EXCI_206_LECTURE_NOTES_2013_nervous_systemp. 57

The all or noneresponse law applies to the sarcomere

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The all-or-none–response law applies to the sarcomere, the myofibril, the muscle fiber, and the motor unit, because all these structural levels of muscle tissue are innervated by a single motor neuron that either carries the message to contract or does not carry the message to contract. However, the all-or-none–response law does not apply to an entire skeletal muscle. A skeletal muscle can have partial contractions.
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75 EXCI 206 2013 A skeletal muscle has a number of motor units within it; some of these motor units may be told to contract by their motor neurons, whereas other motor units within the muscle may be relaxed because their motor neurons are not stimulating them to contract at that time. Therefore an entire skeletal muscle can have partial contractions because the nervous system can order some motor units to contract while others are relaxed. In this manner the nervous system can control the degree of contraction of a muscle. By having the ability to control the degree of contraction, a person can generate just the right amount of force needed for each particular situation. Example: if a person wants to do a bicep curl and lift a 5-lb weight, a partial contraction of the biceps brachii muscle is needed. If however, the person wants to do a bicep curl and lift a 15-lb weight, a stronger partial contraction by the biceps brachii is needed. Again, a partial contraction of a muscle occurs by having some of the motor units of the muscle contract completely and other motor units of the muscle not contract at all (see fig below). In other words, more motor units will contract to lift the 15-lb weight than the 5- lb weight. Illustration of the idea that a muscle contains multiple motor units and that some may contract while others are relaxed. In this figure, two motor units (yellow) are contracting, while the other motor unit (black) is relaxed.
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  • Winter '12
  • ChantalComeau

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