Skeletal Muscles (A Level Only)

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Antagonistic Muscles

Skeletal muscles enable movement by exerting a pulling force on the rigid, incompressible skeleton.
Muscles are connected to bones via tendons, which do not stretch when a muscle contracts and pulls on a bone.
Because muscles can only pull and cannot push, they often act in pairs known as antagonistic pairs.
In an antagonistic pair, one muscle pulls the bone in one direction, and the other pulls it in the opposite direction.
The biceps and triceps in the arm are an example of an antagonistic pair.
To raise the lower arm: the biceps contracts and the triceps relaxes, pulling the bone upwards and bending the arm at the elbow.
To straighten the arm: the triceps contracts and the biceps relaxes, pulling the bone down and straightening the arm.

The Structure of Skeletal Muscle

Skeletal muscle (also called striated or striped muscle) contracts to move the skeleton.
Other muscle types include cardiac muscle in the heart and smooth muscle in the walls of internal organs.
Skeletal muscle is made up of bundles of muscle cells, usually known as muscle fibres.
In a muscle fibre: the cell surface membrane is the sarcolemma, the cytoplasm is the sarcoplasm, and the endoplasmic reticulum is the sarcoplasmic reticulum.
Each muscle fibre is an elongated cell containing many nuclei.
The sarcoplasm contains an organised arrangement of contractile proteins that form myofibrils.
The sarcolemma has many deep tube-like projections called T-tubules.
Many mitochondria in the sarcoplasm generate ATP for muscle contraction.

Myofibrils and Sarcomeres

Myofibrils are long, rod-like structures made of protein filaments.
The protein filaments in myofibrils are myosin (thick filaments) and actin (thin filaments).
Myofibrils are divided into sections called sarcomeres, which shorten during muscle contraction as myosin and actin filaments slide past each other.
Each sarcomere contains the following features: Z-line, M-line, A band, H band, and I band.
The Z-line is the boundary between sarcomeres; actin filaments attach here, and during contraction the Z-lines get closer together.
The M-line is the central point of each sarcomere; myosin filaments attach here.
The A band is the region where myosin filaments extend; it remains the same size whether the muscle is contracted or relaxed.
The H band contains only myosin filaments and shrinks during contraction as the overlap between actin and myosin increases.
The I band contains only actin filaments and also shrinks during contraction as the overlap increases.

Skeletal Muscle Under a Microscope

Pre-prepared slides of skeletal muscle tissue can be examined under an optical microscope.
Optical microscopes have low resolution, so visible features may include only banding and nuclei.
Electron microscopes can view muscle fibres in more detail and reveal the structure of myofibrils.
Using electron microscopes, visible features may include dark lines at each Z-line, light bands corresponding with I bands, and dark bands corresponding with A bands.
The dark bands in skeletal muscle produce a characteristic striped appearance.
The detailed structures of muscle fibres are visible using TEM due to the higher resolution of electron microscopes.

Muscle Contraction: Sliding Filament Theory

During muscle contraction, actin and myosin filaments slide over each other, causing sarcomeres to shorten.
Myosin filaments are fibrous proteins with a globular head.
Actin filaments have actin-myosin binding sites and are associated with tropomyosin, which blocks the binding sites when the muscle is relaxed.
1. An action potential arrives at a neuromuscular junction, and calcium ions are released from the sarcoplasmic reticulum.
2. Calcium ions cause tropomyosin to change position on the actin filaments, exposing the actin-myosin binding sites.
3. Myosin heads bind to actin, forming cross-bridges between the two types of filament.
4. The myosin heads bend, releasing ADP and inorganic phosphate and pulling the actin filaments towards the centre of the sarcomere; this is the power stroke.
5. ATP binds to the myosin heads, causing them to release from actin.
6. ATP hydrolase hydrolyses ATP, providing energy that allows the myosin heads to reset to their original positions.
7. The myosin heads can then bind to new binding sites on the actin filaments, and the process repeats.

