NCERT Class 11-Biology: Chapter – 20 Locomotion and Movement Part 5 (For CBSE, ICSE, IAS, NET, NRA 2022)

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Question 2:

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?


A slipped disc is a condition in which the intervertebral disc is displaced from its original position. The spine gets affected as the intervertebral disc is displaced due to wear and tear of the outer fibrous ring.

It affects our health in the following ways:

(i) It causes muscle weakness.

(ii) It can cause numbness of the body and pain in the muscles of arms, legs, and lower back.

(iii) The activities like walking and standing would become painful.

(iv) The affected area around the slipped disc would cause burning and tingling sense along with severe pain.

(v) Permanent nerve damage and paralysis can occur if it is left untreated in severe cases.

Question 3:

Explain sliding filament theory of muscle contraction with neat sketches.


The sliding filament theory states that contraction of a muscle fibre occurs through the sliding of the thin filaments over the thick filaments. The thin filament is known as actin, and the thick filament is known as myosin.

The contraction of the muscle is initiated when a neural signal is sent to the neuromuscular junction by a motor neuron by the central nervous system (CNS) .

In the muscle fibre it has an alternate light and dark bands which has the specialised contractile protein filaments called actin and myosin.

An elastic fibre called Z-line is present in the muscle fibre which bisects each of the I-bands.

The actin filament gets anchored to the Z-line. The central part of the myosin is not overlapped by actin filament, and it is known as the H-zone.

During the muscle contraction, the head of the myosin filament come in close contact with the actin filament. This results in the actin filament to be pulled or slide towards the middle of the sarcomere.

The Z-line which is attached to the actin filaments which leads to the shortening of the sarcomere.

Thus, the length of the A-band would remain constant while the I-band shortens in length, and the H-zone gets reduced and ultimately disappears.

This mechanism occurs in all the sarcomeres results in the shortening of the myofibrils and thus causes muscle contraction.

Two Sarcomeres

The above diagram shows the contraction of a muscle fibre according to the sliding filament theory by the sliding of the thin filaments over the thick filaments.

Question 4:

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


The muscle shortens during its contraction and returns to its original form during relaxation by the following steps:

(i) The neurotransmitter acetylcholine is released when a neural signal reaches the neuromuscular junction and acetylcholine creates an action potential in the sarcolemma.

(ii) A release of calcium ions occurs due to the action potential, which is spread through the muscle fibres.

(iii) The increase in the calcium ion concentration causes the binding of the calcium ion to a sub-unit of troponin on the actin filaments. This causes the removal of active sites in myosin.

(iv) The myosin head binds with the exposed active sites of actin and forms a cross-bridge.

(v) This causes the attached actin filaments to move towards the centre of A-band.

(vi) The Z-line also moves inwards, and a contraction of the sarcomere occurs in the muscles.

(vii) During the muscle contraction, the I-band gets reduced while the A-band retains its original length.

(viii) Breaking of cross-bridge occurs when the ADP and Pi releasing myosin goes back to its relaxed state and another new ATP binds which breaks the cross-bridge.

(ix) Further sliding occurs when the ATP is hydrolysed again by myosin head, and repetition of cross-bridge formation and breakage occurs.

(x) This process continues as the calcium ion concentration causes the formation of an action potential.

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