17.7 Exercise Problems

Q1. Draw the diagram of a sarcomere of skeletal muscle showing different regions.

Answer: The diagrammatic representation of a sarcomere is as follows:

Q2. Define the sliding filament theory of muscle contraction.

Answer: Sliding filament theory of muscle contraction

1. The sliding filament theory was mainly proposed to explain the process of muscle contraction. This theory proposes that during muscle contraction the thin filaments slide over the thick filaments leading to shortening of the myofibrils.
2. Each muscle fibre possesses alternate light and dark bands, which contains a specialised contractile protein known as actin and myosin respectively.
3. Actin refers to a thin contractile protein present in the light band and is known as the I-band, on the other hand, myosin is a thick contractile protein present in the dark band and is known as the A-band.
4. An elastic fibre called \(z\) line bisects each I-band. to this \(z\) line, the thin filament is firmly anchored. The central part of the thick filament that is not overlapped by the thin filament is known as the \(\mathrm{H}\)-zone.
5. During muscle contraction, the myosin heads or cross bridges come in close contact with the thin filaments causing the thin filaments to be pulled towards the middle of the sarcomere. The \(\mathrm{Z}\) line attached to the actin filaments is also pulled leading to the shortening of the sarcomere. Hence, the length of the band remains constant as its original length and the I-band shortens and the H-zone disappears.

Q3. Describe the important steps in muscle contraction.

Answer: During skeletal muscle contraction, the thick filament slides over the thin filament by a repeated binding and releases myosin along the filament. This whole process occurs in a sequential manner.
Step 1: Muscle contraction is initiated by signals that travel along the axon and reach the neuromuscular junction or motor end plate. The neuromuscular junction is a junction between a neuron and the sarcolemma of the muscle fibre. As a result, Acetylcholine (a neurotransmitter) is released into the synaptic cleft by generating an action potential in sarcolemma.
Step 2: The generation of this action potential releases calcium ions from the sarcoplasmic reticulum in the sarcoplasm.
Step 3: The increased calcium ions in the sarcoplasm leads to the activation of actin sites. Calcium ions bind to the troponin on actin filaments and remove the tropomyosin, wrapped around actin filaments. Hence, active actin sites are exposed and this allows myosin heads to attach to this site.
Step 4: In this stage, the myosin head attaches to the exposed site of actin and forms cross bridges by utilizing energy from ATP hydrolysis. The actin filaments are pulled. As a result, the \(\mathrm{H}\)-zone reduces. It is at this stage that the contraction of the muscle occurs.
Step 5: After muscle contraction, the myosin head pulls the actin filament and releases ADP along with inorganic phosphate. ATP molecules bind and detach myosin and the cross-bridges are broken.
Stage 6: This process of formation and breaking down of cross-bridges continues until there is a drop in the stimulus, which causes an increase in calcium. As a result, the concentration of calcium ions decreases, thereby masking the actin filaments and leading to muscle relaxation.

Q4. Write true or false. If false change the statement so that it is true.
(a) Actin is present in thin filament
(b) The H-zone of striated muscle fibre represents both thick and thin filaments.
(c) The human skeleton has 206 bones.
(d) There are 11 pairs of ribs in man.
(e) Sternum is present on the ventral side of the body.

Answer: (a) True
(b) False
(c) True
(d) False
(e) True

Q5. Write the difference between :
(a) Actin and Myosin
(b) Red and White muscles
(c) Pectoral and Pelvic girdle

Answer: (a) The differences between actin and myosin are as follows:

