Exercises
Draw the diagram of a sarcomere of skeletal muscle showing different regions.
Answer
A sarcomere is the functional unit of a myofibril, bounded by two Z-lines. Key regions: I-band (light band) — contains only thin (actin) filaments; it spans from the Z-line to the edge of the A-band. A-band (dark band) — contains both thick (myosin) and thin filaments in the overlap zone, and only thick filaments in the H-zone. H-zone — the central, lighter region of the A-band containing only thick filaments and no thin filaments. M-line — a dark line at the centre of the H-zone where myosin filaments are cross-linked. Z-line — the boundary of a sarcomere; thin filaments are anchored to it. During contraction, I-band and H-zone shorten; A-band width remains constant.
(Students should draw a diagram labelling: Z-line, I-band, A-band, H-zone, M-line, thin filaments, thick filaments.)
Define sliding filament theory of muscle contraction.
Answer
The sliding filament theory (proposed by Hugh Huxley and Jean Hanson, 1954) states that muscle contraction occurs by the sliding of thin actin filaments over thick myosin filaments, without any change in the length of the filaments themselves. The myosin heads form cross-bridges with actin, undergo a power stroke (using ATP energy), pulling the actin filaments toward the centre of the sarcomere (M-line). This shortens the sarcomere, reducing the I-band and H-zone, while the A-band remains the same width. Repeated cross-bridge cycles produce contraction of the whole muscle fibre.
Describe the important steps in muscle contraction.
Answer
Steps in muscle contraction:
1. A nerve impulse reaches the neuromuscular junction; acetylcholine is released from motor neuron terminals.
2. ACh binds to receptors on the motor end plate, generating an action potential that spreads across the sarcolemma and into transverse tubules (T-tubules).
3. The action potential triggers the sarcoplasmic reticulum (SR) to release Ca2+ ions into the sarcoplasm.
4. Ca2+ binds to troponin (part of the troponin-tropomyosin complex on actin). This causes a conformational change that moves tropomyosin away from the myosin-binding sites on actin, exposing them.
5. Myosin heads (with ADP + Pi attached) bind to the exposed active sites on actin, forming cross-bridges.
6. The power stroke occurs: myosin heads pivot, pulling actin filaments toward the M-line (ADP + Pi are released).
7. A new ATP molecule binds to the myosin head, causing detachment from actin. ATP is hydrolysed to ADP + Pi, re-energising the myosin head (recovery stroke).
8. Cycle repeats as long as Ca2+ and ATP are available, producing continuous contraction.
9. When the nerve impulse stops, Ca2+ is pumped back into the SR; tropomyosin covers the active sites on actin; muscle relaxes.
Write true or false. If false, change the statement so that it is true.
Answer
(a) Actin is present in thin filament
Ans. True.
(b) H-zone of striated muscle fibre represents both thick and thin filaments.
Ans. False. The H-zone represents only thick (myosin) filaments — it is the region in the centre of the A-band where thin filaments do not reach.
(c) Human skeleton has 206 bones.
Ans. True.
(d) There are 11 pairs of ribs in man.
Ans. False. There are 12 pairs of ribs in humans (7 true ribs, 3 false ribs, 2 floating ribs).
(e) Sternum is present on the ventral side of the body.
Ans. True. The sternum (breastbone) is located on the ventral (anterior) side of the thorax.
Write the difference between: (a) Actin and Myosin (b) Red and White muscles (c) Pectoral and Pelvic girdle
Answer
(a) Actin forms the thin filaments (~6–7 nm diameter); composed of two F-actin strands twisted together with tropomyosin and troponin; anchored at the Z-line; has binding sites for myosin heads. Myosin forms the thick filaments (~15 nm diameter); has a globular head (with ATPase activity) and a rod-like tail; heads form cross-bridges with actin; arranged in the centre of the sarcomere (A-band).
(b) Red muscles (slow-twitch): Contain high amounts of myoglobin (giving red colour), many mitochondria, abundant blood supply; rely on aerobic metabolism; contract slowly but are fatigue-resistant; suited for sustained activity (e.g., postural muscles). White muscles (fast-twitch): Less myoglobin, fewer mitochondria; rely on anaerobic glycolysis; contract quickly and powerfully but fatigue rapidly; suited for burst activities (e.g., sprinting).
(c) Pectoral girdle (shoulder girdle) consists of two clavicles and two scapulae; connects upper limbs to the axial skeleton; located in the upper thoracic region. Pelvic girdle (hip girdle) consists of two coxal bones (each formed by ilium, ischium, and pubis fused); connects lower limbs to the axial skeleton; located in the lower abdominal/pelvic region; also helps protect pelvic organs and bears body weight.
