BIOCHEMICAL EVENTS THAT CAUSE MUSCLE CONTRACTION
In the early 1900s, researchers realized that when a muscle contracts the sarcomere shortens, so that whereas the A-band of the sarcomere (see Exercises 1.1) remains the same width, the I-band becomes thinner. The British scientist A.F. Huxley proposed that the proteins of the I-band had “slid” or been pulled into the A-band by the action of the A-band proteins. For this sliding filament theory
The molecular mechanisms involved in the cross-bridge cycle. ATP adenosine triphosphate; ADP = adenosine diphosphate; Pi = phosphate of muscle contraction, Huxley won the Nobel prize for biology. Today there is general consensus that the Huxley hypothesis is essentially correct (Podolsky & Schoenberg, 1983).
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In essence, the thick filaments that constitute the A-band comprise myosin molecules, each of which has a body and a “head” (see Exercises 1.3). The individual myosin heads point toward the actin proteins of the thin filaments, and at each point where the myosin interacts with actin, the tropomyosin molecule is found. The presence of this molecule is necessary because the myosin head has a natural attraction for actin, and the absence of some blocking agent would mean that actin and myosin would be perpetually bound to each other, thereby causing persistent muscle contraction or, in running terms, muscle cramps. Before muscle contraction can occur, therefore, some mechanism must operate to move the tropomyosin molecule from its position between actin and myosin.
The trigger that does this is calcium. When you decide to move a particular muscle, the brain sends a message in the form of an electrical impulse down the spinal cord to the nerve that supplies the muscle. At the junction of the nerve and the particular muscle cell that it serves is a special site known as the motor-nerve end plate. Passage of the current through the motor-nerve end plate causes a special chemical messenger, acetylcholine, to-be released into the gap between the motor-nerve end plate and the outer envelope or membrane of the muscle cell or its extensions that extend to the interior of the cell, the transverse tubules (see Exercises 1.2).
Having traversed this tiny gap, the acetylcholine molecule binds to special acetylcholine receptors on the muscle cell. In a way that we still do not quite understand, this causes calcium that is stored deep inside the cell in another specialized structure in the cell gap, the sarcoplasmic reticulum, to be released very rapidly. The calcium then floods into the cell sarcoplasm surrounding the muscle filaments and binds to specific calcium-binding sites on the special calcium-binding protein, troponin-C, which is attached to both tropomyosin and actin. This binding of calcium causes the troponin-C molecule to undergo a complex twisting movement, the effect of which is to extricate the tropomyosin molecule from its blocking position between actin and myosin (see Exercises 1.3). This allows the myosin head to attach itself to actin, the essential first step in muscular contraction.
The next ingredient required for muscular contraction is a source of energy, which is supplied by ATP molecules produced in the mitochondria that travel to specific ATP-binding sites on the myosin heads. Here the ATP is, in fact, stored as two ATP breakdown products, adenosine diphosphate (ADP) and phosphate (Pi).
When the energy-loaded myosin head attaches itself to actin, almost immediately the Pi and ADP are released in sequence by the myosin-binding site, which causes the myosin head to undergo a complex sequence of events in which its flexible “neck” is believed to “bend” 45° from its normal vertical angle (see Exercises 1.3). This bending action causes the thin filaments to be pulled toward the center of each sarcomere. This movement, repeated in millions of thick and thin filaments in millions of sarcomeres in a single muscle, produces the visible muscle shortening that we recognize as movement.
The fully contracted position, in which the myosin head is bent to the 45° position and is bound to actin, is known as the rigor complex. To break the rigor complex, the mitochondria must supply fresh ATP to the ATP-binding site on the myosin head, and the calcium bound to the troponin-C must be removed so that tropomyosin can again move into its blocking position between the actin and myosin.
A muscle cramp is, of course, nothing more than the development of a rigor complex. Thus, we could speculate that a muscle cramp could occur either because no fresh ATP is available to the myosin head to allow relaxation or because a breakdown occurs in the mechanism whereby calcium is released from tropo-nin-C and pumped back into the sarcoplasmic reticulum.