\(
\begin{array}{|c|c|c|}
\hline & \begin{array}{c}
\text { Actin filaments } \\
\text { (Thin } \\
\text { myofilaments) }
\end{array} & \begin{array}{c}
\text { Myosin filaments } \\
\text { (Thick } \\
\text { myofilaments) }
\end{array} \\
\hline \text { (i) } & \begin{array}{l}
\text { Found in both A } \\
\text { and I bands. }
\end{array} & \begin{array}{l}
\text { Found only } \\
\text { in A band of } \\
\text { sarcomere. }
\end{array} \\
\hline \text { (ii) } & \begin{array}{l}
\text { Thinner ( } 0.005 \\
\text { mm) but shorter } \\
\text { (2-2.6 mm) } \\
\text { than myosin } \\
\text { filaments. }
\end{array} & \begin{array}{l}
\text { Thicker ( } 0.01 \mathrm{~mm}) \\
\text { but longer (4.5 } \\
\text { mm) than actin } \\
\text { filaments. }
\end{array} \\
\hline \text { (iii) } & \begin{array}{l}
\text { Cross bridges } \\
\text { absent, hence } \\
\text { have smooth } \\
\text { surface. }
\end{array} & \begin{array}{l}
\text { Cross bridges } \\
\text { present, hence } \\
\text { have rough } \\
\text { surface. }
\end{array} \\
\hline \text { (iv) } & \begin{array}{l}
\text { More numerous } \\
\text { than myosin } \\
\text { filaments, six of } \\
\text { them surround } \\
\text { each myosin } \\
\text { filament. }
\end{array} & \begin{array}{l}
\text { Fewer than actin } \\
\text { filaments. }
\end{array} \\
\hline \text { (v) } & \begin{array}{l}
\text { Free at one end } \\
\text { and are joined to } \\
\text { Z-line by other } \\
\text { end. }
\end{array} & \begin{array}{l}
\text { Free at both the } \\
\text { ends. }
\end{array} \\
\hline \text { (vi) } & \begin{array}{l}
\text { Consist of } 3 \\
\text { proteins : actin, } \\
\text { tropomyosin } \\
\text { and troponin. }
\end{array} & \begin{array}{l}
\text { Consist of } 2 \\
\text { proteins : myosin } \\
\text { and meromyosin. }
\end{array} \\
\hline \text { (vii) } & \begin{array}{l}
\text { Slide into } \\
\text { H-zone } \\
\text { during muscle } \\
\text { contraction. }
\end{array} & \begin{array}{l}
\text { Do not slide } \\
\text { during muscle } \\
\text { contraction. }
\end{array} \\
\hline
\end{array}
\)

(b) Differences between red muscle fibres and white muscle fibres are given in the following table:

\(
\begin{array}{|c|c|c|}
\hline & \begin{array}{c}
\text { Red muscle } \\
\text { fibres }
\end{array} & \begin{array}{c}
\text { White muscle } \\
\text { fibres }
\end{array} \\
\hline \text { (i) } & \text { They are thin. } & \begin{array}{l}
\text { They are much } \\
\text { thicker. }
\end{array} \\
\hline \text { (ii) } & \begin{array}{l}
\text { They contain } \\
\text { abundant } \\
\text { mitochondria, } \\
\text { low glycogen } \\
\text { content and } \\
\text { poorly formed } \\
\text { sarcoplasmic } \\
\text { reticulum. }
\end{array} & \begin{array}{l}
\text { They are poor in } \\
\text { mitochondria, } \\
\text { and have } \\
\text { abundant } \\
\text { glycogen } \\
\text { granules and } \\
\text { well formed } \\
\text { sarcoplasmic } \\
\text { reticulum. }
\end{array} \\
\hline \text { (iii) } & \begin{array}{l}
\text { They are dark } \\
\text { red as they } \\
\text { contain abundant } \\
\text { pigment } \\
\text { myoglobin. }
\end{array} & \begin{array}{l}
\text { They are light in } \\
\text { colour as they } \\
\text { have very little } \\
\text { myoglobin. }
\end{array} \\
\hline \text { (iv) } & \begin{array}{l}
\text { Their myoglobin } \\
\text { stores } \mathrm{O}_2 \text { as } \\
\text { oxymyoglobin } \\
\text { that releases } \mathrm{O}_2 \\
\text { for oxidation } \\
\text { during muscle } \\
\text { contraction. }
\end{array} & \begin{array}{l}
\text { They have little } \\
\text { or no store of } \\
\text { oxygen. }
\end{array} \\
\hline \text { (v) } & \begin{array}{l}
\text { They get energy } \\
\text { for contraction } \\
\text { by aerobic } \\
\text { respiration. }
\end{array} & \begin{array}{l}
\text { They get energy } \\
\text { for contraction } \\
\text { mainly by } \\
\text { anaerobic } \\
\text { respiration. }
\end{array} \\
\hline \text { (vi) } & \begin{array}{l}
\text { They accumulate } \\
\text { little lactic acid. }
\end{array} & \begin{array}{l}
\text { They } \\
\text { accumulate } \\
\text { lactic acid } \\
\text { during } \\
\text { strenuous work. }
\end{array} \\
\hline \text { (vii) } & \begin{array}{l}
\text { They undergo } \\
\text { slow sustained } \\
\text { contractions for } \\
\text { long periods. }
\end{array} & \begin{array}{l}
\text { They undergo } \\
\text { fast contractions } \\
\text { for short } \\
\text { periods. }
\end{array} \\
\hline \text { (viii) } & \begin{array}{l}
\text { They are not } \\
\text { fatigued with } \\
\text { work }
\end{array} & \begin{array}{l}
\text { They soon get } \\
\text { fatigued with } \\
\text { work. }
\end{array} \\
\hline \text { (ix) } & \begin{array}{l}
\text { They are } \\
\text { innervated } \\
\text { by thin, slow- } \\
\text { conducting nerve } \\
\text { fibres. } \\
\text { Example : } \\
\text { Extensor muscles } \\
\text { of the back in } \\
\text { man. }
\end{array} & \begin{array}{l}
\text { They are } \\
\text { innervated by } \\
\text { thick, fast- } \\
\text { conducting } \\
\text { nerve fibres. } \\
\text { Example: } \\
\text { Eyeball muscles. }
\end{array} \\
\hline
\end{array}
\)

