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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
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
neurons
Neurons
  • Basic components of neurons
    • Cell body
      • Nucleus
    • Dendrites
    • Axon
      • Myelination
      • Nodes of Ranvier
    • Axon terminals
    • Synaptic end bulbs
    • Neurotransmitter
      • Acetylcholine (ACH)
motor unit
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
motor unit1
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
slide6

More precise movements

Less precise movements

motor units and muscle force production
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
gradation of muscle force
Gradation of Muscle Force
  • Two neural mechanisms responsible for force gradations:
      • Recruitment
        • Spacial summation
      • Rate coding
        • Temporal summation
recruitment

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

rate coding
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)
rate coding1

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
rate coding2
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
important
Important!
  • Smaller muscles (ex: first dorsal interosseous) rely more on rate coding
  • Larger muscles of mixed fiber types (ex: deltiod) rely more on recruitment
mms fiber types
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
mms biopsies
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
mms fiber typing
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…
mms fiber typing1
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
mms fiber typing2
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
slide23

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