Y1B4M1L4 Articulations, Joint Motion & Muscle Contraction PDF

Y1B4M1L4 ARTICULATIONS, JOINT MOTION & MUSCLE CONTRACTION Figure 35. Propagation of Action Potential ● The depolarization of the motor end plate initiates an action potential which propagates along the sarcolemma in all directions and down the T Tubules. Figure 36. Calcium release from terminal ci...

Y1B4M1L4 ARTICULATIONS, JOINT MOTION & MUSCLE CONTRACTION Figure 35. Propagation of Action Potential ● The depolarization of the motor end plate initiates an action potential which propagates along the sarcolemma in all directions and down the T Tubules. Figure 36. Calcium release from terminal cisternae ● The AP causes the release of Calcium ions from the terminal cisterns into the cytosol. ● Calcium ions then trigger a contraction of the muscle cell. VII. SLIDING FILAMENT THEORY ● Involves activities of 6 molecules: myosin, actin, tropomyosin, troponin, ATP, Ca2 ● Calcium ions Ca2 link action potentials in a muscle fiber to contraction. ● In resting muscle fibers, Ca2 is stored in the endoplasmic (sarcoplasmic) reticulum. ● Spaced along the plasma membrane (sarcolemma) of the muscle fiber are inpocketings of the membrane that form tubules of the “T system”. ● These tubules plunge repeatedly into the interior of the fiber. ● The tubules of the T system terminate near the calcium filled sacs of the sarcoplasmic reticulum. ● Each action potential created at the neuromuscular junction sweeps quickly along the sarcolemma and is carried into the T system. ● The arrival of the action potential at the ends of the T system triggers the release of Ca2 . ● Because of the speed of the action potential (milliseconds), the action potential arrives virtually simultaneously at the ends of the tubules of the T system, ensuring that all sarcomeres contract in unison. ● When the process is over, the calcium is pumped back into the sarcoplasmic reticulum using a Ca2 ATPase. Figure 37. Diffusion of Ca2 among the thick and thin filaments and cascade of events that follow A. MYOSIN ● The shape of an individual myosin molecule is like a golf club with two heads. ● Head or Cross Bridge – has the ability to move back and forth, and the flexing movement of the head provides the power stroke for muscle contraction. ● In skeletal muscle cells, the myosin molecules are bundled together to form the thick filaments, with the heads jutting in all directions. ● The cross bridge has two important binding sites: ○ Binding site for ATP ○ Binding site of Actin filament Figure 38. Cross Bridge in its low energy conformation ● The binding of ATP transfers energy to the myosin cross bridge as ATP is hydrolyzed into ADP and inorganic phosphate. Figure 39. Cross Bridge in its high energy conformation BARBON, CALIZO, DAQUIL, DULANA, LAYGAN, RODRIGUEZ, SOMBONG, TILOS, TUMACOLE | MG 4 11 of 22 Y1B4M1L4 ARTICULATIONS, JOINT MOTION & MUSCLE CONTRACTION B. ACTIN STEP 2 BINDING OF MYOSIN TO ACTIN ● Major component of thin filament. ● Composed of actin subunits twisted into a double helical chain. ● Each actin subunit has a specific binding site to which the myosin cross bridge binds. ● An energized cross bridge binds to the actin. ● The binding of the myosin to the actin causes a conformational change in the cross bridge, releasing inorganic phosphate and ADP. Figure 40. Actin C. TROPOMYOSIN A regulatory protein Also a part of the thin filament. Entwines around the actin. In the unstimulated muscle, tropomyosin covers the binding sites on the actin and prevents myosin cross bridge binding. ● To expose the binding sites, the tropomyosin molecule must be moved aside. This is facilitated by the presence of Troponin. ● ● ● ● Figure 44. Binding of Myosin to Actin STEP 3 POWER STROKE OF THE CROSS BRIDGE ● At the same time, the cross bridge flexes, pulling the actin inward into the center of the sarcomere in what is known as the power stroke. Figure 41. Tropomyosin D. TROPONIN ● Is attached and spaced periodically along the tropomyosin strand. ● After an action potential, Ca+ ions are released from the terminal cisterns and bind to troponin. This causes conformational change in the tropomyosin-troponin complex, dragging the tropomyosin strands off the binding sites. Figure 45. Power Stroke of the Cross Bridge STEP 4 DISCONNECTING THE CROSS BRIDGE FROM ACTIN ● In order to disconnect the cross bridge from the actin, an ATP molecule must bind to its site on the cross bridge. FIgure 42. Troponin E. PROCESS STEP 1 EXPOSURE OF BINDING SITES ON ACTIN ● An action potential causes the release of Ca ions from the terminal cisterns of the sarcoplasmic reticulum into the cytosol. ● The Ca ions bind with the tropomyosin-troponin complex to expose the binding sites on the actin. Figure 46. Disconnecting the Cross Bridge from Actin STEP 5 RE ENERGIZING AND REPOSITIONING THE CROSS BRIDGE ● The release of the myosin cross bridge from the actin triggers the hydrolysis of ATP into