Chapter 17: Locomotion and Movement Case Study Questions | biology Class 11 CBSE

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Case Set 1

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During a biology practical, students observed stained slides of skeletal muscle fibres under a high-power microscope. They noticed alternating dark and light bands running across each fibre. The teacher explained that these bands arise due to the organised arrangement of two key proteins within the myofibrils. She further pointed out a thin dark line bisecting each light band, and a lighter central zone within the dark band. She asked students to identify the structural components and explain how the arrangement of these proteins gives rise to the banding pattern. One student correctly noted that the functional unit of contraction lies between two of these lines, and that this unit shortens during muscle contraction while one band retains its original length.

Question 1: Name the two proteins responsible for the striated appearance of skeletal muscle and identify which band each occupies.

Actin (thin filament) is responsible for the light I band (Isotropic band), which appears lighter because it contains only thin filaments.
Myosin (thick filament) is responsible for the dark A band (Anisotropic band), which appears darker because it contains thick filaments along with overlapping portions of thin filaments.
Both proteins are arranged as rod-like structures parallel to each other and to the longitudinal axis of the myofibril.

Question 2: Identify the 'thin dark line' and the 'lighter central zone' mentioned in the passage and state the function of each.

The thin dark line bisecting each I band is the Z line — it is an elastic fibre to which the thin actin filaments are firmly attached, anchoring them in place.
The lighter central zone within the A band is the H zone — it represents the central portion of thick myosin filaments that is not overlapped by thin actin filaments in the resting state.
The M line, found in the middle of the A band, holds the thick myosin filaments together within the H zone region.

Question 3: The student mentioned that one band retains its length during contraction. Explain what happens to the I band, A band, H zone, and Z lines during muscle contraction, and relate these changes to the sliding filament theory.

During contraction, thin actin filaments slide over thick myosin filaments towards the centre of the A band — this is the basis of the sliding filament theory.
The I bands shorten because the actin filaments slide inward, reducing the portion of the sarcomere occupied solely by thin filaments.
The A band retains its original length because the thick myosin filaments themselves do not shorten or move — only the thin filaments slide over them.
The H zone reduces and may disappear at maximum contraction as thin filaments from both sides meet at the centre of the A band, fully overlapping the previously uncovered region of the thick filament.
The Z lines are pulled inward (towards each other) as the actin filaments to which they are attached slide inward, causing shortening of the sarcomere — the functional unit of contraction.
The net result is a shorter sarcomere, and since sarcomeres are arranged in series along each myofibril, the entire muscle fibre and muscle shorten.

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Case Set 2

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A physiology professor described the following sequence of events to her class: 'When you decide to pick up a pen, your brain sends a signal that travels along a nerve fibre to reach the muscle. At a specialised junction, a chemical is released that causes electrical changes in the muscle membrane. These electrical changes spread through the muscle cell and trigger the release of a stored ion. This ion binds to a regulatory protein on the thin filament, exposing attachment sites. Another protein uses chemical energy to attach to these sites and pull the thin filaments inward. The process reverses when the signal stops.' She then asked students to identify each component of this pathway and explain what would happen if the chemical released at the junction were blocked.

Question 1: Identify the 'specialised junction' and the 'chemical' released at it during muscle contraction initiation.

The specialised junction is the neuromuscular junction, also called the motor-end plate — it is the junction between a motor neuron and the sarcolemma of the muscle fibre.
The chemical released is acetylcholine, a neurotransmitter that is released when the neural signal from the CNS reaches the neuromuscular junction.
Acetylcholine generates an action potential in the sarcolemma, which then spreads through the muscle fibre to initiate the contraction cascade.

Question 2: Identify the 'stored ion' and the 'regulatory protein' mentioned in the passage, and explain the role of the ion in enabling cross bridge formation.