(c)  Differences between pectoral and pelvic girdles are given in the following table:

\(
\begin{array}{|l|l|l|}
\hline & \text { Pectoral girdle } & \text { Pelvic girdle } \\
\hline \text { (i) } & \begin{array}{l}
\text { It lies on the } \\
\text { postero-lateral } \\
\text { aspect of the } \\
\text { upper region of } \\
\text { the thorax. }
\end{array} & \begin{array}{l}
\text { It is located in } \\
\text { the lower part of } \\
\text { the trunk. }
\end{array} \\
\hline \text { (ii) } & \begin{array}{l}
\text { It consists of 2 } \\
\text { dissimilar bones: } \\
\text { scapula and } \\
\text { clavicle. }
\end{array} & \begin{array}{l}
\text { It consists of 2 } \\
\text { similar bones, } \\
\text { innominate. }
\end{array} \\
\hline \text { (iii) } & \begin{array}{l}
\text { Scapula and } \\
\text { clavicle are not } \\
\text { further divided } \\
\text { into any type of } \\
\text { bone. }
\end{array} & \begin{array}{l}
\text { Each innominate } \\
\text { bone consists } \\
\text { of three bones: } \\
\text { ilium, ischium } \\
\text { and pubis. }
\end{array} \\
\hline \text { (iv) } & \begin{array}{l}
\text { It provides } \\
\text { articulation to the } \\
\text { arm bones. }
\end{array} & \begin{array}{l}
\text { It provides } \\
\text { articulation to } \\
\text { the bones of the } \\
\text { leg. }
\end{array} \\
\hline \text { (v) } & \begin{array}{l}
\text { It has at its lateral } \\
\text { angle a shallow } \\
\text { concavity, the } \\
\text { glenoid cavity, } \\
\text { for articulation } \\
\text { of the head of the } \\
\text { humerus. }
\end{array} & \begin{array}{l}
\text { It has at the } \\
\text { middle of its } \\
\text { lateral surface } \\
\text { a deep, cup- } \\
\text { shaped hollow, } \\
\text { acetabulum. }
\end{array} \\
\hline
\end{array}
\)

Q6. Match Column I with Column II:

\(
\begin{array}{|l|l|l|l|}
\hline & \text { Column I } & & \text { Column II } \\
\hline \text { (a) } & \text { Smooth muscle } & \text { (i) } & \text { Myoglobin } \\
\hline \text { (b) } & \text { Tropomyosin } & \text { (ii) } & \text { Thin filament } \\
\hline \text { (c) } & \text { Red muscle } & \text { (iii) } & \text { Sutures } \\
\hline \text { (d) } & \text { Skull } & \text { (iv) } & \text { Involuntary } \\
\hline
\end{array}
\)

Answer: 

\(
\begin{array}{|l|l|l|l|}
\hline & \text { Column I } & & \text { Column II } \\
\hline \text { (a) } & \text { Smooth muscle } & \text { (iv) } & \text { Involuntary } \\
\hline \text { (b) } & \text { Tropomyosin } & \text { (ii) } & \text { Thin filament } \\
\hline \text { (c) } & \text { Red muscle } & \text { (i) } & \text { Myoglobin } \\
\hline \text { (d) } & \text { Skull } & \text { (iii) } & \text { Sutures } \\
\hline
\end{array}
\)

Q7. What are the different types of movements exhibited by the cells of the human body?

