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Lecture 22 - The Motor Unit

Course Code
Michelle French

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L22; Nov. 04, 11
Motor Unit
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
same type.
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.
Some myoglobin.
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
seat,lift weight
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
act pot
M otoneuron - muscle fiber
Parallel btwn motoneuron diameter & fibres
Small diameter motoneurons innervate slow
oxidative fibers.
Intermediate-sized motoneurons innervate fast
oxidative/glycolytic fibers.
Large diameter motoneurons innervate fast
glycolytic fibers.
Size Principle
Small diameter motoneurons more easily
excited by summated EPSPs than large
diameter motoneurons.
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
descending signals
recruitment order; easy to just sit in a chair w
Recruitment Order
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
Persistent Currents
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
oxidative MUs)
Persistent currents facilitate postural
maintenance. Also manifested as ‘Kohnstamm
Turned on depolarizing membrane to threshold
-50mv, inactivate Na+ gated channels
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