Muscle Contractions

Question 1

describe skeletal muscles

Answer 1

• 40mm
• multinucleated
• striated

Question 2

describe cardiac muscle

Answer 2

• 100um
• uninucleated
• branched

Question 3

describe smooth muscle

Answer 3

• 30-100um
• non-striated
• mono-nucleated

Question 4

what is a contraction?

Answer 4

the interaction of actin and myosin using Ca2+ and ATP

Question 5

how is skeletal muscle organised?

Answer 5

in bundles called fascicles made of fibres

Question 6

what is the function of the sarcoplasmic reticulum?

Answer 6

stores Ca2+

Question 7

what is actin and myosin sometimes referred to?

Answer 7

• actin: thin filament

- myosin: thick filament

Question 8

what proteins are also associated with actin?

Answer 8

• tropomyosin: blocks the active site of actin
• troponin C: binds to Ca2+
• troponin T: binds to tropomyosin

Question 9

describe the structure of myosin

Answer 9

• has a head and tail
• head has 2 domains:
  1) binds to ATPase which allows it to flex
  2) binds to actin

Question 10

how is movement activated?

Answer 10

• ATP hydrolysis occurs which provides energy for the myosin to bind to actin, creating a cross bridge
• cross bridge bends, pulling the thin myofilament inward into the low energy configuration
• cross bridge detaches which requires ATP and returns into it’s original configuration
• cross bridge then binds to more distal actin molecule and cycle repeats

Question 11

how do you initiate the sliding filament mechanism?

Answer 11

• influx of Ca2+ from the sarcoplasmic reticulum sallows Ca2+ to bind to troponin-c
• this causes troponin T to bind to tropomyosin with pulls the tropomyosin out of the active site on actin
• this means that actin is free to bind to myosin

Question 12

how is Ca2+ released from the sarcoplasmic reticulum?

Answer 12

-Ach binds to the nAchR
-this leads to the Na+ channel opening
-the influx of Na+ leads to depolarisation down the t tubules
when the AP reaches the dihydropyridine protein (DHP) in the t tubule, it conforms
-the DHP binds to ryanodine receptor (RYR), the calcium release channel
-this conforms the RYR, causing Ca2+ to be released from the sarcoplasmic reticulum

Question 13

cardiac muscles do not have RYRs, so how do they cause an influx of Ca2+?

Answer 13

• cardiac muscles have Ca2+ channels
• an influx of Ca2+ causes Ca2+ to be released from the SR

=calcium induced calcium release (CICR)

Question 14

how is the contraction of smooth muscle different to skeletal muscle?

Answer 14

• they do not have t tubules and have an irregular myolfilament arrangement
• contraction proteins interact in a mesh
• SM wring out instead of getting shorter
• contains calmodulin and not troponin
• myosin is in a different isoform than in SKM (MYHII bc MYHI)

Question 15

how do smooth muscle get an influx of Ca2+?

Answer 15

-through receptor mediated Ca2+ stores or by the production of a diffusible messenger (IP3)