Lecture 12- Skeletal Muscle Plasticity

Question 1

What does the term plasticity refer to?

Answer 1

Changing properties/structures in order to suit different functions

Question 2

List the 3 key differences between skeletal muscle fiber types…

Answer 2

• Type of Myosin expressed
• Oxidative vs glycolytic energy production
• Type of SERCA pump expressed

Question 3

How does the type of myosin expressed differ across different types of skeletal muscle?

Answer 3

• Fast or slow utilization of ATP
• Alters speed of cross-bridge cycling and therefore contraction speed
• If burn through ATP quickly then prone to fatigue if slow then fatigue resistant (sustain force over long period)

Question 4

How does oxidative or glycolytic energy production differ between muscle fiber types?

Answer 4

-High oxidative activity (mitochondria) can generate ATP continuously using O2 and substrates from blood but only relatively slowly
– High glycolytic activity can generate ATP quickly from muscle glycogen but glycogen stores limited (will run out)

Question 5

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

Answer 5

• Faster or slower clearance of Ca2+ from cytoplasm into SR (against concentration gradient)
• faster or slower drop in tension

Question 6

What are type 1 muscle fibers like?

Answer 6

• Slow Oxidative Fibres
• Slow form of myosin ATPase so can’t form cross bridges as quickly
• Many mitochondria, high levels of oxidative enzymes, rich blood supply as continuously producing energy
• Slow form of SERCA
• If fatigue resistant as can hold max force for a long period

Question 7

What are type 2B muscle fibers like?

Answer 7

• Glycolytic Fibres
• Fast form of myosin ATPase means cross bridge cycle occurs fast
• Few mitochondria, low levels of oxidative enzymes, fewer capillaries as mainly using glycogen stores for energy
• Fast form of SERCA
• Prone to Fatigue because there is period where glycogen will be used up without the chance to replenish it

Question 8

What are type 2A intermediate fibers?

Answer 8

• Fast form of myosin ATPase (why it’s a type 2)
• Mix of oxidative and glycolytic enzymes
• Intermediate speed/fatigue

Question 9

What is the distribution of muscle fibers like in humans (and most other mammals)?

Answer 9

-Humans (and most mammals) have a mixed distribution of fiber types because we are a lot more generalized in our muscle use (specific areas don’t completely align with specific functions)

Question 10

How does the colour/ appearance of the differ muscle fiber types differ?

Answer 10

• Slow twitch, oxidative (Type 1)= red
• Fast twitch, oxidative-glycolytic (Type 2 A)= pink
• Fast twitch, glycolytic (Type 2 B)= white

Question 11

In muscles that are power producing but suspectable to fatigue what proportion of fibers would be highest?

Answer 11

Type 2 (B) glycolytic fibers

Question 12

In muscles that are low power producing but resistant to fatigue what proportion of fibers would be highest?

Answer 12

Type 1 oxidative fibers

Question 13

How come training has an effect on skeletal muscle?

Answer 13

Skeletal muscle is adaptive to life conditions and restructures depending on what you need more of

Question 14

What is the effect of strength training on skeletal muscle as opposed to endurance training?

Answer 14

Strength training:
• More actin & myosin →increased fiber diameter (hypertrophy)
• More cross-bridges → more force

Endurance training → Increased oxidative capacity → increased ability for sustained activity
• more mitochondria (more enzymes to break down oxygen and glucose into ATP)
• more capillaries, myoglobin (store oxygen in cells)
• Increased muscle stores of lipid (store energy that we don’t need at that exact moment)
• Increased ability to use lipids directly from blood

Question 15

Skeletal muscle goes into a state of rigor mortis after death BECAUSE calcium binding to troponin is required for actin/myosin interaction.

For the following question, select:
(A) if both statements (the one before and the one after ‘BECAUSE’)
are true, and are causally related (the fact presented in the first
statement is a result of the fact presented in the second statement)
(B) if both statements are true but are not causally related
(C) if the first statement is true and the second is false
(D) if the first statement is false and the second is true
(E) if both statements are false

Answer 15

B) both statements true but not casually related

Question 16

What is a motor unit?

Answer 16

A motor neuron and all the muscle fibres it innervates

Question 17

How do the type of muscle fibers in a motor unit relate?

Answer 17

Muscle fibers in any one motor unit are the same metabolic type: Type 1, type 2A or type 2B. As it is the motor neuron that determines it.

Question 18

Is motor unit size always the same?

Answer 18

No, it varies depending on what muscle it is leading to/ specific function

Question 19

How does the activation of a motor unit work? Can you activate some parts and not others?

Answer 19

• 1:1 relationship between signals that comes from the motor neuron down and into the fibers themselves.
• Every time motor neuron fires all fibers attached/ branches are activated and fibers contract.
• Because of this max force from a unit depends on its size

Question 20

What are the two ways muscle force is regulated?

Answer 20

• Recruitment

- Summation

Question 21

What is recruitment?

Answer 21

• The number of fibres activated is regulated by how many neurons are active at one time.
• A small number of active neurons tends to produce low force from the muscle, with the amount of force generally increasing as more neurons are activated.
• This process of activating more fibres to make more force is called recruitment.

Question 22

Explain the size principle of recruitment?

Answer 22

• Recruitment is orderly, smallest to largest.
• In general, small units recruited first, so more tonically active.
• Results in fine graded control of small forces.
• Bigger units automatically recruited as required force increases

Question 23

What is summation?

Answer 23

• At a low rate of stimulation have full twitch of activity including excitation and relaxation because calcium has time to leave before next input arrives
• As stimulation increases more action potentials arrive at fibers much more quickly, don’t have time to clear calcium each time so can’t fully relax. Therefore, increase in force production of 1 motor unit due to rapid stimulation= summation.

Question 24

What is tetanus?

Answer 24

Maximum obtainable force of an individual motor unit via summation. Signaling event is happening so fast that all cross-bridges that can form are. Even if the signal increased the amount of force produce would not.

Question 25

What is the length- tension relationship?

Answer 25

• There is a length of muscle that is perfect for creating force. If shorter or longer will produce less force= this occurs due to the way in which actin+ myosin overlap and the effect this has on cross bridge formation.
• At optimum want the myosin heads + actin to overlap enough so cross bridges can form at ease but with a little space for the sarcomere to shorten/ produce force.