The stored ion is calcium (Ca++), which is stored in the sarcoplasmic reticulum of the muscle fibre and released into the sarcoplasm when an action potential spreads through the muscle fibre.
The regulatory protein is troponin — in the resting state, a subunit of troponin masks the active binding sites for myosin on the actin filaments, preventing contraction.
When Ca++ levels increase in the sarcoplasm, calcium binds to the troponin subunit, causing a conformational change that removes the masking of the active sites on actin, allowing myosin heads to bind and form cross bridges.

Question 3: The professor asked what would happen if the chemical at the junction were blocked. Answer this question and also explain how the process reverses when the neural signal stops, relating both to the molecular events at the sarcomere level.

If acetylcholine release were blocked, no action potential would be generated in the sarcolemma, so no signal would spread through the muscle fibre.
Without an action potential, Ca++ would not be released from the sarcoplasmic reticulum; troponin would continue masking the active sites on actin, and myosin heads could not form cross bridges — the muscle would remain in a state of complete paralysis.
When the neural signal stops normally, acetylcholine is degraded and no further action potentials are generated; the stimulus for Ca++ release ceases.
Ca++ ions are then actively pumped back into the sarcoplasmic cisternae, lowering cytoplasmic Ca++ levels.
With Ca++ removed from troponin, tropomyosin returns to its blocking position, re-masking the active sites on actin; cross bridge cycling stops.
The Z lines return to their original positions as the thin filaments slide back, the sarcomere returns to its resting length, and the muscle relaxes completely.

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Case Set 3

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Two athletes — Priya and Ravi — compete in different events. Priya is a 100-metre sprinter who completes her race in under 12 seconds with explosive speed. Ravi is a marathon runner who covers 42 kilometres over several hours at a steady pace. After their respective events, both visit a sports physiologist. The doctor examines muscle biopsy samples from both athletes and notes significant differences in the colour of their muscle tissue, the density of mitochondria, the content of a specific oxygen-storing pigment, and the primary energy pathway used by their muscle fibres. The doctor concludes that the composition of their muscle fibres is adapted to the specific demands of their sport.

Question 1: Name the oxygen-storing pigment found in muscles and identify which type of muscle fibre has it in high quantity.

The oxygen-storing pigment in muscles is myoglobin, which is red in colour.
Muscles with high myoglobin content appear reddish and are called red fibres (also referred to as aerobic muscles).
Red fibres also contain a large number of mitochondria, which utilise the oxygen stored in myoglobin for aerobic ATP production.

Question 2: State the structural and metabolic differences between the muscle fibres predominantly used by Priya (sprinter) and Ravi (marathon runner).

Priya predominantly uses white fibres: these have very little myoglobin (pale/whitish appearance), fewer mitochondria, but a high amount of sarcoplasmic reticulum, and they depend on anaerobic processes for energy.
Ravi predominantly uses red fibres: these have high myoglobin content (reddish appearance), plenty of mitochondria enabling aerobic respiration, and are suited for sustained, prolonged activity.
White fibres have shorter reaction times and produce energy quickly but fatigue rapidly due to lactic acid accumulation; red fibres produce energy more slowly but sustain activity for long durations without early fatigue.

Question 3: After Priya's sprint, she experiences intense muscle soreness and fatigue. After Ravi's marathon, he is tired but does not experience the same acute muscle burn. Relate both observations to the metabolic processes occurring in their respective muscle fibres, as described in the chapter.

Priya's white fibres use anaerobic breakdown of glycogen for rapid energy production during the explosive sprint.
This anaerobic process produces lactic acid as a byproduct, which accumulates in the muscle tissue.
The accumulation of lactic acid is the direct cause of the acute burning sensation, soreness, and rapid onset of fatigue that Priya experiences — this is described in the chapter as fatigue caused by repeated muscle activation.
Ravi's red fibres use aerobic respiration, which efficiently breaks down substrates using the large amount of oxygen stored in myoglobin, aided by the abundant mitochondria.
Aerobic respiration does not produce significant lactic acid accumulation, so Ravi does not experience the same acute muscle burn, though he experiences general tiredness from prolonged energy expenditure.
This difference demonstrates why endurance athletes have muscle compositions dominated by red (aerobic) fibres, while power/sprint athletes have more white (anaerobic) fibres.