ADP and inorganic phosphate. ● Energy is then transferred from ATP to the myosin crossbridge which returns to its high-energy conformation. Figure 43. Exposure of Binding Sites on Actin BARBON, CALIZO, DAQUIL, DULANA, LAYGAN, RODRIGUEZ, SOMBONG, TILOS, TUMACOLE | MG 4 12 of 22 Y1B4M1L4 ARTICULATIONS, JOINT MOTION & MUSCLE CONTRACTION ● Bottom: Maximal contraction with the shortening of the muscle. Notice that the length of the sarcomere shortens, but the lengths of each myofilament does not change. The actin and the myosin are still of the same length. ● However, the width of the H zone changes. ● Events that take place as a muscle contracts: ○ The Z disks come closer together ○ The width of the I band decreases ○ The width of the H zone decreases, but there is no change in the width of the A band ● Conversely, as a muscle is stretched, the width of the I bands and H zones increase but there is still no change in the width of the A band. Figure 47. Re-Energizing and repositioning of the cross bridge VIII. MUSCLE FIBER TYPES Table 4. Classification of Muscle Fiber Types Dr. Licup) STEP 6 REMOVAL OF CALCIUM IONS Table 5. Another Classification of Muscle Fiber Types Dr. Licup) Figure 48. Removal of Calcium ions ● Calcium needs to be removed from the cytosol. Otherwise, muscle contraction will continue. This is done by active transport through specialized ion pumps in the membrane of the sarcoplasmic reticulum using ATP. For this process, ATP needs to bind to receptor sites of the calcium channels so that conformational change will occur. ● Whichever classification scheme is used, it should be remembered that there is actually a continuum of fiber types without exact distinction between types. A. RED MUSCLE FIBERS TYPE I/SLOW FIBERS Figure 49. Several Actin and myosin filaments are interacting to demonstrate the sliding filament theory of muscle contraction ● Top: Myofilaments are in a relaxed state. ● Middle: Cross bridges are starting to form and the muscle is beginning to shorten, as seen in the length of the sarcomere. ● Slow fibers (type I, red muscle) – Muscle contraction is slow approximately, 110 ms/muscle contraction. Muscle fibers are resistant to fatigue and are used in activities involving endurance and continuous contraction. Also they: ○ Are about half the diameter of white fibers. ○ Are dark red in color due to the large quantity of myoglobin. ○ Are surmounted by many capillaries. ○ Contain numerous mitochondria ○ Have low glycogen content ○ Possess a more extensive blood vessel system and capillary supply. ○ Form cross-bridges more slowly than other fiber types (that is why itʼs called Slow fibers). ○ Use the Krebs cycle and oxidative metabolism ○ Respond slowly but are prolonged ○ Innervated by smaller nerve fibers ○ Have extra amounts of oxygen BARBON, CALIZO, DAQUIL, DULANA, LAYGAN, RODRIGUEZ, SOMBONG, TILOS, TUMACOLE | MG 4 13 of 22 Y1B4M1L4 ARTICULATIONS, JOINT MOTION & MUSCLE CONTRACTION B. WHITE MUSCLE FIBERS TYPE II/FAST FIBERS ● Fast fibers (type II or white muscle) – Fast twitch up to 50 ms/muscle contraction. Powerful muscle fibers due to larger diameter, used in activities for power and speed for a short duration. ○ Fast-twitch ○ Large, in diameter, fibers for greater strength of contraction ○ Light in color due to reduced myoglobin ○ Surrounded by few capillaries ○ Have a less extensive blood supply ○ Metabolism is of secondary importance ○ Have relatively few mitochondria ○ Have high glycogen content ○ Form cross-bridges rapidly ○ Use glycolysis for rapid release of energy ○ Can cause the buildup of lactic acid ○ Respond rapidly ○ Have extensive and highly developed sarcoplasmic reticulum for rapid contraction ○ Release of calcium ions to initiate contraction ○ Have large amounts of glycolytic enzymes for the glycolytic process Additional Information: ● Major forms of Type II Fibers: ○ Type IIa Fibers Fast Oxidative Glycolytic Fibers) - High myosin ATPase activity and have high oxidative and glycolytic capacity, and are resistant to fatigue. ○ Type IIb Fibers - High myosin ATPase activity and have low oxidative and glycolytic capacity, and are resistant to fatigue. Commonly found in small mammals. ● On the average, most muscles are composed of roughly 50% Type I fibers and 25% Type IIA fibers. COMPARISON OF MUSCLE FIBER IN TWO ATHLETES Figure 50. Different Types of Muscle Fiber in Athletes Figure 52. Common Combinations of Muscle Fiber Types in Different Athletic Activities IX. ENERGETICS OF MUSCLE CONTRACTION Work output during muscle contraction: ● Work - done when a muscle contracts against a load ● To perform work means that energy is transferred from the muscle to the external load to lift an object to a greater height or to overcome resistance to movement. W=LxD Where: W = work output L = load D = distance of movement against the load A. ADENOSINE TRIPHOSPHATE ● Adenosine triphosphate ATP – the main source of energy for muscle