Chapter 8 Skeletal and Muscular System

• Skeletal and muscular system

  The system in animals that provides support, protection, and movement
• Lesson outcomes


  • Function and types of skeleton
  • Types of joints
  • Organization of animal skeletal muscle
  • Labelled structure and function of sarcomere
  • The Sliding Filament Model
• Functions of animal skeletons

  • Support
  • Protection
  • Movement
• Types of skeletons


  • Hydrostatic skeletons
  • Exoskeletons
  • Endoskeletons
• Hydrostatic skeleton


  Consists of fluid held under pressure in a closed body compartment
• Hydrostatic skeletons are the main type in most cnidarians, flatworms, nematodes, and annelids
• Animals with hydrostatic skeletons control their form and movement by using muscles to change the shape of fluid-filled compartments
• Exoskeleton


  A hard encasement deposited on the surface of an animal
• Exoskeletons are found in most molluscs and arthropods
• Molluscs with exoskeletons


  • Enclosed in a calcium carbonate shell secreted by the mantle
• Arthropod exoskeletons


  • Composed of cuticle (outer layer)
  • 30-50% of cuticle consists of chitin
  • Vary in hardness and degree of flexibility
• Arthropods must shed their exoskeleton and produce a larger one when they have a growth spurt
• Endoskeleton


  Hard supporting elements such as bones, buried within the soft tissue of an animal
• Endoskeletons are found in sponges, echinoderms, and chordates
• Mammalian skeleton


  • Built from more than 200 bones
  • Some fused together and others connected at joints by ligaments that allow freedom of movement
• In addition to the skeleton, muscles and tendons help support large land vertebrates
• Muscle action


  Muscles always contract
• Skeletal muscles


  Attached to the skeleton in antagonistic pairs, with each member of the pair working against each other
• Muscles move skeletal parts

  1. Muscle contracts

  2. Muscle relaxes
• Vertebrate skeletal muscle


  • Characterized by a hierarchy of smaller and smaller units
• Skeletal muscle


  Consists of a bundle of long fibers running parallel to the length of the muscle
• Muscle fiber


  A bundle of smaller myofibrils arranged longitudinally
• Myofibrils


  • Composed of two kinds of myofilaments: thin filaments (actin) and thick filaments (myosin)
• Skeletal muscle


  Also called striated muscle due to the regular arrangement of the myofilaments creating a pattern of light and dark bands
• Sarcomere


  The repeating unit of skeletal muscle, bordered by Z lines
• Sarcomere structure


  • Thick filaments attached at the M lines centered in the sarcomere
  • I band, A band, and H zone contain the myofilaments
• Muscle contraction


  Thick and thin filaments slide past each other longitudinally, producing more overlap
• Sliding of filaments


  Causes the I band and H zone to shrink
• Sliding-filament model


  Explains how the interaction between actin and myosin molecules of the thick and thin filaments causes muscle contraction
• Myosin-actin interactions


  The "head" of a myosin molecule binds to an actin filament, forming a cross-bridge and pulling the thin filament toward the center of the sarcomere
• Regulatory proteins


  Tropomyosin blocks myosin-binding sites on the thin filament when the muscle is at rest
• Muscle fiber contraction


  Calcium ions bind to the troponin complex, causing the proteins bound along the actin strands to shift position and expose the myosin-binding sites
• Stimulus for muscle contraction

  An action potential in a motor neuron that makes a synapse with the muscle fiber
• Nervous control of muscle tension

  1. Motor neuron releases acetylcholine, depolarizing the muscle and causing it to produce an action potential

  2. Action potential travels along T tubules, triggering release of calcium from sarcoplasmic reticulum

  3. Calcium binds to troponin-tropomyosin complex, exposing myosin-binding sites and allowing cross-bridge cycle to proceed