TOPIC 6 - skeletal muscles Flashcards
name the 3 types of muscle in the body and where they are located?
cardiac - heart
smooth - walls of blood vessels and intestines
skeletal - attached to incompressible skeleton by tendons
what does the phrase antagonistic pair of muscles mean?
- muscles work in pairs
- one muscle pulls in one direction and the other muscle pulls in opposite direction
describe gross structure pf skeletal muscle?
- made up of bundles of muscle fibres surrounded by connective tissues
- muscle fibres = multinucleate
- each muscle fibre contains myofibrils made up of sarcomeres
- sarcomeres = consist of actin and myosin arranged in a banded pattern
describe microscopic structure of skeletal muscles?
myofibrils = site of contraction
Sarcoplasm = cytoplasm of muscle cell = contains lots of mitochondria and endoplasmic reticulum
sarcolemma = folds inwards towards sarcoplasm to form T- tubules
draw a diagram to show ultra structure of a myofibril.
Z-line = boundary between sarcomeres
I-band = only actin
A-band = overlap of actin and myosin
H-zone = only myosin
how does each band appear under an optical microscope?
I band - light
A band - dark
How is muscle contraction stimulated?
- NMJ - action potential = voltage-gated Ca2+ channels open
- vesicles move towards and fuse with pre-synaptic membrane
- exocytosis of Ach, which diffuses across the synaptic cleft
- ACh binds to receptor on Na+ channel proteins on skeletal muscle cell membrane
- influx of Na+ = depolarisation
explain the role of Ca2+ ions in muscle contraction.
Ca²⁺ binds to troponin, causing it to change shape.
This moves tropomyosin, exposing myosin-binding sites on actin.
Myosin heads can now bind to actin, forming cross-bridges.
Activates ATP hydrolase, providing energy for the power stroke.
Describe how ATP is used in muscle contraction.
ATP hydrolysis provides energy for myosin head movement (power stroke).
ATP breaks cross-bridge between actin and myosin.
ATP used to re-cock myosin head for the next cycle.
ATP used for active transport of Ca²⁺ back into sarcoplasmic reticulum.
Explain how the structure of a sarcomere changes during muscle contraction.
I-band shortens.
H-zone shortens.
A-band stays the same.
Sarcomere shortens, pulling Z-lines closer together.
Outline the sliding filament theory?
Ca²⁺ released from sarcoplasmic reticulum, binds to troponin.
Troponin changes shape, moving tropomyosin, exposing myosin-binding sites on actin.
Myosin heads bind to actin, forming cross-bridges.
ATP hydrolysis provides energy for power stroke (myosin head pulls actin).
New ATP binds, causing myosin head to detach.
ATP is hydrolyzed, re-cocking myosin head for next cycle.
Repeats, pulling actin over myosin, shortening the sarcomere (muscle contraction).
state 4 pieces of evidence that supports the sliding filament theory.
H-zone narrows
I-band narrows
Z-lines get closer ( sarcomere shortens)
A zone remains same width ( proves that myosin filaments do not shorten)
what happens during muscle relaxation?
Ca2+ is actively transported back into endoplasmic reticulum
tropomyosin once again blocks actin binding site
explain the role of phosphocreatine in muscle contraction?
phosphorylates ADP directly to ATP when oxygen for aerobic respiration is limited
how could a student calculate the length of one sarcomere?
- view thin slice of muscle under optical microscope
- calibrate eyepiece graticule
- measure distance from middle of one light band to middle of another
what are slow twitch and fast-twitch muscle fibres around the body?
slow twitch : sites of sustained contraction eg calf muscle
fast twitch: sites of short term, rapid, powerful contraction eg biceps
explain the role of slow twitch and fast twitch muscle fibres.
slow twitch: long duration contraction; well adapted to aerobic respiration to prevent lactate buildup
fast twitch: powerful short-term contraction; well-adapted to anaerobic respiration
explain the structure and properties of slow-twitch muscle fibres.
- glycogen store = many terminal ends can be hydrolysed to release glucose for respiration
- myoglobin - higher affinity for oxygen than Hb at lower partial pressures
- many mitochondria - aerobic respiration produces more ATP
- surrounded by many blood vessels = high supply of O2 and glucose
explain structure and properties of fast-twitch muscle fibres.
- large store of phosphocreatine
- more myosin filaments
- thicker myosin filaments
- Higher conc of enzymes involved in anaerobic respiration
- extensive sarcoplasmic reticulum : rapid uptake and release of Ca2+
what is a motor unit?
one motor neuron supplies several muscle fibres, which act simultaneously as one functional unit.
