Motor Function and the Motor Unit

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

17. 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…

18. 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

19. 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

23. 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