ATP and Phosphocreatine in Muscle Contraction

A supply of ATP is required for muscle contraction, providing energy to reset the position of myosin heads.
ATP for muscle contraction can come from aerobic respiration, which requires sufficient oxygen and sustains low-intensity exercise.
ATP can also come from anaerobic respiration, which produces small amounts of ATP very quickly but can only be sustained for short periods due to lactate build-up.
Phosphocreatine is stored in muscles and can be used for the rapid production of ATP over short periods.
A phosphate ion from phosphocreatine is transferred to ADP: ADP + phosphocreatine → ATP + creatine.
After contraction, calcium ions are taken up again by the sarcoplasmic reticulum via active transport.
When a muscle is no longer stimulated, calcium ions are taken up by the sarcoplasmic reticulum, the actin-myosin binding sites are blocked, and myosin can no longer bind to actin; filaments can be pulled apart by an antagonistic muscle.

Slow and Fast Skeletal Muscle Fibres

There are two types of muscle fibres in skeletal muscles: fast fibres and slow fibres.
The relative proportion of each fibre type present in muscle tissue determines the properties of the muscle.
Fast muscle fibres contract rapidly, rely on anaerobic respiration for ATP, fatigue quickly due to lactate production, and are pale in colour.
Fast fibres have few capillaries and are low in myoglobin due to their low oxygen requirements.
Fast fibres are suited to short bursts of high-intensity activity, e.g., in the limbs of animals that flee predators, human eyelid muscles, and biceps and triceps.
Slow muscle fibres contract more slowly, rely on aerobic respiration for ATP, fatigue slowly due to reduced lactate production, and are darker in colour.
Slow fibres have a dense capillary network and are high in myoglobin and haemoglobin due to their demand for oxygen.
Slow fibres are suited to sustained, low-intensity activities, e.g., in the limbs of animals that migrate or stalk prey, and human back and leg muscles.
Fast fibres have fewer, smaller mitochondria, large glycogen and phosphocreatine stores, large calcium ion stores in the sarcoplasmic reticulum, and high ATP hydrolase activity.
Slow fibres have many large mitochondria, small glycogen and phosphocreatine stores, small calcium ion stores, and lower ATP hydrolase activity.

Investigating Muscle Fatigue

Repeated contraction of muscles may result in muscle fatigue due to production of lactate during anaerobic respiration and a decrease in the availability of calcium ions after repeated contractions.
Muscle fatigue can be investigated using a hand grip strengthener, a stopwatch, and willing human subjects.
Method: squeeze the handgrip strengthener as many times as possible over 20 seconds, record the number of successful squeezes, rest for 10 seconds, and repeat several times.
The experiment should be repeated with other human subjects.
Results can be plotted on a graph showing how the number of successful squeezes decreases over time.
The same hand muscles are used repeatedly, so fatigue is expected to reduce the ability of these muscles to contract.
Limitations: the experiment only looks at hand muscles, other skeletal muscles may fatigue at different rates, the exercise intensity may not be high enough for some individuals, and the hand grip strengthener may be too difficult for some individuals even at the start.

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Übungsfragen

Gratis-Vorschau — 8 von 62 Fragen. Registriere dich, um alle zu sehen.

