Lab #8: Muscle Physiology

In muscle cells, ATP plays an important role in

(a) disconnecting the myosin head from the binding site on actin after the power stroke; (b) energizing the power stroke of the myosin cross bridge; (c) providing energy to pump calcium back into the terminal cisternae of the sarcoplasmic reticulum

Slow oxidative, fatigue-resistant fibers

A relatively high percentage are found in successful marathon runners. Red fibers, because they contain the oxygen-binding protein myoglobin. Abundant in muscles used to maintain posture because they are fatigue resistant.

What is the definition of a motor unit?

A single neuron and the group of muscle fibers it innervates.

In which phase of the muscle twitch shown in the figure above would the maximum amount of ATP be consumed by myosin heads? Refer to printed page of images. Image #4.

B

Which image shows a cross-section through a portion of muscle that is more likely to be fatigue-resistant and have high endurance? Refer to printed page of images. Image #1.

B

In the figure in part G, why is the strength of muscle contraction greater after stimulus #6, compared to stimulus #4? To compare the strength of muscle contraction, look at the tension development caused by each stimulus in the bottom panel of the figure. Refer to printed page of images. Image #6.

Because additional motor units join in (are recruited) to generate an increase in tension.

Why would a muscle that is overstretched develop less tension when stimulated, compared to a muscle that is at the optimum, resting length?

Because there is less overlap of thick and thin filaments, so less ability to form cross-bridges between myosin heads and actin.

Above are 3 sarcomeres of different lengths. Which of the choices (A, B, or C) demonstrates the sarcomere length that would result in the greatest amount of tension (force) generated when the muscle contracts? Refer to printed page of images. Image #7.

C

In the figure shown in part K above, which image shows a sarcomere at rest (as opposed to contracted or overstretched)? Refer to printed page of images. Image #7.

C

In which phase of the figure would the net movement of Ca2+ into the sarcoplasmic reticulum (SR) be greatest? Refer to printed page of images. Image #4.

C

Which of the following statements correctly describes one of the phases of a muscle twitch? During the latent period, the t-tubule depolarize, Ca2+ is released from the terminal cisternae, but there is no visible shortening of the sarcomere. During relaxation, sarcomeres shorten as a result of myosin cross bridge cycling, and Ca2+ is continuing to be released from the terminal cisternae. During contraction, Ca2+ is actively transported back into the terminal cisternae, cross-bridge cycling comes to an end, and the sarcomere lengthens. During relaxation, the sarcolemma and t-tubules depolarize, Ca2+ is released into the cytosol, and cross-bridges begin to form.

During the latent period, the t-tubule depolarize, Ca2+ is released from the terminal cisternae, but there is no visible shortening of the sarcomere.

Fast glycolytic, fatiguable fibers

Have very fast-acting myosin ATPases and depend upon anaerobic metabolism during concentration. Contain abundant amounts of glycogen so primarily use glycolysis to make APT.

Glucose can be used to synthesize ATP either through the aerobic pathway or anaerobic pathway. Which of the following statements is NOT TRUE? In the aerobic pathway the end-product is lactic acid. The anaerobic pathway does not require oxygen. In the aerobic pathway, pyruvic acid is converted to lactic acid. The aerobic pathway can use oxygen either directly from the blood or from myoglobin stores in the muscle.

In the aerobic pathway, pyruvic acid is converted to lactic acid.

What is the case of temporal or wave summation in regards to muscle contraction?

Increased levels of calcium in the cytosol lead to increased cross-bridge cycling and increased muscle tension.

Which process can generate the most ATP for a muscle cell?

Kreb's cycle & oxidative phosphorylation

Would you expect red muscle fibers to synthesize ATP mainly by glycolysis or by the Kreb's cycle and oxidative phosphorylation?

Kreb's cycle & oxidative phosphorylation

Rank from the first to the last steps in stages involved in a twitch contraction that correlate to the A, B, and C brackets at the top of the graph. Refer to printed page of images. Image #3.

Latent period → contraction period → relaxation period

If both motor neurons shown in this figure were to develop action potentials and stimulate muscle fibers, would all the muscle cells shown here contract? Refer to printed page of images. Image #2.

No, because neurons in this figure do not innervate every muscle cell shown.

What is true regarding large motor units?

One motor neuron innervates a large number of muscle fibers (hundreds).

Fast oxidative (intermediated) fibers

Pink in color - have both myoglobin to store oxygen and glycogen to store glucose.

What type of activity is best controlled by muscles made up of small motor units?

Playing the piano.

How is it possible that the resting length of a skeletal muscle also corresponds to the sarcomere length that allows for optimum overlap of the thick and thin filaments (and maximum tension development)?

Skeletal muscles are maintained near their optimum length by the way they are attached to bones

What is true regarding small motor units?

Small motor units allow for precise control of small movements.

What is true about stimulus #5 in the above figure? Refer to printed page of images. Image #6.

Some of the motor units in the muscle have been activated by stimulus #5, but not all of the motor units.