Answer: Movement is a characteristic feature of living organisms. The different types of movement exhibited by cells of the human body are:
(a) Amoeboid movement: Leucocytes present in the blood show amoeboid movement. During tissue damage, these blood cells move from the circulatory system towards the injury site to initiate an immune response.
(b) Ciliary movement: Reproductive cells such as sperms and ova show ciliary movement. The passage of ova through the fallopian tube towards the uterus is facilitated by this movement.
(c) Muscular movement: Muscle cells show muscular movement.

Q8. How do you distinguish between a skeletal muscle and a cardiac muscle?

Answer: We can distinguish between a skeletal muscle and a cardiac muscle on the basis of the features discussed in the following table:

\(
\begin{array}{|l|c|c|}
\hline & \begin{array}{c}
\text { Skeletal or } \\
\text { striated muscle }
\end{array} & \begin{array}{c}
\text { Cardiac } \\
\text { muscle }
\end{array} \\
\hline \text { (i) } & \begin{array}{l}
\text { They are present } \\
\text { in the limbs, } \\
\text { body walls, } \\
\text { tongue, pharynx } \\
\text { and beginning of } \\
\text { oesophagus. }
\end{array} & \begin{array}{l}
\text { They are } \\
\text { present in wall } \\
\text { of the heart, } \\
\text { pulmonary } \\
\text { veins and } \\
\text { superior vena } \\
\text { cava. }
\end{array} \\
\hline \text { (ii) } & \begin{array}{l}
\text { Fibres } \\
\text { unbranched. }
\end{array} & \begin{array}{l}
\text { Fibres } \\
\text { branched. }
\end{array} \\
\hline \text { (iii) } & \text { Multinucleate } & \text { Uninucleate } \\
\hline \text { (iv) } & \begin{array}{l}
\text { Light and dark } \\
\text { bands present. }
\end{array} & \begin{array}{l}
\text { Faint light and } \\
\text { dark bands } \\
\text { present. }
\end{array} \\
\hline \text { (v) } & \begin{array}{l}
\text { No oblique } \\
\text { bridges and } \\
\text { intercalated discs. }
\end{array} & \begin{array}{l}
\text { Oblique bridges } \\
\text { and intercalated } \\
\text { discs present. }
\end{array} \\
\hline \text { (vi) } & \begin{array}{l}
\text { Nerve supply } \\
\text { from central } \\
\text { nervous system. }
\end{array} & \begin{array}{l}
\text { Nerve supply } \\
\text { from the brain } \\
\text { and autonomic } \\
\text { nervous system. }
\end{array} \\
\hline \text { (vii) } & \begin{array}{l}
\text { Very rapid } \\
\text { contraction. }
\end{array} & \begin{array}{l}
\text { Rapid } \\
\text { contraction. }
\end{array} \\
\hline \text { (viii) } & \begin{array}{l}
\text { They soon get } \\
\text { fatigued. }
\end{array} & \begin{array}{l}
\text { They never get } \\
\text { fatigued. }
\end{array} \\
\hline \text { (ix) } & \text { Voluntary } & \text { Involuntary } \\
\hline
\end{array}
\)

Q9. Name the type of joint between the following:-
(a) atlas/axis
(b) carpal/metacarpal of thumb
(c) between phalanges
(d) femur/acetabulum
(e) between cranial bones
(f) between pubic bones in the pelvic girdle

Answer: (a) Atlas/axis: Pivotal joint
(b) Carpal/metacarpal of thumb: Saddle joint
(c) between phalanges: Hinge joint
(d) femur/acetabulum: Ball and socket joint
(e) between cranial bones: Fibrous joint
(f) between pubic bones in the pelvic girdle: Ball and socket joint

Q10. Fill in the black spaces:

(a) All mammals (except a few) have ________ cervical vertebra.
(b) The number of phalanges in each limb of human is _______.
(c) Thin filament of myofibril contains \(2{ }^{\circ} \mathrm{F}\) ‘ actins and two other proteins namely ________ and _________.
(d) In a muscle fibre \(\mathrm{Ca}^{++}\)is stored in _______.
(e) _______ and ________ pairs of ribs are called floating ribs.
(f) The human cranium is made of ________ bones.

