Chap 7 Muscular System.docx

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Exercise Science
ES 207
Scott Wersinger

Chapter 7: Muscular System 7.1   Functions of the Muscular System Muscle tissue: specialized for contraction 3 types of muscle tissue 1. Skeletal 2. Cardiac 3. Smooth Figure 7.1 Overview of the Muscular System 7.2    Characteristics of Skeletal Muscle Skeletal muscle: • Skeletal muscle fibers have multiple nuclei o Long, parallel fibers • 40% of body weight • Striated muscle • Transverse bands, or striations, can be seen in the muscle under the microscope • 4 important major functions: o Contractility: ability of skeletal muscle to shorten with force o Excitability: capacity of skeletal muscle to respond to a stimulus o Extensibility: skeletal muscles stretch. After a contraction, skeletal muscles can be stretched to their normal resting length and beyond a limited degree o Elasticity: the ability of skeletal muscles to recoil to their original resting length after they have been stretched Connective tissue coverings of muscle: • Epimysium: surrounds the entire muscle o Fibrous connective tissue layer surrounding a skeletal muscle • Muscle fasciculi: the numerous visible bundles that compose a muscle o Surrounded by the perimysium • Perimysium: surrounds muscle fasciculi o Fibrous sheath enveloping each of the skeletal muscle fasciculus • Muscle fibers: several muscle cells that compose a fasciculus • Endomysium: surrounds fibers o Fine connective tissue sheath surrounding a muscle fiber Figure 7.2 Structure of a Muscle Fascia: an outer coating of connective tissue that helps keep all the other structures that comprise the muscle together Muscle fibers: groups of even smaller structures bundles together by connective tissue • Single, cylindrical fiber • Multi-nuclear • Sarcolemma Sarcolemma: (has to do only with muscle cells) cell membrane of a muscle fiber • Regions: o T-tubules  capable of electrical excitation o Sarcoplasmic reticulum o Membrane that surrounds each muscle fiber o Electrically excitable o Can get action potentials here Transverse tubules (T tubules): occur at regular intervals along the muscle fiber and extend inward into it. • Important for conduction of electrical excitation • Carry an action potential • Associated with a highly organized smooth endoplasmic reticulum • Tube-like invaginations on the surface of the sarcolemma • Connects the sarcolemma to the sarcoplasmic reticulum Sarcoplasmic reticulum: endoplasmic reticulum of a muscle fiber • Part of the muscle where calcium is stored • Has a relatively high concentration of Ca2+, playing a major role in muscle contraction • Network of membrane tissue Sarcoplasm: cytoplasm of a muscle fiber • “Sarco” = muscle cells • “Plasm” = fluid Myofibrils: fine, longitudinal fibril within a skeletal muscle fiber • Consists of sarcomeres both thick (myosin) and thin (actin) myofilaments, placed end to end • Filaments within: o Actin o Myosin Actin myofilaments: thin filaments • Pulls together in contraction • Physically attached to Z line • One of 2 kinds of protein fibers that make up a sarcomere • Resembles 2 strands of minute pearls twisted together • Have attachment sites for the myosin myofilaments Myosin myofilaments: thick filaments • Attaches to actin and pulls it together • Resembling a bundle of golf clubs • One of 2 kinds of protein fibers that make up a sarcomere Actin and Myosin Myofilaments • Long filaments • Together, they relate to contraction • Where they overlap is a “dark band” • Very highly concentrated in muscle cells • Made up of 3 components: o Actin o Troponin: molecules attached at specific interval along the actin myofilaments  Have binding sites for Ca2+ o Tropomyosin: filaments located along the groove between the twisted strands of actin myofilaments subunits  Blocks the myosin myofilaments binding sites on the actin myofilaments  When Ca2+ is present, it binds to troponin, causing the Tropomyosin filaments to expose the attachment sites on the actin myofilaments Myosin heads: the parts of the myosin that resemble golf club heads • 3 important properties o The heads can bind to attachment sites on the actin myofilaments o They can bend to attachment sites on the actin myofilaments o They can breakdown ATP, releasing energy Sarcomere: basic structural and functional unit of skeletal muscle • Every one works the same way • Sarcomeres: part of a myofibril formed of actin and myosin myofilaments, extending from Z disk to Z disk; the structural and functional unit of a muscle • Smallest portion of skeletal muscle capable of contracting • Separate components can slide past each other, causing the sarcomere to shorten • When sarcomeres shorten  myofibrils shorten • Each sarcomere extends from one Z disk to an adjacent Z dick • Z Disk / Z line: a network of protein fibers forming an attachment site for actin