Requirements for Movement (HL) (HL IB Biology)

• The effective movement of the human body requires both muscle and an incompressible skeleton
• Bones and exoskeletons provide anchorage for muscles and act as levers
• Vertebrates have internal bones, called an endoskeleton, to support their bodies from the inside with tissues surrounding the bone
• Many organisms, such as arthropods, have external skeletons called exoskeletons which are found on the outside of the organism to protect the internal tissues
• Arthropods that have exoskeletons include:
  • Crustaceans
  • Insects
  • Arachnids
  • Centipedes and millipedes
  • Molluscs
• Exoskeletons are made of polysaccharides called chitin
• Key features of both exo and endo skeletons is that they provide support for the body of the organism whilst also facilitating movement
  • Exoskeletons also provide protection for the body's soft tissues within
• Muscles are anchored to the skeleton either on the inside (as with exoskeletons) or the outside (as with endoskeletons) and the presence of pivot points means that skeletons act as levers transferring the size and direction of force
  • Levers have a point of effort, a point of load and a pivot point called the fulcrum
  • These same three features are seen in skeletons

Levers and Skeleton Diagram

Muscles attach to bones at the joints creating a system of levers

• Arthropod exoskeletons also use the mechanisms of leverage in the same way as animals use with endoskeletons
• They utilise jointed legs and jointed body parts with muscles attached in antagonistic pairs

• Synovial joints are the most common type of joint in the human body
• They are characterised by a joint cavity filled with a lubricating synovial fluid which reduces friction
• The fluid is produced by the synovial membrane, which surrounds the joint
• Synovial joints are capable of a variety of different movements which depends on the structure within the joint including the joint type and the ligaments
• The movements possible at the joint are
  • Flexion
  • Extension
  • Rotation
  • Abduction (the movement of a limb away from the body)
  • Adduction (the movement of a limb towards the body)

Examples of Different Joint Types and their Associated Movements Table

The Human Hip Joint

• The hip joint is a ball and socket synovial joint
• Articulation is between the bones of the femur (the ball), and the pelvis, (the socket)
• Cartilage covers both bones and provides as surface to prevent the bones rubbing against each other
• Synovial fluid is enclosed within the ball and socket by a membrane, it's function is to provide lubrication for smooth movement
• The whole joint is encircled by ligaments which hold the bones in place; ligaments are made of a tough connective tissue
• Skeletal muscles are involved to allow the femur to move within the pelvis socket
• The muscles are connected to each bone via tendons

Hip Ball and Socket Diagram

The structure of the hip synovial joint

• There are over 600 skeletal muscles in the human body
• Muscles are effectors, stimulated by nerve impulses from motor neurones (specialised cells adapted to rapidly carry electrical charges called nerve impulses from sensory neurones to the muscles to bring about movement)
• Lengths of strong connective tissue called tendons, connect muscles to bones
  • They are flexible but do not stretch when a muscle is contracting and pulling on a bone
• Muscles are only capable of contracting or pulling, they cannot push
• As a result of this limitation muscles generally operate in pairs
• One muscle pulls in one direction at a joint and the other muscle pulls in the opposite direction
• This is described as antagonistic muscle action

Internal and external intercostal muscles

• An example of this can be seen in the intercostal muscles of the rib cage
  •  Two sets of intercostal muscles to work antagonistically to facilitate breathing
    • External intercostal muscles, pull the rib cage up
    • Internal intercostal muscles pull the ribcage down
  • During inhalation
    
    • The external set of intercostal muscles contract to pull the ribs up and out:
    • This increases the volume of the chest cavity (thorax)
    • Leading to a decrease in air pressure inside the lungs relative to outside the body
    • Air is drawn in
  • During exhalation
    
    • The external set of intercostal muscles relax so the ribs drop down and in
    • This decreases the volume of the chest cavity (thorax)
    • Leading to an increase in air pressure inside the lungs relative to outside the body
    • Air is forced out
  • Note that the diaphragm also relaxes and contracts during ventilation, but this is not part of an antagonistic muscle pair

• The movement of the rib cage in opposite directions is due to the orientation of the muscle fibres in the internal and external layers of the intercostal muscles
  • As the external intercostal muscles contract, their expansion of the rib cage results in the stretching of the internal intercostal muscles
  • The stretching results in stored potential energy within the titin protein of the sarcomere of the internal intercostal muscles
  • The same is true for the antagonistic action of this muscle pair

Intercostal muscles diagram

The intercostal muscles are an example of an antagonistic muscle pair