skeletal muscles Flashcards
Describe how 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
● Made of many bundles of muscle fibres (cells) packaged together
● Attached to bones by tendons
● Muscle fibres contain:
=Sarcolemma (cell membrane) which folds inwards
(invagination) 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
When sarcomeres contract (shorten)…
= H zones get shorter
= I band get shorter
= A band stays the same
= Z lines get closer
describe the process of a myofibril contraction
1.Depolarisation spreads down sarcolemma via T tubules causing Ca2+ release from sarcoplasmic reticulum, which diffuse to myofibrils
2.Calcium ions bind to tropomyosin, causing it to move → exposing binding sites on actin
3.Allowing myosin head, with ADP attached, to bind to binding sites on actin → forming an actinomyosin crossbridge
4.Myosin heads change angle, pulling actin along myosin, (ADP released), using energy from ATP hydrolysis
5.New ATP binds to myosin head causing it to detach from binding site
6.Hydrolysis of ATP by ATP(hydrol)ase (activated by Ca2+) releases energy for myosin heads to return to original position
7.Myosin reattached to a different binding site further along actin. Process is repeated as long as calcium ion conc. is high
what happens during muscle relaxation
- Ca2+ actively transported back into the endoplasmic reticulum using energy from ATP
- Tropomyosin moves back to block myosin binding site on actin again → no actinomyosin 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
slow twitch general properties
● Specialised for slow, sustained contractions (eg. posture, long distance running)
● Obtain ATP mostly from aerobic respiration → release energy slowly
● Fatigues slowly
structure of slow twitch
● High conc. of myoglobin → stores oxygen for aerobic respiration
● Many mitochondria → high rate of aerobic respiration
● Many capillaries → supply high conc. of oxygen / glucose for aerobic respiration and to prevent build-up of lactic acid causing muscle fatigue
location of slow twitch
● High proportion in muscles used for posture eg. back, calves
● Legs of long distance runners
fast twitch of fast twitch
● Specialised for brief, intensive contractions (eg. sprinting)
● Obtain ATP mostly from anaerobic respiration → release energy quickly
● Fatigues quickly due to high lactate conc.
location of fast twitch
● High proportion in muscles used for fast movement eg. biceps, eyelids
● Legs of sprinters
structure of fast twitch
● 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 (in cytoplasm)
● Store phosphocreatine
