A presentation describing the fundamentals of motor unit physiology as it relates to the control of human skeletal muscle for a PhD comprehensive exam.

1. Motor unit physiology
and function
Comprehensive Exam
Melissa Mazzo
February 20, 2019

2. Recording
motor unit
activity
Motor unit physiology
and function
Factors
influencing
motor unit
activity
Motor neuron properties
Muscle unit properties
Types of synaptic input

3. Motor unit: Basic force-producing element
of the neuromuscular system

4. Motor unit

6. Motor unit: Motor unit pool
~93 motor units
~14 motor units
Jenny and Inukai, 1983
Feinstein et al., 1955

7. Motor unit: Motor unit pool
~4150 motor units
~1271 motor units
Jenny & Inukai, 1983 Torre, 1953

9. Motor unit: Innervation number
Average of ~1750
muscle fibers
Average of 5-10
muscle fibers
Feinstein et al., 1955
Torre, 1953

10. Motor unit: Innervation number
Innervation numbers
ranging from
21 - 1770
Innervation numbers
ranging from
18 - 830
Enoka & Fugelvand, 2001 Rafuse et al., 1997

12. Motor unit: Muscle unit territory
Burke and Tsairis, 1973
~750 total
muscle fibers
~200 total
muscle fibers

13. Motor unit: Muscle unit territory
Harris et al., 2005
~33% of
muscle units
Vieira et al., 2011
~10% of
muscle length
~2.5 cm
Entire
muscle length
~6%

14. Motor unit:
Motor unit size
Motor nucleus size
Muscle fiber properties

15. Motor unit: Muscle fiber properties
Henneman and Olson, 1965
Medial gastrocnemius Soleus
Myosin ATPase activity
I
IIa
IIb

16. Motor unit: Muscle fiber properties
Harridge et al., 1996
Soleus
Vastus
lateralis
Triceps
brachii

17. Motor unit: Muscle fiber properties
Van Cutsem et al., 1997 Harridge et al., 1996
Tibialis anterior

18. Motor unit:
Motor nucleus size
Muscle fiber properties
Motor unit size

19. Motor unit:

20. Motor unit: Muscle anatomy
Barry et al., 2009
Flexion vs. supination
Cranial vs. caudal regions
Falla and Farina, 2008

21. Motor unit:
Synergists & antagonists
Antagonist muscles
Antagonist muscles
Synergist muscles
Cappellini et al., 2006
Stance Swing

22. Motor unit:
Motor nucleus size
Muscle compartments, synergists & antagonists
Motor unit size

23. Motor unit: Motor neuron properties

24. 50,000+ synaptic inputs
~1500 labeled
synapses
Alvarez, 1998

25. Muscle
Motor unit: Motor neuron properties

26. Motor unit: Motor neuron properties
Muscle
Excitatory
Inhibitory
Action potential
+ + + + +
- - - - -
Vm

27. Excitatory
Inhibitory
Action potential
Motor unit: Motor neuron properties
EPSP
IPSP
Voltage-gated
channels

28. Motor end plate
Motor neuron axon
Neuromuscular
synapses
Image: T. Caceci

29. Motor unit: Motor neuron properties
Motor neuron action potential
N
a
+
i
n
f
lu
x
K
+
e
ff
lu
x
Giant squid axon
Hodgkin and Huxley, 1945

30. Adrian and Bronk, 1929
Recording motor unit activity: Electrodes

31. Recording motor unit activity: Electrodes
Enoka et al., 1988

32. Recording motor unit activity: EMG
Macefield et al., 1996
Motor unit
action potential
Motor unit
twitch
Stimulus
Surface EMG
Force

33. Recording motor unit activity: Electrodes
DeLuca et al., 1982

34. Recording motor unit activity: EMG
Data provided by Kevin Keenan, PhD
Interference EMG
Rectified EMG
Smoothed EMG
Force
+
-
Reference
Amplifier

35. Recording motor unit activity: EMG
Day and Hullinger, 2001
Recorded
Summed off-line
10 Motor units
Individually stimulated

36. Recording motor unit activity: EMG
Keenan et al., 2005
Sum
Excitation (% Maximum)
up to 63%

37. Recording motor unit activity: EMG
Keenan et al., 2005
Intramuscular
motor unit
action potentials
Surface motor unit
action potentials
Low-pass filter

38. Recording motor unit activity: EMG
Data provided by Kevin Keenan, PhD
Interference EMG
Rectified EMG
Smoothed EMG
Force
+
-
Reference
Amplifier

