Muscle III

Question 1

How many APs happen in muscle fiber from 1 in alpha motor neuron?

Answer 1

1
Very efficient

Question 2

How does Calcium trigger contraction?

Answer 2

Removes inhibition of cross bridge cycling
- binds to low affinity sites on Troponin C which induces conformational change in troponin complex
- troponin and tropomyosin move revealing myosin binding site on actin

Question 3

Steps of myosin binding to actin and power stroke

Answer 3

1. ATP binds to myosin, myosin drops actin
2. Myosin hydrolyzes ATP. ATP rotates myosin head to cocked position. Myosin binds weakly to actin
3. Power stroke begins when phosphate released. Myosin tightly grabs actin and pulls.
4. Myosin releases ADP at end of power stroke
5. Cycle may continue if calcium still high and tropomyosin out of way
   Cycle may begin at any one of these steps

Question 4

What positions can myosin be in for cycling to begin?

Answer 4

• nothing bound
• down with ATP bound
• cocked forward because of ADP+P and already hydrolyzed ATP

Question 5

What happens during ATP binding

Answer 5

ATP binds to head of myosin heavy chain reducing affinity of myosin for actin
Drops actin

Question 6

What happens during ATP hydrolysis?

Answer 6

ATP is broken down to ADP and inorganic phosphate resulting in myosin head pivoting into cocked state
Cocked head is now aligned with and binds to a new actin molecule on thin filament

Question 7

What happens during the power stroke

Answer 7

Dissociation of Pi from myosin head strengthens bond between actin and myosin and triggers power stroke
A conformational change where myosin head returns to un-cocked state and while doing so pulls actin filament generating force and motion

Question 8

What happens during ADP release

Answer 8

Dissociation of ADP from myosin causes myosin to remain bound to actin until ATP initiates cycle again

Question 9

What does termination of contraction require

Answer 9

Removal of Calcium
Must be removed so myosin binding site on actin can be covered by tropomyosin

Question 10

How is calcium removed?

Answer 10

1. Can be removed into Extracellular space by Na-Ca exchanger or by Ca2+ pump
   - eventually would deplete all calcium so only plays minor role
2. Calcium reuptake in SR

Question 11

How does calcium reuptake to SR?

Answer 11

Mediated by Sarcoplasmic and endoplasmic reticulum Ca2+-ATPase (SERCA)-type Ca2+ pump
- high Ca in SR inhibits pump
- Ca binding proteins exist in SR to delay inhibition

Question 12

What are binding proteins in SR that delay inhibition of Ca reuptake?

Answer 12

Calsequestrin and calreticulin
Maximize Ca uptake by SR
- 50 binding sites per protein

Question 13

How many binding sites for calcium does troponin c have?

Answer 13

2 low affinity binding sites
When Ca is high they will bind
When low unbind

Question 14

What causes sarcomere to return to initial resting position?

Answer 14

Elastic recoil of titin and other connective tissue

Question 15

What is rigor mortis “stiffness of death”?

Answer 15

Development of rigid muscle several hours after death
-2-36 hours after
- Ca leaks into Sarcoplasm and binds to troponin
- myosin tightly grabs actin and muscles can’t move
- ATP production has stopped

Question 16

What is the timing of the excitation-contraction coupling?

Answer 16

• slight delay between motor neuron AP and muscle fibre AP (synaptic release)
• delay between muscle fibre AP and contraction (latent period) (more significant delay)

Question 17

Why is there slight delay between motor neuron AP and muscle fibre AP?

Answer 17

Ca must come in cause exocytosis of ACh, ACh must cross synaptic cleft and activate nicotinic receptors and create end plate potential
- few msec

Question 18

What is latent period?

Answer 18

Delay between motor muscle fibre AP and the contraction
- Ca is being released and binding troponin
- DHP, ryanidine receptors, Ca bind troponin, move tropomyosin, cross bridge starts

Question 19

What is ATP needed for in muscle contraction?

Answer 19

Myosin ATPase (contraction)
Ca2+ ATPase: SERCA (relaxation)
Na+/K+ ATPase (after AP in muscle fibre)

Question 20

What are the sources of ATP

Answer 20

1. Free intracellular ATP (few seconds)
2. ATP formed from phosphocreatine (10 sec)
3. Anaerobic metabolism
4. Aerobic metabolism

Question 21

How is ATP formed from phosphocreatine?

Answer 21

Phosphocreatine is created at rest from creatine and ATP
When active, phosphocreatine and ADP create more ATP using creatine kinase

Question 22

What is needed for a steady supply of ATP for muscle?

Answer 22

• large amount of glucose stored in muscle form glycogen (glycogenesis)
• when ATP needed: glycogen is converted back to glucose (glycogenolysis)

Question 23

What is anaerobic metabolism

Answer 23

One glucose broken down to pyruvate by glycolysis resulting in 2 ATP
- occurs without oxygen
- takes place in sarcoplasm
- pyruvate converted to lactate

Question 24

What is oxidative (aerobic) metabolism?

Answer 24

• oxygen and mitochondria present
• after glycolysis pyruvate enters citric acid cycle producing 2 ATP and high energy electrons and H+
• e- and H+ combine with O2 in electron transport chain to produce 26-28 ATP
• in mitochondria

Question 25

How many ATP molecules after aerobic metabolism

Answer 25

30-32 ATP