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30 Muscle Motor Unit - Motor Unit PSL302 Lecture 30 by Prof...

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Muscle Motor Unit Motor Unit - Def’n: a motoneuron + all the muscle fibers that it innervates - The muscle fibers of a motor unit all contract together - Motoneuron in ventral horn (or motor nucleus in medulla) -> axon thru ventral root -> specific muscle -> terminate on diff muscle fibers = contract together - Jxn bwn terminal + muscle fiber: terminal is broken up, parallel along same muscle fiber = large synaptic summation automatically = a.p. guaranteed in post-jxnal fibre - Neuromuscular jxn very diff from normal synapses Motor Unit Type - The muscle fibers of a motor unit are all same type - There are 3 broad categories of muscle fibers , and hence of motor units 1. Slow oxidative - Slow twitch (long rise-time of force): build-up of force relatively long (in ms) - “Twitch”: contraction caused by 1 a.p. - Rich in myoglobin : dark red colour - Stores oxygen (muscle version of hemoglobin) - Many mitochondria: “oxidative” -> aerobic metabolism : Krebs cycle - Non-fatiguing : efficient generation of ATP; oxygen doesn’t run out - Used in postural support: sustains contraction for hours on end - Small maximal force : enough for antigravity, but not enough to get you out of chair 2. Fast oxidative/glycolytic - Intermediate twitch & rise-time - Moderate maximal force - Some myoglobin: lighter colour - Moderate number of mitochondria - Fatigues slowly (over several minutes) - Mixed glycolytic/oxidative metabolism 3. Fast glycolytic - Fast twitch & rise-time - Large maximal force + large fiber size , i.e. get out of chair - Little or no myoglobin: pale fibers - Few mitochondria: glycolytic metabolism - Fatigues rapidly (in 1 min) due to lactic acid accumulation Diagram: comparing categories of muscle fibers - Twitch tension: size of maximal force - Fibers in muscles not same across species - Chickens: dark meat -> entire muscle is slow oxidative (unlike us: postural support) - In humans: we mix all muscle fibers together, although in diff combos throughout body PSL302: Lecture 30, by Prof. MacKay Friday, Nov. 26, 2010 1 of 4
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Motoneuron - muscle fiber relationship - Small diameter motoneurons innervate
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