Lecture 12: Skeletal muscle performance and plasticity

Question 1

What are the three different types of muscle types?

Answer 1

• Type | (slow twitch)
• Type ||A (fast twitch, oxidative-glycolytic)
• Type ||B (fast twitch)

Question 2

Which of the three different types of muscle fibres has the most oxidative enzymes?

Answer 2

slow twitch (type |)

Question 3

Describe the slow twitch (oxidative) muscle fibres

• enzymes?
• fatigue?
• metabolism?
• big or small?

Answer 3

These have lots of oxidative enzymes and have small, slow twitches. They are fatigue resistant and so remain active for longer as they undergo oxidative metabolism. They are small so there are less myofilaments so they generate less force

Question 4

Describe the fast twitch (glycolytic) muscle fibres

• oxidative enzymes
• how do they make energy?
• big or small?

Answer 4

There are very few oxidative enzymes because they use anaerobic glycolysis to make energy. They are very large fibres and produce a very large, powerful twitch but can fatigue easily

Question 5

As well as fast and slow twitch fibres, there is an oxidative-glycolytic fast twitch fibre (type ||A). Describe this

Answer 5

This is a hybrid of the fast and slow twitch fibres and so it has properties of the fast twitch fibres (it can generate force quickly) but it contains both oxidative and glycolytic enzymes

Question 6

What are the three key differences between the muscle fibre types?

Answer 6

1. the type of myosin expressed
2. the oxidative versus glycolytic energy production
3. the type of SERCA pump expressed

Question 7

Describe how the type of myosin expressed differs between the two fibre types

Answer 7

Myosin is thick with heads to cross-bridge cycling and generating force when flexed. There are two different types of myosin that have different types of ATPases for catalysing the breakdown of ATP to ADP + Pi. These different ATPases alter the speed of cross-bridge cycling and therefore the speed of contraction. A fast twitch fibre has a fast ATPase which utilises ATP quickly but this is fatigue prone. A slow twitch fibre has a slow ATPase which utilises ATP slowly but this is fatigue resistant.

Question 8

Describe how the energy production differs between the two different fibres

Answer 8

The fast twitch fibres uses glycogen and glucose stores within the muscles which generates ATP very quickly but the stores are easily depleted.
The slow twitch fibres have lots of mitochondria to produce ATP continuously using oxygen and substrates from the blood. This process is relatively slow. There are also lots of myoglobin

Question 9

Which muscle type uses

glycolytic energy production?

Answer 9

type ||B - fast twitch

Question 10

Which muscle type uses oxidative energy production?

Answer 10

type | - slow twitch

Question 11

Wy do slow twitch fibres have lots of myoglobin?

Answer 11

to bind oxygen in the muscle

Question 12

How does the type of SERCA pump differ between the two types of muscle fibres?

Answer 12

The SERCA pump terminates skeletal muscle contraction because it is pumping Ca2+ back into the sarcoplasmic reticulum. When there is an action potential, the dihydropyridine receptor opens the Ca2+ channel and Ca2+ enters. Fast twitch fibres have SERCA that clears Ca2+ very quickly from the sarcoplasm into the sarcoplasmic reticulum which means that force can be terminated quickly. In the slow twitch, Ca2+ is cleared slower so it there is the slower twitch

Question 13

What does SERCA stand for?

Answer 13

sarcoplasmic endoplasmic reticulum calcium ATPase pump

Question 14

Describe Type | slow oxidative fibres

• what form of myosin ATPase?
• what are the levels of myoglobin ?
• mitochondria and oxidative enzymes?
• density of ryanodine receptors?
• type of SERCA pump?
• what is in the blood?

Answer 14

• they have the slow form of myosin ATPase
• there are high levels of myoglobin to be a reserve of O2
• there are many mitochondria and high levels of oxidative enzymes
• there is a low density of ryanodine receptors (less Ca2+ released into the sarcoplasm)
• slow SERCA pump
• have a rich blood supply to be able to get glucose, O2 and free fatty acids in for oxidative metabolism

Question 15

What is the effect of a single stimulus for a slow muscle fibre?

Answer 15

If there is a single action potential from a motor neuron, there is very slow force generation and it causes a single twitch

Question 16

What is the effect of lots of stimuli on a slow muscle fibre?

Answer 16

there is repetitive activation of the motor neuron and the force profile is fatigue resistant so it maintains steady level of force generation for a long period of time

Question 17

Describe type || glycolytic fast twitch fibres

Answer 17

• have the fast form of myosin ATPase
• have low levels of myoglobin
• have few mitochondria and low levels of oxidative enzymes
• have a high density of ryanodine receptors
  have a fast SERCA pump
• are less vascularised

Question 18

What is the effect of a single stimulus for a fast muscle fibre?

Answer 18

this generates one single twitch which is very fast

Question 19

What is the effect of lots of stimuli on a fast muscle fibre?

Answer 19

the force peaks quickly but fatigues because it uses up its energy stores

Question 20

Describe type ||A intermediate fibres

Answer 20

these are the fast form of myosin ATPase
there is high oxidative and glycolytic enzymes
there is an intermediate speed and fatigue

Question 21

Give an example of a muscle which have a higher proportion of type || glycolytic fibres

Answer 21

one that is power producing but susceptible to fatigue such as the bicep

Question 22

Give an example of a muscle which has a higher proportion of type A oxidative fibres

Answer 22

one that is resistant to fatigue but has low power such as the soleus muscle

Question 23

What is the effect of strength training?

