Muscular System Lesson - TED-Ed Science PDF

L e s s o n 3  ✓ Body Movement ✓ Maintenance of posture ✓ Respiration ✓ Production of body heat ✓ Communication ✓ Constriction of organs and vessels ✓ Heartbeat  e.g. walking & running is due to the contraction of skeletal muscles 5  e.g. sitting or...

L e s s o n 3  ✓ Body Movement ✓ Maintenance of posture ✓ Respiration ✓ Production of body heat ✓ Communication ✓ Constriction of organs and vessels ✓ Heartbeat  e.g. walking & running is due to the contraction of skeletal muscles 5  e.g. sitting or standing erect is due to skeletal muscles that constantly maintain the tone. 6  muscles of the thorax like the diaphragm & intercostal rib muscles carry out breathing action 7  heat is given off as a by-product of the contraction of skeletal muscles and is needed in thermoregulation 8  speaking, writing, gesturing, texting/typing & facial expressions are due to skeletal muscle contraction 9  The contraction of smooth muscles in the walls of internal organs helps propel & mix food & water in the digestive tract, propel secretions from organs, & regulate blood flow through blood vessels 10  cardiac muscle contraction causes heartbeat & propels blood to all body parts 11  Contractility – the ability of a muscle to shorten forcefully. Excitability – the capacity to respond to stimulus from nerves Extensibility – can be stretched to its normal resting length and beyond Elasticity – the ability to recoil to their original resting length after being stretched Types of Muscle Tissue 13  Are composed of skeletal muscle cells associated with smaller amounts of connective tissue, blood vessels, and 15 nerves.  Epimysium – or Fascia; connective tissue surrounding each skeletal muscle Perimysium – loose connective tissue surrounding a fascicle / fasciculus = muscle bundle composed of several muscle fibers 17 E ndomysium – surrounds Structure of each muscle fiber in a fascicle or fasciculus Skeletal Muscle  Structure of a Muscle Fiber or Muscle Cell Sarcolemma – the cell membrane of a muscle fiber/muscle cell Sarcoplasm – cytoplasm of a muscle fiber T or Transverse Tubules – tubelike invaginations of the sarcolemma that wrap around sarcomeres Sarcoplasmic Reticulum – a highly organized smooth ER; contains a high concentration of calcium ions needed for muscle contraction  Myofibril – threadlike structure contained in the sarcoplasm Myofilaments – protein filaments in the form of actin and myosin contained in each myofibril. Sarcomere – a skeletal muscle's basic structural and functional unit; repeating units along a myofibril; extends from one disk to the next. ✓ I or Light band – consists of only actin myofilaments ✓ A or Dark band – the central region of each sarcomere where actin and myosin overlaps ✓ H zone – second light zone in the center of a sarcomere consisting only of myosin myofilaments ✓ M line – where myosin myofilaments are anchored ✓ Z disk – attachment site for actin myofilaments Thin myofilament resembling two pearl strands twisted together with molecules of: Troponin – attached at specific intervals and provide calcium binding sites on the actin Tropomyosin – filaments located along the groove between the twisted strands of actin; expose attachment  Thick myofilaments resemble bundles of minute golf clubs Its heads can bind to the exposed attachment sites on the actin myofilaments Motor Neuron – nerve cell where action potential travels along in a skeletal muscle fiber Neuromuscular Junction or Synapse – point of contact of a motor neuron with a skeletal muscle; located near the center of a muscle fiber; composed of the presynaptic terminal, synaptic cleft, postsynaptic membrane, synaptic vesicles  Acetylcholine - neurotransmitters found in synaptic vesicles Motor Unit – made up of one motor neuron innervating to several skeletal muscle fibers  Occurs as actin and myosin myofilaments slide past one another, causing the sarcomeres to shorten. Many sarcomeres joined end-to-end form myofibrils. Shortening of the sarcomeres causes myofibrils to shorten. Shortening of myofibrils causes the entire muscle to shorten. 1. Actin and Myosin filaments in a relaxed muscle and a contracted muscle are the same lengths. Myofilaments do not change length during muscle contraction. 2. During contraction, actin myofilaments at each end of the sarcomere slide past the myosin myofilaments toward each other. As a result, the Z disks are brought closer together, and the sarcomere shortens. 3. As the actin myofilaments slide over the myosin filaments, the H zones (yellow) and the I bands (blue) narrow. The A bands, which are equal to the length of the myosin myofilaments, do not narrow, because the length of the myosin myofilaments does not change.  4. In a fully contracted muscle, the ends of the actin myofilaments overlap at the center of the sarcomere and the H zone disappears. 1. During the contraction of a muscle, Ca2+ binds to troponin molecules, and tropomyosin molecules move, causing exposure of myosin attachment sites on myofilaments.  The myosin heads bind to the exposed attachment sites of the actin myofilaments to form cross-bridges, and phosphates are released from the myosin heads.  Energy stored in the myosin heads is used to move the myosin heads (green arrows), causing the actin myofilament to slide past the myosin myofilament (purple arrow), and ADP molecules are released from the myosin heads (black arrows).  ATP molecules bind to the myosin heads.  As ATP is broken down to ADP and phosphates, the myosin heads release from the actin attachment sites.  The heads of the myosin molecules return to their resting position (green arrows), and energy is stored in the heads of the myosin molecules. If Ca2+ is still attached to troponin, cross-bridge formation, and movement are repeated (return to step 2). This cycle occurs many times during a muscle contraction. Not all cross-bridges form and release simultaneously.