Match Column I with Column II
Answer
(a) Smooth muscle — (iv) Involuntary
(b) Tropomyosin — (ii) Thin filament
(c) Red muscle — (i) Myoglobin
(d) Skull — (iii) Sutures
What are the different types of movements exhibited by the cells of the human body?
Answer
The different types of movements exhibited by human body cells are:
Amoeboid movement: Cytoplasmic streaming movements; exhibited by leucocytes (WBCs) and macrophages to engulf pathogens (phagocytosis); involves pseudopodia.
Ciliary movement: Rhythmic beating of cilia on epithelial cells; seen in the respiratory tract (to move mucus and particles), oviduct (to move ovum toward uterus), and in kidney tubules.
Muscular movement: Contraction and relaxation of muscle fibres (skeletal, smooth, and cardiac); responsible for limb movements, peristalsis, heartbeat, etc.
How do you distinguish between a skeletal muscle and a cardiac muscle?
Answer
Skeletal muscle: Striated (shows bands), cylindrical, multinucleate (peripherally placed nuclei); voluntary control; does not have intercalated discs; does not show autorhythmicity; fatigues with prolonged contraction.
Cardiac muscle: Striated, branched, uninucleate (centrally placed nucleus); involuntary (myogenic); connected by intercalated discs (for rapid impulse conduction and mechanical coupling); shows autorhythmicity (spontaneous contraction); does not fatigue under normal conditions.
Q9. Name the type of joint between the following
(a) atlas/axis — Pivot joint (allows rotation of the head)
(b) carpal/metacarpal of thumb — Saddle joint (allows movement in two planes; opposition of thumb)
(c) between phalanges — Hinge joint (allows flexion and extension only)
(d) femur/acetabulum — Ball and socket joint (allows movement in all directions)
(e) between cranial bones — Fibrous joint (suture) (immovable)
(f) between pubic bones in the pelvic girdle — Cartilaginous joint (symphysis pubis) (slightly movable/amphiarthrosis)
Q10. Fill in the blank spaces
(a) All mammals (except a few) have seven (7) cervical vertebrae.
(b) The number of phalanges in each limb of human is 14 (in hand: 2 in thumb + 3 × 4 fingers = 14; same in foot).
(c) Thin filament of myofibril contains 2 'F' actins and two other proteins namely troponin and tropomyosin.
(d) In a muscle fibre Ca2+ is stored in sarcoplasmic reticulum (SR).
(e) 11th and 12th pairs of ribs are called floating ribs.
(f) The human cranium is made of 8 bones.
Important Extra Questions
What is rigor mortis?
Answer
Rigor mortis is the temporary stiffening of muscles that occurs after death. When a person dies, ATP production ceases. Without ATP, myosin heads remain attached to actin (cross-bridges cannot be broken), causing the muscles to stiffen. It begins 2–6 hours after death and resolves within 24–48 hours as muscle proteins begin to degrade.
What are the different types of joints on the basis of mobility?
Answer
(i) Synarthroses (fibrous/immovable joints) — bones joined by fibrous connective tissue (e.g., skull sutures); no movement. (ii) Amphiarthroses (cartilaginous/slightly movable) — bones joined by cartilage (e.g., intervertebral discs, symphysis pubis); limited movement. (iii) Diarthroses (synovial/freely movable joints) — bones separated by a fluid-filled synovial cavity; wide range of movement (e.g., knee, hip, shoulder, elbow).
What is the role of calcium in muscle contraction?
Answer
Ca2+ released from the sarcoplasmic reticulum binds to troponin C (a subunit of troponin on the thin filament). This causes a conformational change that displaces tropomyosin from the myosin-binding sites on actin, allowing the myosin heads to bind and initiate contraction. Without Ca2+, tropomyosin physically blocks the cross-bridge sites, and the muscle remains relaxed.
Name the bones of the human axial skeleton.
Answer
The axial skeleton (80 bones) includes: skull (29 bones: cranium 8 + facial 14 + ossicles 6 + hyoid 1), vertebral column (26 bones: 7 cervical + 12 thoracic + 5 lumbar + 1 sacrum + 1 coccyx), sternum (1), and ribs (24 = 12 pairs).
What is the difference between isometric and isotonic contractions?
Answer
In isometric contraction, the muscle develops tension but does not shorten (no movement occurs); muscle length stays constant (e.g., holding a heavy weight stationary, pushing against a wall). In isotonic contraction, the muscle shortens and does work (produces movement) while tension remains approximately constant (e.g., lifting a weight, walking).