction

Figure 11.8 Excitation of a Muscle Fiber. These events link action potentials in a nerve fiber to the generation of action potentials in the muscle fiber.

5. Opening of voltage-gated ion channels; creation of action potentials

Figure 11.8 Excitation of a Muscle Fiber. These events link action potentials in a nerve fiber to the generation of action potentials in the muscle fiber.

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Chapter 11 Muscular Tissue 419

Chapter 11 Muscular Tissue 419

9. Shifting of tropomyosin; exposure of active sites on actin

Figure 11.9 Excitation-Contraction Coupling. These events link action potentials in the muscle fiber to the release and binding of calcium ions. The numbered steps in this figure begin where the previous figure left off.

being similar to the way you would pull in a boat anchor hand over hand. When the myosin head cocks, it is like your hand reaching out to grasp the anchor rope. When it flexes back into the low-energy position, it is like your elbow flexing to pull on the rope and draw the anchor up a little bit. When you let go of the rope with one hand, you hold onto it with the other, alternating hands until the anchor is pulled in. Similarly, when one myosin head releases the actin in preparation for the recovery stroke, there are many other heads on the same thick filament holding onto the thin filament so that it doesn't slide back. At any given moment during contraction, about half of the heads are bound to the thin filament and the other half are extending forward to grasp the filament farther down. That is, the myosin heads of a thick filament do not all stroke at once but contract sequentially.

As another analogy, consider a millipede—a little wormlike animal with a few hundred tiny legs. Each leg

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420 Part Two Support and Movement

Sliding Filament Theory Tropomyosin

12. Power stroke; sliding of thin filament over thick filament

Figure 11.10 The Sliding Filament Mechanism of Contraction. This is a cycle of repetitive events that cause a thin filament to slide over a thick filament and generate tension in the muscle. The numbered steps in this figure begin where the previous figure left off.

12. Power stroke; sliding of thin filament over thick filament

Figure 11.10 The Sliding Filament Mechanism of Contraction. This is a cycle of repetitive events that cause a thin filament to slide over a thick filament and generate tension in the muscle. The numbered steps in this figure begin where the previous figure left off.

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Sliding Filament Mechanism
17. Loss of calcium ions from troponin

18. Return of tropomyosin to position blocking active sites of actin

Figure 11.11 Relaxation of a Muscle Fiber. These events lead from the cessation of a nerve signal to the release of thin filaments by myosin. The numbered steps in this figure begin where the previous figure left off.

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Insight 11.2 Clinical Application

422 Part Two Support and Movement takes individual jerky steps, but all the legs working together produce smooth, steady movement—just as all the heads of a thick filament collectively produce a smooth, steady pull on the thin filament. Note that even though the muscle fiber contracts, the myofilaments do not become shorter any more than a rope becomes shorter as you pull in an anchor. The thin filaments slide over the thick ones, as the name of the theory implies.

A single cycle of power and recovery strokes by all the myosin heads in a muscle fiber would shorten the fiber by about 1%. A fiber, however, may shorten by as much as 40% of its resting length, so obviously the cycle of power and recovery must be repeated many times by each myosin head. Each head carries out about five strokes per second, and each stroke consumes one molecule of ATP.

Rigor Mortis

Rigor mortis7 is the hardening of the muscles and stiffening of the body that begins 3 to 4 hours after death. It occurs partly because the deteriorating sarcoplasmic reticulum releases calcium ions into the cytosol, and the deteriorating sarcolemma admits more calcium ions from the extracellular fluid. The calcium ions activate myosin-actin cross bridging and muscle contraction. Furthermore, the muscle cannot relax without ATP, and ATP is no longer produced after death. Thus, the fibers remain contracted until the myofilaments begin to decay. Rigor mortis peaks about 12 hours after death and then diminishes over the next 48 to 60 hours.

7 rigor = rigidity + mortis = of death

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