lecture 17

Question 1

2 key molecules required for cross bridge formation and cycling

Answer 1

1. Ca2+ : allows contraction to occur by binding troponin which moves tropomyosin out of the way so myosin to bind to actin (high force cross bridge formation)
2. ATP: for myosin heads to do this they need a source of energy so since myosin has a binding site for ATP, it will hydrolyze ATP and the power stroke motion will occur. Binding to ATP will allow myosin to detach from actin (cross bridge cycling)

Question 2

in absence of calcium explain what will happen

Answer 2

1. we will be in a relax state
2. without Ca2+ there is no signal from troponin to move tropomyosin out of the way therefore myosin binding sites on actin will be covered
3. this will result in low force cross bridge

Question 3

With presence of Ca2+ what will happen

Answer 3

1. increase in Ca2+
2. Ca2+ will bind to troponin
3. troponin pulls tropomyosin out of the way of exposing myosin binding sites on actin
   —> which is what allows high force cross bridge formation
4. myosin binds strongly to actin and stays which allows for cross ridge cycling and power stroke to occur

Question 4

the contraction cycle

Answer 4

1. ATP binds to myosin
2. myosin hydrolizes ATP and uses this energy to swivel into ready position
3. input of Ca2+ signal (explain process), but my myosin binding sites are exposed
4. Myosin binds to actin cause power stroke to occur
5. pulling on neighbouring thin and tick filaments at same time will cause entire muscle to shorten
6. ATP that we’re hydrolized are let go
7. myosin is still bound till we get new ATP to bind and remove it

Question 5

what is a sarcomere

Answer 5

it is a small bit of a myofibril made up of one thick filament and 2 thin filaments

Question 6

What happens during contraction

Answer 6

• thin and thick filaments do not change length but they slide past one another (sliding filament theory)
• I band and H band shorten
• sarcomere shortens (z line is closer)

Question 7

Excitation contraction coupling in skeletal muscle

Answer 7

1. Neurotransmitter acetylcholine is released via exocytosis and binds to nicotinic ACh channels which is always excitatory
2. Channels open and K+ flows out but not to a huge extent and Na+ flows in and has a greater driving force because cell is farther from Ena this causes a depolarizing end plate potential
3. EPP is a supra-threshold and activates voltage gated Na+ channels to trigger an action potential. For action potential to spread through muscle fiber via T-tubules voltage gated Na+ and K+ channels are required
4. Action potential in t-tubule causes conformational change in DHP receptor which triggers opening because the they are voltage gated
5. DHP receptor opens RyR channels because they are physically linked and RyR channel is a mechanically gated channel
6. Ca2+ rushes out of sarcoplasmic reticulum and into sarcoplasm
7. Ca2+ binds to troponin and troponin signals tropomyosin to move actin can bind to myosin
8. Myosin heads will execute powerstroke and actin filaments slide toward center of sarcomere

Question 8

Relaxation

Answer 8

1. Muscle must relax, so Ca2+ is put back into the sarcoplasmic reticulum by a pump which requires an energy input called SERCA
2. With calcium gone it will unbind to troponin
3. Troponin will no longer pull on tropomyosin and it will slip back over myosin binding sites on actin
4. In this case there is no more high force cross bridges or cross bridge cycling and titin will help bring muscle back to resting length

Question 9

a muscle twitch in a singles muscle fiber is

Answer 9

one cycle of contraction followed by one cycle of relaxation

Question 10

summation

Answer 10

stimuli closer together not allowing muscle to fully relax cause more forceful contraction

Question 11

summation leading to unfused tetanus

Answer 11

stimuli are far enough apart to allow muscle to relax slightly between stimuli

Question 12

summation leading to complete tetanus

Answer 12

muscle is working at full capacity
fatigue causes muscle to loose tension

Question 13

a motor unit is

Answer 13

• lower motor unit collaterals and all of the muscle fibers it innervates

Question 14

size principle of motor unit recruitment

Answer 14

• come in different sizes
• smaller motor units are recruited first if they can do the job because they’re more excitable than large motor units

Question 15

length-tension relationship

Answer 15

• sarcomere contract w/ max force when they are at their optimal resting length just prior to contraction
• at long lengths actin are spaced to far apart and myosin can’t reach actin binding sites
• at short lengths actin filaments overlap interfering with some binding sites

Question 16

more muscle fibers…

Answer 16

generate more force