6.3 Skeletal muscles Flashcards
what are the muscle fibres called
myofibrils
name the 3 types of muscles in the body and where they are located
cardiac - exclusively found in heart
Smooth - walls of blood vessels and intestines
Skeletal - attached to incompressible skeleton by tendons
What does the phrase antagonstic pair of muscles mean
muscles can only pull, so they work in pairs to move bones around joints
Pairs pull in opposite directions - agonist contracts while antagonist is relaxed
Describe the gross structure of the skeletal muscle
muscle cells are fused together to form bundles of parallel muscle fibres (myofibrils)
Arrangement ensure there is no point of weakness between cells
Each bundle is surrounded by endomycium - loose connective tissues with many capillaries
Describe the microscopic structure of skeletal muscle
myofibrils
Sacroplasm
Sacrolemma
myofibrils
site of contraction
Sacroplasm
shared nuclei and cytoplasm with lots of mitochondria and endoplasmic reticulum
Sacrolemma
folds inwards towards sacroplasm to form transverse (T) tubules
Draw a diagram to show the ultrastrcutre of myofibril
Z line - bondarary 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?
Neuromuscular junction: Action potential = voltage-gated Ca²⁺ channels open.
Vesicles move towards & fuse with presynaptic membrane.
Exocytosis of acetylcholine (ACh), which diffuses across synaptic cleft.
ACh binds to receptors on Na⁺ channel proteins on skeletal muscle cell membrane.
Influx of Na⁺ = depolarisation.
Explain the role of Ca²⁺ ions in muscle contraction.
1.Action potential moves through T-tubules in the sarcoplasm = Ca²⁺ channels in sarcoplasmic reticulum open.
2- Ca²⁺ binds to troponin, triggering conformational change in tropomyosin.
3- Exposes binding sites on actin filaments so actinomyosin bridges can form.
Outline the ‘sliding filament theory’.
Myosin head with ADP attached forms cross bridge with actin.
Power stroke: Myosin head changes shape & loses ADP, pulling actin over myosin.
ATP attaches to myosin head, causing it to detach from actin.
ATPase hydrolyses ATP → ADP + Pi so myosin head can return to original position.
Myosin head re-attaches to actin further along filament.
How does sliding filament action cause a myofibril to shorten?
Myosin heads flex in opposite directions = actin filaments are pulled towards each other.
Distance between adjacent sarcomere Z-lines shortens.
Sliding filament action occurs up to 100 times per second in multiple sarcomeres.
State 4 pieces of evidence that support the sliding filament theory.
H-zone narrows.
I-band narrows.
Z-lines get closer (sarcomere shortens).
A-band 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 (e.g. during vigorous exercise).
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.
Where are slow and fast-twitch muscle fibres found in the body?
Slow-twitch: Sites of sustained contraction (e.g. calf muscle).
Fast-twitch: Sites of short-term, rapid, powerful contraction (e.g. biceps)
Explain the role of slow 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.
Contain myoglobin: Higher affinity for oxygen than haemoglobin at lower partial pressures.
Many mitochondria: Aerobic respiration produces more ATP.
Surrounded by many blood vessels: High supply of oxygen & glucose.
Explain the structure and properties of fast-twitch muscle fibres.
Large store of phosphocreatine.
More myosin filaments.
Thicker myosin filaments.
High concentration of enzymes involved in anaerobic respiration.
Extensive sarcoplasmic reticulum: Rapid uptake & release of Ca²⁺.
What is a motor unit?
One motor neuron supplies several muscle fibres, which act simultaneously as one functional unit.
what are microfibrils made up of
actin and myosin
actin
is a globular protein
long chain
thinner and consists of two stands twisted around one another
myosin
made up of two types of proteins:
1- fibrous protein arranged into a filament
2- globular protein - two bulbous structure at one end
thicker and consists of long rod- shaped tails with bulbous heads that project to the side
tropomyosin
forms a fibrous strand around the actin filament
what is the neuromuscular junction
point where a motor neuron meets a skeletal muscle fibre
similarities between neuromuscular junction and a synapse
both have
- neurotransmitter that are transported by diffusion
- receptors that on binding with the neurtransmiiter, causes an influx of sodium ions
- both use sodium - potassium pump to repolorise the axon
- use enzymes to breakdown the neurotransmitter
differences between neuromuscular junctions and cholinergic synapse
contraction of muscle involves
the sliding filament mechanism
muscle relaxation
when nervous stimulation end
Ca2+ ions AT back into endoplasmic reticulum - using energy from hydrolysis of ATP
reabsorption of Ca ions allows tropomyosin to block the actin filament again
Myosin head now unable to bind to actin filament
contraction end
energy for muscle contraction
hydrolysis of ATP
for…
movement of myosin head
Reabsorption of ca ions into endoplasmic reticulum by AT
explain the band pattern shown in the diagram
light I band only actin
H zone/ band only myosin
darkest overlapping region actin and myosin
Another group of scientists suggested that a decrease in the force of muscle contraction is caused by an increase in the concentration of inorganic phosphate, Pi, in muscle tissues.
Their hypothesis is that an increase in the concentration of Pi prevents the release of calcium ions within muscle tissues.
Explain how a decrease in the concentration of calcium ions within muscle tissues could cause a decrease in the force of muscle contraction.
less tropomyosin moved from binding site
fewer actinomyosin bridges formed
Myosin head doesn’t move
less ATP hydrolyase
In muscles, pyruvate is converted to lactate during prolonged exercise.
Explain why converting pyruvate to lactate allows the continued production of ATP by anaerobic respiration
regenerates/ produces NAD
so glycolysis continues
Name structures C, D and E.
C- M line/ myosin filament
D - mitochondrion
E - myofibril
Give the name of the structure shown between points A and B.
sacromere
The image shows glycogen granules present in skeletal muscle.
Explain their role in skeletal muscle.
as a store of glucose
to be hydrolysed to glucose
for respiration/ to provide ATP
During vigorous exercise, the pH of skeletal muscle tissue falls. This fall in pH leads to a reduction in the ability of calcium ions to stimulate muscle contraction.
Suggest how.
1- low ph changes shape of calcium ion receptors
2- fewer calcium ions bind to tropomyosin
3- fewer tropomyosin molecules move away
4- fewer binding sites on actin revealed
5- fewer cross - bridges can form
6- fewer myosin heads can bind
Describe the roles of calcium ions and ATP in the contraction of a myofibril. 5
1-Cacium ions diffuse into myofibrils from sacroplasmic reticium
2- causes movement of tropomyosin on actin
3- Movement causes exposure of the binding site on the actin
4- myosin heads attach to binding sites on actin
5- hydrolysis of ATP on myosin head causes myosin heads to bend
6- bending pulling actin molecules
7- attachment of a new ATP molecule to each myosin head causes myosin heads to detach from actin sites
What is the role of ATP in myofibril contraction?
reaction with ATP breaks binding of myosin to actin
2- provides energy to move myosin head
