2. Exercise Science Section 3: The Muscular System An Introduction to Health and Physical Education Ted Temertzoglou Paul Challen ISBN 1-55077-132-9

3. The Sliding Filament Theory  As we have already seen, within the myofibrils, actin and myosin filaments are arranged in such a manner as to be overlapping one another. According to the Sliding Filament Theory, one set of these filaments will “slide” over the other (much like a telescope), thus, shortening the muscle.  The theory proposes that calcium ions are responsible for this reaction.  Muscle tissue is innervated by nerve fibres (axons) which terminate in motor end plates located just under the sarcolemma. A single axon can innervate anywhere from a few to several hundred muscle fibres. The ratio of nerve-to-muscle fibre depends on the refinement of control required by the muscle. In eye muscles, where fine motor control is necessary, the ratio may be 1:3, whereas in large leg muscles, the ratio might be 1:150.

4. ©Thompson Educational Publishing, Inc. 2003. All material is copyright protected. It is illegal to copy any of this material. This material may be used only in a course of study in which Exercise Science: An Introduction to Health and Physical Education (Temertzoglou/Challen) is the required textbook. The Sliding Filament Theory – In Brief  Myosin crossbridges (small “bridges” on the thick filaments that extend to the thin filaments):  Attach, rotate, detach, and reattach in rapid succession  Results in the sliding or overlap of the actin and myosin filaments  Causes sarcomere to contract (muscle contraction)  Known as the sliding filament theory  The 5 stages of this theory are covered in the following slides.

5. The Sliding Filament Theory - REST  Crossbridges on the myosin filament extend toward but do not interact with the actin filament.  ATP molecules are bound to the ends of these crossbridges; this is referred to as an “uncharged” ATP crossbridge complex.  Calcium required to initiate the reaction is stored in vesicles in the sarcoplasmic reticulum of the myofibril.  In the absence of free Calcium, troponin (on actin) does not allow the myosin crossbridges to bind with actin. Thus, actin and myosin are left uncoupled.

6. ©Thompson Educational Publishing, Inc. 2003. All material is copyright protected. It is illegal to copy any of this material. This material may be used only in a course of study in which Exercise Science: An Introduction to Health and Physical Education (Temertzoglou/Challen) is the required textbook. The Sliding Filament Theory - REST  Myosin crossbridges (small “bridges” on the thick filaments that extend to the thin filaments)

7. The Sliding Filament Theory – Excitation-Coupling  Acetylcholine is released into the neuromuscular junction at the end of the axon (motor end plate).  In turn, this releases nerve impulses into the sarcolemma. Depolarization of the membrane now occurs (ie. Calcium is now uncharged).  This is termed action potential.  The action potential now causes Ca++ to be released from the sarcoplasmic reticulum. The troponin molecules on actin take up the available Ca++, causing a structural change in troponin and tropomyosin.  This results in the active sites on actin to be “turned-on”.  The uncharged (“resting”) ATP crossbridge complex now becomes charged. Myosin and actin now attract each other and form actomyosin.

8. ©Thompson Educational Publishing, Inc. 2003. All material is copyright protected. It is illegal to copy any of this material. This material may be used only in a course of study in which Exercise Science: An Introduction to Health and Physical Education (Temertzoglou/Challen) is the required textbook. The Sliding Filament Theory – Excitation-Coupling  Myosin crossbridges  Attach, rotate, detach, and re-attach in rapid succession

9. Sliding Filament Theory – Contraction/Recharging  Contraction  The newly formed actomyosin activates an enzyme called myosin ATPase. This enzyme acts to break ATP down into ADP + P (organic phosphate), releasing large amounts of cellular energy.  This energy causes the crossbridges to rotate, resulting in the actin filament moving toward the center of the sarcomere. In this way, the myofibril and muscle shorten  Recharging  Crossbridges are made and broken many times during each contraction of a muscle. First, the bond between actin and myosin must be broken.  The crossbridge is reloaded with a new ATP, breaking the bond, thus freeing the ATP crossbridge from an actin binding site.

10. ©Thompson Educational Publishing, Inc. 2003. All material is copyright protected. It is illegal to copy any of this material. This material may be used only in a course of study in which Exercise Science: An Introduction to Health and Physical Education (Temertzoglou/Challen) is the required textbook. Sliding Filament Theory – Contraction/Recharging  Myosin crossbridges  Results in the sliding or overlap of the actin and myosin filaments  Causes sarcomere to contract (muscle contraction)

11. Sliding Filament Theory - Relaxation  Motor nerve ceases to create an action potential.  Ca++ is released from troponin; returns to the sarcoplasmic reticulum.  The removal of calcium “turns off” the binding sites on the actin.  ATP crossbridges on myosin are no longer able to form a bond with actin.  Myofilaments return to their original position and the muscle relaxes.

12. ©Thompson Educational Publishing, Inc. 2003. All material is copyright protected. It is illegal to copy any of this material. This material may be used only in a course of study in which Exercise Science: An Introduction to Health and Physical Education (Temertzoglou/Challen) is the required textbook. The Role of Adenosine Triphosphate  The sliding filament theory at the molecular level:  Nerve impulse transmitted through the muscle fibre and releases calcium ions  Calcium (in presence of troponin and tropomyosin) facilitates the interaction of myosin and actin molecules  Adenosine triphosphate (ATP) is the energy source behind the release of calcium  ATP detaches myosin from the actin molecule  ATP must be replaced through food metabolism for process to continue