Muscular System Powerpoint

• Muscles


  Responsible for all types of body movement
• Three basic muscle types found in the body


  • Skeletal muscle
  • Cardiac muscle
  • Smooth muscle

  
• Skeletal and smooth muscle cells


  • Elongated (muscle cell = muscle fiber)
  • Contraction and shortening due to movement of microfilaments

  
• Prefixes myo- and mys-


  Refer to "muscle"
• Prefix sarco-


  Refers to "flesh"
• Terminology shared by all muscles


  • Prefixes myo- and mys- (refer to "muscle")
  • Prefix sarco- (refers to "flesh")
• Skeletal muscle


  • Most skeletal muscle fibers are attached by tendons to bones
  • Skeletal muscle cells are large, cigar-shaped, and multinucleate
  • Also known as striated muscle and voluntary muscle

  
• Skeletal muscle cells


  • Surrounded and bundled by connective tissue
  • Endomysium - encloses a single muscle fiber
  • Perimysium - wraps around a fascicle (bundle) of muscle fibers
  • Epimysium - covers the entire skeletal muscle
  • Fascia - on the outside of the epimysium

  
• Tendons


  Cordlike structures, mostly collagen fibers, often cross a joint because of their toughness and small size
• Aponeuroses


  Sheetlike structures that attach muscles indirectly to bones, cartilages, or connective tissue coverings

  
• Smooth muscle


  • No striations, involuntary, found mainly in the walls of hollow visceral organs, spindle-shaped fibers that are uninucleate, contractions are slow and sustained

  
• Cardiac muscle


  • Striations, involuntary, found only in the walls of the heart, uninucleate, branching cells joined by gap junctions called intercalated discs, contracts at a steady rate set by pacemaker
• Skeletal muscle


  • Maintain posture and body position
  • Stabilize joints
  • Generate heat

  
• Sarcolemma


  Specialized plasma membrane
• Myofibrils


  Long organelles inside muscle cell
• I band


  Light band, contains only thin filaments, Z disc is a midline interruption
• A band


  Dark band, contains the entire length of the thick filaments, H zone is a lighter central area, M line is in center of H zone
• Sarcomere


  Contractile unit of a muscle fiber, structural and functional unit of skeletal muscle

  
• Thick filaments


  Myosin filaments, composed mostly of the protein myosin, contain ATPase enzymes to split ATP to release energy for muscle contractions, possess projections known as myosin heads, myosin heads are known as cross bridges when they link thick and thin filaments during contraction
• Thin filaments


  Actin filaments, composed mostly of the contractile protein actin, actin is anchored to the Z disc
• At rest, within the A band


  There is a zone that lacks actin filaments called the H zone
• During contraction


  H zones disappear as actin and myosin filaments overlap
• Sarcoplasmic reticulum (SR)


  Specialized smooth endoplasmic reticulum that surrounds the myofibril, stores and releases calcium
• Skeletal muscles


  • Irritability (also called responsiveness) - ability to receive and respond to a stimulus
  • Contractility - ability to forcibly shorten when an adequate stimulus is received
  • Extensibility - ability to be stretched
  • Elasticity - ability to recoil and resume resting length after stretching
• Motor unit


  One motor neuron and all the skeletal muscle cells stimulated by that neuron
• Neuromuscular junction


  Association site of axon terminal of the motor neuron and sarcolemma of a muscle
• Neurotransmitter


  Chemical released by nerve upon arrival of nerve impulse in the axon terminal, acetylcholine (ACh) is the neurotransmitter that stimulates skeletal muscle
• Synaptic cleft


  Gap between nerve and muscle filled with interstitial fluid, nerve and muscle do not make contact
• Events at the neuromuscular junction

  1. Nerve impulse reaches the axon terminal

  2. Calcium channels open, and calcium ions enter the axon terminal

  3. Calcium ion entry causes some synaptic vesicles to release acetylcholine (ACh)

  4. ACh diffuses across the synaptic cleft and attaches to receptors on the sarcolemma

  5. If enough ACh is released, the sarcolemma becomes temporarily more permeable to sodium ions (Na+), more sodium ions enter than potassium ions leave, entry of sodium ions produces an imbalance in which interior has more positive ions (depolarization), thereby opening more Na+ channels

  6. Depolarization opens more sodium channels that allow sodium ions to enter the cell, an action potential is created, acetylcholinesterase (AChE) breaks down acetylcholine into acetic acid and choline

  7. Cell returns to its resting state when potassium ions (K+) diffuse out of the cell and the sodium-potassium pump moves sodium and potassium ions back to their original positions
• A single nerve impulse


  Produces only one contraction
• Once begun, the action potential is unstoppable
• The action potential conducts the electrical impulse from one end of the cell to the other
• Acetylcholinesterase (AChE)


  Ends muscle contraction
• Comparing the action potential to a flame consuming a dry twig
• Neuromuscular junction


  The site where a motor neuron connects with a muscle fiber
• Events at the neuromuscular junction


  Shown in Figures 6.5(1-8)
• Sliding filament theory


  Mechanism of muscle contraction
• Mechanism of muscle contraction

  1. Calcium ions (Ca2+) bind regulatory proteins on thin filaments and expose myosin-binding sites

  2. Myosin heads on thick filaments attach

  3. Cross bridges pivot, causing thin filaments to slide toward center of sarcomere

  4. Contraction occurs, cell shortens

  5. Cross bridges attach and detach several times

  6. ATP provides energy for sliding process, continues as long as calcium ions are present
• Muscle fiber contraction is "all-or-none"
• Graded responses


  Different degrees of skeletal muscle shortening