1.What is the name of the cytoplasm of a muscle fibre?
Easy
- ASarcoplasm
- BSarcolemma
- CSarcoplasmic reticulum
- DMyofibril
2.Which protein filaments are the thick filaments in a myofibril?
Easy
- AMyosin
- BActin
- CTropomyosin
- DCollagen
3.What is an antagonistic muscle pair?
Easy
- AA pair of muscles that work together to pull a bone in opposite directions
- BA pair of muscles that both contract at the same time to move a bone
- CA pair of muscles that push a bone in the same direction
- DA pair of muscles that are attached to each other by a tendon
4.During muscle contraction, what happens to the length of the A band and the sarcomere?
Medium
- AThe A band remains the same length and the sarcomere shortens
- BThe A band shortens and the sarcomere remains the same length
- CBoth the A band and the sarcomere shorten
- DBoth the A band and the sarcomere remain the same length
5.Which of the following is a function of calcium ions in muscle contraction?
Medium
- AThey cause tropomyosin to move, exposing the actin-myosin binding sites
- BThey hydrolyse ATP to provide energy for the power stroke
- CThey bind to myosin heads and pull the actin filaments
- DThey transfer a phosphate ion to ADP to form ATP
6.Which of the following correctly describes a feature of slow muscle fibres?
Medium
- AThey have many capillaries and a high myoglobin content
- BThey have few capillaries and a low myoglobin content
- CThey rely mainly on anaerobic respiration
- DThey fatigue rapidly due to lactate production
7.Which of the following statements about fast muscle fibres are correct? (Select all that apply.)
Medium
- AThey contract rapidly
- BThey rely mainly on anaerobic respiration
- CThey have many large mitochondria
- DThey have large glycogen and phosphocreatine stores
- EThey fatigue slowly due to reduced lactate production
8.Which of the following are parts of the sliding filament theory of muscle contraction? (Select all that apply.)
Medium
- ACalcium ions cause tropomyosin to move, exposing binding sites on actin
- BMyosin heads bind to actin forming cross-bridges
- CThe myosin heads bend, pulling actin filaments towards the centre of the sarcomere
- DATP binds to actin, causing it to detach from myosin
- EThe A band shortens as myosin filaments contract