Answer: (a) All mammals (except a few) have Seven cervical vertebra.
(b) The number of phalanges in each limb of a human is 14.
(c) Thin filament of myofibril contains 2 ‘F’ actins and two other proteins, namely troponin and tropomyosin.
(d) In a muscle fibre, \(\mathrm{Ca}^{++}\) is stored in the sarcoplasmic reticulum.
(e) The 11th and 12th pairs of ribs are called floating ribs.
(f) The human cranium is made up of eight bones.

Exemplar Section

VERY SHORT ANSWER TYPE QUESTIONS

Q1. Name the cells/tissues in human body which
a. exhibit ameboid movement
b. exhibit ciliary movement

Answer: a. Macrophages and leucocytes
b. Trachea, fallopian tube and bronchiole

Q2. Locomotion requires a perfect coordinated activity of muscular, _____, _______ systems.

Answer: Skeletal and Neural

Q3. Sarcolemma, sarcoplasm and sarcoplasmic reticulum refer to a particular type of cell in our body. Which is this cell and to what parts of that cell do these names refer to?

Answer: Each muscle fibre or muscle cell is lined by the plasma membrane called sarcolemma enclosing the sarcoplasm. A muscle fibre is a syncytium as the sarcoplasm (cytoplasm) contains many nuclei. The endoplasmic reticulum, i. e., sarcoplasmic reticulum of the muscle fibres is the store house of calcium ions.

Q4. Label the different components of actin filament in the diagram given below

Answer: 

Q5. The three tiny bones present in middle ear are called ear ossicles. Write them in correct sequence beginning from ear drum. 

Answer: Malleus, incus and stapes.

Q6. What is the difference between the matrix of bones and cartilage?

Answer: Bones have a hard and non-pliable ground substance rich in calcium salts and collagen fibres which give bone its strength. The inter-cellular material of cartilage is solid and pliable which resists compression. Cell of cartilage are called chondrocytes which are enclosed in a small cavities (lacunae) within the matrix secreted by them.

Q7. Which tissue is afflicted by Myasthenia gravis? What is the underlying cause?

Answer: Myasthenia gravis: Auto-immune disorder affecting neuromuscular junction leading to fatigue, weakening and paralysis of skeletal muscle.

Q8. How do our bone joints function without grinding noise and pain?

Answer: Our bone joints function without grinding noise and pain due to the presence of synovial fluid between bones.

Q9. Give the location of a ball and socket joint in a human body.

Answer: Ball and socket joint: Between humerus and pectoral girdle (shoulder joint). Between femur and acetabulum of pelvic girdle (hip joint). Total 4 ball and socket joints present in human body -2 shoulder joint and 2 hip joint.

Q10. Our fore arm is made of three different bones. Comment.

Answer: The bones of the forearm are humerus, radius and ulna.

SHORT ANSWER TYPE QUESTIONS

Q1. With respect to rib cage, explain the following:
a. Bicephalic ribs
b. True ribs
c. Floating ribs

Answer: a. Bicephalic ribs: Each rib is a thin flat bone connected dorsally to the vertebral column and ventrally to the sternum. It has two articulation surfaces on its dorsal end and is hence called bicephalic.
b. True ribs: First seven pairs of ribs are called true ribs. Dorsally, they are attached to the thoracic vertebrae and ventrally connected to the sternum with the help of hyaline cartilage.
c. Floating ribs: Last 2 pairs (11th and 12th) of ribs are not connected ventrally and are therefore, called floating ribs.

Q2. In old age, people often suffer from stiff and inflamed joints. What is this condition called? What are the possible reasons for these symptoms?

Answer: In old age, people suffer from stiff and inflamed joints, it is due to rheumatoid arthritis (autoimmune disorder) Causes:

(i) Inflammation of synovial membrane
(ii) Genetic factors (50% cases) .
(iii) Smoking
(iv) Vitamin-D deficiency

Q3. Exchange of calcium between bone and extracellular fluid takes place under the influence of certain hormones
a. What will happen if more of \(\mathrm{Ca}^{++}\)is in extracellular fluid?
b. What will happen if very less amount of \(\mathrm{Ca}^{++}\)is in the extracellular fluid?

