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• A motor unit is a motor neuron and all of the muscle cells (muscle fibers) it stimulates. The strength of a muscle contraction is determined by the size and number of motor units being stimulated.
Page 2. Goals• To examine the components of a motor unit. • To understand the relationship between motor unit size and precision of muscle movement. • To explore the relationship of motor units to muscle tone.
Page 3. The Motor Unit
• A motor unit is a motor neuron and all of the muscle cells (muscle fibers) it stimulates. When one neuron fires, all of the muscle cells which are stimulated by that oneneuron will contract. • On this diagram each of the two motor neurons shown have only a few junctions with the muscle cells, however the actual number can vary from four to several hundred muscle cells per motor neuron. The average number of muscle cells in a motor unit is 150. ** Note the following important points: • When the nerve impulse gets down to the branches, it separates and goes downeach branch. • Only the muscle cells that have neuromuscular junctions with one of the branches of that particular neuron will contract.
• Motor neuron A stimulates fewer muscle cells than motor neuron B. When you clicked on motor unit A, the force of contraction was less than with motor unit B as evidenced by the height the red weight moved toward the bell. • Noticethat motor neuron A & B do not stimulate muscle cells in just one location within the muscle. The muscle cells stimulated are spread out. This causes an even muscle contraction.
** Now is a good time to go to quiz question 1:
• Click the Quiz button on the left side of the screen. • After answering question 1, click the Back to Topic button on the left side of the screen. • To get back to whereyou left off, click on the scrolling page list at the top of the screen and choose "4. Recruitment".
• Recruitment occurs when a strong contraction is needed and additional motor units are stimulated.
** Note the following important
points: • When you click on motor unit A, which is a small motor unit, the weight trave ls about one-third of the way to the bell.When you click on motor unit B, which is a larger motor unit, the weight travels about two-thirds of the way to the bell. By stimulating both motor units to contract at once (recruitment), enough energy is provided to have the weight hit the bell.
Page 5. Question on Motor Unit Size
• Motor units which contain only a few muscle cells allow for precise muscle movements.
• Variables affecting the strength and degree of muscle movement: 1. number of motor units firing 2. the number of muscle cells per motor unit • Precise movements are created by small motor units.
Large Motor Units
• Gross movements are created by large motor units.
** Now is a good time to go to quiz questions 2-4:
• Click the Quiz button on the left side of thescreen • Click on the scrolling page list at the top of the screen and choose "2. Precise Movements". • Work through questions 2,3, & 4. • After answering question 4, click the Back to Topic button on the left side of the screen. • To get back to where you left off, click on the scrolling page list at the top of the screen and choose "8. Muscle Tone".
• Muscle tone is dueto random, asynchronous motor unit contractions. ** The narration states that muscle tone is due to minute contractions which are maintained by activities of the spinal cord. Note that many muscles of the face and some muscles of the neck are controlled by cranial nerves, as opposed to spinal nerves. In this case, the impulses would come from the brain, not the spinal cord.
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