Mehr Fragen

9.The A band remains the same length during muscle contraction.
Easy
True or false?
10.Tendons stretch when a muscle contracts and pulls on a bone.
Easy
True or false?
11.Phosphocreatine can be used for the rapid production of ATP over short periods.
Easy
True or false?
12.Match each sarcomere feature with its correct description.
Medium
- Z-line
- M-line
- A band
- H band
- I band
- The boundary between sarcomeres; actin filaments attach here
- The central point of each sarcomere; myosin filaments attach here
- The region across which the myosin filaments extend; does not change length during contraction
- The region containing only myosin filaments; shrinks during contraction
- The region containing only actin filaments; shrinks during contraction
13.Place the events of the sliding filament theory in the correct order.
Hard
- An action potential arrives at the neuromuscular junction and calcium ions are released from the sarcoplasmic reticulum
- Calcium ions cause tropomyosin to move, exposing the actin-myosin binding sites
- Myosin heads bind to actin, forming cross-bridges
- The myosin heads bend, pulling the actin filaments towards the centre of the sarcomere
- ATP binds to the myosin heads, causing them to detach from actin
- ATP hydrolase hydrolyses ATP, allowing the myosin heads to recock
14.Which of the following statements about slow muscle fibres are correct? (Select all that apply.)
Medium
- AThey have a dense capillary network.
- BThey contain a large store of phosphocreatine.
- CThey rely mainly on aerobic respiration for ATP.
- DThey fatigue rapidly due to lactate production.
- EThey contain many large mitochondria.
15.Which of the following are roles of ATP in the sliding filament theory of muscle contraction? (Select all that apply.)
Hard
- AIt binds to myosin heads, causing them to detach from actin.
- BIt is hydrolysed to provide energy for the power stroke.
- CIt is hydrolysed to allow the myosin heads to reset to their original positions.
- DIt causes tropomyosin to move and expose the actin-myosin binding sites.
- EIt is required for the active transport of calcium ions back into the sarcoplasmic reticulum.
16.Match each muscle fibre component with its correct description.
Medium
- Sarcolemma
- Sarcoplasm
- Sarcoplasmic reticulum
- T-tubules
- Cell surface membrane of a muscle fibre
- Cytoplasm of a muscle fibre
- Endoplasmic reticulum of a muscle fibre
- Deep tube-like projections that fold in from the outer surface
17.Place the following events of the sliding filament theory in the correct order.
Medium
- Calcium ions are released from the sarcoplasmic reticulum.
- Tropomyosin moves, exposing actin-myosin binding sites.
- Myosin heads bind to actin, forming cross-bridges.
- The myosin heads bend, pulling actin towards the centre of the sarcomere.
- ATP binds to myosin heads, causing them to detach from actin.
- ATP is hydrolysed, allowing myosin heads to reset.
18.Which of the following correctly describes the H band of a sarcomere?
Medium
- AThe region containing only myosin filaments; it shrinks during contraction.
- BThe region containing only actin filaments; it shrinks during contraction.
- CThe region containing both actin and myosin filaments; it remains constant.
- DThe boundary between adjacent sarcomeres; it moves closer together during contraction.
19.Which of the following is a correct difference between fast and slow muscle fibres?
Medium
- AFast fibres have more capillaries than slow fibres.
- BFast fibres have larger glycogen and phosphocreatine stores than slow fibres.
- CFast fibres have more mitochondria than slow fibres.
- DFast fibres have a larger store of calcium ions in the sarcoplasmic reticulum than slow fibres.
20.During intense exercise, the pH of skeletal muscle tissue falls. This decrease in pH reduces the ability of calcium ions to stimulate muscle contraction. Which of the following best explains this effect?
Medium
- ACalcium ions are less able to bind to tropomyosin, so the actin-myosin binding sites remain blocked.
- BCalcium ions are less able to bind to myosin heads, so cross-bridges cannot form.
- CCalcium ions are less able to bind to actin filaments, so the power stroke cannot occur.
- DCalcium ions are less able to be released from the sarcoplasmic reticulum, so tropomyosin does not move.
21.In an experiment using a hand grip strengthener, a student squeezes the handles as many times as possible in 20 seconds, rests for 10 seconds, and repeats this cycle several times. The number of successful squeezes decreases over time. Which of the following best explains this decrease?
Medium
- ALactate builds up during anaerobic respiration and calcium ions become less available, causing muscle fatigue.
- BThe muscle fibres run out of actin and myosin filaments, so they can no longer contract.
- CThe tendons connecting the muscle to the bone stretch permanently and reduce the force produced.
- DThe number of mitochondria in the muscle fibres decreases during the experiment.
22.Slow muscle fibres contain a large amount of myoglobin because they have a high demand for oxygen.
Easy
True or false?
23.Figure 1 shows a bundle of muscle fibres. In the bundle there are 24 slow fibres and 56 fast fibres. Calculate the ratio of slow to fast muscle fibres. Give your answer in the form 1 : n.
