Muscle III Flashcards
How many APs happen in muscle fiber from 1 in alpha motor neuron?
1
Very efficient
How does Calcium trigger contraction?
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
Steps of myosin binding to actin and power stroke
- ATP binds to myosin, myosin drops actin
- Myosin hydrolyzes ATP. ATP rotates myosin head to cocked position. Myosin binds weakly to actin
- Power stroke begins when phosphate released. Myosin tightly grabs actin and pulls.
- Myosin releases ADP at end of power stroke
- Cycle may continue if calcium still high and tropomyosin out of way
Cycle may begin at any one of these steps
What positions can myosin be in for cycling to begin?
- nothing bound
- down with ATP bound
- cocked forward because of ADP+P and already hydrolyzed ATP
What happens during ATP binding
ATP binds to head of myosin heavy chain reducing affinity of myosin for actin
Drops actin
What happens during ATP hydrolysis?
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
What happens during the power stroke
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
What happens during ADP release
Dissociation of ADP from myosin causes myosin to remain bound to actin until ATP initiates cycle again
What does termination of contraction require
Removal of Calcium
Must be removed so myosin binding site on actin can be covered by tropomyosin
How is calcium removed?
- Can be removed into Extracellular space by Na-Ca exchanger or by Ca2+ pump
- eventually would deplete all calcium so only plays minor role - Calcium reuptake in SR
How does calcium reuptake to SR?
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
What are binding proteins in SR that delay inhibition of Ca reuptake?
Calsequestrin and calreticulin
Maximize Ca uptake by SR
- 50 binding sites per protein
How many binding sites for calcium does troponin c have?
2 low affinity binding sites
When Ca is high they will bind
When low unbind
What causes sarcomere to return to initial resting position?
Elastic recoil of titin and other connective tissue
What is rigor mortis “stiffness of death”?
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
What is the timing of the excitation-contraction coupling?
- slight delay between motor neuron AP and muscle fibre AP (synaptic release)
- delay between muscle fibre AP and contraction (latent period) (more significant delay)
Why is there slight delay between motor neuron AP and muscle fibre AP?
Ca must come in cause exocytosis of ACh, ACh must cross synaptic cleft and activate nicotinic receptors and create end plate potential
- few msec
What is latent period?
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
What is ATP needed for in muscle contraction?
Myosin ATPase (contraction)
Ca2+ ATPase: SERCA (relaxation)
Na+/K+ ATPase (after AP in muscle fibre)
What are the sources of ATP
- Free intracellular ATP (few seconds)
- ATP formed from phosphocreatine (10 sec)
- Anaerobic metabolism
- Aerobic metabolism
How is ATP formed from phosphocreatine?
Phosphocreatine is created at rest from creatine and ATP
When active, phosphocreatine and ADP create more ATP using creatine kinase
What is needed for a steady supply of ATP for muscle?
- large amount of glucose stored in muscle form glycogen (glycogenesis)
- when ATP needed: glycogen is converted back to glucose (glycogenolysis)
What is anaerobic metabolism
One glucose broken down to pyruvate by glycolysis resulting in 2 ATP
- occurs without oxygen
- takes place in sarcoplasm
- pyruvate converted to lactate
What is oxidative (aerobic) metabolism?
- 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
How many ATP molecules after aerobic metabolism
30-32 ATP
