Muscles Flashcards
1
Q
What are the three types of muscle
A
1) Cardiac muscle
2) Smooth muscle
3) Skeletal muscle
2
Q
Where is smooth muscle found
A
In the walls of blood vessels and in the gut
3
Q
What is the similarity between cardiac muscle and smooth muscle
A
Neither of these types of muscle is under conscious control and we remain largely unaware of their contractions.
4
Q
What is skeletal muscle
A
The muscle that makes up the bulk of body muscle in vertebrates- it is attached to bone and acts under voluntary, conscious control.
5
Q
What are muscles made up of
A
Millions of tiny muscle fibres called myofibrils
6
Q
Why is muscle not made up of individual cells joined end to end
A
- If muscle was made up of individual cells joined end to end it would not be able to contract efficiently.
- This is because the junction between adjacent cells would be a point of weakness that would reduce the overall strength of the muscle.
7
Q
Describe the basic structure of a muscle fibre
A
- Muscle fibres consist of cells fused together.
- These muscle fibres share nuclei and a cytoplasm, called the sarcoplasm, which is mostly found around the circumference of the fibre.
- Within the sarcoplasm is a large concentration of mitochondria and endoplasmic reticulum.
- Muscle fibres are made up of myofibrils.
8
Q
What are the two protein filaments that myofibrils are made up of
A
- Actin, which is thinner and consists of two strands twisted around one another.
- Myosin, which is thicker and consists of long rod-shaped tails with bulbous heads that project to the side.
9
Q
Why do myofibrils appear striped
A
Myofibrils appear striped due to their alternating light-coloured and dark-coloured bands.
10
Q
What are the light bands in a myofibril called and why do they appear light
A
- I bands (isotropic bands).
- They appear lighter because the thick and thin filaments (myosin and actin) do not overlap in this region.
11
Q
What are the dark bands in a myofibril called and why do they appear darker
A
- A bands (anisotropic bands)
- They appear darker because the thick and thin filaments overlap in this region.
12
Q
What is present at the centre of each A (anisotopic) band
A
A lighter coloured region called the H-zone
13
Q
What is at the centre of each I band
A
A line called the Z line
14
Q
What is a sarcomere
A
The distance between adjacent Z lines
15
Q
What happens to the sarcomeres during muscle contraction
A
The sarcomeres shorten and the pattern of light and dark bands changes
16
Q
What is tropomyosin
A
An important protein found in muscle fibres which forms a fibrous strand around the actin filament
17
Q
What are the two types of muscle fibres
A
1) Slow twitch fibres
2) Fast twitch fibres
18
Q
What are slow twitch fibres
A
- Slow twitch fibres contract more slowly than fast twitch fibres and provide less powerful contractions but over a longer period of time.
- They are therefore more adapted to endurance work.
- In humans they are more common in muscles like the calf muscle, which must contract constantly to maintain the body in an upright position.
19
Q
How are slow twitch fibres adapted to their role
A
- They are adapted for aerobic respiration in order to avoid a build up of lactic acid, which would cause them to function less effectively and prevent long-duration contraction.
- They have a large store of myoglobin which is a bright red molecule that stores oxygen. It accounts for the red colour of slow-twitch fibres.
- They have a rich supply of blood vessels to deliver oxygen and glucose for aerobic respiration.
- They have numerous mitochondria which produce ATP.
20
Q
What are fast-twitch fibres
A
- Fast twitch fibres contract rapidly and produce powerful contractions but only for a short period.
- They are therefore adapted for intense activity/exercise.
- They are more common in muscles which need to do short bursts of intense activity, such as the bicep muscle of the upper arm.
21
Q
How are fast twitch fibres adapted to their role
A
- They have thicker and more numerous myosin filaments
- They have a high concentration of glycogen
- They have a high concentration of enzymes involved in anaerobic respiration which provides ATP rapidly.
- They have a store of phosphocreatine, a molecule that can rapidly generate ATP from ADP in anaerobic conditions and so provide energy for muscle contraction.
22
Q
What is a neuromuscular junction
A
The point where a motor neurone meets a skeletal muscle fibre
23
Q
Why are there many neuromuscular junction spread throughout a muscle
A
- If there were only one junction, it would take time for a wave of contraction to travel across the muscle.
- In this case, not all of the fibres would contract simultaneously and the movement would be slow.
- As rapid and coordinated muscle contraction is frequently essential for survival, there are many neuromuscular junctions spread throughout a muscle.
- This ensures that contraction of a muscle is rapid and powerful when it is simultaneously stimulated by action potentials.
24
Q
What is a motor unit
A
All of the muscle fibres supplied by a single motor neurone which act together and a functional unit.
25
Why is the motor unit arrangement important
- It gives control over the force that the muscle exerts.
- If only slight force is needed, only a few units are stimulated.
- If a greater force is required, a larger number of units are stimulated.
26
Describe simply what happens when a nerve impulse is received at the neuromuscular junction
- When a nerve impulse is received at the neuromuscular junction, the synaptic vesicles fuse with the presynaptic membrane and release their acetylcholine.
- The acetylcholine diffuses to the postsynaptic membrane- which is the membrane of the muscle fibre- and alters its permeability to Na+ ions.
- This causes the Na+ ions to enter rapidly, depolarising the membrane.
- This leads to contraction according to the sliding filament mechanism.
- The acetylcholine is broken down by acetylcholinesterase to ensure that the muscle is not overstimulated.
- The resulting choline and ethanoic acid diffuse back into the neurone, where they are recombined to form acetylcholine using energy provided by the mitochondria found there.
27
List the similarities between a neuromuscular junction and cholinergic synapse
- Both have neurotransmitters that are transported by diffusion.
- Both have receptors, that on binding with the neurotransmitter cause an influx of sodium ions.