Answer: a. If more of \(\mathrm{Ca}^{++}\)is in extracellular fluid then it will be accumulated on the bones under the influence of thyrocalcitonin (TCT).
b. If very less amount of \(\mathrm{Ca}^{++}\)is in the extracellular fluid then parathyroid hormone (PTFI) acts on bones and stimulates the process of bone resorption (dissolution/demineralisation). PTH also stimulates reabsorption of \(\mathrm{Ca}^{2+}\) by the renal tubules and increases \(\mathrm{Ca}^{2+}\) absorption from the digested food.

Q4. Name at least two hormones which result in fluctuation of \(\mathrm{Ca}++\) level.

Answer: Thyrocalcitonin (TCT) and Parathyroid Hormone (PTH).

Q5. Rahul exercises regularly by visiting a gymnasium. Of late he is gaining weight. What could be the reason? Choose the correct answer and elaborate.
a. Rahul has gained weight due to accumulation of fats in body.
b. Rahul has gained weight due to increased muscle and less of fat.
c. Rahul has gained weight because his muscle shape has improved.
d. Rahul has gained weight because he is accumulating water in the body

Answer: Rahul has gained weight due to increased muscle and less of fat.

Q6. Radha was running on a treadmill at a great speed for 15 minutes continuously. She stopped the treadmill and abruptly came out. For the next few minutes, she was breathing heavily/fast. Answer the following questions.
a. What happened to her muscles when she did strenuously exercised?
b. How did her breathing rate change?

Answer: a. Repeated activation of the muscles can lead to the accumulation of lactic acid due to anaerobic breakdown of glycogen in them, causing fatigue.
b. During strenuous exercise demand of oxygen also increases so breathing rate has been changed.

Q7. Write a few lines about Gout.

Answer: When metabolic waste-uric acid crystals are accumulated in bones, then it results into inflammation of bone and joints thereby causing pain. This disorder of skeletal system is called gout.

Q8. What is the source of energy for muscle contraction? 

Answer: ATP (Adenosine Triphosphate)

Q9. What are the points for articulation of Pelvic and Pectoral girdles?

Answer: The components of pelvic girdle are ilium, ischium and pubis. It articulates with, femur through acetabulum. The components of pectoral girdle are scapula and clavicle. It is the glenoid cavity of pectoral girdle in which head . of humerus articulates.

LONG ANSWER TYPE QUESTIONS

Q1. Calcium ion concentration in blood affects muscle contraction. Does it lead to tetany in certain cases? How will you correlate fluctuation in blood calcium with tetany?

Answer: Muscle contraction is initiated by a signal sent by the central nervous system (CNS) via a motor neuron. A neural signal reaching this junction releases a neurotransmitter (acetyl choline) which generates an action potential in the sarcolemma. This spreads through the muscle fibre and causes the release of calcium ions into the sarcoplasm. Increase in \(\mathrm{Ca}^{++}\)level leads to the binding of calcium with a subunit of troponin on actin filaments and thereby remove the masking of active sites for myosin. Utilising the energy from ATP hydrolysis, the myosin head now binds to the exposed active sites on actin to form a crossbridge. This pulls the attached actin filaments towards the centre of ‘ \(A\) ‘ band. The ‘ \(Z\) ‘ line attached to these actins are also pulled inwards thereby causing a shortening of the sarcomere, i.e., contraction. The process continues till the \(\mathrm{Ca}^{++}\)ions are pumped back to the sarcoplasmic cisternae resulting in the masking of actin filaments.

Tetany: Rapid spasms (wild contractions) in muscle due to low Ca in body fluid.

Q2. An elderly woman slipped in the bathroom and had severe pain in her lower back. After X-ray examination doctors told her it is due to a slipped disc. What does that mean? How does it affect our health?

Answer: Displacement of intervertebral disc from’ their normal position is called slipped disc Effects:
i. Neck or lower back pain
ii. Muscular weakness
iii. Paralysis
iv. Sciatica

Q3. Explain sliding filament theory of muscle contraction with neat sketches. 

