Download - Ch13
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THE MOTOR SYSTEM, part I
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SOMATIC MOTOR SYSTEM
Muscles and neurons that control musclesRole: Generation of coordinated movements
Parts of motor controlSpinal cord coordinated muscle contractionBrain motor programs in spinal cord
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SOMATIC MOTOR SYSTEM
Types of MusclesSmooth: digestive tract, arteries, related structuresStriated: Cardiac (heart) and skeletal (bulk of body muscle mass)In each muscle there are 100 of muscle fibers innervated by a single axon from the CNS
muscle fibers
Axon from CNS
muscle
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SOMATIC MOTOR SYSTEM
Somatic MusculatureAxial muscles: Trunk movementProximal muscles: Shoulder, elbow, pelvis, knee movementDistal muscles: Hands, feet, digits (fingers and toes) movement
Flexors
Extensors
SynergistAntagonist
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Ventral hornLower motor neuron
Ventral root
Muscle fiberSpinal nerve
The Lower Motor NeuronLower motor neuron: Innervated by ventral horn of spinal cordUpper motor neuron: Supplies input to the spinal cord
THE SPINAL CORD
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Alpha Motor NeuronsTwo lower motor neurons: Alpha and GammaAlpha Motor Neurons directly trigger the contraction of the muscleMotor Unit: muscle fibers + 1 alpha motor neuronMotor neuron pool: all alpha motor neuron that innervate a single muscle
Graded Control of Muscle Contraction by Alpha Motor NeuronsVarying firing rate of motor neurons (temporal summation)Recruit additional synergistic motor units. More motor units in a muscle allow for finely controlled movement by the CNS
THE SPINAL CORD
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Inputs to Alpha Motor Neurons1) Information about muscle lenght2) Voluntary control of movement 3) Excitatory or inhibitory in order to generate a spinal motor program
3 1
2
THE SPINAL CORD
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THE MOTOR UNITS
Types of Motor UnitsRed muscle fibers: Large number of mitochondria and enzymes, slow to contract, can sustain contractionWhite muscle fibers: Few mitochondria, anaerobic metabolism, contract and fatigue rapidlyFast motor units: Rapidly fatiguing white fibersSlow motor units: Slowly fatiguing red fibers
Normal innervation
Crossedinnervation
slow fast slow fast
slow fast Slow likeFast like
Hypertrophy: Exaggerated growth of muscle fibersAtrophy: Degeneration of muscle fibers
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Muscle fiber structure Sarcolemma: external membraneMyofibrils: cylinders that contract after an APSarcoplasmic reticulum: reach of Ca2+T tubules: network that allow the AP to go through
Mitochondria Myofibrils
T tubules
Sarcoplasmic reticulum
Opening of T tubules
Sarcolemma
THE MOTOR UNITS
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The Molecular Basis of Muscle ContractionZ lines: Division of myofibril into segments by disksSarcomere: Two Z lines and myofibrilThin filaments: Series of bristles. Contains actinThick filaments: Between and among thin filaments. Contains myosin
Sliding-filament model: Binding of Ca2+ to troponin causes myosin to bind to actin. Myosin heads pivot, cause filaments to slide
THE MOTOR UNITS
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Muscle contractionAlpha motor neurons release AChACh produces large EPSP in muscle fibers (via nicotinic ACh receptors)EPSP evokes action potential. Action potential triggers Ca2+ release, leads to fiber contractionRelaxation, Ca2+ levels lowered by organelle reuptake
THE MOTOR UNITS
Excitation: Action potential, ACh release, EPSP, action potential in muscle fiber, depolarizationContraction: Ca2+, myosin binds actin, myosin pivots and disengages, cycle continues until Ca2+ and ATP presentRelaxation: EPSP end, resting potential, Ca2+ by ATP driven pump, myosin binding actin covered
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SPINAL CONTROL
Muscle spindles: specialized structures inside the skeletal muscle. They informabout the sensory state of the muscle (proprioception)
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SPINAL CONTROLThe Myotatic ReflexStretch reflex: Muscle pulled tendency to pull backFeedback loop. MonosynapticDischarge rate of sensory axons: Related to muscle lengthExample: knee-jerk reflex (stretching the quadriceps and consequent contraction)
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SPINAL CONTROL
Intrafusal fibers: gamma motor neuron Extrafusal fibers: alpha motor neuron
Gamma LoopProvides additional control of alpha motor neurons and muscle contractionCircuit: Gamma motor neuron intrafusal muscle fiber Ia afferent axon alpha
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SPINAL CONTROLProprioception from Golgi Tendon Organ.In series with the muscle fibers. Information about the tension applied to the muscleReverse myotatic reflex function: Regulate muscle tension within optimal range
Golgi Tendon Organ
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SPINAL CONTROLSpinal InterneuronsSynaptic inputs1)Primary sensory axons2)Descending axons from brain3)Collaterals of lower motor neuron axonsSynaptic outputs: alpha motor neuron
Reciprocal inhibition: Contraction of one muscle set accompanied by relaxation of antagonist muscle Example: Myotatic reflex
Crossed-extensor reflex: Activation of extensor muscles and inhibition of flexors on opposite side
flex flex
extend extend
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MOTOR PROGRAM