Lecture 7

Question 1

Epimysium

Answer 1

connective tissue that surrounds the entire muscle tissue

Question 2

Perimysium

Answer 2

connective tissue surrounding a bundle of muscle fibers

Question 3

Endomysium

Answer 3

a network of connective tissue, which surrounds individual muscle fibres

Question 4

Fascicle

Answer 4

a group of muscle fibers is “bundled” as a unit within a muscle

Question 5

Sarcolemma

Answer 5

the plasma membrane of the muscle cell

• surface membrane of 1 muscle fibre

Question 6

Sarcomere

Answer 6

the basic contractile unit of muscle fibre

Question 7

What activates muscle contraction?
I.e. leads to Ca2+ release

Answer 7

At the neuromuscular junction:
(1) a nerve impulse from the axon comes into the nerve ending and releases Ach into the synaptic cleft
- Ach diffuses across the synaptic cleft and attaches to Ach receptors on the sarcolemma

(2) muscle action potential depolarizes transverse tubules at the sarcomere’s A-I junction

(3) T-tubule system depolarization causes Ca2+ release from the sarcoplasmic reticulum lateral sacs

(4) Ca2+ binds to troponin in actin filaments and tropomyosin (“rope”) moves away from the binding sites on actin
- allowing for myosin to bind

Question 8

What is the Cross Bridge Cycle

Answer 8

a series of molecular events that triggers muscle contraction

• Sarcomere shortens when myosin heads in thick myofilaments form cross-bridges with actin molecules and thin myofilaments

Question 9

What has to happen before the Cross Bridge Cycle can begin?

Answer 9

Myosin head must be activated

• this occurs when ATP binds to the myosin head and is hydrolyzed to ADP and inorganic phosphate the energy liberated from the hydrolysis of ATP activated myosin head – forcing it into the cocked position

Question 10

What happens to initiate the Cross Bridge Cycle

Answer 10

Formation of a cross bridge is initiated when calcium ions released from the sarcoplasmic reticulum bind to troponin
- binding causes troponin to change shape

Tropomyosin moves away from the myosin binding sites on actin, allowing the myosin head to bind actin and form a cross bridge

Question 11

What are the 4 steps of the Cross Bridge Cycle?

Answer 11

1. Cross-bridge formation
2. Power Stroke
3. Cross-bridge detachment
4. Reactivation of the Myosin head

Question 12

Cross Bridge Cycle:
Step 1 - Cross-bridge formation

Answer 12

Activated myosin head binds to actin forming a cross bridge

Inorganic phosphate is released and the bond between myosin and actin becomes stronger

Question 13

Cross Bridge Cycle:
Step 2 - Power Stroke

Answer 13

ADP is released and the activated myosin head pivots

Sliding the thin myofilament toward the center of the sarcomere

Question 14

Cross Bridge Cycle:
Step 3 - Cross-bridge detachment

Answer 14

When another ATP binds to the myosin head the link between the myosin head and actin weakens and the myosin head detaches

Question 15

Cross Bridge Cycle:
Step 4 - Reactivation of the Myosin head

Answer 15

ATP is hydrolyzed to ADP and inorganic phosphate

The energy release during hydrolysis reactivates the myosin head, returning it to the cocked position

Question 16

Key points of the Cross Bridge Cycle

Answer 16

As long as the binding sites on actin remain exposed, the cross-bridge cycle will repeat

As the cycle repeats the thin myofilaments are pulled toward each other and the sarcomere shortens
- This shortening causes the whole muscle to contract

Cross-bridging ends when calcium ions are actively transported back into the sarcoplasmic reticulum

Troponin returns to its original shape allowing tropomyosin to glide over and cover the myosin binding site on actin

Question 17

What is muscle relaxation?

Answer 17

When the second ATP binds to the myosin head, weakening the link between myosin and actin
- when 1 myosin head detaches

Question 18

What is the definition of neuromuscular fatigue?

Answer 18

A failure to maintain the required force during a given task;
- An exercise-induced decline in maximal muscle force or power production capacity

• Central fatigue
• Peripheral fatigue

Question 19

Central NS

Answer 19

Brain
Spinal Cord

Question 20

Peripheral NS

Answer 20

Spinal nerves (autonomic and somatic)
Muscle units

• note: e-c coupling = excitation contraction coupling

Question 21

What is voluntary activation (VA)

Answer 21

the amount of recruitment of muscles during a voluntary contraction effort

Question 22

What is the Twitch Interpolation technique (ITT)?

