3.6.3 Skeletal muscles are stimulated to contract by nerves and act as effectors Flashcards

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1
Q

Describe how muscles work

A

• work in antagonistic pairs - pull in opposite directions e.g biceps
- one muscle contracts pulling on bone
- one muscle relaxes
• skeleton is incompressible so muscle can transmit force to bone

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2
Q

Describe an advantage of muscle fibres working antagonistically

A

the second muscle required to reverse movement caused by the first and contraction of both muscles helps to maintain posture

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3
Q

Describe the gross and microscopic structure of a skeletal muscle

A
  • made of many bundles of muscle fibres packaged together
  • attached to bones by tendons
  • muscle fibres contain :
    • sarcolemma which folds inwards to form transverse (T) tubules
    • sarcoplasm (cytoplasm)
    • multiple nuclei
    • many myofibrils
    • sarcoplasmic reticulum ( endoplasmic reticulum)
    • many mitochondria
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4
Q

describe the ultra structure of a myofibril

A

• made up of two types of long protein filaments , arranged in parallel
- myosin = thick filament
- actin = thin filament
• arranged in functional units called sarcomeres
- ends - Z line
- middle - M line
- H zone - contains only myosin

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5
Q

Explain the banding pattern to be seen in myofibrils

A

• I bands = light bands containing only thin actin filaments
• A bands = dark bands containing thick myosin filaments
- H zone contains only myosin
- Darkest region contains overlapping of actin and myosin

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6
Q

Give an overview of muscle contraction

A
  • myosin heads slide actin along myosin causing the sarcomere to contract
  • Simultaneous contraction of many sarcomeres cause myofibrils and muscle fibrils to contract
  • when sarcomeres contract :
    • H zones get shorter
    • I band gets shorter
    • A band stays the same
    • Z lines get closer
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7
Q

Describe the role of actin , myosin , calcium ions , tropomyosin and ATP in myofibril contraction

A

1) Depolarisation spreads down sarcolemma via T tubules causing Ca2+ release from sarcoplasmic reticulum which diffuse to myofibrils ( calcium ions , )
2) Calcium ions bind to tropomyosin causing it to move - exposing binding sites on actin
( calcium ions , actin , tropomyosin)
3) Allowing myosin head , with ADP attached , to bind to binding sites on actin - forming an actinomyosin cross bridge.
( Calcium ions , actin , tropomyosin , myosin )
4) Myosin heads change angle , pulling actin along myosin , ADP released , using energy from ATP hydrolysis
( ATP , actin , myosin)
5) New ATP binds to myosin head causing it to detach from binding site
( Actin , myosin , ATP)
6) Hydrolysis of ATP by ATPhydrolase (activated by Ca2+) releases energy for myosin heads to return to original position
( calcium ions , myosin , ATP)
7) Myosin reattached to a different binding site further along actin.
Process is repeating as long as calcium conc is high.
( Actin , myosin)

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8
Q

what happens during muscle relaxation

A

1) Ca2+ actively transported back into the endoplasmic reticulum using energy from ATP
2) Tropomyosin moves back to block myosin binding site on actin again - no actinmyosin cross bridges

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9
Q

Describe the role of phosphocreatine in muscle contraction

A

• a source of inorganic phosphate (Pi) => rapidly phosphorylates ADP to regenerate ATP
- ADP + phosphocreatine => ATP + creatine
• runs out after a few seconds => used in short bursts of vigorous exercise
• anaerobic and alactic

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10
Q

Compare the structure , location and general properties of slow and fast skeletal muscle fibres

A

SLOW TWITCH :
- General properties
• specialised for slow , sustained contractions (e.g long distant running )
• Produce more ATP slowly from aerobic respiration
• fatigues slowly
Location :
• high proportion in muscles used for posture
• legs of long distance runners
Structure :
• high conc of myoglobin => stores oxygen for aerobic respiration
• many mitochondria => high rate of aerobic respiration
• many capillaries => supply high conc of oxygen or glucose for aerobic respiration and to prevent build up of lactic acid causing muscle fatigue
FAST TWITCH :
General properties :
• specialised for brief , intensive contractions (e.g sprinting )
• Produce less ATP rapidly from anaerobic respiration
• fatigues quickly due to high lactate concentration
Location :
• high proportion in muscles used for fast movement e.g. biceps
• legs of sprinters
Structure :
• Low levels of myoglobin
• Lots of glycogen => hydrolysed to provide glucose for glycolysis/ anaerobic respiration which is inefficient so large quantities of glucose required.
• High conc of enzymes involved in anaerobic respiration
• Store phosphocreatine

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