muscle overview 3 different types of muscle tissue provide movement: –skeletal (attached to the...

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  • Slide 1
  • Muscle Overview 3 different types of muscle tissue provide movement: Skeletal (attached to the bones of the skeleton) controlled consciously (voluntary) Cardiac (heart) controlled unconsciously (involuntary) Smooth (airways of the lungs, blood vessels, the digestive, urinary, and reproductive tracts) controlled unconsciously (involuntary) Prefixes sarco- flesh sarcoplasm = cytoplasm of a muscle cell (fiber) my- muscle myocyte = muscle fiber
  • Slide 2
  • Characteristics of Muscle Tissue Irritability the ability to receive and respond to stimuli Conductivity the ability to conduct an electrical impulse called an action potential along the cell membrane an action potential (AP) is caused by the diffusion of ions (typically Na + and K + ) across the cell membrane through opened gated ion channels Contractility the ability to shorten forcibly Extensibility the ability to be stretched or extended Elasticity the ability to recoil after being stretched
  • Slide 3
  • Microscopic Anatomy of a Skeletal Muscle Fiber Each fiber is long (up to 30 cm) and cylindrical with multiple nuclei just beneath the cell membrane the cell membrane contains ion channels capable of generating an action potential portions of the cell membrane called transverse (t) - tubules fold inward toward the center of the fiber Occupying most of the space within the cell are long filamentous contractile proteins that are organized into bundles called myofibrils each myofibril is composed of 2 types of proteins (myofilaments) that overlap and slide past one another during contraction and relaxation thin thick
  • Slide 4
  • Microscopic Anatomy of a Skeletal Muscle Fiber
  • Slide 5
  • Striations of Skeletal Muscle When viewed longitudinally, the overlapping arrangement of myofilaments creates a repeating pattern of dark and light striations (stripes) called sarcomeres the contractile unit of skeletal (and cardiac) muscle
  • Slide 6
  • Sarcomeres
  • Slide 7
  • Segments of a Sarcomere Z disc a protein that creates a thin, dark vertical line in the middle of a light vertical band the distance between successive Z discs represents the length of a single sarcomere anchors the thin filaments during contraction A band the length of the thick filaments I band the length of thin filaments within a sarcomere that is not overlapping with the thick filaments H (bare) zone the length of thick filaments within in a sarcomere that is not overlapping with the thin filaments
  • Slide 8
  • Motor Unit: The Nerve-Muscle Functional Unit In order to contract, a skeletal muscle must be stimulated by a motor neuron The location where the end of a motor neuron and a skeletal muscle fiber meet is called the neuromuscular junction (NMJ) A single motor neuron is capable of stimulating multiple skeletal muscle fibers to contract simultaneously one neuron branches allowing it to stimulate multiple muscle fibers simultaneously the anatomical relationship between a single motor neuron and all skeletal fibers that it controls is called a motor unit
  • Slide 9
  • Motor Unit: The Nerve-Muscle Functional Unit
  • Slide 10
  • Slide 11
  • The number of muscle fibers per motor unit can range: few (small motor unit) control fine, precise movements (fingers, eyes) several hundred (large motor unit) control gross movements (arms, legs) large weight-bearing muscles (back)
  • Slide 12
  • Muscle Twitch The contraction followed by the relaxation of a muscle fiber to a single, brief stimulus by a motor neuron is called a twitch There are three phases of a muscle twitch Latent (lag) period time between the stimulation by a motor neuron and the beginning of contraction (few milliseconds) Contractile period contractile proteins within the fiber hydrolyze ATP causing the fiber to shorten resulting in an increase in tension (force) Relaxation period fiber lengthens causing tension to decrease
  • Slide 13
  • Muscle Twitch
  • Slide 14
  • The Neuromuscular Junction Between the motor neuron and the skeletal muscle fiber is a small space called a symaptic cleft A motor neuron stimulates the contraction of a skeletal muscle fiber by exocytosing a chemical messenger called a neurotransmitter into the synaptic cleft The specific neurotransmitter released onto skeletal muscle fibers is called acetylcholine (ACh) Acetylcholine diffuses through the ECF within the cleft and binds to integral membrane proteins of the skeletal muscle fiber called ACh receptors The binding of ACh to ACh receptors creates an action potential in the cell membrane of the skeletal muscle fiber which will ultimately cause the cell to elicit a twitch Linking the action potential to the contraction of a muscle fiber is called excitation-contraction coupling
