6.3 Skeletal muscles

Question 1

What are the 3 types of muscle and where are they found?

Answer 1

• skeletal muscle is attached to bone, and acts under voluntary, conscious control.
• smooth muscle is found in the walls of blood vessels and the gut
• cardiac muscle is found only in the heart
  Neither of these are under conscious control.

Question 2

What are myofibrils?

Answer 2

Individual muscles are made up of tiny muscle fibres called myofibrils.
Alone, they produce almost no force, but collectively can be extremely powerful.
The myofibrils are arranged to give maximum force.
Muscle is composed of smaller units bundled into progressively larger ones.

Question 3

Why is the structure of muscles not regular?

Answer 3

If muscle was individual cells joined end to end, the junction in between would be a point of weakness that would reduce the overall strength.

Question 4

What is the structure of the muscle?

Answer 4

• The separate cells have become fused together into muscle fibres.
  The muscle fibres share a nuclei and cytoplasm, the sarcoplasm, mainly found around the circumference of the fibre.
• There is a large concentration of mitochondria and endoplasmic reticulum in the sarcoplasm

Question 5

What are myofibrils made up of?

Answer 5

Two types of protein filament:
Actin, thinner and consists of two strands twisted around one another.
Myosin, thicker and consists of long, rod-shaped tails with bulbous heads that project to the side.

Question 6

What are the myofibril bands?

Answer 6

Myofibrils appear striped due to alternating light-coloured and dark-coloured bands.
The light bands are I-bands (isotropic), and are lighter because the thick and thin filaments don’t overlap here.
The dark bands are A bands (anisotropic), and the thick and thin bands overlap here.

Question 7

What is the structure inside the myofibril bands?

Answer 7

At the centre of each A band is a lighter coloured region the H-zone.
At the centre of each I band is the Z-line.
The distance between adjacent Z-lines is the sarcomere.
When a muscle contracts, the sarcomere shortens, and the pattern of light and dark bands changes.

Question 8

How does each band appear under an optical microscope?

Answer 8

I-band - light
A-band - dark

Question 9

What is tropomyosin?

Answer 9

An important protein in the muscle which forms a fibrous strand around the actin filament.

Question 10

What are slow-twitch muscle fibres?

Answer 10

• Slower, less powerful contractions but over a longer period than fast twitch.
• Adapted to endurance work.
• Adapted for aerobic respiration to avoid a build up of lactic acid, which would cause them to function less effectively, and prevent long-duration contraction.

Question 11

How are slow twitch muscle fibres adapted for aerobic respiration?

Answer 11

• A large store of myoglobin (red molecule that stores oxygen).
• A rich supply of blood vessels to deliver oxygen and glucose.
• Numerous mitochondria to produce ATP.

Question 12

What are fast twitch muscle fibres?

Answer 12

• Contract more rapidly and powerfully, but for a short period.
• Adapted for intense exercise, such as weight lifting.

Question 13

How are fast twitch fibres adapted for anaerobic activity?

Answer 13

• Thicker and more numerous myosin filaments.
• A high concentration of glycogen.
• A high concentration of enzymes involved in anaerobic respiration, which provides ATP rapidly.
• A store of phosphocreatine, which rapidly regenerates ATP from ADP in anaerobic conditions.

Question 14

What does the phrase ‘antagonistic pair of muscles’ mean?

Answer 14

Muscles can only pull, so they work in pairs to move bones around joints
Pair pull in opposite directions: agonist contracts while antagonist is relaxed

Question 15

What are neuromuscular junctions?

Answer 15

• The point where a motor neurone meets a skeletal muscle fibre.
• There are many junctions along the muscle, to ensure the muscle fibres contract simultaneously and the movement not slow.

Question 16

What is a motor unit?

Answer 16

• All muscle fibres supplied by a single motor neurone act together as a single functional unit.
• This arrangement gives control over the force that the muscle exerts.
• For small force, only a few units are stimulated.

Question 17

How does a nerve impulse travel in a neuromuscular junction?

Answer 17

When a nerve impulse is recieved, the synaptic vesicles fuse with the presynaptic membrane, and release acetylcholine.
The acetylcholine diffuses to the postsynaptic membrane (of the muscle fibre), altering its permeability to sodium ions, which enter rapidly, depolarising the membrane.

Question 18

How does acetylcholinesterase work?

