3.6.3 - Skeletal muscles are stimulated to contract by nerves and act as effectors Flashcards
Topic 6
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
Advantage to how muscles work (posture)
The second muscle is required to reverse movement caused by the first (muscles can only pull) and contraction of both muscles helps maintain posture
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
Give an 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
1) Depolarisation spreads down sarcolemma via T tubules causing Ca2+
release from sarcoplasmic reticulum, which diffuse to myofibrils
2) Calcium ions bind to troponin causing tropomyosin 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+ ions) releases
energy for myosin heads to return to original position
7) Myosin reattaches to a different binding site further along actin
8) Process is repeated as long as calcium ion concentration is high
During muscle relaxation:
- When nervous stimulation stops, Ca2+ ions are actively transported back into the sarcoplasmic reticulum using energy from the hydrolysis of ATP
- This reabsorption of Ca2+ ions allows Tropomyosin to move back to block the myosin binding site on actin again → no actinomyosin cross bridges form as myosin heads are now unable to bind to actin filaments
3) So contraction stops and muscle relaxes
> In this state, force from antagonistic muscles can pull actin filaments out from between myosin (to a point)
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
General properties of Slow twitch skeletal muscle fibres
> General properties
● Specialised for slow, sustained
contractions (eg. posture, long
distance running)
● Produce more ATP slowly (mostly) from
aerobic respiration
● Fatigues slowly
Location of slow twitch skeletal muscle fibres
● High proportion in muscles used for posture eg. back, calves
● Legs of long distance runners
Structure of slow twitch skeletal muscle fibres
● 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
General properties of fast twitch skeletal muscle fibres
● Specialised for brief, intensive contractions (eg. sprinting)
● Produce less ATP rapidly (mostly) from
anaerobic respiration
● Fatigues quickly due to high lactate
concentration
Location of fast twitch skeletal muscle fibres
● High proportion in muscles used for fast
movement eg. biceps, eyelids
● Legs of sprinters
Structure of fast twitch skeletal muscle fibres
● 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
Two types of skeletal muscle fibres
- Slow twitch skeletal muscle fibres
- Fast twitch skeletal muscle fibres