What result would be expected if an additional stimulus, equal in intensity to the first, were to be applied to the muscle at the 60 millisecond (ms) time point? Refer to printed page of images. Image #4.

The muscle would increase in tension to a level greater than that measured at the beginning of phase C.

If white muscle fibers have large glycogen stores and lower levels of myoglobin, which of the following statements would you expect to be TRUE? Glucose is not available in these muscle fibers. These muscle fibers rely mostly on the aerobic pathway to generate ATP which can result in lactic acid build up. Oxygen is readily available in these muscle fibers. These muscle fibers rely mostly on the aerobic pathway to generate ATP.

These muscle fibers rely mostly on the aerobic pathway to generate ATP which can result in lactic acid build up.

Which of the following describes unfused or incomplete tetanus?

When a muscle is stimulated more than once in a relatively short time span, the force of contraction due to later stimuli is greater than the force of contraction with the first stimulus.

How is energy released from ATP?

a hydrolytic enzyme breaks the high energy bond at the terminal phosphate group

muscle tone

a nearly constant state of low level tension in a resting muscle due to a random motor unit contractions

When a whole muscle is stimulated, it shows variations in the development of tension, or the force of contraction depending on number of motor units recruited degree of muscle stretch frequency of stimulation

all of the above

What is myoglobin, and what is its role in muscle tissue?

an oxygen-binding protein found in muscle tissue

What is the name of the muscle response when multiple stimuli of identical intensity are delivered to a muscle and the muscle contraction or tension is sustained at a high level, with no evidence of relaxation occurring between stimuli?

complete (fused) tetanus

complete (fused) tetanus

continued sustained contraction without any relaxation due to rapid stimulation

Which is the phase in which Ca2+ is continuing to be released from the terminal cisternae and peak tension develops in the muscle as a result of myosin cross bridge cycling?

contraction phase

Muscle cells can obtain glucose

either directly from the blood or by the breakdown of glycogen stores in the muscle cell

motor unit recruitment

how a smooth increase in muscle force is produced by activation of adding motor units

A muscle twitch is a single stimulus-contraction-relaxation sequence in a muscle fiber. An examination of a myogram (recording) of a muscle twitch shows 3 phases. What are the three phases of a muscle twitch?

latent period, contraction phase, relaxation phase

Each neuron shown in this figure innervates a group of muscle fibers. What is the term for a group of muscle fibers innervated by a single neuron? Refer to printed page of images. Image #2.

motor unit

Even in a muscle's relaxed state, random motor unit contractions provide a nearly constant state of low-level tension in the muscle. This is called:

muscle tone

The oxygen-binding protein found in muscle cells is

myoglobin

When ATP supplies are low, muscle cells can use 3 different processes to synthesize additional ATP. Which of the following is NOT one of the ways a muscle cell can generate ATP? pumping calcium into the terminal cisternae of the sarcoplasmic reticulum glycosis Kreb's cycle & oxidative phosphorylation hydrolysis of creatine phosphate

pumping calcium into the terminal cisternae of the sarcoplasmic reticulum

If both of the neurons in this figure were activated, more muscle fibers would contract than if either neuron alone were active. This mechanism for control of the force of muscle contraction is known as Refer to printed page of images. Image #2.

recruitment

Increasing stimulus from the CNS to motor neurons that innervate a skeletal muscle will result in increased tension due to additional motor units in the muscle being activated. This response is called

recruitment

During which phase are calcium ions (Ca2+) being actively pumped back into the terminal cisternae of the sarcoplasmic reticulum?

relaxation phase

Red muscle fibers are known as

slow twitch oxidative fibers

Which of the stimuli (in the figure) are either equal to or greater than the maximal stimulus that is needed to recruit all of the motor units in the indicated skeletal muscle? Refer to printed page of images. Image #6.

stimuli 7, 8, 9, 10

The relationship between the length of a muscle and the amount of tension that can be developed during contraction is called

the length-tension relationship

Which of the following regions best identifies where myosin would have maximum cross-bridge access to actin? Refer to printed page of images. Image #8.

the region between 2 and 3

temporal (wave) summation

the situation in which additional contractions become stronger (increased tension) due to stimulation before complete relaxation occurs

maximal stimulus

the stimulus above which no stronger contraction can be elicited, because all motor units are firing in the muscle

How many motor units are shown in the picture above? Refer to printed page of images. Image #2.

two

In the above figure, the myogram (recording of muscle tension) demonstrates the tension developed in the muscle when two identical stimuli are delivered to the muscle in quick succession (the second stimulus was delivered to the muscle before the first muscle twitch had completely relaxed). The muscle twitch recorded due to the second stimulus was larger than the first muscle twitch and is positioned on the falling phase of the first muscle twitch. This phenomenon, where a second identical stimulus causes a larger twitch is called Refer to printed page of images. Image #5.

wave or temporal summation

Which muscle fiber type has large diameter fibers, reduced myoglobin, and high glycogen content?

white muscle fibers

Which type of muscle fiber is best suited for activities requiring power and speed over a short duration?

white muscle fibers