Answer 18

The acetylcholine is broken down by acetylcholinesterase to ensure the muscle is not over-stimulated.
The resulting choline and ethanoic acid diffuse back into the neurone, where they are recombined to form acetylcholine using energy from ATP.

Question 19

What are the similarities of the neuromuscular junction and a synapse?

Answer 19

they both:
- have neurotransmitters that are transported by diffusion
- Have receptors, that on binding with the neurotransmitter, cause an influx of sodium ions
- use a sodium-potassium pump to repolarise the axon
- use enzymes to breakdown the neurotransmitter

Question 20

What are the characteristics of the neuromuscular junction?

Answer 20

• only excitatory
• only links neurones to muscles
• only motor neurones are involved
• the action potential ends here
• acetylcholine binds to receptors on membrane of muscle fibre

Question 21

What are the characteristics of the cholinergic synapse?

Answer 21

• may be excitatory or inhibitory
• links neurones to neurones, or neurones to other effector organs
• motor, sensory and intermediate neurones may be involved
• a new action potential may be produced along another neurone
• acetylcholine binds to receptors on membrane of post-synaptic neurone

Question 22

What is the evidence of the sliding filament mechanism?

Answer 22

there will be more overlap of actin and myosin in a contracted muscle than in a relaxed one; the I-band narrows, the Z-lines move closer together/the sarcomere shortens, the H-zone narrows

Question 23

What is the evidence that discounts against other mechanisms?

Answer 23

• the A-band remains the same width. As the width of this band is determined by the length of the myosin filaments, it follows that the myosin filaments have not become shorter

Question 24

What is myosin?

Answer 24

• a fibrous protein arranged into a filament made up of several hundred molecules
• a globular protein formed into two bulbous structures at one end

Question 25

What is actin?

Answer 25

a globular protein whose molecules are arranged into long chains that are twisted around one another to form a helical strand

Question 26

What is the sliding filament mechanism of muscle contraction?

Answer 26

• the theory that actin and myosin filaments slide past one another during muscle contractions
• myosin heads from cross bridges with actin
• Myosin head changes shape and 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

Question 27

What is the muscle stimulation of the sliding filament mechanism?

Answer 27

• an action potential reaches many neuromuscular junctions simultaneously, causing calcium ion protein channels to open and calcium ions to diffuse into the synaptic knob
• Calcium ions cause the synaptic vesicles to fuse with the presynaptic membrane and release their acetylcholine into the synaptic cleft
• acetylcholine diffuses across the synaptic cleft and binds with receptors on the muscle cell-surface membrane, causing it to depolarise

Question 28

What is the role of calcium ions in muscle contraction - sliding filament theory

Answer 28

• action potential moves through T-tubules in the sarcoplasm = sodium ion channels in sarcoplasmic reticulum open
• Sodium ions binds to troponin, triggering conformational change in tropomyosin
• Exposes binding sites on actin filaments so actinomyosin bridges can form

Question 29

How does sliding filament action cause a myofibril to shorten?

Answer 29

• myosin heads flex in opposite direction = actin filaments are pulled towards each other
• distance between adjacent sarcomere Z lines shortens
• siding filament action occurs up to 100 times per second in multiple sarcomeres

Question 30

What happens during muscle contraction?

Answer 30

• sodium ions actively transported back into endoplasmic reticulum
• tropomyosin once again blocks actin binding site

Question 31

What is the role of phosphocreatine in muscle contraction?

Answer 31

• phosphorylates ADP directly to ATP when oxygen for aerobic respiration is limited e.g. during vigorous exercise

Question 32

How could a student calculate the length of one sarcomere?

Answer 32

• view thin slice of muscle under optical microscope
• calibrate eyepiece graticule
• measure distance from middle of one light band to middle of another

Question 33

What is muscle relaxation of the sliding filament mechanism?

Answer 33

• when nervous stimulation ceases, calcium ions are actively transported back into endoplasmic reticulum using energy from the hydrolysis of ATP
• this reabsorption of the calcium ions allows tropomyosin to block the actin filament again
• myosin heads are now unable to bind to actin filaments and contractions ceases, muscles relaxes
• force from antagonistic muscles can pull actin filaments out from between myosin

Question 34

What is energy supply during muscle contraction?

Answer 34

The energy released is needed for
- movement of the myosin heads
- the reabsorption of calcium ions into the endoplasmic reticulum by active transport