Muscle & Motility

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Adaptations for Movement

Movement is a unifying feature of all living organisms, but its form varies hugely.
Motile organisms can move from place to place; most animals and many predatory bacteria are motile.
Motile organisms also show internal movement in response to stimuli, e.g. peristalsis in the digestive system.
Sessile organisms cannot move from place to place but can move parts of their body in response to environmental stimuli.
Examples of sessile organisms: sponges, corals, anemones, most fungi and all plants.
Sessile movement includes orientation of plant stems towards the sun and movement of cytoplasm within a unicellular organism.
You need to know one named example of a motile species and one of a sessile species for the exam.

Skeletal Muscle Structure

Skeletal muscles are attached to the skeleton and aid movement; they appear striated (stripy) under a microscope.
Striated muscle cells are bundled into fibres — highly specialised cell-like units.
Each muscle fibre contains an organised arrangement of contractile proteins, many nuclei, and a specialised endoplasmic reticulum called the sarcoplasmic reticulum (SR).
The SR stores calcium and conveys signals to all parts of the fibre at once using protein pumps in its membranes.
The specialised cytoplasm is called sarcoplasm and contains mitochondria and myofibrils.
Mitochondria carry out aerobic respiration to generate the ATP required for muscle contraction.
Myofibrils are bundles of actin and myosin filaments that slide past each other during contraction.

Myofibril Structure and Bands

Each myofibril is made of two protein filaments: thick filaments of myosin and thin filaments of actin.
H band: only thick myosin filaments are present.
I band: only thin actin filaments are present.
A band: contains areas where only myosin is present and areas where myosin and actin overlap.
M line: attachment point for myosin filaments.
Z line: attachment point for actin filaments.
A sarcomere is the section of myofibril between two Z lines.

Sliding Filament Model

Myosin molecules are fibrous proteins with a globular head; the fibrous part anchors the molecule into the thick filament.
In the thick filament, many myosin molecules lie next to each other with their globular heads pointing away from the M line.
Actin molecules are globular proteins; many link together to form a chain, and two chains twist together to form one thin filament.
Tropomyosin is a fibrous protein twisted around the two actin chains; troponin is attached to the actin chains at regular intervals.
During contraction, myosin heads form cross-bridges by binding to sites on the actin filaments.
The myosin heads then change orientation, pulling the actin filaments so they slide next to the myosin — this is the power stroke.
Sarcomeres shorten as the Z lines are pulled closer together; the A band stays the same length while the I band and H zone shorten.

Muscle Relaxation and Titin

Muscles can only contract or pull — they cannot push.
Muscles therefore generally operate in antagonistic pairs: one pulls in one direction at a joint, the other pulls in the opposite direction.
Isometric contraction is a muscle contraction without motion; antagonistic muscles both contract at a joint to maintain posture.
Titin is a large protein that joins the ends of the myosin filaments to the Z line.
The many folds in titin give it spring-like properties; when a muscle is relaxed the sarcomere lengthens and titin is stretched out.
Stretched titin stores chemical energy and prevents overstretching.
During contraction, sarcomeres shorten and titin recoils, releasing stored energy that adds to the force of contraction.

Motor Units and the Neuromuscular Junction

Skeletal muscle contracts when it receives an impulse from a motor neurone via the neuromuscular junction.
Neuromuscular junctions work in a very similar way to synapses and are located between a motor neurone and a muscle cell.
A motor unit is the collection of a motor neurone, the skeletal muscle fibres, and the neuromuscular junctions between them.
There are multiple neuromuscular junctions spread across several muscle fibres within a motor unit, varying the intensity of contraction.
A low intensity contraction activates a low number of motor units.
A high intensity contraction involves more motor units receiving impulses.

Skeleton, Levers and Joints

Effective movement of the human body requires both muscle and an incompressible skeleton.
Bones and exoskeletons provide anchorage for muscles and act as levers.
Vertebrates have an internal endoskeleton; arthropods and molluscs have an external exoskeleton made of the polysaccharide chitin.
Skeletons provide support and facilitate movement; exoskeletons also protect soft tissues.
Levers have a point of effort, a point of load, and a pivot called the fulcrum — the same three features are seen in skeletons.
Synovial joints are the most common joint type; they have a joint cavity filled with lubricating synovial fluid that reduces friction.
The fluid is produced by the synovial membrane, which surrounds the joint.
Movements possible at synovial joints include flexion, extension, rotation, abduction (away from the body) and adduction (towards the body).