Medium
- A1 : 2.3
- B1 : 0.43
- C1 : 2.4
- D1 : 3.0
24.A TEM image of skeletal muscle is taken at a magnification of ×12,000. A single A band on the image measures 31 mm. Calculate the actual length of the A band in μm.
Hard
- A2.6 μm
- B0.39 μm
- C372 μm
- D0.0026 μm
25.A muscle fibre is 40 mm long when relaxed and 32 mm long when fully contracted. Calculate the percentage decrease in length.
Medium
- A20%
- B25%
- C8%
- D12.5%
26.A contracted sarcomere measures 2.0 μm. A muscle fibre contains 15,000 sarcomeres in series. Calculate the length of the contracted muscle fibre in mm.
Hard
- A30 mm
- B3 mm
- C7,500 mm
- D0.03 mm
27.A student views a skeletal muscle slide under an optical microscope at ×400. The eyepiece graticule has 100 divisions. The stage micrometer shows that 40 graticule divisions equal 100 μm. Calculate the actual width of a muscle fibre that measures 60 graticule divisions.
Medium
- A150 μm
- B60 μm
- C240 μm
- D15 μm
28.Scientists studied capillaries in slow and fast muscle fibres of mice diaphragms. They took samples from 10 female and 11 male mice, selected several fields of view at random, and counted capillaries per fibre. Which of the following precautions would improve the reliability of their mean values? (Select all that apply.)
Hard
- AUsing a large number of mice in each group
- BSelecting fields of view at random
- CCounting the same field of view multiple times for each mouse
- DUsing a larger magnification so that more fibres are visible in each field
- EEnsuring all mice are the same age and sex within each group
29.The table below shows the mean number of capillaries associated with slow and fast muscle fibres in male and female mice. Which conclusions are supported by the data? (Select all that apply.)
Medium
- AMale mice have more capillaries per slow fibre than female mice
- BFast fibres have more capillaries than slow fibres in both sexes
- CThe standard deviations for male mice are smaller than for female mice
- DThere is no overlap between the standard deviations for male and female slow fibres
- EFemale mice have more capillaries per fast fibre than male mice
30.A student investigates muscle fatigue using a hand grip strengthener. They squeeze the handles as many times as possible in 20 seconds, rest for 10 seconds, and repeat this cycle five times. Which of the following would be valid conclusions from this experiment? (Select all that apply.)
Hard
- AThe number of successful squeezes decreases over time due to fatigue
- BFatigue is caused by a decrease in calcium ion availability after repeated contractions
- CThe results can be generalised to all skeletal muscles in the body
- DLactate production during anaerobic respiration may contribute to fatigue
- EThe experiment measures the maximum grip strength of the subject
31.During muscle contraction, the H band and I band both become shorter.
Medium
True or false?
32.In an experiment investigating muscle fatigue, the number of successful squeezes is expected to increase over time as the muscle warms up.
Medium
True or false?
33.A muscle fibre that is 25 mm long when relaxed and 20 mm long when contracted has decreased in length by 20%.
Medium
True or false?
34.Match each experimental observation with the correct explanation.
Medium
- Fast muscle fibres have fewer capillaries
- Slow muscle fibres contain a large amount of myoglobin
- Muscle fatigue occurs after repeated contractions
- Phosphocreatine is used rapidly during short bursts of exercise
- They rely on anaerobic respiration and have low oxygen requirements
- They have a high demand for oxygen for aerobic respiration
- Calcium ion availability decreases and lactate builds up
- It can transfer a phosphate ion to ADP to form ATP quickly
35.A student examines a pre-prepared slide of skeletal muscle using an optical microscope. Which of the following features would be visible?
Medium
- ABanding and nuclei
- BZ-lines and H bands
- CIndividual actin and myosin filaments
- DT-tubules and sarcoplasmic reticulum
36.Which of the following are reasons why an electron microscope is more useful than an optical microscope for studying skeletal muscle fibres? (Select all that apply.)
Medium
- AHigher resolution allows individual myofilaments to be seen
- BIt can show the detailed structure of myofibrils
- CIt allows living muscle tissue to be observed in real time
- DIt can reveal the arrangement of Z-lines and bands
- EIt uses a beam of light to produce a magnified image
37.During muscle contraction, which of the following correctly describes the change in the sarcomere?
Medium
- AThe A band shortens, and the I band remains the same length
- BThe A band remains the same length, and the I band shortens
- CBoth the A band and the I band shorten
- DThe A band lengthens, and the I band shortens
38.A scientist is investigating muscle fatigue using a hand grip strengthener. After several sets of squeezing, the number of successful squeezes per set decreases. Which of the following best explains this observation?
Medium
- AIncreased availability of calcium ions in the sarcoplasm
- BA decrease in the availability of calcium ions and a build-up of lactate
- CAn increase in the number of mitochondria in the muscle fibres
- DA decrease in the amount of ATP hydrolase enzyme in the muscle fibres
