5.5.16 The Sliding Filament Model

Question 1

Structure of filaments in myofibril

Answer 1

• Thick filaments made of myosin
• Fibrous protein
• Globular head
• Fibrous part of myosin anchors molecule
• Globular heads point away from M line
• Thin filaments made of actin
• Globular proteins
• Link forming a chain
• 2 actin chains twist forming 1 thin filament
• Fibrous protein tropomyosin twisted around actin chain
• Troponin attached to actin chain

Question 2

How does muscle contraction occur?

Answer 2

• Action potential arrives at neuromuscular junction
• Calcium ions released from sarcoplasmic reticulum into sarcoplasm by diffusion
• Calcium ions bind to troponin molecules stimulating them to change shape
• Troponin and tropomyosin changes position on actin filament
• Myosin binding sites exposed on actin molecule
• Globular heads of myosin molecule bind with these sites forming cross bridges between the 2 filaments
• Myosin heads bend and pull actin filament towards centre of sarcomere, causing muscle to contract a small distance
• ADP released
• ATP binds to myosin head letting it detach from actin
• Myosin head acts as ATPase enzyme, hydrolysing ATP into ADP and Pi
• Energy released during this allows myosin head to return to original position
• Myosin head now binds to site on actin filament closer to Z line
• Myosin head moves again pulling actin filaments even closer to centre of sarcomere and causing sarcomere to shorten further
• As long as troponin and tropomyosin are not blocking myosin binding sites, and the muscle has a supply of ATP, the process repeats til muscle is fully contracted

Question 3

What is the role of ATP ?

Answer 3

• ATP binding allows myosin to detach from actin
• ATP hydrolysis lets myosin heads return to original position
• Return of calcium ions back to sarcoplasmic reticulum occurs via active transport

Question 4

What is the role of phosphocreatine?

Answer 4

• Resting muscle only has small amount of ATP stored only lasting 3-4 seconds of intense exercise
• Mitochondria present in muscle fibres respire aerobically to produce ATP but is slow
• Anaerobic respiration is faster but takes 10 seconds before it produces ATP
• PHOSPHOCREATINE stored in muscles can rapidly produce ATP
• Phosphate ion transferred to ADP
• ADP + phosphocreatine = ATP + Creatine
• Lets muscles recontract for short period of time until mitochondria can resupply ATP