Joint Types and the Hip Joint

Knee and elbow are hinge joints allowing flexion and extension.
Hip is a ball and socket joint allowing flexion, extension, rotation, and sideways and backwards movement.
Shoulder allows abduction and adduction, flexion and extension.
The hip joint articulates between the femur (the ball) and the pelvis (the socket).
Cartilage covers both bones, providing a surface that prevents the bones rubbing against each other.
Synovial fluid is enclosed within the ball and socket by a membrane and lubricates for smooth movement.
The joint is encircled by ligaments (tough connective tissue) that hold the bones in place.
Skeletal muscles move the femur within the pelvis socket and are connected to each bone via tendons.

Antagonistic Muscles: Intercostal Example

There are over 600 skeletal muscles in the human body; muscles are effectors stimulated by nerve impulses from motor neurones.
Tendons are lengths of strong connective tissue that connect muscles to bones; they are flexible but do not stretch when a muscle contracts.
The intercostal muscles of the rib cage are an example of an antagonistic pair.
External intercostal muscles pull the rib cage up; internal intercostal muscles pull the rib cage down.
During inhalation, the external intercostals contract to pull the ribs up and out, increasing thorax volume, decreasing lung air pressure, so air is drawn in.
During exhalation, the external intercostals relax so the ribs drop down and in, decreasing thorax volume, increasing lung air pressure, so air is forced out.
The opposite rib movements are due to the orientation of muscle fibres in the internal and external layers.
Contraction of the external intercostals stretches the internal intercostals, storing potential energy in the titin protein of their sarcomeres.

Movement of Joints: Skills and Locomotion

The range of motion of a joint can be measured using a goniometer, which measures the distance and direction a joint can move in degrees.
Computer analysis can track and measure joint movement, and phone apps can simulate a goniometer.
Locomotion is the movement or ability to move from one place to another; it is particularly important in the animal kingdom.
Reasons for locomotion include foraging for food, escaping from danger, searching for a mate and migration.
Examples: guinea pigs forage for food; jumping spiders (Salticidae) jump away from predators; salmon swim from ocean to freshwater rivers to mate; caribou migrate over 3,000 miles a year.
Marine mammals are adapted for swimming: streamlined bodies, front limbs modified into flippers for steering, and rear legs lost for streamlining.
Their tails form a fluke that moves up and down for propulsion, and a blowhole allows periodic breathing between dives and can be sealed to keep water out.

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Practice questions

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1.Which of the following is an example of a sessile organism?
Easy
- AA sponge
- BA salmon
- CA jumping spider
- DA caribou
2.Which of the following is NOT a reason for locomotion in animals?
Easy
- AForaging for food
- BEscaping from danger
- CSearching for a mate
- DPhotosynthesis
3.The A band of a sarcomere contains areas where only myosin filaments are present and areas where myosin and actin filaments overlap.
Easy
True or false?
4.Which of the following describes a function of ATP in muscle contraction?
Medium
- ATo actively transport calcium ions into myofibrils from the sarcoplasmic reticulum
- BTo move tropomyosin and expose myosin binding sites
- CTo allow crossbridge formation
- DTo cause cocking of the myosin head
5.Which of the following are features of a muscle fibre? (select all that apply)
Medium
- AMany nuclei
- BSarcoplasmic reticulum that stores calcium
- CSarcoplasm containing mitochondria and myofibrils
- DA single nucleus
- ENo mitochondria
6.Match each part of the myofibril with its correct description.
Medium
- H band
- I band
- A band
- Z line
- Only thick myosin filaments present
- Only thin actin filaments present
- Contains areas of only myosin and areas of overlap
- Attachment for actin filaments
7.Place the following structures in descending order of size, from largest to smallest.
Medium
- Muscle fibre
- Myofibril
- Sarcomere
- Myofilament
8.Which of the following are adaptations of marine mammals for swimming? (select all that apply)
Medium
- AStreamlined body shape
- BFront limbs modified into flippers
- CPresence of a blowhole
- DRear limbs enlarged for propulsion
- ETails forming a fluke for up-and-down movement

