11.2 - Movement

Essential Idea: The roles of the musculoskeletal system are movement, support and protection. 11.2 Movement Understandings: Bones and exoskeletons provide anchorage for muscles and act as levers. Synovial joints allow certain movements but not others. Movement of the body requires muscles to work in antagonistic pairs. Skeletal muscle fibres are multinucleate and contain specialized endoplasmic reticulum. Muscle fibres contain many myofibrils. Each myofibril is made up of contractile sarcomeres. The contraction of the skeletal muscle is achieved by the sliding of actin and myosin filaments. ATP hydrolysis and cross bridge formation are necessary for the filaments to slide. Calcium ions and the proteins tropomyosin and troponin control muscle contractions. Application: Skills: Antagonistic pairs of muscles in an insect leg. Annotation of a diagram of the human elbow. Drawing labelled diagrams of the structure of a sarcomere. Analysis of electron micrographs to find the state of contraction of muscle fibres.

A. Provide support and attachment points for muscles – act as levers Skeletons A. Provide support and attachment points for muscles – act as levers B. Exoskeletons – shell, usually of chitin 1. Exoskeletons are found in many invertebrates including arthropods, corals, and molluscs. 2. In animals with exoskeletons, the muscles for movement are attached to the inner surface of the exoskeleton.

Endoskeleton – inside the body 1. Found in vertebrates 2. Acts as an attachment site for muscles and provides a means to transmit muscular force. 3. Muscles pull on the skeleton to create movement about joints. 4. Muscles work in opposing (antagonistic) pairs to create opposing movement.

Antagonistic pairs A. Pairs of muscle that accomplish opposite movements

Joints – provide movement A. Joints are points of contact between bones or between cartilage and bones. B. Bones are too rigid to bend - to allow movements, the human skeletal system consists of bones held together at joints by flexible connective tissues called ligaments. C. Joints may be classified structurally as fibrous, cartilaginous or synovial.

D. Parts of a Synovial Joint

D. Parts of a Synovial Joint

Muscle fibres (muscle cells) Contain multiple nuclei Plasma membrane = sarcolemma 1. Contains tunnel like extensions going into the cell called T tubules Cytoplasm = sarcoplasm 1. Organelles that store glycogen 2. Myoglobin – like hemoglobin, stores oxygen Myofibrils – stringy sections of a muscle cell 1. Lots of mitochondria packed between them 2. Composed of sarcomeres (contractile units)

V. Sarcomeres

VI. Steps of a Muscle Contraction 1. At rest, Na+ is actively pumped out of the cell and K+ is actively pumped into the cell (requires ATP).

2. Neurotransmitter (chemical messenger in nervous system) called acetylcholine (Ach) is released by exocytosis at the neuromuscular junction (NMJ)

3. Ach diffuses across the NMJ and attaches to receptors on the muscle cell, causing sodium gates on the sarcolemma to open and let Na+ into the cell which causes K+ to leave the cell

4. If enough Ach is released, an “action potential” is generated (can’t stop once it has started) 5. At rest, actin and myosin are kept apart by troponin and tropomyosin and Ca++ is being actively transported into the SR

6. Change in Na+/K+ concentrations causes the SR to release Ca++ 7 6. Change in Na+/K+ concentrations causes the SR to release Ca++ 7. Ca++ attaches to troponin and tropomyosin, pulling them away from actin and exposing the myosin binding sites

8. Myosin heads bind to actin and start pulling the actin inward, shortening the sarcomere (a contraction)

9. As this is occurring, Ach is being broken down and its components are returning to the nerve cell 10. Once Ach is broken down, sodium gates and potassium gates close, and Na+ is actively pumped out and K+ is pumped into the muscle cell

11. Return of Na+/K+ concentrations causes SR to start taking Ca++ back in 12. Without Ca++, troponin and tropomyosin block myosin and actin from binding 13. Sarcomere relaxes

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