39.A student plans an experiment to investigate muscle fatigue using a hand grip strengthener. Which of the following are appropriate precautions to improve the reliability of the results? (Select all that apply.)
Medium
- AUse the same hand grip strengthener for all trials
- BAllow the same rest period between each set of squeezes
- CUse different subjects for each trial without controlling for age or sex
- DRecord the number of successful squeezes in each 20-second period
- EAsk subjects to squeeze as hard as they can but not count the squeezes
40.Scientists investigated the time taken for phosphocreatine to be re-formed in leg muscles after exercise in people of different ages. They found a large variation in re-formation time even among people of very similar age. Which of the following is the most likely explanation for this variation?
Hard
- ADifferences in the number of muscle fibres between individuals
- BDifferences in the proportion of slow and fast fibres and in fitness levels
- CDifferences in the length of the leg muscles between individuals
- DDifferences in the type of exercise performed before the measurement
41.Complete the following paragraph about the structure of skeletal muscle.
Easy
Muscle fibres consist of many muscle cells fused together. As well as sharing nuclei, muscle fibres share ____, which contains many mitochondria. Each fibre is made up of myofibrils which contain 2 types of protein; ____ which is thinner and ____ which is thicker.
42.Complete the table by naming the molecules that carry out each function.
Easy
Function: Hydrolyses ATP → ____ ; Covers the binding site on the actin filament (when the muscle is relaxed) → ____ ; Required for recocking of the myosin head → ____
43.Name the protein that blocks the actin-myosin binding sites when a muscle is relaxed.
Medium
44.What is the name of the structure that forms the boundary between adjacent sarcomeres?
Medium
45.A muscle fibre is 30 mm long when relaxed and 22.5 mm long when contracted. Calculate the percentage decrease in length of the contracted muscle.
Medium
46.If each sarcomere is 2.5 μm long in a contracted muscle fibre, calculate how many sarcomeres would be found along the whole length of a contracted muscle fibre that is 22.5 mm long.
Medium
47.A bundle of muscle fibres contains 30 slow fibres and 70 fast fibres. Calculate the ratio of slow to fast fibres in the bundle. Give your answer as a decimal to two decimal places.
Medium
48.Complete the following paragraph about the roles of molecules in muscle contraction.
Medium
During the sliding filament theory, the molecule that hydrolyses ATP to reset the myosin head is ____. When the muscle is relaxed, the binding sites on actin are blocked by ____. The ion that diffuses across the sarcolemma and triggers contraction is ____.
49.Complete the sentence about the effect of pH on muscle contraction.
Medium
During intense exercise, the pH of skeletal muscle tissue falls. This decrease in pH reduces the ability of ____ ions to stimulate muscle contraction because it changes the shape of the ____ proteins on the actin filaments.
50.Complete the sentence about the sliding filament theory.
Medium
In the sliding filament theory, the myosin heads bend and pull the actin filaments towards the centre of the sarcomere. This is known as the ____ stroke. The myosin heads then bind to new binding sites on the actin filaments after ATP is hydrolysed by the enzyme ATP ____.
51.A muscle fibre is 36 mm long when relaxed and 27 mm long when contracted. Calculate the percentage decrease in length.
Medium
52.A contracted muscle fibre is 20 mm long. Each sarcomere in the fibre is 2.5 μm long. Calculate the number of sarcomeres along the length of this muscle fibre.
Hard
53.Define the term antagonistic muscle action.
Medium
54.Explain why a decrease in the availability of calcium ions would result in a decrease in the strength of muscle contraction.
Hard
55.Describe the role of phosphocreatine in supplying energy during muscle contraction.
Hard
56.Explain why one action potential arriving at a neuromuscular junction might not cause a contraction, but several arriving in succession might. (3 marks)
Hard
57.Describe the events in the cycle of actinomyosin bridge formation. (5 marks)
Hard
58.A muscle fibre is 30 mm long when relaxed and 22.5 mm long when contracted. Calculate the percentage decrease in length of the contracted muscle.
Medium
- A22 %
- B28 %
- C25 %
- D31 %
59.If each sarcomere is 2.5 μm long in a contracted muscle fibre, calculate how many sarcomeres would be found along the whole length of a contracted muscle fibre that is 22.5 mm long.
Medium
- A8100 sarcomeres
- B9000 sarcomeres
- C10800 sarcomeres
- D9900 sarcomeres
60.A bundle of muscle fibres contains 30 slow fibres and 70 fast fibres. Calculate the ratio of slow to fast fibres in the bundle. Give your answer as a decimal to two decimal places.
Medium
- A1.43
- B-0.57
- C2.43
- D0.43
61.A muscle fibre is 36 mm long when relaxed and 27 mm long when contracted. Calculate the percentage decrease in length.
Medium
- A25 %
- B22 %
- C31 %
- D28 %
62.A contracted muscle fibre is 20 mm long. Each sarcomere in the fibre is 2.5 μm long. Calculate the number of sarcomeres along the length of this muscle fibre.
Hard
- A8800 sarcomeres
- B9600 sarcomeres
- C7200 sarcomeres
- D8000 sarcomeres

