Muscles Flashcards
How do muscles work
● Work in antagonistic pairs → pull in opposite directions eg. biceps / triceps
○ One muscle contracts (agonist), pulling on bone / producing force
○ One muscle relaxes (antagonist)
● Skeleton is incompressible so muscle can transmit force to bone
Describe the gross and microscopic structure of skeletal muscle
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 ultrastructure of a myofibril
● Made 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 / disc
○ 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
(and some actin filaments)
○ H zone contains only myosin
○ Darkest region contains overlapping actin and
myosin
Overview of muscle contraction
● Myosin heads slide actin along myosin causing the sarcomere to contract
● Simultaneous contraction of many sarcomeres causes myofibrils and muscle fibres to contract
● When sarcomeres contract (shorten)…
○ H zones get shorter
○ I band get shorter
○ A band stays the same
○ Z lines get closer
Describe the roles of actin, myosin, calcium ions, tropomyosin and ATP in myofibril contraction
Depolarisation spreads down sarcolemma via T tubules causing Ca2+ release from sarcoplasmic reticulum, which diffuse to myofibrils
Calcium ions bind to tropomyosin, causing it to move → exposing binding sites on actin
Allowing myosin head, with ADP attached, to bind to binding sites on actin → forming an actinomyosin crossbridge
Myosin heads change angle, pulling actin along myosin, (ADP released), using energy from ATP hydrolysis
New ATP binds to myosin head causing it to detach from binding site
Hydrolysis of ATP by ATP(hydrol)ase (activated by Ca2+) releases energy for myosin heads to return to original position
Myosin reattaches to a different binding site further along actin Process is repeated as long as calcium ion concentration is high
During muscle relaxation
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 actinomyosin cross bridges
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
Composition of Skeletal Muscle
single cell forms one muscle fibre - its surface membrane is known as sarcolemma
t tubules - invaginations of sarcolemma - conduct a charge when cell depolarised
saroplasmic reticulum - contains large store of Ca ions