Answer: Mechanism of muscle contraction: Mechanism of muscle contraction is best explained by the sliding filament theory which states that contraction of a muscle fibre takes place by the sliding of the thin filaments over the thick filaments. Muscle contraction is initiated by a signal sent by the Central Nervous System (CNS) via a motor neuron. A motor neuron alongwith the muscle fibres connected to it constitute a motor unit. The junction between a motor neuron and the sarcolemma of the muscle fibre is called the neuromuscular junction or motor-end plate. A neural signal reaching this junction releases a neurotransmitter (acetyl choline) which generates an action potential in the sarcolemma. This spreads through the muscle fibre and causes the release of calcium ions into the sarcoplasm. Increase in \(\mathrm{Ca}^{++}\)level leads to the binding of calcium with a subunit of troponin on actin filaments and thereby remove the masking of active sites for myosin.Utilising the energy from ATP hydrolysis, the myosin head now binds to the exposed active sites on actin to form a cross-bridge. This pulls the attached actin filaments towards the centre of ‘A’ band. The ‘Z’ line attached to these actins are also pulled inwards thereby causing a shortening of the sarcomere, i.e., contraction. It is clear from the above steps, that during shortening of the muscle, i.e., contraction, the ‘I’ bands get reduced, whereas the ‘A’ bands retain the length. The myosin, releasing the ADP and P, goes back to its relaxed state. A new ATP binds and the cross-bridge is broken. The ATP is again hydrolysed by the myosin head and the cycle of cross-bridge formation and breakage is repeated causing further sliding. The process continues till the \(\mathrm{Ca}^{++}\)ions are pumped back to the sarcoplasmic cistemae resulting in the masking of actin filaments. This causes the return of ‘ \(Z\) ‘ lines back to their original position, i.e., relaxation.

Q4. How does a muscle shorten during its contraction and return to its original form during relaxation?

Answer: Relaxation of muscles happens, when actin and myosin filaments slide away from each other. The striations appear in the striated muscles, due to the alternate bands of myosin and actin. The band of actin, i.e. the I band, is light in colour and with an elastic band called \(Z\) line, it is held in the middle. The myosin band has a darker colour and is called the \(A\) band and by an elastic band called \(M\) line, it is held in the middle.

During contraction, the position of \(Z\) line changes in relation to \(M\), line making the muscle fibre shorter. During relaxation, the actin filaments move to their original position and the muscle fibre becomes longer.

Q5. Discuss the role of \(\mathrm{Ca}^{2+}\) ions in muscle contraction. Draw neat sketches to illustrate your answer.

Answer: Muscle contraction is initiated by a neural signal, which after reaching the neuromuscular junction or motor end plate releases a neurotransmitter, as a result, an action potential in the sarcolemma is generated. Action potential spreads through muscle fibre and causes the release of calcium ions into the sarcoplasm. An increase in \(\mathrm{Ca}^{2+}\) level leads to the binding of calcium with a subunit of troponin on actin filaments and thereby removes the masking of active sites for myosin. Utilising the energy from ATP hydrolysis, the myosin head now binds to the exposed active site on actin to form a cross-bridge. This pulls the attached actin filaments towards the centre of ‘ \(A\) ‘ band. The ‘ \(Z\) ‘ line attached to these actions is also pulled inwards thereby causing the shortening of the sarcomere, i.e., contraction.

A new ATP binds to the myosin head and the cross-bridge is broken. The ATP is again hydrolysed by the myosin head and the cycle of cross-bridge formation and breakage is repeated causing further sliding. The process continues till the \(\mathrm{Ca}^{++}\) ions are pumped back to the sarcoplasmic cisternae resulting in the masking of actin filaments and the breakage of all cross-bridges. This cause the return of ‘Z’ lines along with filaments back to their original position, i.e., relaxation.

Q6. Differentiate between Pectoral and Pelvic girdle

Answer: Pectoral and pelvic girdle help in the articulation of upper and lower limbs respectively. Each girdle is made of two equal halves. Each half of a pectoral girdle consists of clavicle and scapula. Scapula is a large triangular flat bone. There is glenoid cavity at the joint of scapula, clavicle and acromian process, which articulates with the head of humerus to form the shoulder joint. Each half of pelvic girdle is formed by three bones-ilium, ischium and pubis. At the point of their fusion; there is a cavity called acetabulum to which the head of femur articulates.