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Chapter 12

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University of Waterloo
BIOL 273
Dragana Miskovic

Chapter 12: Muscles o Our muscles have two common functions: to generate motion and to generate force o Three types of muscle tissues: Skeletal, cardiac, and smooth o Skeletal muscles: attached to the bones of the skeleton, enabling these muscles to control body movement  Fibers are large, multinucleate cells o Cardiac muscle: is found only in the heart and moves blood through the circulatory system  Fibers are striated, small, branched and uninucleate. Cells are joined by junctions called intercalated disks o Striated muscles: alternating light and dark bands seen under the light microscope (Skeletal and cardiac muscles only) o Smooth muscle: primary muscles of internal organs and tubes (e.g. stomach, urinary bladder, blood vessels)  Primary function is to influence the movement of material into, out of, and within the body  Lacks striated muscles: results from the less organized arrangement of contractile fiber Skeletal Muscle o Make up the bulk of muscle in the body o Skeletal muscles are usually attached to bones by tendons made of collagen o The origin of a muscle is the end of the muscle that is attached closest to the trunk or to the more stationary bone o The insertion of the muscle is more mobile o Flexor: the centers of the connected bones are brought closer together when the muscle contracts o Extensor: the bones move away from each other when the muscle contracts o Antagonistic muscle groups: flexor-extensor pairs that exert opposite effects. Skeletal Muscles Are Composed of Muscle Fibers o A skeletal muscle is a collection of muscle cells, or muscle fibers  Long cylindrical cell with up to several hundred nuclei near the surface of the fiber  Are the largest cells in the body, created by the fusion of many individual embryonic muscle cells  Satellite cells: lie outside the muscle fiber membrane, activate and differentiate into muscle when needed for muscle growth and repair  Each skeletal muscle fiber is sheathed in connective tissue, with groups of adjacent muscle fibers bundled together into units called fascicles. o Muscle Fiber Anatomy  Sarcolemma: the cell membrane of a muscle fiber  Sarcoplasm: the cytoplasm of a muscle fiber  Myofibrils: the main intracellular structures in striated muscles, highly organized bundles of contractile and elastic proteins that carry out the work of contraction  Sarcoplasmic reticulum: a form of modified endoplasmic reticulum that wraps around each myofibril like a piece of lace.  Consists of longitudinal tubules with enlarged end regions called the terminal cisternae  Concentrates and sequesters Ca with the help of Ca-ATPase in the SR membrane  T-tubules: a branching network that allow action potentials to move rapidly from the cell surface onto the interior of the fiber so that they reach the terminal cisternae nearly simultaneously.  One t-tubule and its two flanking terminal cisternae are called a triad. Myofibrils Are Muscle Fiber Contractile Structures o Myosin: a motor protein with the ability to create movement. o Each myosin molecule is composed of protein chains that intertwine to form a long tail and a pair of tadpole-like heads o The tail is stiff but the myosin heads have an elastic hinge region where the heads join the rods. o Each myosin head has two protein chains: a heavy chain and a smaller light chain  The heavy chain is the motor domain that binds ATP and uses the energy from ATP’s high-energy phosphate bond to create movement  The heavy chain contains a binding site for actin o In skeletal muscle, about 250 myosin molecules join to create a thick filament. Each thick filament is arranged so that the myosin heads are clustered at each end of the filament, and the central region of the filament is a bundle of myosin tails. o Actin: is a protein that makes up the thin filaments of the muscle fibers o The parallel thick and think filaments of the myofibril are connected by myosin crossbridges that span the space between the filaments o The arrangement of thick and thin filaments in a myofibril creates a repeating pattern of alternating light and dark bands  Z disks: zigzag protein structures that serve as the attachment site for thing filaments  I bands: lightest color bands and represents a region occupied only by thin filaments. Half of an I band belongs to a different sacromere  A band: darkest bands and encompasses the entire length of a thick filament  H zone: this central