Topic 7 Muscular System: Motor Unit

Question 1

• A motor unit is a neuron and the muscle fibers it innervates. Each muscle fiber (cell) forms synapses with only one motor neuron, but each motor neuron typically synapses with many muscle fibers. The force of contraction depends on the number and size of active motor units and frequency of action potentials.

Answer 1

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Question 2

• Smaller motor units tend to be activated first, and larger ones are recruited as needed. There is a smooth increase in force generated — from slow twitch to fast twitch. A twitch occurs when one muscle fiber contracts in response to a stimulus.

Answer 2

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Question 3

• Recruitment occurs when a greater quantity of muscle fibers are activated by the brain rather than an increase in frequency of action potentials that stimulate muscle fiber contraction. Intricate movements tend to use smaller motor units (such as in the finger), whereas muscles requiring greater force (like the back) have larger motor units.

Answer 3

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