Motor Unit Physiology and Function

For my comprehensive exam to transition from a PhD student to candidate, I summarized the basic physiology of motor units and the variation in the neuromuscular system that allows for the breadth of human movement, from precision tasks to powerful actions. Taking inspiration from the hand-drawn figures of early seminal papers in the field, I drew some original figures to represent my interpretations of fundamental concepts.

Transcript

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: Motor nucleus size Muscle compartments, synergists & antagonists Motor unit size
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 Na+ influx K+ efflux 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
49. Motor unit: Size principle Henneman, 1957
50. Motor unit: Size principle 1 2 3 4 5 Henneman et al., 1965
51. Motor unit: Recruitment threshold Desmedt, 1978 Force EMG
52. Motor unit: Properties Milner-Brown, 1973
53. Motor unit: Size principle Monster and Chan, 1977 (Absolute Recruitment Threshold)
54. Motor unit: Properties Enoka & Fugelvand, 2001 FDI Motor Unit Number
55. Motor unit: Muscle unit properties Burke and Tsairis, 1974 Gastrocnemius Soleus G S
56. Motor unit: Muscle fiber properties Data from Bottinelli, 1996 Aagard et al., 2001
57. Motor unit: Controlling muscle force
58. Motor unit: Controlling muscle force Person and Kudina, 1972 Recruitment Discharge Rate
59. Motor unit: Controlling muscle force Kukulka and Claman, 1981 Upper limit of recruitment
60. Motor unit: Recruitment threshold Kukkulka and Claman, 1981 Discharge rate
61. Motor unit: Discharge rate Fugelvand et al., 2015
62. Motor unit: Discharge rate Adrian and Bronk, 1929 Minimal discharge rate
63. Motor unit: Motor neuron properties Moritz et al., 2005 Maximal discharge rate Finger force (% MVC)
64. Motor unit: Motor neuron properties Farina and Falla, 2009
65. Motor unit: Discharge rate Van Cutsem et al., 1998
66. Motor unit: Discharge rate Bawa and Calancie, 1983
67. Motor unit: Discharge rate Fugelvand et al., 1999
68. Motor unit: Discharge rate & ISI Moritz et al., 2005 Interspike Intervals Variability in ISI
69. Motor unit: Adjustments Desmedt, 1980
70. Motor unit: Adjustments Westgaard and DeLuca, 1999
71. Synaptic input: Types
73. Synaptic input: Types EPSPs Efferent Input
74. Synaptic input: Multiple actions Desmedt and Godaux, 1981
75. Synaptic input: Types EPSPs Afferent Input Efferent Input
76. Synaptic input: Types EPSPs IPSPs Afferent Input Efferent Input
77. Synaptic input: Afferent and efferent input Gesslbauer, 2017
78. Synaptic input: Afferent input Botterman et al., 1978 Golgi tendon organ Muscle spindle
79. Synaptic input: Afferent input Botterman et al., 1978 Golgi tendon organ Muscle spindle
80. Synaptic input: Afferent input Kokkorogiannis, 2004
81. 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
82. Synaptic input: Afferent input Macefield, 1993 Deafferented Normally innervated
83. Synaptic input: Types Presynaptic input EPSPs IPSPs
84. Synaptic input: Types Presynaptic facilitation 5-HT GABA Presynaptic inhibition Post-synaptic cell (motor neuron) Axon Increased neurotransmitter release Decreased neurotransmitter release
85. Synaptic input: Types EPSPs IPSPs Neuromodulation
86. 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:
87. Synaptic input: Types Persistent Inward Currents Neuromodulators G-coupled protein receptors Second messenger cascades Na+ or Ca2+ Monoamines 5-HT NE
88. Synaptic input: Non-linear properties Powers and Heckman, 2017 Johnson et al., 2017
89. Synaptic input: Non-linear properties Wienecke, 2009 Current Discharge Rate Membrane potential
90. Synaptic input: Sources of input Sherrington, 1925
91. Synaptic input: Synchronization Nordstrom et al., 1992 Reference
92. Synaptic input: Synchronization Nordstrom et al., 1992 Synchronous discharge Expected due to chance
93. Synaptic input: Sources of input
94. Synaptic input: Sources of input
95. Synaptic inputs: Sources of input Powers & Heckman, 2017 Johnson, 2017
96. Synaptic inputs: Sources of input Powers & Heckman, 2017 Johnson, 2017
97. Synaptic input: Sources of input Christou et al., 2004 Fluctuations in force Bandwidth important for force control
98. Synaptic inputs: Sources of input Powers & Heckman, 2017 Johnson, 2017
99. Synaptic inputs: Sources of input Powers & Heckman, 2017 Johnson, 2017
100. Synaptic input: Common input Cumulative spike train Force Motor nucleus 400 ms Hanning window Farina, 2016
101. Synaptic input: Common input
102. Synaptic input: Common input
103. Negro et al., 2009 Synaptic input: Common input First principal component:
104. Synaptic input: Common input Negro et al., 2009
105. Synaptic input: Synaptic noise Additional variation Influences force Influences variability in discharge rate
106. Synaptic input: Common input Maximal force correlation from EMG estimates Number of trials with adequate # of motor units Thompson et al., 2018
107. Synaptic input: Common noise Feeney et al., 2018 Q: Index of common noise
108. Recording motor unit activity: Advances in technology
109. Recording motor unit activity: Grids Merletti et al., 2009
110. Recording motor unit activity: Grids Farina et al., 2016 Intramuscular Surface
111. 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
112. Recording motor unit activity: Tracking Martinez-Valdes et al., 2017 Before After
113. Recording motor unit activity: Dynamic contractions Glaser and Holobar, 2019
114. 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
115. Thanks!