More questions

9.Match each joint movement with its correct description.
Medium
- Flexion
- Extension
- Abduction
- Adduction
- Decreasing the angle at a joint
- Increasing the angle at a joint
- Movement of a limb away from the body
- Movement of a limb towards the body
10.Place the events of the sliding filament model in the correct chronological order.
Medium
- Myosin heads bind to actin forming cross-bridges
- Myosin heads change orientation, pulling actin filaments (power stroke)
- ATP binds to myosin heads, breaking cross-bridges
- Myosin heads return to their original position (cocking)
11.Which of the following are true of skeletal muscle fibres? (select all that apply)
Medium
- AThey contain many nuclei.
- BThey have a specialised endoplasmic reticulum called sarcoplasmic reticulum.
- CThey are striated.
- DThey are individual cells with a single nucleus.
- EThey contain myofibrils in their cytoplasm.
12.Which of the following best explains why a muscle movement is smooth rather than jerky?
Medium
- AAll motor units contract simultaneously.
- BDifferent motor units contract at different times, so the pulls overlap.
- CThe power stroke is a long, continuous movement.
- DTitin prevents any movement until many cross-bridges form.
13.During muscle contraction, the A band shortens.
Easy
True or false?
14.Which of the following statements correctly describes the events observed during a muscle contraction?
Medium
- AThe length of the sarcomere stays the same, the dark band gets longer and the light bands stay the same
- BThe length of the sarcomere gets shorter, the dark band gets shorter and the distance between the two Z lines gets shorter
- CThe length of the sarcomere stays the same, the dark band gets longer and the distance between the two Z lines gets shorter
- DThe length of the sarcomere gets shorter, the dark band stays the same and the light bands get shorter
15.Which of the following is not a range of movement demonstrated by a synovial joint such as an elbow or knee joint?
Medium
- AAbduction
- BFlexion
- CExtension
- DInversion
16.A student measures the length of a muscle fibre as 50 mm when relaxed and 40 mm when contracted. What is the percentage decrease in length?
Medium
- A10%
- B20%
- C25%
- D80%
17.Which of the following statements about the role of ATP in muscle contraction are correct? (select all that apply)
Medium
- AATP is required for the myosin head to detach from actin.
- BATP hydrolysis provides energy for the power stroke.
- CATP is used to pump calcium ions back into the sarcoplasmic reticulum.
- DATP is necessary for the binding of calcium to troponin.
- EATP is broken down to ADP and phosphate to cock the myosin head.
18.A student uses a goniometer to measure the range of motion at a knee joint. Which of the following is the unit used by a goniometer to record joint movement?
Medium
- ADegrees
- BNewtons
- CMillimetres
- DJoules
19.Which of the following best describes the role of the sarcoplasmic reticulum in a skeletal muscle fibre?
Medium
- AIt stores calcium and conveys signals to all parts of the fibre at once using protein pumps in the membranes.
- BIt carries out aerobic respiration to generate the ATP required for muscle contraction.
- CIt is a bundle of actin and myosin filaments that slide past each other during contraction.
- DIt is a tough connective tissue that connects muscle to bone.
20.Titin is a large protein that joins the ends of the myosin filaments to the Z line.
Easy
True or false?
21.Identify the molecules present in the regions labelled 1 and 2 in the image.
Medium
- A1 = actin only; 2 = myosin only
- B1 = myosin only; 2 = actin only
- C1 = actin and myosin; 2 = actin only
- D1 = actin only; 2 = actin and myosin
22.Which row correctly identifies the parts of a sarcomere? I = the line that attaches actin filaments, II = the section between two Z lines, III = the band containing only thick myosin filaments, IV = the band containing only thin actin filaments.
Easy
- AI = Z line, II = sarcomere, III = H zone, IV = I band
- BI = H zone, II = A band, III = Z line, IV = I band
- CI = sarcomere, II = H zone, III = I band, IV = Z line
- DI = sarcomere, II = I band, III = A band, IV = Z line
23.Which numbers represent a pair of antagonistic muscles at the elbow?
Easy
- AI and II
- BV and IV
- CVI and II
- DII and III
24.Which of the following are reasons for locomotion in animals? I. Foraging for food II. Escaping from danger III. Searching for a mate
Easy
- AI and II only
- BI and III only
- CII and III only
- DAll of the above
25.The diagram shows the sarcomere of a muscle fibre. Which of the following statements correctly describes the events observed during a muscle contraction?
Medium
- AThe length of the sarcomere stays the same, the dark band gets longer and the light bands stay the same
