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Chapter 8

Chapter 8- Control of Movement.pdf

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Britt Anderson

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Chapter 8Control of Movement the ultimate function of the nervous system is control movement Skeletal Muscles Skeletal muscleOne of the striated muscles attached to bonesSkeletal muscles are the ones that move us our skeletons around and are responsible for our actionsAttached to our bones via tendons strong bands of connective tissueThere are many different moves two specific types are flexion and extension FlexionContraction of a flexor muscle a movement of a limb that tends to bend its joints the opposite of extension Eg dog lifts pawExtensionContraction of extensor muscle a movement of a limb that tends to straighten its joints the opposite of flexion Eg dog puts paw down Anatomy consists of two types of muscle fibers Extrafusal muscle fibersOne of the muscle fibers that are responsible for the force exerted by contraction of a skeletal musclesServed by the Alpha motor neuronsContractions of these neurons provide muscles motive forcesingle myelinated axon of an alpha motor neuron serves several extrafusal muscle fibers Alpha motor neuron A neuron whose axon forms synapses with extrafusal muscle fibers of a skeletal muscle activation contracts the muscle fibers Intrafusal muscle fibera muscle fiber that functions as a stretch receptor arranged parallel to the extrafusal muscle fibers thus detecting changes in muscle lengthCentral region capsule contains sensory endings that are sensitive to stretch applied to the muscle Two types of intrafusal muscle fibers one being Gamma motor neuronA neuron whose axons form synapses with intrafusal muscle fiberscauses the intrafusal muscle fiber to contract however this contraction contributes an insubstantial amount of force In primates the number of muscle fibers served by a single axon varies depending on the precision with which the muscle can be controlledMotor unitA motor neuron and its associated muscle fibersconsist of alpha motor neuron its axon and associated extrafusal muscle fibers Myofibrilan element of muscle fibers that consists of overlapping strands of actin and myosin responsible for muscular contractions ActinOne of the proteins with myosin that provide the physical basis for muscular contraction MyosiOne of the proteins with actin that provide the physical basis for muscular contractionregions in which the actin and myosin filaments overlap produce dark stripes or striations hence skeletal muscle is referred to as striated muscle Striated muscleSkeletal muscle muscle that contains striationsThe Physical Basis of Muscular Contraction Neuromuscular junctionThe synapse between the terminal buttons of an axon and a muscle fiber Motor endplate The postsynaptic membrane of a neuromuscular junctionterminal buttons of the neurons synapse on this membrane located in groves along the surface of the muscle Endplate potential The postsynaptic potential that occurs in the motor endplate in response to release of acetylcholine by this terminal button Processan axon fires acetylcholine is liberated by the terminal buttons and produces a depolarization of the postsynaptic membraneendplates potential is much larger than an excitatory postsynaptic potential its potential always causes the muscle fiber to fire circulating the potential along its length inducing a contraction or twitch of the muscle fiberdepolarization of a muscle fiber opens the gates of voltagedependent calcium channels allowing calcium ions to enter the cytoplasm triggering the contraction Calcium acts as a cofactor allowing the myofibrils to extract energy from the ATP that is present in the cytoplasm The myosin cross bridges alternately attach to the actin strands bend in one direction detach themselves bend back reattach to the actin at a point farther down the strand etc and form cross bridges row along the actin filaments
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