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

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

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