- BThe length of the sarcomere gets shorter, the dark band gets shorter and the distance between the two Z lines gets shorter
- CThe length of the sarcomere stays the same, the dark band gets longer and the distance between the two Z lines gets shorter
- DThe length of the sarcomere gets shorter, the dark band stays the same and the light bands get shorter
26.One myosin powerstroke requires hydrolysis of one molecule of ATP and moves one actin filament 20 nm. How many ATP molecules would be required to move a single actin filament 0.3 μm when the sarcomere contracts?
Medium
- A150 ATP
- B15 ATP
- C20 ATP
- D1.5 ATP
27.Match each band or line of a myofibril with the filaments present there.
Medium
- H band
- I band
- A band
- M line
- Z line
- Only thick myosin filaments present
- Only thin actin filaments present
- Contains areas of myosin only and areas where actin and myosin overlap
- Attachment point for myosin filaments
- Attachment point for actin filaments
28.Match each joint with the movements it is capable of demonstrating.
Medium
- Knee (hinge)
- Elbow (hinge)
- Hip (ball and socket)
- Shoulder (ball and socket)
- Flexion and extension
- Flex, extend, rotate, sideways and backwards movement
- Abduction and adduction, flexion and extension
29.Match each component of a synovial joint with its role.
Medium
- Synovial fluid
- Synovial membrane
- Cartilage
- Ligaments
- Tendons
- Lubricates the joint to reduce friction
- Produces the synovial fluid surrounding the joint
- Covers bone surfaces to prevent them rubbing together
- Tough connective tissue that holds the bones in place
- Connect muscle to bone
30.Match each reason for locomotion with the animal example given for it.
Medium
- Foraging for food
- Escaping from danger
- Searching for a mate
- Migration
- Guinea pigs, which spend at least 70% of their waking time searching for and eating plants
- Salticidae (jumping) spiders, which use their legs to jump away from predators
- Salmon, which swim from the ocean to freshwater rivers to spawn
- Caribou, which can travel over 3,000 miles a year between winter ranges and calving grounds
31.Match each protein with its correct description.
Medium
- Myosin
- Actin
- Tropomyosin
- Troponin
- Titin
- Fibrous molecule with a globular head that forms the thick filament
- Globular molecules that link into chains, two of which twist to form a thin filament
- Fibrous protein twisted around the two actin chains
- Protein attached to the actin chains at regular intervals
- Large protein joining the ends of myosin filaments to the Z line
32.Place the following structures in ascending order of size, from smallest to largest.
Medium
- Actin molecule
- Thin filament
- Myofibril
- Muscle fibre
33.Place the events of ventilation during inhalation in the correct sequence.
Medium
- External intercostal muscles contract and pull the ribs up and out
- Volume of the chest cavity (thorax) increases
- Air pressure inside the lungs decreases relative to outside the body
- Air is drawn into the lungs
34.Place the following events in the correct order for one cycle of the sliding filament mechanism.
Hard
- Myosin head binds to a site on the actin filament forming a cross-bridge
- Myosin head changes orientation, pulling the actin filament (power stroke)
- Actin filament slides past the myosin filament
- Sarcomere shortens as the Z lines are pulled closer together
35.Which of the following statements about the motor unit of skeletal muscle are correct? (select all that apply)
Medium
- AIt consists of a motor neurone, muscle fibres and the neuromuscular junctions between them
- BNeuromuscular junctions work in a very similar way to synapses
- CA low intensity contraction activates a low number of motor units
- DA high intensity contraction activates fewer motor units than a low intensity contraction
- EMotor units are spread across several muscle fibres to vary the intensity of contraction
36.Which of the following are properties of titin? (select all that apply)
Medium
- AIt joins the ends of the myosin filaments to the Z line
- BIts many folds give it spring-like properties
- CIt stores chemical energy when the sarcomere is stretched
- DIt prevents overstretching of the muscle
- EIt forms the thin filaments of the myofibril
37.Which of the following are examples of technology being used to analyse joint motion? (select all that apply)
Medium
- AUsing a computer to analyse an image of a joint
- BUsing a goniometer to measure the angle of a joint
- CUsing a phone app to measure joint angles
- DUsing a microscope to count the nuclei in a muscle fibre
38.Which of the following statements about antagonistic muscle action are correct? (select all that apply)
Hard
- AMuscles can only contract or pull, they cannot push
