6.6 Skeletal muscles

Question 1

How do muscles work?

Answer 1

• In antagonistic pairs (one muscle contracts, other muscle relaxes)
• Skelton is incompressible so muscle can transmit force to bind

Question 2

How is skeletal muscle structured?

Answer 2

• Made of many bundles of muscle fibres packaged together
• Muscle fibres contain sarcolemma, sarcoplasm, sarcoplasmic reticulum, many nuclei, mitochondria and myofibrils
• Myofibrils contain parallel actin and myosin filaments, arranged in sarcomeres

Question 3

What is the banding pattern seen in myofibrils?

Answer 3

• I bands are light bands containing actin only
• A bands are dark bands containing myosin and some overlapping actin
• H zone contains myosin only

Question 4

How does muscle contraction occur?

Answer 4

• Depolarisation spreads down sarcolemma
• Causing Ca2+ release from sarcoplasmic reticulum which diffuse into myofibrils
• Calcium ions bind to tropomyosin, causing it to move, exposing binding sites on actin
• Allowing myosin head to bind to binding sites on actin, forming an actin-myosin cross bridge
• Myosin heads bend, pulling actin along, using energy from ATP hydrolysis
• New ATP binds to myosin head, causing it to detach from binding site
• Hydrolysis of ATP releases energy for myosin heads to return to original position
• Myosin reattaches to a different binding site further along actin

Question 5

What happens after muscle contraction?

Answer 5

• Ca2+ actively transported back into sarcoplasmic reticulum using energy from ATP
• Tropomyosin moves back to block myosin binding site on actin

Question 6

What is the role of phosphocreatine in muscle contraction?

Answer 6

• Provides Pi, rapidly phosphorylating ADP
• Producing ATP and creatine
• Used in short bursts of vigorous exercise

Question 7

Slow twitch

Answer 7

Specialised for slow, sustained contractions
Produces more ATP but slowly by aerobic respiration
Fatigues slowly
High concentration of myoglobin, storing oxygen for aerobic respiration
Many mitochondria
Many capillaries
Stores less phosphocreatine and glycogen

Question 8

Fast twitch

Answer 8

Specialised for brief, intensive contractions
Produces less ATP but rapidly by anaerobic respiration
Fatigues rapidly
Low concentration of myoglobin
Fewer mitochondria
Fewer capillaries
Store phosphocreatine and glycogen