Answer 23

This increases the amount of actin and myosin in the fibre which increases the fibre diameter (hypertrophy). This means that there is more actin-myosin interactions and this generated more force

Question 24

What is the effect of endurance training?

this increases the __________ capacity and increases the ability for ________ activity because it causes there to be more m________ and more o________ ________s, more c__________ and m__________, more m______ s________ of l_____, increased ability to use ______ directly from the _______

Answer 24

this increases the oxidative capacity and increases the ability for sustained activity because it causes there to be more mitochondria and more oxidative enzymes, more capillaries and myoglobin, more muscle stores of lipid, increased ability to use lipids directly from the blood

Question 25

What is contraction?

Answer 25

the generation of tension

Question 26

What are the two types of contraction?

Answer 26

isotonic and isometric

Question 27

What is isotonic contraction?

Answer 27

this is contraction with force held constant while the muscle length changes

Question 28

What is isometric contraction?

Answer 28

this is contraction with the length held constant while the force changes

Question 29

Describe a concentric contraction

Answer 29

for example, when doing a bicep curl and you’re lifting the dumbbell up, the force of contraction is greater than the external force (the dumbbell) and so the muscle shortens

Question 30

What is the opposite of a concentric contraction?

Answer 30

an eccentric contraction

Question 31

Describe an eccentric contraction

Answer 31

for example, when doing a bicep curl and you’re bringing the dumbbell back down, the external force is greater than the muscle force and the muscle lengthens

Question 32

Concentric and eccentric contractions are what type of contractions?

Answer 32

isotonic contractions

Question 33

What does the amount of force that a muscle can generate depend on?

Answer 33

the length of skeletal muscle and the speed of contraction

Question 34

The length-tension relationship can be shown on what type of contraction?

Answer 34

isometric contraction - muscle length is fixed at certain points

Question 35

What two things can we plot from the force and sarcomere length graph?

Answer 35

the passive and active tension

Question 36

Describe the passive tension

Answer 36

When we stretch the skeletal muscle, it wants to recoil due to its elastic components. The more we stretch it, the more it wants to recoil. When we stretch the skeletal muscle, the longer it is stretched, the more force we have to put on it to stretch it more - this is the passive tension, it is not due to cross-bridge cycling, it is only due to the elastic components of muscle

Question 37

Describe the active tension

Answer 37

When we electrically stimulate the muscle and we get a twitch. The size of the twitch changes based on the length of the muscle.
The active tension is due to the cross bridges between actin and myosin

Question 38

What happens to the force as you decrease the length of the muscle below its resting length?

Answer 38

the force goes down

Question 39

What happens to the force as you increase the length of the muscle from its resting length?

Answer 39

the force goes down

Question 40

What happens to the force generated as you shorten the skeletal muscle? Why is this?

Answer 40

the force decreases because the actin filaments start crossing over each other and you start blocking the ability of some of the myosin heads to form cross bridges with actin

Question 41

What happens to the force generated as you lengthen the skeletal muscle

Answer 41

it decreases because now non of the myosin heads are opposed to any actin so you can’t form any cross bridges

Question 42

What is the velocity and force generated when there is no load?

Answer 42

there is maximal velocity but there is no force generated

Question 43

What is the velocity and force generated when there is a medium load?

Answer 43

there is a reduction in the velocity but there is more force generated

Question 44

What happens when the velocity of shortening drops to zero?

Answer 44

the person cannot shorten their muscle at all

Question 45

If given such a heavy weight, what is the only thing that that they can do with it?

Answer 45

you can only let that weight down and you are getting lengthening of the muscle so you get an eccentric contraction

Question 46

The speed or velocity of muscle determines what?

Answer 46

how much force we can generate

Question 47

Why does the speed of contraction determine the force?

Answer 47

because if there is a really fast contraction, there is not a lot of time for them to form so not many cross bridges can form and this reduces the force
as you slow down the contraction, the cross bridges have more time to form and so more of them do form and so the force generated is larger

Question 48

Define a motor unit

Answer 48

a motor neuron and all the muscle fibres it innervates

Question 49

What are the properties of motor units

Answer 49

• all the muscle fibres in one unit are the same metabolic type (eg. all fast of all slow)
• the motor unit size varies from small (about 6 fibres) to large (more than 2000 fibres)
• all fibres in the unit are active at once so the maximum force from one unit depends on size
• the size of the motor neuron cell body depends on the number of muscle fibres in the motor unit

Question 50

What two processes allow us to increase the force generated?

Answer 50

• changing the rate of activity in each unit (rate modulation/mechanical summation) (rate principle)
• changing number of units active (unit principle)

Question 51

Describe the rate principle which can change the force generated

Answer 51

As you stimulate a muscle fibre, you need to overcome the elastic properties of the fibre so some of the cross bridge cycling goes into just overcoming the elastic properties before you can generate force. The initial twitch expends energy stretching the muscle and then when you have later twitches, those act on the pre-stretched muscle which are able to then generate more force. At high rates of stimuli, the Ca2+ cannot be cleared by the SERCA so there is continued cross bridge cycling and force generation. You can stimulate at such as high frequency of APs in the motor neuron so you get a fused tetanus

Question 52

Describe the recruitment principle which can change the force generated

Answer 52

We recruit more more motor units based on the size principle
this means we recruit the smallest motor units first (the ones that only innervate a few muscle fibres) to give us fine motor control of force generation and as we need to generate more force, we recruit larger motor unit