Chapter 40 BSC 1005 Tidwell Action and Support The Muscles and Skeleton Alternating contractions of muscles with opposing actions called antagonistic muscles move arms and legs back and forth 3 forms of skeletons o Hydrostatic Basically a sac or tube filled with liquid o Exoskeleton Encased by rigid outside skeletons EX cnidarians mollusks EX arthropods Movement of an exoskeleton typically only occurs at joints bends a joint when contracted Flexor Extensor straightens a joint when contracted Because the exoskeleton cannot expand an arthropod must periodically molt its exoskeleton so that it can grow o Endoskeleton Have rigid structures inside the body EX Echinoderms chordates 3 types of muscle o Skeletal Moves the skeleton striated Located at the heart striated o Cardiac o Smooth Surrounds large blood vessels and most hollow organs producing slow sustained contractions that cannot be controlled voluntarily Tendons connect muscles to bones Muscles Bundles of muscle fibers Myofibrils Sarcomeres Each myofibril consists of repeating subunits called sarcomeres These are the contractual unit of the muscle The thin and thick filaments of myofibrils are composed primarily of two proteins actin and myosin The myosin head is hinged to the shaft and can move back and forth Muscle fibers move through interactions between thin and thick filaments Page 1 of 3 Chapter 40 BSC 1005 Tidwell o When a muscle contracts tropomyosin moves aside exposing the binding sites o The myosin head then binds to these sites temporarily linking the thick and thin on the active proteins filaments o The myosin heads flex pulling on the thin filaments and causing them to slide a o The myosin heads on the two ends of each thick filament pull the thin filaments tiny distance along the thick filament toward the middle of the sarcomere Muscle contraction requires ATP because the energy is needed not to flex the myosin head but to extend it and store the energy in this stretched position o When ATP binds to a myosin head it causes the head to release actin Only then can the energy be used to extend the head storing that energy to use during the pull on the think filament A skeletal muscle s reserves of ATP are used up after only a few second of high intensity exercise o Skeletal muscles also stock a supply of creatine phosphate an energy storage molecule that can donate a high energy phosphate to ADP thus regenerating ATP however this is also depleted rapidly o During brief high intensity exertion muscle cells generate a bit more ATP using glycolysis which does not require oxygen but is also not very efficient o For prolonged or low intensity exercise muscle cells produce ATP from glucose and fatty acids using cellular respiration which requires oxygen The nervous system controls contraction of skeletal muscles Axons communicate with muscle fibers at specialized synapses called neuromuscular junctions Each action potential in a motor neuron releases enough acetylcholine to produce a huge excitatory postsynaptic potential in the muscle fiber bringing its membrane potential above threshold and triggering an action potential A motor neuron and all the muscle fibers that it innervates are called a motor unit Bones store calcium and phosphorus Axial skeleton includes the bones of the head vertebral column and rib cage Ligaments hold bones together at joints Page 2 of 3 Chapter 40 BSC 1005 Tidwell A bone consists of a hard outer shell of compact bone and encloses spongy bone in its interior Osteoblast bone forming cells Osteocytes mature bone cells Osteoclasts bone dissolving cells Bone remodeling allows skeletal repair and adaptation to stresses 5 10 is replaced each year o Bones that carry heavy loads or are subjected to extra stress become thicker o Everyday stresses such as walking and weightlifting help to maintain bone strength o In the aging body the activity of osteoclasts exceeds the activity of osteoblasts and bones become more fragile as a result osteoporosis o Callus a porous mass of bone and cartilage that surrounds the break Joints on bones are held together by ligaments Tendons attach muscles to bones The tendon at one end of each muscle called the origin is fixed to a bone that remains stationary while the other end the insertion is attached to the bone on the far side of the joint which is moved by the muscle Ball and socket joints allow movement in several directions but do not provide a lot of joint stability Page 3 of 3