Answer 22

A test that determines central vs. peripheral fatigue
- necessary to discriminate between central and peripheral fatigue mechanisms

Consists of stimulating a representative sample of the muscle belly through an electric shock both during a voluntary contraction and at rest
- Stimulation to the femoral nerve

Question 23

Explanation of ITT

Answer 23

MVC force output is the maximal voluntary force that your central nervous system + your muscles (i.e., peripheral system) can produce
- (MVC - maximum voluntary contraction)

Reduction in MVC does NOT inform us whether the deficit is from the brain or the muscle

Larger SIT after fatigue means a reduction of CNS to drive muscle voluntarily

Smaller resting twitch after fatigue = a reduction of the exercised muscle’s ability to produce force.
- This is peripheral fatigue (this is not voluntary)

Decreased voluntary activation means central fatigue

*To measure voluntary activation, you need BOTH SIT and RT

Question 24

Superimposed Twitch (SIT)

Answer 24

The size of SIT indicates central fatigue

During ITT, SIT is measured by stimulating the femoral nerve after (during) voluntary contraction

Question 25

Potential or Resting Twitch (RT)

Answer 25

Size of RT indicates peripheral fatigue
- measured by looking at RT before and after exercise

Question 26

How do you calculate Voluntary activation?

Answer 26

Voluntary activation = 1–SIT/RT*100

ex. Voluntary activation = [1– (2/300)]*100 = 99.3%
* this means there is a 0.7% deficit

Question 27

Central Fatigue

Answer 27

When the brain region related to fatigue and pain sensations is activated and the brain sends fewer voluntary motor signals to the skeletal muscles
- The sensation of fatigue and pain reduces muscle contraction and thus maximal voluntary force or power production capacity decreases

Question 28

Why do we have central fatigue?

Answer 28

Afferent feedback theory:

• High-intensity exercise results in the accumulation of metabolic by-products such as lactate and hydrogen ions in your muscles
• These metabolites activate sensory afferent neurones, called group III/IV afferent, which in return convey pain- and fatigue-related sensory signals to the brain

Question 29

Central fatigue in simple terms

Answer 29

Metabolites activate afferent receptors in your muscles and send signals to the brain that generate pain and fatigue
- Brain “panics” and reduces signals to muscles (central motor output)

Question 30

What happens when fentanyl is injected into the spinal cord?

Answer 30

It blocks the group III/IV afferents and maintains the voluntary motor drive to the exercising muscles

• same as an epidural injection during labour

Question 31

What is shown in this graph?

Answer 31

That power output was significantly higher in participants with fentanyl
- but only in the 1st half
- 2nd half: fentanyl participants had significantly less power output (why? peripheral fatigue)

Control & placebo participants had steady power output and steady central & peripheral fatigue

Question 32

What are the Mechanisms of peripheral fatigue?

Answer 32

Neuromuscular junction

Calcium availability

Metabolic alterations

Slowing of relaxation

Question 33

Mechanisms of peripheral fatigue: Neuromuscular junction

Answer 33

Inhibition of pre- and post-synaptic areas

Inadequate Ach release

Question 34

Mechanisms of peripheral fatigue:
Calcium availability

Answer 34

decreased release of Ca+

Question 35

Mechanisms of peripheral fatigue: Metabolic alterations

Answer 35

decreased the level of ATP and PCr

Question 36

Mechanisms of peripheral fatigue: Slowing of relaxation

Answer 36

slowed down Ca+ reuptake

slowed down actomyosin detachment

Question 37

What activates muscle contraction?
I.e. leads to Ca2+ release

Answer 37

At the neuromuscular junction:
(1) a nerve impulse from the axon comes into the nerve ending and releases Ach into the synaptic cleft
- Ach diffuses across the synaptic cleft and attaches to Ach receptors on the sarcolemma

(2) muscle action potential depolarizes transverse tubules at the sarcomere’s A-I junction

(3) T-tubule system depolarization causes Ca2+ release from the sarcoplasmic reticulum lateral sacs

(4) Ca2+ binds to troponin in actin filaments and tropomyosin (“rope”) moves away from the binding sites on actin
- allowing for myosin to bind

Question 38

What activates muscle contraction?
I.e. leads to Ca2+ release

Answer 38

At the neuromuscular junction:
(1) a nerve impulse from the axon comes into the nerve ending and releases Ach into the synaptic cleft
- Ach diffuses across the synaptic cleft and attaches to Ach receptors on the sarcolemma

(2) muscle action potential depolarizes transverse tubules at the sarcomere’s A-I junction

(3) T-tubule system depolarization causes Ca2+ release from the sarcoplasmic reticulum lateral sacs

(4) Ca2+ binds to troponin in actin filaments and tropomyosin (“rope”) moves away from the binding sites on actin
- allowing for myosin to bind