chapter 15 - muscle tissue

• types of muscle cell types

  1. skeletal
  2. cardiac
  3. smooth
• muscle tissue is made up of cells and extracellular matrix
• endomysium: extracellular matrix that surrounds an individual muscle fiber

  perimysium: connective tissue sheath the encloses a bundle of many muscle fibers called a fascicle

  epimysium: all the fascicles in a muscle are surrounded by this layer of connective tissue
• skeletal muscle characteristics:

  • long, cylindrical striated muscle fibers
  • multinucleated
  • attached to skeleton
  • voluntary
  • produces movement
• cardiac muscle characteristics:

  • short, wide, branching striated muscle fibers joined by gap junctions
  • uni- or bi- nucleated
  • located in heart
  • involuntary
  • produces heartbeat
• smooth muscle characteristics:

  • thin, smooth muscle cells joined by gap junctions
  • uni-nucleated
  • located on walls of hollow organs, skin, eyes
  • involuntary
  • changes diameter of hollow organs
  • causes hair to stand up
  • adjusts shape of lens and pupil of the eye
• muscle cell characteristics:

  • contractility: ability of protein fibers within myocytes to draw together
  • excitability: responds to stimuli (electrical or chemical)
  • conductivity: conducts stimulus (electricity)
  • extensibility: can be stretched up to 3x resting length
  • elasticity: ability to regain original state after stretching
• myocyte: muscle cell
• sarcolemma: myocyte's plasma membrane
• sarcoplasmic reticulum (SR): modified endoplasmic reticulum

  • forms web-like network surrounding myofibrils
• myofibrils: unique structures found in all muscle cells

  • contain 100s to 1000s cylindrical organelles making up 50-80% of cell volume
  • allow for contraction
• label

  A) sarcoplasmic

  B) reticulum

  C) sarcolemma

  D) sarcoplasm

  E) myofibril

  F) mitochondrion

  G) nucleus

  
• skeletal muscles are fascicles connected by perimysium and multiple fascicles are bundled together by epimysium
• label parts of skeletal muscle fiber.

  A) muscle fiber

  B) endomysium

  C) sarcoplasmic reticulum

  D) transverse (T) tubule

  E) terminal cisternae

  F) sarcolemma

  G) myofilaments

  H) myofibrils

  I) cytosol

  J) mitochondrion

  K) nucleus

  L) openings of T-tubules

  
• transverse tubules (T-tubules): deep inward extensions of sarcolemma; surround each myofibril

  • form tunnel-like network
  • continuous with exterior of cell; filled with extracellular fluid
• Terminal cisternae: enlarged sections of SR; on sides of T-tubules
• two terminal cisternae plus corresponding T-tubules form triad
• 3 types of myofilaments

  1. thick filaments: bundles of contractile protein myosin
  2. thin filaments: proteins actin, tropomyosin, and troponin
  3. elastic filaments: single massive, spring-like structural protein (titin); stabilizes myofibril structure; resists excessive stretching
• filament proteins:

  1. actin
  2. myosin
  3. titin
  4. tropomosin
  5. troponin
• l band: light regions; only thin filaments
• Z disc: in middle of I band; composed of structural proteins that:

  • anchor thin filaments in place and to one another
  • serve as attachment points for elastic filaments
  • attach myofibrils to one another across entire diameter of muscle fiber
• A band: dark regions; contains zone of overlap with both thick and thin filaments; generate tension during contraction
• H zone: middle of A band where only thick filaments exist
• M line: dark line in middle of A band; structural proteins hold thick filaments in place; serve as anchoring point for elastic filaments
• actin filaments slide along myosin towards M line, pulling Z-discs closer together
• electrical gradient: separation of charged particles (electrolyte pairs) across the plasma membrane (Sarcolemma)
• electrical potential: potential energy due to barrier (sarcolemma) maintaining gradient
• voltage (V): difference in electrical potential between two points
• membrane potential: electrical potential difference either side of the cell membrane
• resting membrane potential for myocytes = -85 mV
• sarcolemmas contain millions of sodium-potassium pumps that help generate resting membrane potentials
• what is an action potential and how is it generated?

  quick, local, temporary change in membrane potential

  1. resting place: gates to channels are closed and maintained by leak channels
  2. depolarization phase: gates open and Na+ enter
  3. repolarization phase: Na+ gates close, K+ gates open and K+ exit down its gradient
  4. Na+/K+ pump restores resting potential
• motor neurons stimulate skeletal muscle to contract
• all skeletal muscle fibers are innervated: supplied with nerves
• describe the neuromuscular junction and how it generates an action potential in the sarcolemma.
• excitation/contraction coupling

  1. action potential from neuron results in acetylcholine secretion in synaptic cleft
  2. ligand gated Na+ channels in sarcolemma open when bound to acetylcholine
  3. generates action potential wave that travels down T-tubules of sarcolemma
  4. T-tubule depolarization causes Ca2+ channels in the terminal cisternae of sarcoplasmic reticulum to open
  5. Ca2+ interacts with troponin, releases tropomyosin and allows myosin heads to interact with actin, resulting in sarcomere contraction
  6. muscle relaxes when acetylcholine no longer present and Ca2+ in sarcoplasm returns to normal
• Contraction phase: The Crossbridge Cycle

  1. ATP hydrolysis "cocks" the myosin heads
  2. myosin heads bind to actin active sites (crossbridge)
  3. myosin releases ADP and P, power stroke pulls actin towards M line
  4. Binding of a new ATP "breaks" crossbridge
  5. cycle starts again
• muscle relaxation:

  1. acetylcholinesterase degrades acetylcholine in snaptic cleft
  2. sacrolemma returns to resting potential, and calcium channels in sarcoplasmic reticulum close
  3. calcium ion pumps return calcium to sarcoplasmic reticulum
  4. troponin and tropomyosin block active sites on actin
• types of energy sources

  1. immediate sources (10s): creatine phosphate (CP) +ADP --> ATP + creatine
  2. anaerobic glucose catabolism (1m): glycolysis; glucose + 2 ATP--> pyruvate--> 2 lactic acid
  3. aerobic glucose catabolism: if the 2 pyruvate made from glycolysis has low oxygen then the Krebs cycle starts and makes 36 ATP
• what is the functional contractile unit of the myofibril?

  sarcomere