Ch. 9 Muscles and Muscle Tissue

Marieb - Human Anatomy & Physiology 7th Edition

Marieb, Hoehn7th EditionHuman Anatomy & PhysiologyISBN: 9780805359091Not the one you use?Change textbook

Marieb, Hoehn 7th Edition

Ch. 9 Muscles and Muscle Tissue

Problem 24

Chapter 9, Problem 24

Explain how a slight (but smooth) contraction differs from a vigorous contraction of the same muscle. Use the concepts of multiple motor unit summation.

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Begin by defining a motor unit: it consists of a single motor neuron and all the muscle fibers it innervates. The size and number of motor units recruited determine the strength of a muscle contraction.

Explain that a slight (but smooth) contraction involves the activation of only a few motor units. This means fewer muscle fibers are contracting, resulting in a gentle and controlled muscle tension.

Describe multiple motor unit summation (also called recruitment) as the process by which the nervous system increases the number of active motor units to produce greater muscle force.

Contrast this with a vigorous contraction, where many motor units are recruited simultaneously. This leads to a larger number of muscle fibers contracting, generating a stronger and more forceful muscle contraction.

Emphasize that the smoothness of the contraction is maintained because motor units are recruited asynchronously and progressively, preventing jerky movements even as contraction strength increases.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Motor Units

A motor unit consists of a single motor neuron and all the muscle fibers it innervates. The size and number of motor units activated determine the strength of a muscle contraction, with smaller units controlling fine movements and larger units generating more force.

Multiple Motor Unit Summation

Multiple motor unit summation refers to the process of increasing muscle contraction strength by recruiting more motor units. A slight contraction involves activating fewer motor units, while a vigorous contraction recruits many more, resulting in greater force.

Smooth vs. Vigorous Contraction

A slight but smooth contraction occurs when motor units are gradually and orderly recruited, allowing controlled force increase. In contrast, a vigorous contraction involves rapid and extensive recruitment of motor units, producing a stronger and more forceful muscle response.

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Related Practice

Textbook Question

Describe some cause(s) of muscle fatigue and define this term clearly.

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Textbook Question

When a suicide victim was found, the coroner was unable to remove the drug vial clutched in his hand. Explain the reasons for this. If the victim had been discovered three days later, would the coroner have had the same difficulty? Explain.

Muscle-relaxing drugs are administered to a patient during major surgery. Which of the two chemicals described next would be a good skeletal muscle relaxant and why?

• Chemical A binds to and blocks ACh receptors of muscle cells.

• Chemical B floods the muscle cells' cytoplasm with Ca²⁺.

a. Describe the structure of a sarcomere and indicate the relationship of the sarcomere to myofilaments.

b. Explain the sliding filament model of contraction using appropriately labeled diagrams of a relaxed and a contracted sarcomere.

Smooth muscle has some unique properties, such as low energy usage, and the ability to maintain contraction over long periods. Tie these properties to the function of smooth muscle in the body.

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