Chapter 11 (1)

• Movement is a fundamental characteristic of all living organisms
• Three types of muscular tissue

  • Skeletal
  • Cardiac
  • Smooth muscle
• Universal Characteristics of Muscle

  • Excitability (responsiveness) to chemical signals, stretch, and electrical changes across the plasma membrane
  • Conductivity—local electrical excitation sets off a wave of excitation that travels along the muscle fiber
  • Contractility—shortens when stimulated
  • Extensibility—capable of being stretched between contractions
  • Elasticity—returns to its original rest length after being stretched
• Skeletal Muscle


  • Voluntary, striated muscle usually attached to bones
  • Striations—alternating light and dark transverse bands
  • Results from arrangement of internal contractile proteins
  • Voluntary—usually subject to conscious control
  • Muscle cell is a muscle fiber (myofiber)—as long as 30 cm
• Skeletal Muscle Fibers


  1. Connective tissue wrappings: Endomysium (connective tissue around muscle cell), Perimysium (connective tissue around muscle fascicle), Epimysium (connective tissue surrounding entire muscle)

  2. Tendons are attachments between muscle and bone matrix
• The Muscle Fiber


  • Sarcolemma—plasma membrane of a muscle fiber
  • Sarcoplasm—cytoplasm of a muscle fiber
  • Myofibrils: long protein cords occupying most of sarcoplasm
  • Glycogen: carbohydrate stored to provide energy for exercise
  • Myoglobin: red pigment; provides some oxygen needed for muscle activity
• The Muscle Fiber 2

  • Multiple nuclei—flattened nuclei pressed against the inside of the sarcolemma
  • Myoblasts: stem cells that fused to form each muscle fiber early in development
  • Satellite cells: unspecialized myoblasts remaining between the muscle fiber and endomysium
  • Play a role in regeneration of damaged skeletal muscle tissue
  • Mitochondria—packed into spaces between myofibrils
• The Muscle Fiber 3

  • Sarcoplasmic reticulum (SR)—smooth ER that forms a network around each myofibril: Terminal cisterns—dilated end-sacs of SR which cross the muscle fiber from one side to the other
  • Acts as a calcium reservoir; it releases calcium through channels to activate contraction
  • T tubules—tubular infoldings of the sarcolemma which penetrate through the cell and emerge on the other side
  • Triad—a T tubule and two terminal cisterns associated with it
• Marginal utility is the additional utility (satisfaction) gained from the consumption of an additional product. If you add it up for each unit you get total utility
• Ch blocking six or seven active sites on G actin subunits
• Troponin molecule: small, calcium-binding protein on each tropomyosin molecule
• Elastic filaments


  • Titin: huge, springy protein
  • Run through core of thick filament and anchor it to Z disc and M line
  • Help stabilize and position the thick filament
  • Prevent overstretching and provide recoil
• Contractile proteins—myosin and actin do the work of contraction
• Regulatory proteins—tropomyosin and troponin
• Contraction activation


  1. Activated by release of calcium into sarcoplasm and its binding to troponin

  2. Troponin changes shape and moves tropomyosin off the active sites on actin
• Other proteins associated with myofilaments

  • Dystrophin—clinically important protein
  • Links actin in outermost myofilaments to membrane proteins that link to endomysium
  • Transfers forces of muscle contraction to connective tissue ultimately leading to tendon
  • Genetic defects in dystrophin produce disabling disease muscular dystrophy
• Striations result from the precise organization of myosin and actin in cardiac and skeletal muscle cells
• Striations components


  • A band: dark; “A” stands for anisotropic
  • H band: not as dark; middle of A band; thick filaments only
  • M line: middle of H band
  • I band: light; “I” stands for isotropic
  • Z disc: provides anchorage for thin filaments and elastic filaments
• Sarcomere—segment from Z disc to Z disc
• Muscle cells shorten because their individual sarcomeres shorten
• Neither thick nor thin filaments change length during shortening
• Only the amount of overlap changes during shortening
• During shortening, dystrophin and linking proteins also pull on extracellular proteins
• Transfers pull to extracellular tissue
• Skeletal muscle cannot contract unless stimulated by a nerve
• If nerve connections are severed or poisoned, a muscle is paralyzed
• Denervation atrophy: shrinkage of paralyzed muscle when nerve remains disconnected
• Motor Neurons and Motor Units

  • Somatic motor neurons
  • Nerve cells whose cell bodies are in the brainstem and spinal cord that serve skeletal muscles
  • Somatic motor fibers—their axons that lead to the skeletal muscle
  • Each nerve fiber branches out to a number of muscle fibers
  • Each muscle fiber is supplied by only one motor neuron
• Motor unit—one nerve fiber and all the muscle fibers innervated by it
• Muscle fibers of one motor unit contract in unison
• Muscle fibers of one motor unit produce weak contraction over wide area
• Muscle fibers of one motor unit provide ability to sustain long-term contraction as motor units take turns contracting
• Muscle fibers of one motor unit are dispersed throughout the muscle
• Muscle fibers of one motor unit produce weak contraction over a wide area
• Muscle fibers of one motor unit provide the ability to sustain long-term contraction as motor units take turns contracting
• Effective contraction usually requires the contraction of several motor units at once
• The average motor unit contains 200 muscle fibers
• Small motor units have a fine degree of control with three to six muscle fibers per neuron, found in eye and hand muscles
• Large motor units have more strength than control, with powerful contractions supplied by large motor units with hundreds of fibers, such as in the quadriceps femoris and gastrocnemius which have 1,000 muscle fibers per neuron
• The neuromuscular junction is the point where a nerve fiber meets its target cell