region of the A and is lighter than the outer edges of the A band  M line: represents proteins that form the attachment site for thick filaments. Divides an A band in half. o Titin has two functions: it stabilizes the position of the contractile filaments and its elasticity returns stretched to their resting length. Titin is helped by nebulin, an inelastic giant protein that lies alongside thin filaments and attaches to the Z disk It helps align the actin filaments of the sarcomere. Muscle Contraction Creates Force o Muscle tension: the force created by contracting muscle o Load: a weight or force that opposes contraction of a muscle o Contraction: the creation of tension in a muscle, is an active process that requires energy input from ATP. o Relaxation: the release of tension created by contraction o Steps leading to skeletal muscle contraction 1. Events at the neuromuscular junction convert an acetylcholine signal from a somatic motor neuron into an electrical signal in the muscle fiber 2. Excitation-contraction coupling is the process in which muscle action potentials initiate calcium signals that in turn activate a contraction-relaxation cycle 3. At the molecular level a contraction-relaxation cycle can be explained by the sliding filament theory of contraction. In intact muscles, one contraction-relaxation cycle is called a muscle twitch. Actin and Myosin Slide Past Each Other During Contraction o Sliding filament theory of contraction: overlapping actin and myosin filaments of fixed length slide past one another in an energy-requiring process, resulting in muscle contraction o It explains how a muscle can contract and create force without creating movement o Tension generated in a muscle fiver is directly proportional to the number of hgh-force crossbridges between the thick and thin filaments o The movement of myosin crossbridges provides force that pushes the actin filament during contraction o Myosin heads bind to actin molecules. A calcium signal initiates the power stroke, when myosin crossbridges swivel and push the actin filaments toward the center of the sarcomere. At the end of the power stoke, each myosin head releases actin, then swivels back and binds to a new actin molecule, ready to start another contractile cycle. Calcium Signals Initiate Contraction o Troponin: a calcium-binding complex of three proteins. It controls the positioning of an elongated protein polymer, tropomyosin. o In resting skeletal muscle, tropomyosin wraps around actin filaments and partially covers actin’s myosin-binding sites. o Myosin is blocked from completing its power stroke. o The off-on positioning of tropomyosin is regulated by troponin  Troponin C binds reversibly to Ca  The calcium-troponin C complex pulls tropomyosin completely away from actin’s myosin- binding sites  This “on” position enables the myosin heads to form strong, high-force crossbridges and carry out their power strokes  Moving the actin filament  Contractile cycles repeat as long as the binding sites are uncovered Myosin Heads Step Along Actin Filaments o Rigor state: the myosin heads are tightly bound to G-actin molecules. The rigor state occurs for only a very brief period. Then:  ATP binds and myosin detaches.  ATP hydrolysis provides energy for the myosin head to rotate and reattach to actin  The power stroke begins after Ca binds to troponin to uncover the rest of the myosin- binding site. Release of Pi allows the myosin head to swivel toward the M lone, sliding the attached actin filament along with them.  Myosin release ADP. With ADP gone, the myosin head is again tightly bound to actin in the rigor state. Acetylocholine Initiates Excitation-Contraction Coupling o Excitation-contraction coupling is the combination of electrical and mechanical events ina muscle fiber o Four major events: 1. ACh is released from the somatic motor neuron  Released into the synapse at the neuromuscular junction binds to Ach receptor- channels on the motor end plate of the muscle fiber. 2. ACh initiates an action potential in the muscle fiber  The addition of net positive charge to the muscle fiber depolarizes the membrane, creating an end-plate potential  Net entry of Na+ through Ach receptor-channel initiates a muscle action potential. 3. The muscle action potential triggers calcium release from the SR.  The action potential that moves down the t-tubules causes CA release from the SR. When cytosolic CA levels are high, Ca binds to troponin, tropomyosin moves to the “on” position and contraction occurs
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