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Motor Function and the Motor Unit

Motor Function and the Motor Unit. Organization of the Nervous System Central nervous system (CNS) Brain Spinal cord Peripheral nervous system (PNS) Afferent (sensory) division [periphery  CNS] Efferent (motor) division [CNS  periphery] Somatic [motor neurons] Autonomic

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Motor Function and the Motor Unit

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  1. Motor Function and the Motor Unit • Organization of the Nervous System • Central nervous system (CNS) • Brain • Spinal cord • Peripheral nervous system (PNS) • Afferent (sensory) division [periphery  CNS] • Efferent (motor) division [CNS  periphery] • Somatic [motor neurons] • Autonomic • Sympathetic • Parasympathetic

  2. Neurons • Basic components of neurons • Cell body • Nucleus • Dendrites • Axon • Myelination • Nodes of Ranvier • Axon terminals • Synaptic end bulbs • Neurotransmitter • Acetylcholine (ACH)

  3. Motor Unit • The motor neuron and all the muscle fibers it innervates. • Motor neuron determines fiber type • Only ONE fiber type per motor unit • FG • FOG • SO

  4. MU Classifications

  5. Motor Unit • The number of muscle fibers in a motor unit (innervated by 1 motor neuron) varies • Gastrocnemius • 2,000 muscle fibers per motor neuron • Extraocular muscles • < 10 muscle fibers per motor neuron • Ratio of muscle fibers to motor neurons • Affects the precision of movement

  6. More precise movements Less precise movements

  7. Motor Units and Muscle Force Production • The All-or-None Law (Bowditch’s Law) for motor units • Applies to individual motor units, but not the entire muscle. • The all-or-none law is based upon the difference between graded potentials and action potentials • Stimulation threshold • A motor unit is either activated completely or is not activated at all • If there is enough graded potential to create an action potential that travels down the α-motor neuron of a motor unit, then all of the fibers in that motor unit will contract. • The level of force production of a single motor unit is independent of the intensity of the stimulus, but it is dependent on the frequency of the stimulus • This law implies a stimulation threshold  important for the Size Principle

  8. Gradation of Muscle Force • Two neural mechanisms responsible for force gradations: • Recruitment • Spacial summation • Rate coding • Temporal summation

  9. Number & Size of Motor Units Recruited Small motor units Low stimulus threshold Larger motor units Higher stimulus threshold Largest motor units Highest stimulus threshold ↓ ↑ Amount of Force Required During Movement Recruitment • Varying the number of motor units activated. The Size Principle

  10. Rate Coding • Rate coding refers to the motor unit firing rate. • Active motor units can discharge at higher frequencies to generate greater tensions. • Recruitment vs. rate coding • Smaller muscles (ex: first dorsal interosseous) rely more on rate coding • Larger muscles of mixed fiber types (ex: deltiod) rely more on recruitment • The firing of individual motor units occurs as a stochastic process • Firing rate is a better term to describe the global changes in firing frequency (i.e., rate coding)

  11. Rate coding 100 Larger muscles Smaller muscles % Maximal Voluntary Motor Unit Recruitment Motor unit recruitment Motor unit recruitment 50 Motor unit firing frequency 0 0 100 50 % Maximal Voluntary Force Production Rate Coding

  12. Rate Coding • Rate coding occurs in two stages • Treppe (the treppe effect) • A phenomenon in cardiac muscle first observed by H.P. Bowditch; • If a number of stimuli of the same intensity are sent into the muscle after a quiescent period, the first few contractions of the series show a successive increase in amplitude (strength) • Tetanus • A state of sustained muscular contraction without periods of relaxation • Caused by repetitive stimulations of the α-motor neuron trunk (axon) at frequencies so high that individual muscle twitches are fused and cannot be distinguished from one another, also called tonic spasm and tetany • Two forms of tetanus • Incomplete tetanus – occurs when there are relaxation phases allowed between twitches • Complete tetanus – occurs when the relaxation phases are completely eliminated between twitches

  13. Important! • Smaller muscles (ex: first dorsal interosseous) rely more on rate coding • Larger muscles of mixed fiber types (ex: deltiod) rely more on recruitment

  14. MMS Fiber Types • Three general methods to determine or estimate muscle fiber type composition • Invasive sampling of skeletal MMS tissue • Biopsies • Invasive and noninvasive analysis of motor unit recruitment strategies • Needle, fine wire, and/or surface electromyography (EMG) • Noninvasive field techniques for estimating fiber type composition • Thorstensson test • Based upon a fatigue index

  15. MMS Biopsies • From the biopsy sample, serial slices of the tissue can be treated • Histochemical and Immunocytochemical treatments • Histochemistry: • Incubations with substrates or stains • Immonocytochemistry • The reaction between specific protein isoforms with an antibody to that isoform • A common procedure is to characterize fibers based upon how different antibodies bind to different myosin heavy chain (MHC) isoforms • Positive relationship between Myosin ATPase activity within a muscle fiber and contraction velocity (R. Close, 1965; M. Barany, 1967) • Maximum velocity of shortening (dynamic) • Time to peak tension (isometric) • There are exceptions to this relationship (injury, distributional extremes, etc.); therefore, an immunoassay may simply be a test of Myosin ATPase activity, rather than contraction velocity • Fast-twitch fibers react dark with Myosin ATPase when preincubated under alkaline conditions (i.e., pH ~10.3) • “Acid-reversal” occurs when the reaction is reversed; fast-twitch fibers react light with Myosin ATPase when preincubated under acidic conditions (pH ~4.3) • Staining with a succinic dehydrogenase (SDH) reactant can identify the oxidative fibers • Fiber characteristics are then determined by light microscopy

  16. MMS Fiber Typing • TRADITIONALLY, four identified skeletal muscle fiber types • Based upon MHC isoform reactants and enzymatic activity • Type I • Type IIa • Type IIx • Type IIb • More sophisticated techniques, however, have identified more…

  17. MMS Fiber Typing • Comparison of fiber typing methods • Histochemistry • Qualitative, not quantitative • False dichotomy • Fiber typing exists on a continuum • Gel electrophoresis and immunoblotting reveals a large number of separate MHC isoforms as well as myosin light chain (MLC) isoforms • The combinations of MHC and MLC isoforms are numerous, but a more complex continuum has been suggested by Pette & Vrbova (1992): • Type I • Type Ic • Type IIc • Type IIac • Type IIa • Type IIab • Type IIb

  18. MMS Fiber Typing • Genes are present to change MHC isoforms based upon a training stimulus • The direction of fiber type transition seems to be from IIb  IIa • Regardless of the training modality (Fry, JSCR, 2003) • Baumann et al. 1987 • Dudley, Tesch, Fleck, Kraemer, and Baechle, 1986 • Fry, Schilling, Staron, Hagerman, et al. in press • Staron et al. JAP, 1994, 1991, and 1990

  19. Displacement Sensor Accelerometer Laser Beam Bipolar EMG Electrodes Force Transducer Orizio, C., Gobbo, M., Diemont, B., Esposito, F., Veicsteinas, A. Eur J Appl Physiol. 2003. Isometric muscle action at 30% MVC

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