Muscular Responses page 311

1. Threshold stimulus is the minimal stimulus needed to elicit a muscular contraction.

2. Recording a muscle contraction a. A myogram is a recording of an electrically stimulated isolated muscle pulling a lever.

b. A twitch is a single, short contraction reflecting stimulation of some motor units in a muscle.

c. The latent period is the time between stimulus and responding muscle contraction.

d. During the refractory period immediately following contraction, a muscle cannot respond.

3. All-or-none response a. If a muscle fiber contracts at all, it will contract completely.

b. Increasing the strength of the stimulus does not affect the strength of contraction.

4. Staircase effect a. An inactive muscle undergoes a series of contractions of increasing strength when subjected to a series of stimuli.

b. This staircase effect seems to be due to failure to remove calcium ions from the sarcoplasm rapidly enough.

5. Summation a. A rapid series of stimuli may produce summation of twitches and sustained contraction.

b. Forceful, sustained contraction without relaxation is a tetanic contraction.

6. Recruitment of motor units a. Muscles whose motor units contain small numbers of muscle fibers produce finer movements.

b. Motor units respond in an all-or-none manner.

c. At low intensity of stimulation, relatively small numbers of motor units contract.

d. At increasing intensities of stimulation, other motor units are recruited until the muscle contracts with maximal tension.

7. Sustained contractions a. When contractions fuse, the strength of contraction may increase due to recruitment of fibers.

b. Even when a muscle is at rest, its fibers usually maintain tone—that is, remain partially contracted.

8. Types of contractions a. When a muscle contracts and its ends are pulled closer together, the contraction is called isotonic.

b. Another type of isotonic contraction occurs when the force a muscle generates is less than that required to move or lift an object. This lengthening contraction is called an eccentric contraction.

c. When a muscle contracts but its attachments do not move, the contraction is called isometric.

d. Most body movements involve both isometric and isotonic contractions.

9. Fast and slow muscle fibers a. The speed of contraction is related to a muscle's specific function.

b. Slow-contracting, or red, muscles can generate ATP fast enough to keep up with ATP breakdown and can contract for long periods.

c. Fast-contracting, or white, muscles have reduced ability to carry on aerobic respiration and tend to fatigue relatively rapidly.

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