Lernnotizen

## Antagonistic Muscles - Skeletal muscles **pull** on the rigid, incompressible skeleton to enable movement. - Muscles attach to bones via **tendons**, which do not stretch when the muscle contracts and pulls on a bone. - Muscles can only pull, not push, so they act in **antagonistic pairs**: one muscle pulls the bone one way, the other pulls it the opposite way. - Example: **biceps and triceps**. - Raise lower arm: biceps contracts, triceps relaxes → arm bends at elbow. - Straighten arm: triceps contracts, biceps relaxes → arm straightens. ## The Structure of Skeletal Muscle - Skeletal (striated/striped) muscle contracts to move the skeleton. - Made of bundles of muscle cells = **muscle fibres**. - In a muscle fibre: - cell surface membrane = **sarcolemma** - cytoplasm = **sarcoplasm** - endoplasmic reticulum = **sarcoplasmic reticulum** - Features: elongated cell with many nuclei; sarcoplasm contains contractile proteins forming **myofibrils**; sarcolemma has deep tube-like projections (**T-tubules**); many mitochondria generate ATP. - **Myofibrils** are long, rod-like structures made of protein filaments: - **myosin** = thick filaments - **actin** = thin filaments - Myofibrils are divided into **sarcomeres**, which shorten during contraction as actin and myosin filaments slide past each other (**sliding filament model**). ### Sarcomere features - **Z-line**: boundary between sarcomeres; actin attaches here; z-lines get closer together during contraction. - **M-line**: central point of each sarcomere; myosin attaches here. - **A band**: region across which myosin filaments extend; myosin does not change length, so **A band remains the same size** whether contracted or relaxed. - **H band**: region containing only myosin filaments; **shrinks** during contraction as overlap increases. - **I band**: region containing only actin filaments; **shrinks** during contraction as overlap increases. ## Skeletal Muscle Under a Microscope - Optical microscope: too low resolution to see details; visible features include **banding** and **nuclei**. - Electron microscope (TEM): higher resolution reveals myofibril structure — **dark lines at each Z-line**, **light bands = I bands**, **dark bands = A bands**. - Dark bands produce the characteristic striped appearance. ## Muscle Contraction (Sliding Filament Theory) - Myosin filaments are fibrous proteins with a **globular head**. - Actin filaments have **actin-myosin binding sites**; **tropomyosin** blocks these sites when the muscle is relaxed. - Steps: 1. Action potential arrives at neuromuscular junction; **calcium ions released from sarcoplasmic reticulum**. 2. Calcium ions cause **tropomyosin to change position**, exposing actin-myosin binding sites. 3. Myosin heads bind to actin, forming **cross-bridges**. 4. Myosin heads bend, releasing ADP and inorganic phosphate, pulling actin towards the centre of the sarcomere — the **power stroke** (occurs spontaneously with release of ADP and Pi, not directly from ATP hydrolysis). 5. ATP binds to myosin heads, causing them to release from actin. 6. **ATP hydrolase** hydrolyses ATP, providing energy to reset myosin heads. 7. Myosin heads bind to new sites; process repeats. - When no longer stimulated: calcium ions are taken up again by the sarcoplasmic reticulum (via active transport), binding sites are blocked, and filaments are pulled apart by an antagonistic muscle. ### ATP and phosphocreatine - ATP needed to reset myosin heads. - ATP sources: aerobic respiration (low-intensity, needs oxygen), anaerobic respiration (fast but short due to lactate), **phosphocreatine** (rapid ATP over short periods). - **ADP + phosphocreatine → ATP + creatine** ## Slow & Fast Skeletal Muscles | Feature | Fast fibres | Slow fibres | |---|---|---| | Contraction | Rapid | Slow | | ATP supply | Mostly anaerobic | Mostly aerobic | | Fatigue | Rapidly (high lactate) | Slowly (reduced lactate) | | Mitochondria | Fewer, smaller | Many, large | | Glycogen & phosphocreatine | Large stores | Small stores | | Calcium store in SR | Large | Small | | Capillaries | Fewer | More | | ATP hydrolase activity | High | Lower | | Colour | Pale | Dark | | Myoglobin/haemoglobin | Low | High | - Fast fibres suit short bursts of high-intensity activity (e.g. eyelid muscles, biceps/triceps). - Slow fibres suit sustained, low-intensity activity (e.g. back and leg muscles). ## Investigating Muscle Fatigue - Fatigue due to **lactate production** and **decreased calcium ion availability** after repeated contractions. - Method: squeeze hand grip strengthener as many times as possible over 20 s; record successful squeezes; rest 10 s; repeat several times; repeat with other subjects. - Results: number of successful squeezes decreases over time. - Limitations: only hand muscles studied; other muscles may fatigue differently; exercise intensity may be too low for some; hand grip may be too difficult for some even at start.

