Lecture 12

Question 1

What are the three key differences between fiber types:

Answer 1

1. Type of myosin expressed
2. Energy production
3. Type of SERCA pump expressed

Question 2

What fiber types are there?

Answer 2

1. Type I - slow twitch (oxidative)
2. Type IIA - fast twitch (oxidative-glycolytic)
3. Type IIB - fast twitch (glycolytic)

Question 3

How do types of myosin differ?

Answer 3

different types of myosin have different ATPase. Fast ATPase means fast ATP breakdown, while slow ATPase means slow ATP breakdown. This alters the speed of cross-bridge cycling, as speed of contraction differs.

Question 4

How does energy production differ?

Answer 4

High oxidative activity has a lot of mitochondria and can generate ATP continuously using O2 but it relatively slow.
high glycolytic activity can generate ATP quickly from muscle glycogen, but has limited stores.

Question 5

How does the type of SERCA pump differ?

Answer 5

faster or slower clearance of Ca2+ from sarcoplasm into SR affects the speed of drop in tension.

Question 6

Characteristics of Type I (slow oxidative) fibres:

Answer 6

• slow form of myosin ATPase
• high levels of myoglobin - O2 reserve
• many mitochondria, high level of oxidative enzymes
• lower density of RyR, so the response to a single AP results in less Ca2+ release
• slow SERCA pump
• rich blood supply
  contraction is slow, but sustained. it is fatigue resistant.

Question 7

Characteristics of Type IIB (glycolytic) fibres:

Answer 7

• fast form of myosin ATPase
• low myoglobin levels
• few mitochondria, low levels of oxidative enzymes
• high density of RyR - a lot of Ca2+ is released
• Fast SERCA pump
• uses glycogen, which has limited stores
  Contraction is fast, but it runs out and is easily fatigued.

Question 8

Characteristics of Type IIA fibres:

Answer 8

• fast form myosin ATPase
• high oxidative and glycolytic enzymes
• intermediate speed/fatigue

Question 9

TRUE or FALSE - muscle are either one type of fiber or the other

Answer 9

FALSE - muscle might have a higher proportion of one muscle type, but there is always a mix

Question 10

What type of fiber is dominant in the soleus?

Answer 10

It uses low power but is resistant to fatigue - higher proportion of Type I oxidative fibers

Question 11

What type of fibre is dominant in the biceps?

Answer 11

Power producing but susceptible to fatigue - higher proportion of Type II glycolytic fibres

Question 12

What are the effects of strength training?

Answer 12

It produces more actin and myosin, so increases fiber diameter. more actin-myosin interactions can form, so more force can be produced.

Question 13

What are the effects of endurance training?

Answer 13

It increases oxidative capacity, which increases the ability for sustained activity. it results in more mitochondria, more capillaries (and myoglobin), increased muscle stores of lipid (for energy production), and increased ability to use lipids directly from blood.

Question 14

What is contraction?

Answer 14

Generation of tension

Question 15

What is isotonic contraction?

Answer 15

Contraction with force held constant while muscle length changes.

Question 16

What is isometric contraction?

Answer 16

Contraction with length held constant

Question 17

What are the two physical limits to muscle performance?

Answer 17

1. Length-tension relationship

2. Load-velocity relationship

Question 18

What is the Length-tension relationship?

Answer 18

At normal resting length, we can generate peak force for a muscle twitch. When shortening or lengthening the fiber, the force from a twitch drops. this is due to the cross-bridge interactions that go on.
as we shorten the skeletal muscle, the actin filaments start crossing over each other, and it blocks the ability of the myosin heads to form cross-bridges. on the other hand, when the fiber is stretched too far apart, myosin heads cannot reach any actin and no cross bridges can form. changing the muscle length directly affects the amount of force that can be produced.

Question 19

What is passive tension?

Answer 19

The more we stretch a muscle, the more force we have to apply to it, and the more it wants to recoil

Question 20

What is the load-velocity relationship?

Answer 20

As we increase the load on a muscle, the speed in which we can contract the muscle decreases. sometimes, the load is so heavy that the muscle is only capable of holding it in place (V=0).
when there is little load, we contract so quickly most cross-bridges don’t even contract. as we slow down contraction, cross-bridges have more time to form and can generate more force.

Question 21

what is eccentric contraction?

Answer 21

It when the load is so heavy we can only put it back down. velocity is negative and the muscle is in place.

Question 22

When can we generate the most force?

Answer 22

when the muscle stops shortening and we can generate the most cross-bridges.

Question 23

What is a motor unit?

Answer 23

It is a motor neuron and the muscle fibres it innervates

Question 24

What are the properties of a motor neuron?

Answer 24

• all muscle fibers in one unit are same metabolic type
• motor unit size varies
• all fibers in unit active at once, so maximum force from a unit depends on a unit’s size
• size of motor neuron cell body depends on number of muscle fibres in the motor unit

Question 25

What is muscle force regulated by?

Answer 25

Changing rate of activity in each unit and the changing number of units active.

Question 26

What is the rate modulation of motor units?

Answer 26

It is the change in force produced by one motor unit

Question 27

How can we generate more force?

Answer 27

1. By having a higher frequency of action potentials

2. by activating more motor units

Question 28

What is the initial muscle twitch used for?

Answer 28

to stretch the muscle and overcome its elastic properties. following twitches work on the already stretched muscle

Question 29

Do we activate small motor units or big motor units first?

Answer 29

We active small units first, for fine motor control. as we need more forced, we activate larger motor units.