- BAntagonistic muscles generally operate in pairs
- CWhen one muscle of a pair pulls in one direction at a joint, the other pulls in the opposite direction
- DBoth muscles of an antagonistic pair must always contract at the same time to produce movement
- EThe diaphragm forms an antagonistic pair with the intercostal muscles
39.Which of the following are features of the sarcoplasm of a muscle fibre? (select all that apply)
Medium
- AIt contains mitochondria
- BIt contains myofibrils
- CIt carries out aerobic respiration to generate ATP for contraction
- DIt stores calcium ions
- EIt is the specialised cytoplasm of the muscle fibre
40.Which of the following organisms are sessile? (select all that apply)
Medium
- ASponges
- BCorals
- CAnemones
- DMost fungi species
- ESalmon
41.Complete the sentence about the sliding filament model.
Easy
The thin filaments of a myofibril are made of the protein ____, while the thick filaments are made of the protein ____.
42.One myosin power stroke requires the hydrolysis of one molecule of ATP and moves one actin filament 20 nm. How many ATP molecules would be required to move a single actin filament 0.3 μm when the sarcomere contracts?
Medium
43.State the word used to describe a pair of muscles that act against each other to perform opposite actions.
Easy
44.A structure is 30 mm long when relaxed and 22.5 mm long when contracted. Calculate the percentage decrease in length.
Medium
45.Name the protein that joins the ends of the myosin filaments to the Z line and provides spring-like properties.
Easy
46.Complete the sentence about the role of ATP in muscle contraction.
Medium
ATP is required to ____ the myosin head, allowing it to bind to actin, and to break the cross-bridge by ____ to the myosin head.
47.One myosin power stroke requires hydrolysis of one molecule of ATP and moves one actin filament 20 nm. How many ATP molecules would be required to move a single actin filament 0.3 μm when the sarcomere contracts?
Medium
48.A sarcomere is 2.5 μm long when relaxed. During contraction, it shortens by 20%. What is the new length of the sarcomere in μm?
Medium
49.A goniometer is used to measure the range of motion at a knee joint. The angle recorded is 135°. Express this angle in degrees as a whole number.
Medium
50.During a muscle contraction, a myosin power stroke moves an actin filament by 20 nm. If a single actin filament is moved a total distance of 0.4 μm, how many power strokes are required?
Medium
51.What is the name of the protein that forms the thin filaments in a myofibril?
Easy
52.A muscle fibre is 50 mm long when relaxed and 40 mm when contracted. Calculate the percentage decrease in length.
Medium
53.Outline how the filaments within muscle fibres slide past each other in the sliding filament mechanism of muscle contraction.
Hard
54.Explain how the structure of a myofibril is related to its function in muscle contraction.
Hard
55.Identify structures X, Y and Z.
Medium
56.State the word which is used to describe a pair of muscles that act against each other to perform opposite actions.
Easy
57.The diagram shows a myofibril strand in a relaxed muscle fibre. Describe the effect of muscle fibre contraction on the length of the A-band and the sarcomere.
Medium
58.One myosin power stroke requires the hydrolysis of one molecule of ATP and moves one actin filament 20 nm. How many ATP molecules would be required to move a single actin filament 0.3 μm when the sarcomere contracts?
Medium
- A14 ATP molecules
- B17 ATP molecules
- C15 ATP molecules
- D16 ATP molecules
59.A structure is 30 mm long when relaxed and 22.5 mm long when contracted. Calculate the percentage decrease in length.
Medium
- A31 %
- B22 %
- C25 %
- D28 %
60.One myosin power stroke requires hydrolysis of one molecule of ATP and moves one actin filament 20 nm. How many ATP molecules would be required to move a single actin filament 0.3 μm when the sarcomere contracts?
Medium
- A16 ATP
- B14 ATP
- C17 ATP
- D15 ATP
61.A sarcomere is 2.5 μm long when relaxed. During contraction, it shortens by 20%. What is the new length of the sarcomere in μm?
Medium
- A3 μm
- B4 μm
- C1 μm
- D2 μm
62.A goniometer is used to measure the range of motion at a knee joint. The angle recorded is 135°. Express this angle in degrees as a whole number.
Medium
- A135°
- B121°
- C149°
- D163°
63.During a muscle contraction, a myosin power stroke moves an actin filament by 20 nm. If a single actin filament is moved a total distance of 0.4 μm, how many power strokes are required?
Medium
- A22 power strokes
- B24 power strokes
- C20 power strokes
- D18 power strokes
64.A muscle fibre is 50 mm long when relaxed and 40 mm when contracted. Calculate the percentage decrease in length.
Medium
- A24 %
- B20 %
- C18 %
- D22 %

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