Movement Information about the environment is useless unless an animal can respond to it in an appropriate way usually by moving Locomotion is the movement of an entire animal however it is only one type of movement in animals Sessile animals rely on internal movements such as those of the heart muscle or gills to accomplish internal processes In order to produce movement muscles must pull against resistance such as that provided by a skeleton 2011 Pearson Education Inc Skeletons Skeletons provide attachment sites for muscles and a support system for the body s soft tissues Three types of skeletons are found in animals 1 Exoskeletons are hard hollow structures that envelop the body 2 Hydrostatic skeletons use the pressure of internal body fluids to support the body 3 Endoskeletons are hard structures inside the body 2011 Pearson Education Inc Endoskeleton Function Muscles can only exert force by contracting so pairs of muscles work together to move a bone back and forth Flexors swing two long bones toward each other extensors straighten them out Locomotion in animals with exoskeletons and hydrostatic skeletons is also based on antagonistic muscle groups Many animals with hydrostatic skeletons have both circumferential muscles and longitudinal muscles that work in concert to shorten and lengthen parts of the body 2011 Pearson Education Inc Endoskeleton Function In all vertebrate animals the movements of paired muscles are coordinated by motor neurons that originate in the brain or spinal cord These motor neurons project from processing centers that receive input from sensory systems Motor neuron activity changes in response to information about balance smells sights and sounds Thus an animal s movements are directly tied to its sensory systems The interplay of sensory input and motor output results in coordinated movements 2011 Pearson Education Inc Muscle Types Skeletal muscle consists of unbranched multinucleate cells It is also known as striated muscle because of its striated or striped appearance Cardiac muscle contains branched cells whose ends are connected via intercalated discs which contain protein lined openings called gap junctions that provide a direct cytoplasmic connection between adjacent muscle cells Intercalated discs are critical to the flow of electrical signals from cell to cell and to the coordination of the heartbeat 2011 Pearson Education Inc Muscle Types Smooth muscle is unbranched lacks the internal strands called myofibrils that are found in skeletal and cardiac muscle and is often organized into thin sheets It is essential to the function of the lungs blood vessels digestive system urinary bladder and reproductive system 2011 Pearson Education Inc How Do Muscles Contract Vertebrate skeletal and cardiac muscle tissue is composed of long slender cells called muscle fibers Within each muscle fiber are many small strands called myofibrils 2011 Pearson Education Inc Myofibril Structure Each myofibril contains many functional units called sarcomeres The sarcomeres in a myofibril shorten as the cell contracts They lengthen when the cell relaxes and an external force exerted by the antagonistic muscle of the pair stretches the muscle 2011 Pearson Education Inc 2011 Pearson Education Inc The Sliding Filament Model Sarcomeres are made up of two types of filaments thick filaments and thin filaments They appear to change width during a contraction Hugh Huxley and Jean Hanson proposed the sliding filament model The hypothesis was that the banding patterns in the sarcomere are caused by thick and thin filaments and that they do not change width but rather slide past one another during a contraction 2011 Pearson Education Inc 2011 Pearson Education Inc The Sliding Filament Model Thin filaments are composed of two coiled chains of the globular protein actin One end of each thin filament is bound to the Z disk which forms the end of the sarcomere and anchors the filament The other end is free to interact with the thick filament 2011 Pearson Education Inc The Sliding Filament Model Thick filaments are composed of multiple strands of myosin a long protein Thick filaments are anchored to the middle of the sarcomere both ends are free to interact with thin filaments The myosin head has binding sites for both actin and ATP it can catalyze the hydrolysis of ATP into ADP and a phosphate ion Myosin is the site of active movement 2011 Pearson Education Inc 2011 Pearson Education Inc How Do Actin and Myosin Interact Myosin and actin interact in a series of four steps 1 When relaxed the myosin head of a thick filament is attached to ATP but not to actin in a thin filament 2 When ATP is hydrolyzed to ADP and inorganic phosphate Pi the myosin neck straightens and the head pivots and binds to a new actin subunit farther down the thin filament 2011 Pearson Education Inc How Do Actin and Myosin Interact 3 When Pi is released the myosin neck bends back into its original position and the head pivots moving the entire thin filament this is called the power stroke 4 ADP is released and a new ATP molecule binds to the myosin causing myosin to release from actin and the cycle is ready to repeat As ATP binding hydrolysis and release continue the two ends of the sarcomere are pulled closer together 2011 Pearson Education Inc 2011 Pearson Education Inc How Does Relaxation Occur In addition to containing actin thin filaments contain proteins called troponin and tropomyosin When these proteins form a complex and block the myosin binding sites on actin actin and myosin cannot slide past each other When calcium ions bind to troponin the troponin tropomyosin complex moves so that actin can interact with myosin In this way the troponin tropomyosin complex regulates muscle activity 2011 Pearson Education Inc 2011 Pearson Education Inc 2011 Pearson Education Inc 2011 Pearson Education Inc Events at the Neuromuscular Junction When an action potential from a motor neuron arrives at the muscle cell it initiates a contraction 1 Action potentials trigger the release of the neurotransmitter acetylcholine ACh from the motor neuron into the synaptic cleft 2 ACh binds to ACh receptors on the muscle cell s plasma membrane triggering membrane depolarization that leads to action potentials 3 The action potentials propagate along the membrane of the muscle fiber and spread into its interior via invaginations called T tubules 2011 Pearson Education Inc Events at the Neuromuscular Junction 4 Ca2 channels in the