- both use a sodium-potassium pump to repolarise the axon
- Both use enzymes to breakdown the neurotransmitter
28
What are the differences between a cholinergic synapse and neuromuscular junction
- A neuromuscular junction can only be excitatory whereas a cholinergic synapse may be excitatory or inhibitory.
- A neuromuscular junction only links neurones to muscles whereas a cholinergic synapse links neurones to neurones, or neurones to other effector organs.
- Only motor neurones are involved in the neuromuscular junction whereas motor, sensory and intermediate neurones may be involved in a cholinergic synapse.
- The action potential ends at the neuromuscular junction (it is the end of a neural pathway) whereas a new action potential may be produced along another postsynaptic neurone after a cholinergic synapse.
- In a neuromuscular junction, acetylcholine bonds to receptors on the membrane of muscle fibre whereas in a cholinergic synapse, acetylcholine binds to receptors on the membrane of the post-synaptic neurone.
29
How do skeletal muscles occur
In antagonistic pairs- these pairs pull in opposite directions and when one is contracted the other is relaxed
30
What is the sliding filament mechanism
The theory of muscle contraction which involves the actin and myosin filaments sliding past one another.
31
What is the evidence for the sliding filament mechanism
- If the sliding filament mechanism is correct, then there will be more overlap of actin and myosin in a contracted muscle than in a relaxed one.
- We know that myofibrils appear darker where the actin and myosin filaments overlap and lighter where they do not,
- When a muscle contracts, the I band of the sarcomere becomes narrower.
- The z-lines move closer together: the sarcomere shortens
- The H zone becomes narrower.
- The A band remains the same widely so the myosin filaments have not become shorter.
- This evidence therefore converges to support the sliding filament mechanism.
32
What are the three main proteins involved in the sliding filament mechanism
1) Myosin. Myosin is made up of two types of protein: a fibrous protein arranged into a filament made up of several hundred molecules (the tail) and a globular protein formed into two bulbous structures at one end (the head).
2) Actin which is a globular protein whose molecules are arranged into long chains that are twisted around one another to form a helical strand.
3) Tropomyosin which forms long thin threads that are wound around actin filaments.
33
Summarise how the sliding filament mechanism of muscle contraction works
- The bulbous heads of the myosin filaments form cross-bridges with the actin filaments.
- They do this by attaching themselves to binding sites on the actin filaments, and then flexing in unison, pulling the actin filaments along the myosin filaments.
- They then become detached and, using ATP as a source of energy of energy, return to their original angle and re-attach themselves further along the actin filaments.
34
What three stages can the continuous sliding filament mechanism be separated into
1) Muscle stimulation
2) Muscle contraction
3) Muscle relaxation
35
Describe the first stage of the sliding filament mechanism of muscle contraction- muscle stimulation
- An action potential reaches many neuromuscular junctions simultaneously, causing calcium ion protein channels to open and calcium ions to diffuse into the synaptic knob.
- The 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.
36
Describe the second stage of the sliding filament mechanism of muscle contraction: muscle contraction
- The action potential travels deep into the fibre through a system of tubules (T-Tubules) that are extensions of the cell-surface membrane and branch throughout the cytoplasm of the muscle (sarcoplasm).
- The tubules are in contact with the endoplasmic reticulum of the muscle (sarcoplasmic reticulum) which has actively transported calcium ions from the cytoplasm of the muscle.
- This leads to a very low Ca 2+ concentration in the cytoplasm.
- The action potential opens the calcium ion protein channels on the endoplasmic reticulum and calcium ions diffuse into the muscle cytoplasm down a concentration gradient.
- The calcium ions cause the tropomyosin molecules that were blocking the binding sites on the actin filament to pull away.
- ADP molecules attached to the myosin heads mean they are in a state to bind to the actin filament and form a cross-bridge.
- Once attached to the actin filament, the myosin heads change their angle, pulling the actin filament along as they do so and releasing a molecule of ADP.
- An ATP molecule attaches to each myosin head, causing it to become detached from the actin filament.
- The calcium ions then activate the enzyme ATPase, which hydrolyses the ATP to ADP.
- The hydrolysis of ATP to ADP provides the energy for the myosin head to return to its original position.
- The myosin head, once more with an attached ADP molecule, then reattaches itself further along the actin filament and the cycle is repeated as long as the concentration of calcium ions in the myofibril remains high.
- As the myosin molecules are joined tail to tail in two oppositely facing sets, the movement of one set of myosin heads is in the opposite direction to the other set.
- This means that actin filaments to which they are attached also move in opposite directions.
- The movement of actin filaments in opposite directions pulls them towards each other, shortening the distance between the two adjacent Z-lines.
- The overall effect of this process taking place repeatedly and simultaneously throughout a muscle is to shorten it and so bring about movement of a part of the body.
37
Describe the third stage of the sliding filament mechanism of muscle contraction: muscle relaxation
- When nervous stimulation ceases, calcium ions are actively transported back into the 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 contraction ceases, so the muscle relaxes.
- In this state force from antagonistic muscles can pull actin filaments out from between myosin.
38
How is the energy for muscle contraction supplied
From the hydrolysis of ATP
39
What two things is the energy released from the hydrolysis of ATP needed for in muscle contraction
1) The movement of myosin heads
2) the reabsorption of calcium ions into the endoplasmic reticulum by active transport
40
What are the two ways that a muscle can generate ATP anaerobically
- Using a chemical called phosphocreatine
- By glycolysis
41
How does phosphocreatine regenerate ATP
- Phosphocreatine is stored in muscle and acts as a reserve supply of phosphate, which is available immediately to combine with ADP and so re-form ATP.
- The phosphocreatine store is replenished using phosphate from ATP when the muscle is relaxed.
