6.3 Skeletal muscles are stimulated to contract by nerves and act as effectors 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
Advantage - the second muscle 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
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
general properties of slow twitch fibres
● Specialised for slow, sustained
contractions (eg. posture, long
distance running)
● Produce more ATP slowly (mostly) from
aerobic respiration
● Fatigues slowly
general properties of fast twitch 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 slow twitch fibres
● High proportion in muscles used for posture eg. back, calves
● Legs of long distance runners
location of fast twitch fibres
● High proportion in muscles used for fast
movement eg. biceps, eyelids
● Legs of sprinters
structure of slow twitch 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
structure of fast twitch 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
