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Neuromuscular integration

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  1. Neuromuscular integration Tom Burkholder thomas.burkholder@ap.gatech.edu 4-1029 Weber 123 http://www.ap.gatech.edu/burkholder/8813/

  2. Technical Frog anatomy Muscle mechanics Force transducer Feedback control Conceptual Muscle physiology Proprioceptors Sensorimotor integration Learning goals Develop a closed loop hybrid system to investigate some aspect of neuromuscular control. Ideally, the structure or parameters of the computational system will test a model of biological control

  3. References • Gasser HS and Hill AV. The dynamics of muscular contraction. Proc R Soc Lond (B) 96: 398-437, 1924. • Rack PM and Westbury DR. The effects of length and stimulus rate on tension in the isometric cat soleus muscle. J Physiol (Lond) 204: 443-460, 1969. • Nichols TR and Houk JC. Improvement in linearity and regulation of stiffness that results from actions of stretch reflex. J Neurophysiol 39: 119-142, 1976. • McCrea DA. Spinal circuitry of sensorimotor control of locomotion. J Physiol 533: 41-50, 2001. • Lutz GJ and Rome LC. Built for jumping: the design of the frog muscular system. Science 263: 370-372, 1994. • Rome LC, Swank D, and Corda D. How fish power swimming. Science 261: 340-343, 1993. • Chizeck HJ, Crago PE, and Kofman LS. Robust closed-loop control of isometric muscle force using pulsewidth modulation. IEEE Trans Biomed Eng 35: 510-517, 1988.

  4. Control of Motion • Phylogenic background • Motor proteins • Muscle properties • Control systems

  5. Protista motility • RNA Polymerase • Mitosis • Swimming • Flagella • Cilia • Crawling • Rolling • Pseudopod formation • Chemotactic • Receptor mediated activation of myosin

  6. Nematodes • Large scale swimming • Cyclical • Force/motion phase • Specialized organs • Sensors • Motors • Wiring • Complex behavior • Avoidance Muscle activation Muscle activation

  7. Active force Passive force Stimulation Applied length Insect Flight • Indirect flight muscles • Activated less than once per cycle • Molecular kinetics • Springlike, but positive work • Stretch activation

  8. Mammalian locomotion • Multiple limbs • Ballistic

  9. Muscular work during gait • Positive work • Passive elastic mechanisms Daley, M. A. et al. J Exp Biol 2003;206:2941-2958

  10. Terrestrial posture • Support body against gravity • Perturbation control • External (wind) • Internal (respiration, muscle) • Small movements

  11. Motor proteins • Kinesin, dynein, myosin • Globular head • Filament binding & ATPase • Cargo-carrying tail Myosin Kinesin

  12. Myofilament structure • Myosin polymers arrange motor domains to maximize interaction with actin filament 200 nm

  13. Structural homogeneity • Structural order yield functional consistency • Narrow range of sarcomere “strength” • Minimizes intra-muscular force loss

  14. Z I A I Sliding filament theory • Force varies in proportion to crossbridge binding

  15. Crossbridge cycle • ATP driven, ratchet motion • Mechanochemical coupling by crossbridge elasticity

  16. Crossbridge Cycle Hydrolysis of ATP energizes myosin; moves crossbridge ATP binding to myosin displaces actin Energized myosin binds actin Myosin binds actin strongly (rigor) Release of inorganic phosphate triggers power stroke

  17. Fundamental reactions • Actin-myosin association • Slow (20 ms) • All or none change in force • Power stroke • Fast (1 ms) • Modulatory A rapid shortening pushes crossbridges through the power stroke. These crossbridges rapidly accommodate the change and are slowly displaced by new crossbridges

  18. Isotonic shortening • Muscle can shorten against less load than it can hold. • Stimulate muscle • Allow force to stabilize • Release against constant load Magnetic catch Counterweight Muscle

  19. 1.8 1.6 1.4 1.2 Po 100 00 1.0 Force Force response 0.8 500 0.6 0 0.4 0.2 0.2 0 (mm) Vmax L 0.0 D 0 100 200 300 400 -0.5 0 0.5 1 Time (ms) Applied length Shortening Velocity Dynamic response of muscle • Isotonic force velocity relation • Stretch and hold response Force (mN)

  20. 1.8 1.6 1.4 1.2 Po Force 1.0 0.8 0.6 0.4 0.2 Vmax 0.0 -0.5 0 0.5 1 Shortening Velocity Engineering analog • “Force-length” is like stiffness • “Force-velocity” is like viscosity F=Fo-bv F=kx

  21. Phenomenological (Hill) Model • Linear model • Force-length spring constant • Force-velocity viscosity • Standard linear solid analogy • Contractile Force-length • Contractile Force-velocity • Elastic elements account for dynamics

  22. Control of activation • Troponin/tropomyosin complex • Bind actin • Block myosin • Calcium dependent

  23. Calcium control • Contractile dynamics are calcium dependent • Efficient contraction requires homogeneous calcium transients • Sarcoplasmic reticulum • T-Tubules

  24. Excitation contraction coupling • Synaptic discharge initiates action potential • V-gated Ca2+ channels open • Ca2+ bind TnC • Force generation • Recovery Action potential Calcium Force

  25. Force summation • Nonlinear addition of subsequent APs

  26. Force Frequency • Muscle & species dependent • Myosin kinetics • Calcium kinetics

  27. Whole muscle organization • Physical • Fiber • Fascicle • Muscle • Agonist • Neural • Motor unit • Compartment • Muscle • Synergy

  28. Motor Unit • Alpha motorneuron • Large (12-20 um) • High CV (70-120 m/s) • Innervated muscle fibers • 10-1000 fibers/neuron • Generally proportional to axon size • Generally of similar function • 5-1000s per muscle MN Innervated fibers

  29. Whole muscle force modulation • Rate • Force-frequency • Continuous control • Recruitment • Select subpopulation of MU • Force sharing • Metabolic optimization • Size principle

  30. Motor unit control • Smooth force generation • Individual MUs sub-tetanic • Rate & phase variation

  31. Electrical stimulation • Recruitment • Axonal input resistance • Capacitance • Synchrony • Recruitment modulation • Intensity • Pulse width • High frequency block