6.3 Skeletal Muscles (A Level only)

Figure 1 below shows a cross section through a skeletal muscle.

Figure 1

Identify structures A-D labelled on Figure 1.

The structures labelled E in Figure 1 above are called myofibrils. When myofibrils are viewed in longitudinal section they have a striped, or striated, appearance and are made up of manysarcomeres. Two sarcomeres are shown in Figure 2 below. 

Figure 2

The myofibril is 4.5cm in length when it is relaxed, while a contracted sarcomere measures 1.9μm. 

Use the scale on Figure 2 to calculate the length of a contracted myofibril (the image measurements are provided for you). Give your answer in mm.

The striated pattern visible on the sarcomeres is due to the presence of two types of filament within the myofibril. These filaments are known to be involved with muscle contraction, but a relatively recent discovery has shown that a third protein filament known as titin, is also present. The role played by titin is still being investigated, but one study was carried out in mice that looked at the effects of removing titin from their muscles.

To remove the titin protein, the mice had their muscles modified so that their titin filaments contained a cutting site for a plant enzyme called TEV protease.

Suggest why the plant enzyme TEV was used rather than an animal protease.

Figure 3 below shows the location of titin within the sarcomere, as well as some of the results gained from the study described in part c) above. 

Figure 3

Use Figure 3 and your existing knowledge to suggest 3 possible functions of titin filaments

Figure 1 below shows the relationship between muscle fibre diameter and oxidative capacity (ability to use oxygen).

Figure 1

Suggest an explanation for this relationship.

Research has shown that larger animals tend to have a higher proportion of muscle fibres with a larger diameter, and that these large-diameter fibres are fast muscle fibres, while slow fibres are narrower in diameter.

Figure 2 shows the relationship between body mass in mammals and reptiles and their running speed. 

Figure 2

Suggest an explanation for the relationship shown in Figure 2 between body mass and speed in mammals.

Reptiles often have a different angle between their legs and body compared to mammals, and this is often suggested as a reason for the difference in average maximum speed attained by mammals and reptiles of equivalent body mass.

Suggest and explain one other reason for the difference in average maximum speed between mammals and reptiles of equivalent body mass.

A researcher used a light microscope to calculate the mean diameter of the muscle fibres in rats. The microscope used had a field of view with a circular area of 1.5mm², and 12 muscle fibres were visible in cross-section when looking down the microscope.

Use this information and the equation provided to calculate the mean diameter of a rat muscle fibre. Give your answer in µm.

The area of a circle = π r²

Curare is the name given to a collection of plant compounds that have been used historically as both poisons and as muscle relaxants for medical procedures. Curare is known to act at neuromuscular junctions to bring about paralysis.

Suggest how curare might affect the neuromuscular junctions to bring about muscle paralysis.

Figure 1 below shows a set of structures called the T-tubules which play an important role at the neuromuscular junction. Use Figure 1 to suggest how the T-tubules help in the initiation of a muscle contraction.

Figure 1

Figure 2 shows the events that take place after a muscle is stimulated and contraction begins.

Figure 2

Use the information in Figure 2 and your own knowledge to explain the role played by ATP in muscle contraction. 

Each round of myosin binding, power stroke and resetting requires the hydrolysis of one molecule of ATP. If a single round moves an actin filament by 35 nm and a full muscle contraction moves an actin filament by 0.49 µm, calculate the number of ATP molecules required for this full contraction to take place.

A group of scientists wanted to find out about the effect of a protein known as TWEAK on exercise capabilities in mice. They compared 6 mice with a normal TWEAK gene (described as wild-type) with 6 mice in which the TWEAK gene function had been removed (described as TWEAK-KO). Some of their results are shown below. 

Figure 1

* = numbers vary significantly between wild-type and TWEAK-KO

The researchers concluded that TWEAK-KO mice had improved exercise tolerance. 

Use Figure 1 and the information provided to evaluate this conclusion.

