Chapter 10.1 – Movement and Muscle Tissue (pages 332-342)
The Mechanism of Muscle Fibre Contraction Two types of myofilaments make up muscle fibres: • actin myofilament: • two strands of protein wrapped around each other • like to strands of beads
myosin myofilament: • two strands of protein wrapped around each other • 10 x longer than actin • one end consists of long rod • one end consists of double head
Contraction The Steps: • Heads of the myosin move, like flexing your hand towards your wrist • Myosin heads are chemically bonded to actin the actin filament and move it along • ATP is used to re-position the myosin head to flex again • The myosin bonds to actin farther down the actin filament • The actin filament is “walked along” in the direction of the flex
The Sliding Filament Model • One end of the actin myofilament is anchored at a position called the Z line • The movement of the actin along the myosin pulls the “anchor” with it • The Z lines on opposite ends of the myosin filament are pulled towards each other • This results in a shortening of the muscle fibre (contraction)
The Role of Calcium Ions in Contraction • Ca2+ ions are pumped into the microfibrils when a contraction nerve impulse is signaled • High concentrations of Ca2+ are required to expose the binding site between actin and myosin • ATP is used in the process • Ca2+ ions are pumped out when the nerve signal stops; the binding site no longer accessible
1. Explain how the myofilaments produce muscle contraction even though the length of each myofilament does not change. • During contraction, the myofilaments slide past each other, decreasing the distance between the Z lines along the entire length of the muscle fibre. Because there are millions of Z lines along a muscle fibre, its length decreases.
2. Measurements show that the distance between the two Z lines varies between 1.5 µm and 3.0 µm. How do these measurements relate to the sliding filament model as shown above? • The distance between the Z lines is 3.0 µm at rest and 1.5 µm when contracted.
Use the diagram to explain why there is a limit to how much a muscle can shorten as it contracts. • a muscle contracts, the Z lines are pulled closer to the ends of the myosin filaments. When the Z line bumps into the myosin, further shortening of the muscle is impossible.
As ATP is spent in causing the myofilaments to slide, some of its energy is released as “wasted” heat. In what way is this heat wasted? In what way is it useful to muscle contraction? • The heat from ATP breakdown is wasted in the sense that it does not power the reactions between actin and myosin as chemical energy does. However, the heat is useful in warming the muscle, reducing the friction between the myofilaments and increasing the force exerted by the entire muscle as it warms up.
Energy for Muscle Contraction • ATP reserves are used up very fast during strenuous activity • Muscles may acquire ATP three ways: • breakdown of creatine phosphate (anaerobic process) • aerobic cellular respiration • fermentation (anaerobic process)
Creatine Phosphate Breakdown • creatine phosphate is a high energy molecule that is used to regenerate ATP from ADP • occurs in the sliding filaments • fastest way to make ATP available to muscles • provides enough energy for 8 seconds of intense activity • rebuilt while muscle is resting
Aerobic Cellular Respiration • Takes place in the mitochondria • provides most of the ATP to the muscle • heat produced during cellular respiration helps to warm the entire body • 2/3 of heat used to maintain constant body temperature comes from aerobic cellular respiration in skeletal muscles.
Fermentation • occurs in the absence of oxygen • accumulation of lactic acid causes enzymes to break down • prolonged fermentation causes cramping and fatigue
Oxygen Deficit • Muscle tissues can function without oxygen for extended periods, unlike brain tissue • Muscle cells build up an oxygen deficit • To replenish an oxygen deficit: creatine phosphate must be replenished, lactate is disposed of