Lecture 22 - The Motor Unit
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L22; Nov. 04, 11
•Def’n: a motoneuron & all of muscle fibers it
•Muscle fibers of a motor unit all contract
•Project from spinal nerve from ventral root,
branches at terminal, each branch contacts dif
muscle fibre (many cells fused together)
•Note: Striping pattern
•A given motoneuron innervates 1 type of
•Large muscles – 1000s of muscle fibres
•Tiny muscle – 10 fibres, ex. Eye
•Note: actual neuromuscular junction = end
plate = set of synapses much stronger
connection btwn axon & muscle fibre
Motor Unit Type
•The muscle fibers of a motor unit are all of the
○3 broad categories of muscle fibers & of motor
○Overlap btwn them, spectrum
○Speed of contraction + metabolism
1. Slow oxidative
•Slow twitch (long rise-time of force).
•Rich in myoglobin: dark red – store a lot of
•Many mitochondria: oxidative metabolism.
•Non-fatiguing – maintain contraction all day long
•Small maximal force.
○Adequate for postural maintenance, semi-
forces that go on & on
1. Fast oxidative/glycolytic
•Intermediate twitch rise-time.
•Moderate maximal force.
•Moderate n# of mitochrondria.
•Fatigues slowly (over several minutes)
1. Fast glycolytic
•Fast twitch rise-time.
•Large maximal force, & large fiber size.
○Big burst of force in muscle;ex. Jump out of
•Little or no myoglobin: pale fibers.
•Few mitochondria: glycolytic metabolism.
○Quite inefficient to produce ATP
•Fatigues rapidly (in 1 min) due to lactic acid
•Twitch – contraction
produced by a single
M otoneuron - muscle fiber
•Parallel btwn motoneuron diameter & fibres
•Small diameter motoneurons innervate slow
•Intermediate-sized motoneurons innervate fast
•Large diameter motoneurons innervate fast
•Small diameter motoneurons more easily
excited by summated EPSPs than large
• Slow oxidative motor units recruited by
relatively low levels of excitatory synaptic
•Fast glycolytic motor units recruited by high levels
of excitatory synaptic input – need more
• recruitment order; easy to just sit in a chair w
1) gamma-motoneurons; smallest size &
conduct’n velocity, innervate & contract intrafusal
muscle fibers (no work, have sensory f’n to
sensitize, collect data about muscle state)
2) slow motor units; oxidative extrafusal muscle
3) fast oxidative/glycolytic motor units
4) fast glycolytic motor units; for powerful burst of
•Motoneurons contain both Na & Ca channels
– inactivate very slowly, giving rise to ‘persistent
currents’ that maintain depolarization &
spiking activity after synaptic activity ceases
•Most prominent in small motoneurons (slow
•Persistent currents facilitate postural
maintenance. Also manifested as ‘Kohnstamm
•Turned on depolarizing membrane to threshold
-50mv, inactivate Na+ gated channels
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