Exam 2: Lab 4 Contraction of Motor Units & Contraction of Whole Muscle

Question 1

What are the groups of muscle cells called, within a muscle, that are innervated by one motor neuron and contracted together and stimulated by that motor neuron?

Answer 1

motor unit

Question 2

What important factors determine the strength of contraction of a muscle?

Answer 2

the size and number of motor units being stimulated

Question 3

What is a motor unit composed of?

Answer 3

motor neuron and all of the muscle cells it stimulates

Question 4

What is the junction between the branch of a neuron and a muscle cell called?

Answer 4

Neuromuscular junction

Question 5

When a motor neuron fires, how many cells are stimulated?

Answer 5

all of muscle cells

Question 6

What is the relationship between the strength of the muscle contraction and the number of motor units which are stimulated?

Answer 6

stronger contraction more number of motor units to be stimulated

Question 7

What is recruitment

Answer 7

the stimulation of additional motor units for increased strength of contraction

Question 8

What is the general function of motor units that contain only a few muscle cells?

Answer 8

generates precise movement

Question 9

what two factors effect the strength and degree o f muscle movement?

Answer 9

number of motor units firing and the number of cells per motor unit

Question 10

Describe the motor units in large muscles which exhibit gross movements.

Answer 10

They have a large motor units in which a single neuron is connected to a large number of muscle cells

Question 11

Tell if the following muscles would have a small or large motor units:

a. muscles of the fingers
b. biceps brachii mucle
c. gastrocnemius muscle in speech
d. muscles of the throat involved

Answer 11

a. small
b. large
c. small
d. large

Question 12

Muscle A contains 12,000 muscle cells and 30 motor neurons which innervate these cells. Muscle B contains 2,000 muscle cells and 400 motor neurons which innervate these cells.:

a. On average, how many muscle cells are there in each motor unit for both of these muscles?
b. Which of these muscles would you expect to contract with precise control?

Answer 12

a. Muscle A: 400 muscle cells
Muscle B: 5 muscle cells

b. B

Question 13

what is muscle tone?

Answer 13

resistance to stretch and (low level tension)

Question 14

What causes muscle tone?

Answer 14

random asynchronous motor unit contraction (provide a nearly constant state of low level tension)

Question 15

What happens to a muscle if all the motor neurons to that muscle are cut?

Answer 15

it will become flaccid

Question 16

What is a motor unit?

Answer 16

groups of muscle cells, within a muscle, that are innervated by one motor neuron and contracted together.

Question 17

What is recruitment?

Answer 17

the stimulation of additional motor units for increased strength of contraction

Question 18

Describe the different types of contraction that occur in motor units of different sizes.

Answer 18

small motor unit - precise movement

large motor unit - gross movement

Question 19

what is muscle tone and how is it produced?

Answer 19

Muscle tone: resistance to stretch and produced by random asynchronous motor unit contraction

Question 20

The level of muscle tension that can be developed within a given muscle is always constant.

a. true
b. false

Answer 20

b. false

Question 21

What are the three factors which affect muscle tension in a whole muscle?

Answer 21

1. Frequency of stimulation
2. Number of motor units recruited
3. Degree of muscle stretch

Question 22

Muscle twitch

Answer 22

a muscle contraction in response to a single stimulus of adequate strength

Question 23

What are the three phases of a muscle twitch?

Answer 23

1. latent period
2. contraction period
3. relaxation period

Question 24

latent period

Answer 24

• no visible shortening of the muscle

- the sarcolemma and the T tubules depolarize

Question 25

Contraction period

Answer 25

myosin cross bridge cycling causes sarcomeres to shorten

Question 26

relaxation period

Answer 26

1. calcium ions are actively transported back into the terminal cisternae
2. cross bridge cycling decreases and ends
3. muscle to return to its original length

Question 27

What two factors will cause the speed of the contraction phase of a muscle twitch to vary?

Answer 27

1. slow-twitch fibers

2. fast-twitch fibers

Question 28

Temporal (wave) summation

Answer 28

when a second stimulus of the same intensity is applied to a muscle before the completion of the relaxation period of the first stimulus resulting in increased muscle tension

Question 29

In temporal summation, why is the second peak is higher than the first?

Answer 29

Additional influx of Ca+ promotes a second contraction, which is added to the first contraction.

Question 30

If two stimuli are given to the same muscle, which will result in a more intense second contraction?

a. if the stimuli are given 50 milliseconds apart
b. if the stimuli are given 70 milliseconds apart

Answer 30

a.

Question 31

If you wait until relaxation is complete from the first stimulus, then give a second stimulus to the same muscle, why wont temporal summation occur?

Answer 31

1. cross bridge cycling has stopped; there is no activity to be summed
2. The Ca+ ions from the first contraction have already been actively transported back into the terminal cisternae

Question 32

complete tetanus

Answer 32

Muscle stay contracted, with no apparent relaxation

Question 33

treppe

Answer 33

the muscle relax totally between stimulations

Question 34

temporal summation

Answer 34

strength of contraction increase with each stimulation because of a gradual accumulation of calcium ions in the cytosol

Question 35

incomplete tetanus

Answer 35

strength of contraction decrease with each stimulation

Question 36

reasons for muscle fatigue

Answer 36

• ionic imbalance
• relative but not total lack of ATP
• a build-up of acidic compounds (lactic acid)

Question 37

Factors that affects the development of muscle tension

Answer 37

a. number of motor units recruited
b. degree of muscle stretch
c. frequency of stimulation

Question 38

a given muscle always uses the same number of motor units, no matter what the muscle is trying to accomplish.

a. true
b. false

Answer 38

b. false

Question 39

subthreshold stimulus

Answer 39

the minimum stimulus (voltage) which can evoke a response is applied to a muscle

Question 40

maximal stimulus

Answer 40

the stimulus (voltage) is applied to a muscle. when another, more intense stimulus does not evoke greater tension the original stimulus would be called maximal stimulus.

Question 41

Temporal summation:
How does the time interval between stimuli affect he muscle contraction?
Does relaxation occur?

Answer 41

• smaller time frequency = increase contraction

- no relaxation occur

Question 42

What happens to the graph of muscle contraction when the frequency of stimulation is increased?

Answer 42

reaches maximal stimulus

Question 43

How does the number of motor units recruited affect the strength of contraction?

Answer 43

increase number of motor units = greater strength of contraction

Question 44

multiple motor unit summation:

• how is threshold determined?
• how is this different than the threshold in vivo?

Answer 44

• in vivo: happen naturally

- pick up something/or move hit threshold

Question 45

multiple motor unit summation:

• how are motor units recruited?
• how is this different from recruitment in vivo?

Answer 45

• varied voltage - varied wt pick up

- brain estimates number of motor unit needed and activates it.