Whole Muscle Contraction

I. The tension produced by whole-muscle contraction depends on the amount of tension developed by each fiber and the number of active fibers in the muscle (Table 11-4).

III.

VII.

VIII.

Muscles that produce delicate movements have a small number of fibers per motor unit, whereas large postural muscles have much larger motor units. Fast-glycolytic motor units not only have large-diameter fibers but also tend to have large numbers of fibers per motor unit.

Increases in muscle tension are controlled primarily by increasing the number of active motor units in a muscle, a process known as recruitment. Slow-oxidative motor units are recruited first during weak contractions, then fast-oxidative motor units, and finally fast-glycolytic motor units during very strong contractions.

Increasing motor-unit recruitment increases the velocity at which a muscle will move a given load. The strength and susceptibility to fatigue of a muscle can be altered by exercise.

a. Long-duration, low-intensity exercise increases a fiber's capacity for oxidative ATP production by increasing the number of mitochondria and blood vessels in the muscle, resulting in increased endurance.

b. Short-duration, high-intensity exercise increases fiber diameter as a result of increased synthesis of actin and myosin, resulting in increased strength.

Movement around a joint requires two antagonistic groups of muscles: one flexes the limb at the joint, and the other extends the limb. The lever system of muscles and bones requires muscle tensions far greater than the load in order to sustain a load in an isometric contraction, but the lever system produces a shortening velocity at the end of the lever arm that is greater than the muscle-shortening velocity.

SECTION A KEY TERMS

skeletal muscle

sliding-filament mechanism

smooth muscle

cross-bridge cycle

cardiac muscle

rigor mortis

muscle fiber

troponin

myoblast

tropomyosin

satellite cell

excitation-contraction

muscle

coupling

tendon

sarcoplasmic reticulum

striated muscle

lateral sac

myofibril

transverse tubule (T tubule)

sarcomere

motor neuron

thick filament

motor unit

myosin

motor end plate

thin filament

neuromuscular junction

actin

acetylcholine (ACh)

A band

end-plate potential (EPP)

Z line

acetylcholinesterase

I band

tension

H zone

load

M line

isometric contraction

titin

isotonic contraction

cross bridge

lengthening contraction

contraction

twitch

relaxation

latent period

PART TWO Biological Control Systems

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART TWO Biological Control Systems contraction time summation tetanus optimal length (lo) creatine phosphate oxygen debt muscle fatigue central command fatigue fast fiber slow fiber oxidative fiber myoglobin red muscle fiber glycolytic fiber white muscle fiber slow-oxidative fiber fast-oxidative fiber fast-glycolytic fiber recruitment hypertrophy flexion extension antagonist

SECTION A REVIEW QUESTIONS

List the three types of muscle cells and their locations. Diagram the arrangement of thick and thin filaments in a striated-muscle sarcomere, and label the major bands that give rise to the striated pattern. Describe the organization of myosin and actin molecules in the thick and thin filaments. Describe the four steps of one cross-bridge cycle. Describe the physical state of a muscle fiber in rigor mortis and the conditions that produce this state. What three events in skeletal-muscle contraction and relaxation are dependent on ATP? What prevents cross bridges from attaching to sites on the thin filaments in a resting skeletal muscle? Describe the role and source of calcium ions in initiating contraction in skeletal muscle. Describe the location, structure, and function of the sarcoplasmic reticulum in skeletal-muscle fibers. Describe the structure and function of the transverse tubules.

Describe the events that result in the relaxation of skeletal-muscle fibers.

Define a motor unit and describe its structure. Describe the sequence of events by which an action potential in a motor neuron produces an action potential in the plasma membrane of a skeletal-muscle fiber.

What is an end-plate potential, and what ions produce it?

Compare and contrast the transmission of electrical activity at a neuromuscular junction with that at a synapse.

Describe isometric, isotonic, and lengthening contractions.

What factors determine the duration of an isotonic twitch in skeletal muscle? An isometric twitch? What effect does increasing the frequency of action potentials in a skeletal-muscle fiber have upon the force of contraction? Explain the mechanism responsible for this effect.

Describe the length-tension relationship in striated-muscle fibers.

Describe the effect of increasing the load on a skeletal-muscle fiber on the velocity of shortening. What is the function of creatine phosphate in skeletal-muscle contraction?

What fuel molecules are metabolized to produce ATP during skeletal-muscle activity?

List the factors responsible for skeletal-muscle fatigue. What component of skeletal-muscle fibers accounts for the differences in the fibers' maximal shortening velocities?

Summarize the characteristics of the three types of skeletal-muscle fibers.

Upon what two factors does the amount of tension developed by a whole skeletal muscle depend? Describe the process of motor-unit recruitment in controlling (a) whole-muscle tension and (b) velocity of whole-muscle shortening. During increases in the force of skeletal-muscle contraction, what is the order of recruitment of the different types of motor units? What happens to skeletal-muscle fibers when the motor neuron to the muscle is destroyed? Describe the changes that occur in skeletal muscles following a period of (a) long-duration, low-intensity exercise training; and (b) short-duration, high-intensity exercise training.

How are skeletal muscles arranged around joints so that a limb can push or pull?

What are the advantages and disadvantages of the muscle-bone-joint lever system?

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