myofilaments o Where 2 sarcomeres connect • I band: consists of only actin myofilaments o LIGHT part o Spans each Z disk and ends at the myosin myofilaments o Half of each I band is in a different sarcomere o I band goes away during contraction • H band: consists of only myosin myofilaments o Second light zone in the center of a sarcomere o H zone goes away during contraction • M line: the dark-staining band in the center of the sarcomere where the myosin myofilaments are anchored • A band: overlap of the bands • Striations occur from alternating I bands and A bands of the sarcomeres Figure 7.3 Skeletal Muscle Excitability of Muscle Fibers • Similar to a neuron o Polarization across cell membrane • Polarization o Separation of charges across physical space • Resting membrane potential o When the muscle is sitting there doing nothing, there is going to be a certain separation of charge Polarized: To create a difference in potential (charge) between two points, as between the inside and outside of a cell membrane. Resting membrane potential: Charge difference across the membrane of a resting cell (i.e., a cell that has not been stimulated to produce an action potential). Control of Movement • Motor neuron sends a signal to the skeletal muscle fiber • If these signals cant get through, you cant move because skeletal muscles cannot contract • These motor neurons are excited by signals from our brain to activate motor neurons, causing muscle cells to contract Figure 7.4 Ion Channels and the Action Potential Depolarization: Na+ channels close, and additional K+ channels open Repolarization: the change back to the resting membrane potential Action potential: All-or-none change in membrane potential in an excitable tissue that is propagated as an electrical signal Motor neurons: specialized nerve cells that stimulate muscles to contract • Generate action potentials that travel to skeletal muscle fibers Neuromuscular junction: synaptic junction between a nerve axon and a muscle fiber Synapse: the cell-to-cell junction between a nerve cell and either another nerve cell or an effector cell, such as in a muscle or a gland Motor unit: single motor neuron and all the skeletal muscle fibers it innervates • The spinal motor neuron and all the muscle fibers it innervates • When active, the whole motor unit contracts • Ex: Quad  very large motor unit • Larger motor unit vs. smaller motor unit o Larger motor unit: nice big powerful contraction  Ex: running o Smaller motor unit: fingertips, lips, mouth, tongue  Ex: eating, talking Figure 7.5 Neuromuscular Junction Neuromuscular junction: formed by a cluster of enlarged axon terminals resting in indentation of the muscle fiber’s cell membrane • Connection between a spinal motor neuron and a muscle fiber • Specialization  connection between one branch and muscle fiber is a dark spot called the motor end plate • Presynaptic terminal: an enlarged axon terminal Synaptic cleft: the space between the presynaptic terminal and the muscle fiber membrane Postsynaptic membrane: the muscle fiber membrane Postsynaptic receptors: when acetylcholine binds with these receptors, sodium flows into the sarcolemma, causing excitation, action potential, then contraction Synaptic vesicles: many small vesicles in each presynaptic terminal • Contains acetylcholine (Ach) which functions as a neurotransmitter • Neurotransmitter: Chemical that is released by a presynaptic cell into the synaptic cleft and that acts on the postsynaptic cell to cause a response. Acetylcholinesterase: Enzyme that breaks down acetylcholine to acetic acid and choline. • Stops contractions • Breaks down acetylcholine • Located in phones • Grabs acetylcholine and breaks it apart • When released, it binds to proteins on the sarcolemmic receptors • These receptors cause an action potential in the sarcolemma, eventually resulting in the release of calcium from the sarcoplasmic reticulum • As long as there is calcium and ATP in the sarcomere, there will be a contraction • To relax, I need to get rid of all calcium and neurotransmitter Sequence of events 1. Action potential reaches terminal 2. Neurotransmitter (acetylcholine) is released 3. Ach diffuses across the cleft and binds to nicotinic acetylcholine receptors (nAChRs) 4. Sodium flows into the muscle fiber, resulting in an action potential in the muscle fiber 5. Muscle contracts (sliding-filament model) 6. ACh is broken down by acetylcholinesterase a. Action potentials go away 7. nAChrs close, and the muscle relaxes Figure 7.6 Function of the Neuromuscular Junction Muscle contraction: occurs as actin and myosin myofilaments slide past one another, causing the sarcomeres to shorten. Sliding filament model: Mechanism by which actin and myosin myofilaments slide over one another during muscle contraction. • Myofilaments are composed of strings of the proteins; actin & myosin • Actin