  • Slide 15
  • NMJ Function
  • Slide 16
  • Muscle Fiber Relaxation After ACh creates an action potential in the fiber it is rapidly hydrolyzed into acetate and choline by the enzyme Acetylcholine esterase located in the synaptic cleft of the NMJ prevents prolonged stimulation (and contraction) of a skeletal muscle fiber allowing it to relax
  • Slide 17
  • Skeletal muscle fibers contain an elaborate, smooth sarcoplasmic (endoplasmic) reticulum (SR) which is the storage site of intracellular calcium (Ca +2 ) Action potentials travel along the sarcolemma into the t-tubules which open Ca 2+ channels in the SR to open resulting in the diffusion of Ca 2+ out of the SR into the sarcoplasm
  • Slide 18
  • Sliding Filament ModelSliding Filament Model of Contraction An increase in the amount of Ca 2+ in the sarcoplasm, allows the thick filaments to pull the thin filaments toward the center of the sarcomere causing the sarcomere to shorten As all of the sarcomeres in a muscle shortens, the entire muscle shortens
  • Slide 19
  • Structure of Thick Filaments Thick filaments are composed of many molecules of the protein myosin Each myosin protein has a rodlike tail and two heads Myosin heads: hydrolyze a molecule of ATP uses the chemical energy to contract attach to and pull on the protein actin of thin filaments causing the sarcomere to shorten
  • Slide 20
  • Structure of Thick Filaments
  • Slide 21
  • Structure of Thin Filaments Thin filaments are composed of 3 proteins F (fibrous) Actin is a helical polymer of G (globular) actin protein subunits each subunit contains a binding site for the protein myosin of the thick filaments Tropomyosin blocks the interaction between actin and myosin prevents an unstimulated muscle from contracting Troponin C is attached to tropomyosin binds to Ca 2+ in the sarcoplasm during contraction
  • Slide 22
  • Structure of Thin Filaments
  • Slide 23
  • Excitation-Contraction Coupling Ca 2+ in the sarcoplasm binds to troponin C changes the position of troponin C moves tropomyosin away from the myosin binding site on actin promoting the interaction between myosin and actin (CONTRACTION) Linking the action potential to the contraction of a muscle fiber is called excitation-contraction coupling
  • Slide 24
  • Molecular Events of Contraction Myosin pulls on actin in a repetitive (cyclic) fashion progressively moving the thin filaments toward the center of the sarcomere Each cycle consists of 4 steps 1.Activation of the myosin head a molecule of ATP is hydrolyzed and the energy is used by the myosin head to change the shape of myosin into the high-energy state 2.Cross bridge formation myosin cross bridge attaches to actin filament 3.Power stroke myosin head pivots and pulls thin filament 4.Cross bridge detachment the binding of a molecule of ATP to the myosin head causes it to detach from actin
  • Slide 25
  • (Cross Bridge Cycling)Cross Bridge Cycling
  • Slide 26
  • Muscle Fiber Relaxation Within the membrane of the SR is a primary active transporting pump called the Ca 2+ -ATPase The Ca 2+ -ATPase constantly pumps Ca 2+ out of the sarcoplasm into the SR During an action potential, Ca 2+ diffuses into the sarcoplasm faster than the Ca 2+ -ATPase can remove it. However, when the action potential is over the Ca 2+ -ATPase pumps the Ca 2+ back into the SR ending contraction
  • Slide 27
  • Contraction of Skeletal Muscle The two types of muscle contractions are: Isometric contraction = same length muscle contracts and produces tension, but does not shorten trying to lift a car Isotonic contraction = same tension muscle contracts and produces tension shortens as it contracts lifting a pencil
  • Slide 28
  • Isometric Contractions Isometric contraction = same length muscle contracts and produces tension, but the muscle but does not shorten or lengthen
  • Slide 29
  • Isotonic Contractions Isotonic contraction = same tension muscle contracts and produces tension shortens as it contracts, but maintains a constant tension as it shortens
  • Slide 30
  • Types of Skeletal Muscle Fibers There are 3 different types skeletal muscle fibers slow oxidative fibers fast oxidative fibers fast glycolytic fibers Slow fibers have a slow twitch speed (use ATP slowly) Fast fibers have a fast twitch speed (use ATP quickly) Oxidative fibers contain an iron complexed protein called myoglobin (provides a darker color to fibers) which binds oxygen to maintain

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