Antagonistic muscle action: Muscles act in pairs; one muscle pulls the bone in one direction, and the other pulls the bone in the opposite direction.
Tendon: Connects muscle to bone; does not stretch when a muscle is contracting and pulling on a bone.
Sarcolemma: The cell surface membrane of a muscle fibre.
Sarcoplasm: The cytoplasm of a muscle fibre.
Sarcoplasmic reticulum: The endoplasmic reticulum of a muscle fibre; stores and releases calcium ions.
T-tubules: Deep tube-like projections that fold in from the sarcolemma's outer surface.
Myofibril: Long, rod-like structure made of protein filaments (myosin and actin) found in muscle fibres.
Myosin: Thick protein filament; fibrous protein with a globular head that binds to actin.
Actin: Thin protein filament; has actin-myosin binding sites and is associated with tropomyosin.
Sarcomere: Repeating section of a myofibril that shortens during muscle contraction as myosin and actin filaments slide past each other.
Z-line: Boundary between sarcomeres; actin filaments attach here; z-lines get closer together during contraction.
M-line: Central point of each sarcomere; myosin filaments attach here.
A band: Region of the sarcomere across which the myosin filaments extend; remains the same size whether the muscle is contracted or relaxed.
H band: Region of the sarcomere containing only myosin filaments; shrinks during muscle contraction.
I band: Region of the sarcomere containing only actin filaments; shrinks during muscle contraction.
Sliding filament model: During muscle contraction, actin and myosin filaments slide over each other, causing sarcomeres to shorten.
Tropomyosin: Protein associated with actin that blocks the actin-myosin binding sites when the muscle is relaxed.
Power stroke: The myosin heads bend, releasing ADP and inorganic phosphate and pulling the actin filaments towards the centre of the sarcomere.
ATP hydrolase: Enzyme that hydrolyses ATP, providing energy that allows the myosin heads to reset to their original positions.
Phosphocreatine: Molecule stored by muscles that can be used for the rapid production of ATP over short periods: ADP + phosphocreatine → ATP + creatine.
Fast muscle fibres: Contract rapidly, rely on anaerobic respiration, fatigue quickly due to lactate production, are pale in colour, have few capillaries and low myoglobin.
Slow muscle fibres: Contract more slowly, rely on aerobic respiration, fatigue slowly due to reduced lactate production, are darker in colour, have a dense capillary network and high myoglobin and haemoglobin.
Muscle fatigue: Repeated contraction of muscles may result in fatigue due to production of lactate during anaerobic respiration and a decrease in the availability of calcium ions after repeated contractions.

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