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Case Set 4

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A medical student is studying a patient who has been diagnosed with an autoimmune condition. The patient complains of progressive weakness in his arms and legs and difficulty chewing and swallowing. A nerve conduction study shows that the motor neurons are functioning normally and conducting signals at the appropriate speed. However, the signals are not effectively translated into muscle contraction. The muscles themselves appear structurally normal on biopsy, and there is no genetic history of muscle disease. The attending physician explains to the student that the problem lies at a specific interface between the nervous and muscular systems, and that the patient's immune system is responsible for the damage at that site.

Question 1: Based on the clinical description, identify the disorder the patient is suffering from and name the specific site that is being damaged.

The patient is suffering from myasthenia gravis, an autoimmune disorder described in the chapter under disorders of the muscular and skeletal system.
The specific site being damaged is the neuromuscular junction (motor-end plate) — the junction between a motor neuron and the sarcolemma of the muscle fibre.
The immune system attacks this junction, impairing the normal transmission of neural signals to the muscle, leading to weakness and paralysis of skeletal muscles.

Question 2: Explain why the patient's muscles appear structurally normal on biopsy, yet fail to contract effectively, using your knowledge of the neuromuscular junction.

Because the disease targets only the neuromuscular junction and not the muscle fibres themselves, the internal structure of the muscles — sarcomeres, myofibrils, actin-myosin arrangement — remains intact on biopsy.
However, when the junction is damaged, acetylcholine cannot effectively generate an action potential in the sarcolemma, so the contraction cascade never begins.
Without an action potential, no Ca++ is released from the sarcoplasmic reticulum, active sites on actin remain masked by troponin, and no cross bridges form — so structurally normal muscles remain functionally paralysed.

Question 3: How does myasthenia gravis differ from muscular dystrophy in terms of cause, site of pathology, and nature of the disease? Why is this distinction important for treatment?

Myasthenia gravis is an autoimmune disorder — the patient's own immune system attacks the neuromuscular junction, disrupting signal transmission from nerve to muscle.
Muscular dystrophy, by contrast, is a genetic disorder causing progressive degeneration of the skeletal muscle fibre itself — the primary pathology is within the muscle, not at the junction.
In myasthenia gravis, the motor neurons function normally (as confirmed by nerve conduction studies) and the muscle fibres are structurally intact; in muscular dystrophy, the muscle tissue progressively degenerates.
This distinction is critical for treatment: myasthenia gravis may be managed by targeting the immune response (immunosuppressants) or by improving neuromuscular transmission, since the muscles themselves are intact.
Muscular dystrophy requires approaches aimed at slowing muscle degeneration at the genetic or cellular level, as the damage is intrinsic to the muscle fibre.
Confusing the two would lead to inappropriate treatment — for example, immunosuppression would not benefit a muscular dystrophy patient whose muscles are degenerating due to a genetic defect.

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Case Set 5

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A Class 11 student is given a model of a human skeleton and asked to identify its components. She identifies two major divisions of the skeletal system. In the first division, she counts the bones along the main body axis, including those forming the protective case for the brain, the column of bones running down the back, the flat bone on the front of the chest, and the curved bones enclosing the chest cavity. In the second division, she identifies the bones of all four limbs and the two sets of girdle bones that connect the limbs to the first division. Her biology teacher then asks her specific questions about the number of bones in each region and the type of connective tissue found in the skull joints.

Question 1: Name the two major divisions of the human skeletal system and state the number of bones in the axial skeleton.