40. Motor unit: Muscle properties
Low-pass filter

41. Motor unit: Muscle properties
Low-pass filter
Milner-Brown et al., 1973 Christakos, 1982
40% MVC
Force

43. Motor unit: Motor neuron properties
Macefield et al., 1996

44. Motor unit: Motor neuron properties

45. Motor unit: Motor neuron properties
ΔVm
Current
Conductance
=

46. Motor unit: Motor neuron properties
ΔVm
I
gin
=

47. Motor unit: Motor neuron properties
Kernell, 1981 a
Input Conductance
“Leakiness”

48. Motor unit: Motor neuron properties
Fleshman, 1981
Rheobase Current

49. Motor unit: Motor neuron properties

50. Cullheim, 1978
(Axon
Hillock)
Motor unit: Motor neuron properties

51. McPhedran, 1965
Motor unit: Motor neuron properties
Axonal Conduction Velocity
Cullheim, 1978,

52. Motor unit: Size principle
Henneman, 1957

53. Motor unit: Size principle
1
2
3
4 5
Henneman et al., 1965

54. Motor unit: Size principle
Somjen, 1965

55. Motor unit: Recruitment threshold
Desmedt, 1978
Force
EMG

56. Motor unit: Properties
Milner-Brown, 1973

57. Motor unit: Size principle
Monster and Chan, 1977
(Absolute Recruitment Threshold)

58. Motor unit: Properties
Enoka & Fugelvand, 2001
FDI
Motor Unit Number

59. Motor unit: Muscle unit properties
Burke and Tsairis, 1974
Gastrocnemius
Soleus
G
S

60. Motor unit: Muscle fiber properties
Data from Bottinelli, 1996
Aagard et al., 2001

61. Motor unit: Controlling muscle force

62. Motor unit: Controlling muscle force
Person and Kudina, 1972
Recruitment
Discharge Rate

63. Motor unit: Controlling muscle force
Kukulka and Claman, 1981
Upper limit of recruitment

64. Motor unit: Recruitment threshold
Kukkulka and Claman, 1981
Discharge
rate

65. Motor unit: Discharge rate
Fugelvand et al., 2015

66. Motor unit: Discharge rate
Moritz et al., 2005
Fugelvand et al., 2015

67. Motor unit: Discharge rate
Adrian and Bronk, 1929
Minimal discharge rate

68. Motor unit: Motor neuron properties
Moritz et al., 2005
Maximal discharge rate
Finger force (% MVC)

69. Motor unit: Motor neuron properties
Farina and Falla, 2009

70. Motor units: Properties
Barry, 2007

71. Motor unit: Discharge rate
Van Cutsem et al., 1998

72. Motor unit: Discharge rate
Bawa and Calancie, 1983

73. Motor unit: Discharge rate
Fugelvand et al., 1999

74. Motor unit: Discharge rate & ISI
Moritz et al., 2005
Interspike Intervals
Variability in ISI

75. Motor unit: Discharge rate & ISI
Data from Barry, 2007
ISI
DR

76. Motor unit: Discharge rate & ISI
Bellemare et al., 1983
ISI ~30 ms ISI ~40 ms
Discharge
Rate

77. Motor unit: Adjustments
Desmedt, 1980

78. Motor unit: Adjustments
Westgaard and DeLuca, 1999

79. Synaptic input: Types

81. Synaptic input: Types
EPSPs
Efferent Input

82. Synaptic input: Multiple actions
Desmedt and Godaux, 1981

83. Synaptic input: Types
EPSPs
Afferent Input
Efferent Input

84. Synaptic input: Types
EPSPs
IPSPs
Afferent Input
Efferent Input

85. Synaptic input: Afferent and efferent input
Gesslbauer, 2017

86. Synaptic input: Afferent input
Botterman et al., 1978
Golgi
tendon
organ
Muscle
spindle

87. Synaptic input: Afferent input
Botterman et al., 1978
Golgi
tendon
organ
Muscle
spindle

88. Synaptic input: Afferent input
Kokkorogiannis, 2004

89. Synaptic input: Afferent input
Kokkorogiannis, 2004
156 spindles
390 grams
80 spindles
6.2 grams
155 spindles
7.6 grams
266 grams
1193 spindles
408 spindles
433 grams

90. Synaptic input: Afferent input
Macefield, 1993
Deafferented
Normally innervated

91. Synaptic input: Types
Presynaptic input
EPSPs
IPSPs

92. Synaptic input: Types
Presynaptic
facilitation
5-HT GABA
Presynaptic
inhibition
Post-synaptic cell
(motor neuron)
Axon
Increased
neurotransmitter
release
Decreased
neurotransmitter
release