filament  has 2 strands of actin molecules wrapped together • Myosin filament  has many myosin proteins packed together o Each myosin protein has a globular “head” region that protrudes from the filament • Myofilaments can contract/shorten, due to interactions between the myosin “heads” and the actin filaments • Contraction begins with the head of myosin molecules bound to actin on the actin filament • While still bound to actin the myosin head flexes, pulling the actin filament along with it o This causes the actin filament to slide by the myosin filament • Next, the myosin head releases from the actin and unflexes, a change that is powered by ATP o This frees the myosin head to bind with a different actin molecule, farther up the actin filament • ATP is not used when myosin attaches, but when we “let go” o Getting myosin to let go is what costs us energy Sequence of events • Action potential reaches neuromuscular junction • Ca++ is released from the sarcoplasmic reticulum o Sarcoplasmic reticulum spits out calcium • Ca++ binds to troponin • Tropomyosin moves, exposing myosin attachment sites • Myosin heads bind to actin o Results in cross-bridge • Myosin ratchets, pulling actin filament o Also known as “power stroke” • ATP is used to release myosin head from actin o ATP binds, changes shape of myosin, releasing actin  Without ATP, myosin cannot release, causing rigidity • Repeat until out of calcium or ATP o If we run out of calcium, the Tropomyosin is hiding the binding sites on the actin filament, not allowing contraction o If we run out of ATP, we never let go EXAMPLE QUESTIONS • If we took a myofibril and injected a whole bunch of calcium into it, what would happen? o It is going to contract because calcium is going to bind from troponin, exposing myosin attachment sites, etc. • If a person doesn’t have enough calcium? o Trouble with muscular contraction, because calcium is a key component of contraction Figure 7.7 Sarcomere Shortening Cross-bridges: when the exposed attachment sites on the actin myofilaments bind to the heads of the myosin myofilaments to form cross-bridges between the actin and myosin myofilaments Figure 7.8 Summary of Skeletal Muscle Contraction Rigor mortis: Condition in which ATP is not available, and the cross-bridges that have formed are not released, causing the muscles to become rigid Figure 7.9 Breakdown of ATP and Cross-Bridge Movement During Muscle Contraction Muscle twitch: the contraction of a muscle fiber in response to a single stimulus • Typically, the entire motor unit contracts simultaneously • Has 3 phases o Lag phase/Latent period: the time between the application of a stimulus and the beginning of contraction  Action potentials are produced in one or more motor neurons o Contraction phase: the time during which the muscle contracts  Force strengthens until peak is reached o Relaxation phase: the time during which the muscle relaxes  Ca2+ is actively transported back into the sarcoplasmic reticulum Figure 7.10 Phases of a Muscle Twitch The strength of muscle contractions varies from weak to strong All-or-None Law: applies to individual myofibrils NOT the entire muscle • This is because if you pick up something light, you don’t need as much force as if you were picking up a tire • Strength of muscle’s contraction has to do with summation of an individual myofibril’s contraction as well as the number of myofibrils that contract Summation: the force of contraction of individual muscle fibers is increased rapidly stimulating them • As stimulus frequency increases, there is not enough time between contractions for muscle fibers to relax completely • Weak force of contraction  only a few myofibrils are contracting • Strong force lots of myofibrils are contracting • If we keep adding force, you will eventually reach a maximum point to the point where your muscles are contracting at full force (tetany) o We don’t want this to happen, because it will rip muscles off of bone o Tendons could potentially pull off from bones Tetanus: a sustained contraction that occurs when the frequency of stimulation is so rapid that no relaxation occurs • Full contraction of muscle Figure 7.11 Multiple-Wace Summation Recruitment: the number of muscle fibers contracting is increased by increasing the number of motor units stimulated, and the muscle contracts with more force Aerobic respiration: requires O2 and breaks down glucose to produce ATP, CO2, and H2O • Takes place in mitochondria located within the muscle fiber sarcoplasm between the myofibrils • Muscle fibers depend on large amounts of O2 and thus contain large numbers of mitochondria • Much more efficient that anaerobic respiration • We don’t always use aerobic respiration because it requires O2, and we can’t always get enough oxygen to get to our muscles Anaerobic respiration: does NOT r
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