The researchers also carried out tests on the muscle cells of the mice, testing for the presence of an enzyme called succinate dehydrogenase (SDH) that is found in the mitochondria. They stained the muscle tissue using a stain that attaches to SDH. Their results are shown in Figure 2.

Figure 2

Muscle fibre X, and those that look like it, are slow muscle fibres. Explain how this can be concluded only from information found in Figure 2

The SDH stain shows as being darker in colour in areas where more SDH is present. 

State what Figure 2 shows about SDH distribution inside muscle fibres, and suggest an explanation for this distribution.

Explain how the difference between wild-type and TWEAK-KO muscle fibres shown in Figure 2 can account for the results shown in Figure 1.

Scientists investigated whether the blood supply to slow and fast muscle fibres in a muscle differed between sexes. They used diaphragms taken from mice as the diaphragm is constantly in use for breathing. Diaphragms were taken from groups of adult male and female mice. A sample of muscle tissue was taken from each diaphragm and examined under an optical microscope. For each sample, they selected several fields of view at random. In each field of view, the number of capillaries associated with each type of muscle fibre was counted and recorded.

This allowed scientists to calculate the mean number of capillaries for each type of muscle fibre, for males and females. Table 1 below shows the results which include standard deviation (SD).

Table 1

Give three precautions that the scientists took to ensure their calculations of mean number of capillaries per fibre were reliable.

The researchers examined the muscle of an animal in the female age group. They found one field of view contained only slow muscle fibres. They counted 63 capillaries in this field of view. Use a calculation to estimate how many slow muscle fibres were visible in this field of view. Show your working.

A research assistant concluded that sex had a significant effect on the mean number of capillaries per muscle fibre. How do the results support this conclusion?

Give four differences between slow and fast muscle fibres.

Describe the role of phosphocreatine in supplying energy during muscle contraction.

Researchers investigated the time taken for phosphocreatine (PC) to be re-formed in leg muscles after a certain amount of exercise in healthy people of different ages. The exercise involved short, fast contractions of the leg muscles. Figure 1 below shows the results. Each dot is the result of a single individual.

Figure 1

There is a large degree of variation in the time taken for phosphocreatine to be re-formed in people of a very similar age. Suggest three possible reasons for this variation.

Describe and explain the data seen in Figure 1.

Describe what a tendon is and its function.

Describe the events in the cycle of actinomyosin bridge formation.

Describe what happens to the length of the A band and the I band when the sarcomeres contract. Explain your answer. 

Complete Table 1 below by identifying the appropriate molecules.

Table 1

Describe an antagonistic pair of muscles.  Give an example.

It is thought that each human is born with a certain proportion of slow and fast muscle fibres in their skeletal muscles. Most individuals have roughly half “fast fibres” and half “slow fibres”.

A sports researcher predicted that these proportions would change over time depending on the type of activity/sport a person trained in. 

The following facts were established before the researcher’s investigation: the number of muscle fibres in a skeletal muscle remains constant over time but the number of mitochondria within a fibre can change and the diameter of fibres can also change.

She calculated the mean percentages of fast and slow fibres in skeletal muscles of different athletes. The results are shown in Figure 1 below in the form in which he presented them.

Figure 1

Predict which athlete type the sports scientist would expect to find muscle fibres with the lowest number of mitochondria? Justify your answer.

A research assistant reviewed the results and stated that “regular powerlifting changes your proportion of slow and fast skeletal muscle fibres.” Do you agree with this statement? Explain your answer.

The arm muscles of long-distance swimmers are usually larger than the arm muscles of non-athletes. Suggest why.

Ultra long-distance swimmers may suffer from muscle fatigue when the contraction mechanism of muscles is disrupted. One reason for this thought to be a decrease in the availability of calcium ions within muscle fibres. Explain how a decrease in the availability of calcium ions could disrupt the contraction of muscles.