93. Synaptic input: Types
EPSPs
IPSPs
Neuromodulation

94. Synaptic input: Types
Metabotropic
Receptors
Neuromodulators
G-coupled
protein
receptors
Second
messenger
cascades
Monoamines
5-HT NE
• Induce persistent inward currents
• Reduce resting leak conductance
• Induce membrane depolarization
• Enhance or decrease hyperpolarization
Neuromodulators can:

95. Synaptic input: Types
Persistent
Inward
Currents
Neuromodulators
G-coupled
protein
receptors
Second
messenger
cascades
Na+ or Ca2+
Monoamines
5-HT NE

96. Synaptic input: Non-linear properties
Powers and Heckman, 2017 Johnson et al., 2017

97. Synaptic input: Non-linear properties
Wienecke, 2009
Current
Discharge
Rate
Membrane
potential

98. Synaptic input: Non-linear properties
Wienecke, 2009
Current
Discharge
Rate
Membrane
potential

99. Synaptic input: Sources of input
Sherrington, 1925

100. Synaptic input: Synchronization
Nordstrom et al., 1992
Reference

101. Synaptic input: Synchronization
Nordstrom et al., 1992
Synchronous discharge
Expected due to chance

102. Synaptic input: Synchronization
5 indices based on different normalization methods:
(f) The index of common input strength (CIS) (Nordstrom et al. 1992)

103. Synaptic input: Sources of input

104. Synaptic input: Sources of input

105. Synaptic inputs: Synchronization
Sears and Stagg, 1978
Shared
Shared
Independent
Independent
Synchronized
Independent
Independent

106. Synaptic inputs: Sources of input
Powers & Heckman, 2017 Johnson, 2017

107. Synaptic inputs: Sources of input
Powers & Heckman, 2017 Johnson, 2017

108. Synaptic input: Sources of input
Christou et al., 2004
Fluctuations in force
Bandwidth important for force control

109. Synaptic inputs: Sources of input
Powers & Heckman, 2017 Johnson, 2017

110. Synaptic inputs: Sources of input
Powers & Heckman, 2017 Johnson, 2017

111. Synaptic input: Common input
Cumulative spike train Force
Motor nucleus
400 ms Hanning
window
Farina, 2016

112. Synaptic input: Common input

113. Synaptic input: Common input

114. Negro et al., 2009
Synaptic input: Common input
First principal component:

115. Synaptic input: Common input
Negro et al., 2009

116. Synaptic input: Synaptic noise
Additional variation
Influences force
Influences variability
in discharge rate

117. Synaptic input: Common input
Maximal force correlation from EMG estimates
Number of trials with adequate # of motor units
Thompson et al., 2018

118. Synaptic input: Common noise
Feeney et al., 2018
Q:
Index of
common noise

119. Synaptic input: Common noise
Feeney et al., 2018

120. Synaptic input: Common input
Negro, 2016

121. Synaptic input: Common input & force
Castronovo, 2018

122. Synaptic input: Common input
Negro et al., 2016

123. Recording motor unit activity:
Advances in technology

124. Recording motor unit activity: Grids
Merletti et al., 2009

125. Recording motor unit activity: Grids
Farina et al., 2016
Intramuscular Surface

126. High density surface EMG
4 x 8 Grid of recording sites
Estimated
motor
unit
action
potential
IDR
(pps)
Extracted MU Pulse Train
Recording motor unit activity: Decomposition

127. Recording motor unit activity: Grids
Farina, 2016

128. Recording motor unit activity: Tracking
Martinez-Valdes et al., 2017
Before
After

129. Recording motor unit activity: Tracking
Martinez-Valdes, 2017

130. Recording motor unit activity:
Dynamic contractions
Glaser and Holobar, 2019

131. Moving forward: Projects
1. Self-massage with therapy balls improves stretching outcomes in
young and middle-aged adults
Second Author, Published February 2019, Journal of Sports Sciences
2. Poor estimates of motor variability are associated with longer
grooved pegboard times for middle-aged and older adults
Second Author, Published February 2019, Journal of Neurophysiology
3. The effect of stretching and self-massage on motor unit discharge
characteristics
First Author, Data analysis in progress
4. The influence of sensory feedback on estimates of variance in
common synaptic input
5. Differences in the common input to motor neurons during
various types of contractions
Motor unit
tracking
Estimates
of common
input and
common
noise

132. Thanks!