3.6.3 Skeletal muscles are stimulated to contract by nerves and act as effectors Flashcards
Describe how muscles work
• 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
Describe an advantage of muscle fibres working antagonistically
the second muscle required to reverse movement caused by the first and contraction of both muscles helps to maintain posture
Describe the gross and microscopic structure of a skeletal muscle
- 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
describe the ultra structure of a myofibril
• 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
Explain the banding pattern to be seen in myofibrils
• 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
Give an overview of muscle contraction
- 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
Describe the role of actin , myosin , calcium ions , tropomyosin and ATP in myofibril contraction
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)
what happens during muscle relaxation
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
Describe the role of phosphocreatine in muscle contraction
• 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
Compare the structure , location and general properties of slow and fast skeletal muscle fibres
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