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  1. 1. THE MOTOR SYSTEM, part I
  2. 2. SOMATIC MOTOR SYSTEMMuscles and neurons that control musclesRole: Generation of coordinated movementsParts of motor control Spinal cord coordinated muscle contraction Brain motor programs in spinal cord
  3. 3. SOMATIC MOTOR SYSTEMTypes 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
  4. 4. SOMATIC MOTOR SYSTEMSomatic MusculatureAxial muscles: Trunk movementProximal muscles: Shoulder, elbow, pelvis, knee movementDistal muscles: Hands, feet, digits (fingers and toes) movement Antagonist Synergist Flexors Extensors
  5. 5. THE SPINAL CORDThe Lower Motor NeuronLower motor neuron: Innervated by ventralhorn of spinal cordUpper motor neuron: Supplies input to thespinal cord Ventral root Lower motor Ventral horn Spinal neuron nerve Muscle fiber li
  6. 6. THE SPINAL CORDAlpha Motor NeuronsTwo lower motor neurons: Alpha and GammaAlpha Motor Neurons directly trigger the contractionof the muscleMotor Unit: muscle fibers + 1 alpha motor neuronMotor neuron pool: all alpha motor neuron thatinnervate a single muscleGraded Control of Muscle Contraction byAlpha Motor NeuronsVarying firing rate of motor neurons (temporalsummation)Recruit additional synergistic motor units.More motor units in a muscle allow for finelycontrolled movement by the CNS
  7. 7. THE SPINAL CORDInputs to Alpha Motor Neurons1) Information about muscle lenght2) Voluntary control of movement3) Excitatory or inhibitory in order to generate a spinal motor program 3 1 2
  8. 8. THE MOTOR UNITSTypes 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 fibersHypertrophy: Exaggerated growth of muscle fibersAtrophy: Degeneration of muscle fibers Normal Crossed innervation innervation slow fast slow fast slow fast Fast like Slow like
  9. 9. THE MOTOR UNITSMuscle fiber structure Mitochondria MyofibrilsSarcolemma: external membraneMyofibrils: cylinders that contract after an APSarcoplasmic reticulum: reach of Ca2+T tubules: network that allow the AP to gothrough T tubules Sarcoplasmic reticulum Opening of T tubules Sarcolemma
  10. 10. THE MOTOR UNITSThe 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. ContainsmyosinSliding-filament model: Binding of Ca2+ to troponin causesmyosin to bind to actin. Myosin heads pivot, cause filamentsto slide
  11. 11. THE MOTOR UNITSMuscle contraction Excitation: Action potential, ACh release, EPSP,Alpha motor neurons release ACh action potential in muscle fiber, depolarizationACh produces large EPSP in muscle fibers (via Contraction: Ca2+, myosin binds actin, myosinnicotinic ACh receptors) pivots and disengages, cycle continues untilEPSP evokes action potential. Ca2+ and ATP presentAction potential triggers Ca2+ release, leads to Relaxation: EPSP end, resting potential, Ca2+ byfiber contraction ATP driven pump, myosin binding actin coveredRelaxation, Ca2+ levels lowered by organellereuptake
  12. 12. SPINAL CONTROLMuscle spindles: specialized structures inside the skeletal muscle. They informabout the sensory state of the muscle (proprioception)
  13. 13. 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)
  14. 14. SPINAL CONTROLIntrafusal fibers: gamma motor neuronExtrafusal fibers: alpha motor neuron Gamma Loop Provides additional control of alpha motor neurons and muscle contraction Circuit: Gamma motor neuron intrafusal muscle fiber Ia afferent axon alpha
  15. 15. 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 rangeGolgiTendonOrgan
  16. 16. SPINAL CONTROLSpinal InterneuronsSynaptic inputs1)Primary sensory axons2)Descending axons from brain3)Collaterals of lower motor neuron axonsSynaptic outputs: alpha motor neuronReciprocal inhibition: Contraction of one muscle set Crossed-extensor reflex: Activation of extensoraccompanied by relaxation of antagonist muscle muscles and inhibition of flexors on opposite sideExample: Myotatic reflex flex flex extend extend