Skeletal

1
Skeletal Muscular Systems

• Chapter 49

2
Types of Skeletons

• hydrostatic skeleton fluid-filled closed
  chamber
• internal pressures generated by muscle
  contractions change shape of fluid-filled chamber
  to control shape movement
• well suited for life in aquatic environment
• animals cnidarians, flatworms, roundworms,
  annelids
• exoskeleton hard encasement deposited on
  surface of animal
• animals mollusks (shells made of calcium
  carbonate) arthropods (cuticle made of chitin)
• endoskeleton hard supporting elements buried
  within soft tissue of animal
• animals sponges (spicules), echinoderms
  (ossicles), chordates (cartilage and/or bone)

3
Mammalian Skeleton

• more than 200 bones
• divided into two main parts
• axial skeleton
• skull, backbone, rib cage
• appendicular skeleton
• limb bones, pelvic girdle, pectoral girdle

4
Joints

• provide flexibility for body movements
• ball-and-socket joints enable rotation by
  allowing for movement in several planes
• hinge joints restrict movement to a single
  plane provide back forth movement
• pivot joints allow for side-to-side rotation

5
Types of Muscle

• skeletal muscle
• attached to bones responsible for bone movement
• striated
• contracts only when stimulated by a motor neuron
• smooth muscle
• found in walls of hollow organs (blood vessels,
  digestive tract organs)
• no striations
• cardiac muscle
• only found in heart
• striated
• can trigger action potentials without input from
  nervous system

6
Skeletal Muscle

• consists of bundles of long muscle fibers running
  parallel to the length of the muscle
• each muscle fiber is a single cell with multiple
  nuclei composed of a bundle of myofibrils
• myofibrils are made of two types of myofilaments
• thin filaments (actin) thick filaments (myosin)

• skeletal muscle looks striated because
  myofilaments create a pattern of light dark
  bands

7
Sarcomeres

• myofibrils are divided into sarcomeres the
  basic contractile units of the muscle
• the borders between each sarcomere Z lines
• actin myosin filaments do not overlap
  completely when the muscle is at rest which
  creates
• I bands regions at edge of sarcomere that
  contain only actin filaments
• A bands region that corresponds to the length
  of the myosin filaments
• H zone region in middle of sarcomere that
  contains only myosin filaments

8
Sarcomere Structure
9
Muscle Contraction

• muscle contraction is caused by the shortening of
  sarcomeres
• this is not caused by the shortening of either
  actin or myosin filaments
• sliding-filament model
• actin myosin filaments slide past each other
  causing the I and H zones to shrink
• sliding is a result of interactions between actin
  myosin molecules (see next slide)

10
(myosin)
(actin)
11
Regulating Muscle Contraction

• at rest, the myosin-binding sites on the actin
  filaments are blocked by a regulatory protein
  called tropomyosin
• upon stimulation by a motor neuron, Ca2 binding
  to another regulatory protein complex called
  troponin causes the tropomyosin to move, exposing
  the myosin-binding sites

12
Neuro-Muscular Junction

• action potentials are triggered in muscle fibers
  when acetylcholine (a neurotransmitter) is
  released by a motor neuron and binds to receptor
  proteins on the muscle fibers plasma membrane
• the action potential reaches interior muscle
  fibers by traveling through transverse (T)
  tubules
• the action potential triggers the release of Ca2
  ions from specialized endoplasmic reticulum
  called sarcoplasmic reticulum (SR)
• Ca2 binding to troponin exposes mysosin-binding
  sites on actin filaments
• once action potential ends, Ca2 is removed by SR
  so myosin filaments actin filaments can no
  longer interact (contraction stops muscle fiber
  relaxes)

13
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