The two major divisions of the human skeletal system are the axial skeleton and the appendicular skeleton.
The axial skeleton comprises 80 bones distributed along the main axis of the body.
It includes the skull, vertebral column, sternum, and ribs — these form the central supporting framework of the body.

Question 2: Identify the 'protective case for the brain' referred to by the student. State the total number of bones in the skull and distinguish between cranial and facial bones.

The protective case for the brain is the cranium, formed by 8 cranial bones that fuse to create a hard outer protective covering around the brain.
The skull in total is composed of 22 bones: 8 cranial bones (forming the cranium) and 14 facial bones (forming the front part of the skull).
The joints between these flat skull bones are fibrous joints called sutures, formed by dense fibrous connective tissue — they permit no movement, ensuring the brain is firmly protected.

Question 3: Describe the complete structure of the rib cage, including the classification of ribs, their attachments, and the role of the sternum. Also name the bones that together form the rib cage.

The rib cage is formed by the thoracic vertebrae (dorsally), the 12 pairs of ribs, and the sternum (a flat bone on the ventral midline of the thorax).
The first 7 pairs of ribs are true ribs — attached dorsally to thoracic vertebrae and ventrally connected directly to the sternum with hyaline cartilage.
The 8th, 9th, and 10th pairs are false (vertebrochondral) ribs — they do not connect directly to the sternum but join the 7th rib via hyaline cartilage; they are thus indirectly attached.
The 11th and 12th pairs are floating ribs — attached dorsally to thoracic vertebrae but not connected to anything ventrally.
Each rib is a thin flat bone described as bicephalic because it has two articulation surfaces on its dorsal end for attachment to the vertebral column.
The rib cage as a whole protects the thoracic organs (heart and lungs) and plays a role in breathing mechanics.

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Chapter 17: Locomotion and Movement | Case Study Questions

Passage

Question 1: Name the two proteins responsible for the striated appearance of skeletal muscle and identify which band each occupies.

Question 2: Identify the 'thin dark line' and the 'lighter central zone' mentioned in the passage and state the function of each.

Question 3: The student mentioned that one band retains its length during contraction. Explain what happens to the I band, A band, H zone, and Z lines during muscle contraction, and relate these changes to the sliding filament theory.

Passage

Question 1: Identify the 'specialised junction' and the 'chemical' released at it during muscle contraction initiation.

Question 2: Identify the 'stored ion' and the 'regulatory protein' mentioned in the passage, and explain the role of the ion in enabling cross bridge formation.

Question 3: The professor asked what would happen if the chemical at the junction were blocked. Answer this question and also explain how the process reverses when the neural signal stops, relating both to the molecular events at the sarcomere level.

Passage

Question 1: Name the oxygen-storing pigment found in muscles and identify which type of muscle fibre has it in high quantity.

Question 2: State the structural and metabolic differences between the muscle fibres predominantly used by Priya (sprinter) and Ravi (marathon runner).

Question 3: After Priya's sprint, she experiences intense muscle soreness and fatigue. After Ravi's marathon, he is tired but does not experience the same acute muscle burn. Relate both observations to the metabolic processes occurring in their respective muscle fibres, as described in the chapter.

Passage

Question 1: Based on the clinical description, identify the disorder the patient is suffering from and name the specific site that is being damaged.

Question 2: Explain why the patient's muscles appear structurally normal on biopsy, yet fail to contract effectively, using your knowledge of the neuromuscular junction.

Question 3: How does myasthenia gravis differ from muscular dystrophy in terms of cause, site of pathology, and nature of the disease? Why is this distinction important for treatment?

Passage

Question 1: Name the two major divisions of the human skeletal system and state the number of bones in the axial skeleton.

Question 2: Identify the 'protective case for the brain' referred to by the student. State the total number of bones in the skull and distinguish between cranial and facial bones.

Question 3: Describe the complete structure of the rib cage, including the classification of ribs, their attachments, and the role of the sternum. Also name the bones that together form the rib cage.

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