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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
Sympathetic
Parasympathetic
Neurons
Basic components of
neurons
– Cell body
Nucleus
– Dendrites
– Axon
Myelination
Nodes of Ranvier
– Axon terminals
– Synaptic end bulbs
– Neurotransmitter
Acetylcholine (ACH)
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
MU Classifications
Motor Unit Classification
Slow (S)
Fast fatigue-resistant (FR)
FR & FF combination
Fast fatigable (FF)
Fiber Type Classification
Type I
Type IIa
Type IIx
Type IIb
Metabolic Fiber Type Classification
Slow oxidative (SO)
Fast oxidative glycolytic (FOG)
FOG & FG combination
Fast glycolytic (FG)
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
More precise movements
Less precise movements
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
Two neural mechanisms
responsible for force
gradations:
1. Recruitment
 Spacial summation
2. Rate coding
 Temporal summation
Recruitment
Varying the number of motor units activated.
Larger motor units
Largest motor units
Low stimulus threshold
Higher stimulus
threshold
Highest stimulus
threshold
The Size Principle
Amount of Force Required During Movement
↑
Number & Size of Motor Units Recruited
↓
Small motor units
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 Coding
Rate
coding
100
Larger
muscles
Smaller
muscles
% Maximal
Voluntary
Motor Unit
Recruitment
50
Motor unit
firing
frequency
0
0
50
% Maximal Voluntary Force Production
100
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!
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
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
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
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 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 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
Isometric
muscle
action at
30% MVC
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.