contraction. ● Concentration of ATP in the muscle fiber, about 4 mM, is sufficient to maintain a full contraction for only 1 to 2 seconds at most. ● ATP is split to form ADP, which transfers energy from the ATP molecule to the contracting machinery of the muscle fiber. ● ADP is re-phosphorylated to form a new ATP which allows the muscle to continue its contraction. ● Functions of ATP include: ○ For maintenance of cross-bridging. ○ Most of the energy required for muscle contraction is used to actuate the walk-along mechanism by which the cross-bridges pull the actin filaments. ○ For pumping calcium ions from the sarcoplasm into the sarcoplasmic reticulum after the contraction is over. ○ For pumping sodium and potassium ions through the muscle fiber membrane to maintain an appropriate ionic environment for the propagation of muscle fiber action potentials. Figure 51. Different Types of Muscle Fiber in Athletes BARBON, CALIZO, DAQUIL, DULANA, LAYGAN, RODRIGUEZ, SOMBONG, TILOS, TUMACOLE | MG 4 14 of 22 Y1B4M1L4 ARTICULATIONS, JOINT MOTION & MUSCLE CONTRACTION B. SOURCES OF ENERGY C. PHASES OF ATP PRODUCTION DURING EXERCISE PHOSPHOCREATINE ● first source of energy ● A high-energy phosphate bond similar to but which carries a slightly higher amount of free energy than that of ATP is found in phosphocreatine. ○ Therefore, phosphocreatine is instantly cleaved, and its released energy causes the bonding of a new phosphate ion to ADP to reconstitute the ATP. ● The total amount of phosphocreatine is about 5 times as much as ATP. ● The combined energy of both the stored ATP and the phosphocreatine in the muscle can cause maximal muscle contraction for only 5 to 8 seconds. MUSCLE GLYCOGEN Figure 53. Phases of ATP Production During Exercise X. THE MOTOR UNIT ● Next important source of energy. ● Rapid enzymatic breakdown of the glycogen to pyruvic acid and lactic acid liberates energy that is used to convert ADP to ATP. ○ This actually occurs even in the absence of glycogen. Additional notes: ● There is a faster rate of ATP formation (2.5 times). ● End products accumulate in muscle cells ○ Can sustain maximum contraction for only 1 minute (because of end products such as lactic acid) ● ATP can then be used directly to energize additional muscle contraction and to re-form the stores of phosphocreatine. ● Two-fold importance of glycolysis of muscle glycogen is as follows: ○ Anaerobic respiration may be used to sustain muscle contraction up to more than one minute. ○ Glycolytic process is faster than foodstuffs reacting with oxygen in ATP formation; end products accumulate in the muscle cells that its capability to sustain maximum muscle contraction is only for about a minute. OXIDATIVE METABOLISM ● Oxidative mechanism – final source of energy ○ Provides 95 % of all energy used by muscles for sustained, long-term contraction. ○ Oxygen + end product of glycolysis Oxygen combines with various cellular foodstuffs to provide ATP . ○ From macromolecules CHO, fats, and CHON . Additional notes: ● Macromolecules used in oxidative mechanism are: ○ Carbohydrates – contributes half of the energy contributed for a 2 to 4-hour period ○ Fats – contribute to long-term maximal muscle activity over a long period of time → Provide the greatest proportion of energy produced ○ Proteins – only utilized when CHO and fats are used up Figure 54. The Motor Unit ● Motor unit – Consists of a motor neuron and the group of muscle fibers it innervates. Motor units vary according to the size of the cell body, diameter of the axons and the number of muscle fibers and the fiber type. This variation affects the function of the motor units. ● A single motor axon may branch to innervate several muscle fibers that function together as a group. ● Although each muscle fiber is innervated by a single motor neuron, an entire muscle may receive input from hundreds of different motor neurons. ● There are different kinds of motor units. ○ Small motor units produce precise movements. → Axons are small. → Fibers are fewer and primarily Type I. → These are recruited first in most activities. → Small motor units are found where precise movements are needed like the eye which may contain as few as 6 muscle fibers. → Small muscles that react rapidly and whose control must be exact have more nerve fibers for fewer muscle fibers (for instance, as few as two or three muscle fibers per motor unit in some of the laryngeal muscles). ○ Large motor units produce gross movements. → Axon is large. → There are plenty of fibers, primarily Type II. → These are recruited for forceful contractions. → Example: Muscles of the thigh wherein a single motor neuron is connected to a large number of muscle cells. → Large muscles that do not require fine control, such as the soleus muscle, may have several hundred muscle fibers in a motor unit (vary in size from muscle to muscle). BARBON, CALIZO, DAQUIL, DULANA, LAYGAN, RODRIGUEZ, SOMBONG, TILOS, TUMACOLE | MG 4 15 of 22