Lecture 16 – Muscle Physiology IV

Controlled in the nervous system in a standard order

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Controlled in the nervous system in a standard order Some somatic motor neurons have low thresholds and other have high threshold Low thresholds are recruited earlier This determines order of recruitment 1. Weak stimulus directed to a group f somatic motor neurons in the CNS activates only  those SM neurons with the lowest threshold and these typically innervate motor units  consisting of  fatigue resistant, slow twitch (oxidative) fibers  which generate little  force 2. As stimulation increases, SM neurons with higher thresholds are recruited because  these neurons innervate  fatigue resistant, fast twitch (oxidative glycolytic) fibers  Because more motor units are contracting, more force is generated 3. As the stimulation increases to higher levels, SM neurons with the highest thresholds  begin to fire and these innervate  glycolytic, fast twitch fibers.  At this point the contraction is generating near maximum force This is the biggest boost in tension but also increase likelihood of fatigue Summary Fast twitch fibers generate more force than slow twitch fibers But they fatigue ore rapidly than slow twitch fibers Sustained contraction in a muscle requires a continuous stream of APs from the CNS Summation If he muscle is easily fatigued, summation leads to fatigue and loss of tension
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CNS avoids fatigue in sistaniend contractions by asynchronous recruitment  Alteration of motor units so to all are used at the same time This works when you don’t need maximal contraction In maximacl contraction, they are all in fused tentnaus, mecule fibers cycle between  maximal contrction and partial relaxation Barely notice the change in contraction and can continue for a while So long as motor units can drop in and out How much can a fiber shorten? Hwo much can a sarcomere shorten times the number of sarcomeres? The more sarcomeres, the more a fiber can shorten If each sarcoemere shortens the sam amount This affects the velocity of shortening Longer muscle fibers have larger shortening range and faster shortening rates Velocity of fiber shortening = velocity of shortening of 1 sarcomere * # sarcomeres This does NOT relate to tension All skeletal muscles are part of some lever and fulcrum series Fulcrum les between the effort and the load Increases the speed of movement and overcomes resistant In the neck Load lies between the fulcrum and the effort Overcomes resistance, not very quickly ankle Effort lies between the fulcrum and the load A load can be moved rapidly over a large distance, while the point of application moves  over a short distance  Purpose is to obtain rapid movement Operates at a mechanical disadvantage In the biceps
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