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Motor Neurons
Eric Lang Systems Neuroscience Course 2012
NYU
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Why should we study motor neurons? (only ~0.0003 % of all neurons, ~300,000)
1. Know their function • to contract muscle (generate tension,
shorten) 2. MNs are the “final common path”
• Bottleneck (333,333:1) 3. Accessibility
• Large size, so easy to record • Easy to identify • Muscles are targets, so use them as
surrogate (MUAP) 4. Historical
• Sherrington (defined basic concepts of synapse, inhibition, excitation, reflex loops, recruitment)
• Eccles (among first vertebrate CNS intracellular recordings) measurement of synaptic delay, mechanism of synaptic inhibition in the CNS.
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What are motor neurons?
How do you know whether you are recording a MTN?
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How Are Motor Neurons Organized in the Spinal Cord?
1. Columnar Nuclei (Pools) 2. Overall topography
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Motor neuron A1
Muscle A
Motor nucleus A Muscle B
Motor nucleus B
Motor neurons and muscles
A1 A2
A3 A4
MN pool-2
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MtN pool distibution
Topography of motor neuron pools
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Types of motor neurons innervating skeletal muscle
1. alpha motor neurons
2. gamma motor neurons
3. beta motor neurons
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Innervation/Function of MN types
1. alpha motor neurons extrafusal muscle fibers, motor
2. gamma motor neurons intrafusal muscle fibers, sensory-related
3. beta motor neurons extrafusal and intrafusal muscle, both
sensory and motor? fibers
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Historical/ Early Observations
Red vs white muscles Slow vs fast contraction
S-fibers (human muscle) 33%triceps 40%brachioradialis 80%adductor pollicis 88%soleus (100%cat)
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Motor unit fiber distribution
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Muscle unit classification-1
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75% 25%
5% 45% 25%
Fast FF
F(int) FR Slow
Muscle Fiber Contractile Properties
Tension
Fatigue resistance Twitch
contraction time
Fiber diameter
MU properties-1
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Muscle unit classification-2
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Motor neuron properties correlate with motor unit properties
Slow M.U.s as compared to Fast M.U.s have smaller motor neuron axon smaller motor neuron (body plus dendrites) more oxidative enzymes, mitochondria distinct electrical properties
S FR FF
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MN properties summary
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Motor neuron electrophysiology
1nA
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AHP of MN sets minimum firing rate and
is matched to twitch time of muscle fibers
100 ms 50 ms
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Control of Force
• Recruitment gradation (number of motor units activated)
• Rate gradation (firing rate changes of active units)
• Size principle
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Motor unit recruitment
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Size principle-2
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Motor unit recruitment-2
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Recruitment order
Size Principle
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Large vs Small to same stim
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Why are small MNs more excitable?
• Higher input resistance (Rin) – Less membrane area – Higher membrane resistivity
• Smaller Vthresh / and intrinsic properties
• Rheobase lower
So if other factors equal, small MNs activated first
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When does the size principle apply?