Locomotion and Movement | CBSE Class 11 Biology Notes

This note covers NCERT Class 11 Biology Chapter 17, Locomotion and Movement: the types of movement, the three kinds of muscle, the structure of a skeletal muscle and its contractile proteins, the sliding filament theory, the human skeleton, the joints, and the disorders of the muscular and skeletal system. The bone counts are set out as sums that you can check.

How are movement and locomotion related?

Movement is one of the significant features of living beings. Streaming of protoplasm in unicellular organisms like Amoeba is a simple form of movement. Many organisms show movement of cilia, flagella and tentacles. Human beings can move limbs, jaws, eyelids and tongue.

Definition: Locomotion is a voluntary movement that results in a change of place or location. Walking, running, climbing, flying and swimming are forms of locomotory movement.

Locomotory structures need not be different from those that bring about other types of movement.

So movements and locomotion cannot be studied separately. The two are linked by one statement: all locomotions are movements, but all movements are not locomotions.

Methods of locomotion vary with the habitat of the animal and the demand of the situation. Locomotion is generally for the search of food, shelter, mate, suitable breeding grounds or favourable climatic conditions, or to escape from enemies and predators.

What types of movement do the cells of the human body show?

Cells of the human body exhibit three main types of movement: amoeboid, ciliary and muscular.

Cilia and flagella are outgrowths of the cell membrane. Flagellar movement helps in the swimming of spermatozoa, the maintenance of water current in the canal system of sponges, and the locomotion of protists like Euglena.

Locomotion requires a perfectly coordinated activity of the muscular, skeletal and neural systems.

What are the three types of muscles?

Muscle is a specialised tissue of mesodermal origin. About 40 to 50 per cent of the body weight of a human adult is contributed by muscles. Muscles have four special properties: excitability, contractility, extensibility and elasticity.

Muscles have been classified using different criteria: location, appearance and the nature of regulation of their activities. Based on their location, three types of muscles are identified.

Note: Cardiac muscle is the one that mixes the two patterns. It is striated like skeletal muscle but involuntary like visceral muscle, and it is branched.

What is the structure of a skeletal muscle and of a sarcomere?

A skeletal muscle is built in levels, from the whole muscle down to the filaments.

1. Each organised skeletal muscle is made of a number of muscle bundles or fascicles, held together by a common collagenous connective tissue layer called the fascia.
2. Each muscle bundle contains a number of muscle fibres.
3. Each muscle fibre is lined by the plasma membrane, called the sarcolemma, which encloses the sarcoplasm.
4. The sarcoplasm contains a large number of parallelly arranged filaments called myofilaments or myofibrils.
5. Each myofibril has alternate dark and light bands, due to the distribution pattern of two proteins, actin and myosin.

A muscle fibre is a syncitium, because the sarcoplasm contains many nuclei. The endoplasmic reticulum of the muscle fibre, the sarcoplasmic reticulum, is the store house of calcium ions.

A muscle is cut across to show that it is made of several rounded bundles. The labels are: fascicle (muscle bundle), muscle fibre (muscle cell), sarcolemma and blood capillary. Each fascicle contains many muscle fibres packed side by side.

See Fig. 17.1 in your NCERT textbook

The bands of a myofibril

Both proteins are arranged as rod-like structures, parallel to each other and to the longitudinal axis of the myofibrils. Actin filaments are thinner than myosin filaments, so they are called the thin and thick filaments respectively. The A and I bands are arranged alternately throughout the length of the myofibrils.

Part (a) is a picture of a muscle fibre showing alternate dark and light bands. Part (b) is a drawing of one sarcomere. Two vertical Z lines mark its ends. Thin filaments are attached to each Z line and point towards the centre. Thick filaments lie in the middle, overlapping the thin filaments at their ends. The labels are: Z line, A band (the full length of the thick filaments), I band (the region with thin filaments only, next to the Z line), H zone (the central region with thick filaments only) and sarcomere.

See Fig. 17.2 in your NCERT textbook

What are the contractile proteins made of?

The HMM component, that is the head and short arm, projects outwards at a regular distance and angle from the surface of a polymerised myosin filament. It is known as the cross arm.

Part (a) shows the thin filament as two twisted strands of beads, the F actins, with tropomyosin lying along them and troponin at intervals. Part (b) shows a meromyosin as a long tail ending in a globular head on a short arm. The labels on the head are: actin binding sites and ATP binding sites; the head and the cross arm are also labelled.

See Fig. 17.3 in your NCERT textbook

How does a muscle contract?

Definition: The sliding filament theory states that contraction of a muscle fibre takes place by the sliding of the thin filaments over the thick filaments.

A motor neuron along with the muscle fibres connected to it constitutes 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.

1. Contraction is initiated by a signal sent by the central nervous system (CNS) via a motor neuron.
2. The neural signal reaching the neuromuscular junction releases a neurotransmitter (acetylcholine), which generates an action potential in the sarcolemma.
3. The action potential spreads through the muscle fibre and causes the release of calcium ions into the sarcoplasm.
4. The increase in Ca²⁺ level leads to the binding of calcium with a subunit of troponin on the actin filaments, which removes the masking of the active sites for myosin.
5. Utilising the energy from ATP hydrolysis, the myosin head binds to the exposed active sites on actin to form a cross bridge.
6. This pulls the attached actin filaments towards the centre of the A band. The Z lines attached to these actins are also pulled inwards, causing a shortening of the sarcomere, that is contraction.
7. The myosin, releasing the ADP and Pi, goes back to its relaxed state. A new ATP binds and the cross bridge is broken.
8. The ATP is again hydrolysed by the myosin head, and the cycle of cross bridge formation and breakage is repeated, causing further sliding.

Relaxation: the process continues till the Ca²⁺ ions are pumped back to the sarcoplasmic cisternae, which results in the masking of the actin filaments. This causes the return of the Z lines to their original position.

Four stages are arranged in a circle, each showing an actin filament above a myosin filament with one myosin head. At the top the head carries ADP and P. On the right the head is attached to actin: formation of cross bridge. At the bottom the head has turned and ADP and P are released: sliding or rotation. On the left ATP is bound to the head, which is detached from actin: breaking of cross bridge. Arrows lead from each stage to the next.

See Fig. 17.4 in your NCERT textbook

What happens to the bands during contraction?

Two sarcomeres are drawn in three states, one below the other: relaxed, contracting and maximally contracted. In the relaxed state the H zone, I band and A band are all labelled and the Z lines are far apart. In the contracting state the Z lines are closer and the I band and H zone are smaller. In the maximally contracted state the Z lines are closest and the thin filaments reach the centre, while the A band has stayed the same length in all three.

See Fig. 17.5 in your NCERT textbook

Fatigue, and red and white fibres

The reaction time of the fibres can vary in different muscles. Repeated activation of the muscles can lead to the accumulation of lactic acid, due to the anaerobic breakdown of glycogen in them. This causes fatigue.

Muscle contains a red coloured oxygen-storing pigment called myoglobin.

What is the axial skeleton made of?

The skeletal system consists of a framework of bones and a few cartilages. Bone and cartilage are specialised connective tissues. Bone has a very hard matrix due to calcium salts in it, and cartilage has a slightly pliable matrix due to chondroitin salts. In human beings the skeletal system is made up of 206 bones and a few cartilages, grouped into two principal divisions: the axial and the appendicular skeleton.

The axial skeleton comprises 80 bones distributed along the main axis of the body. The skull, vertebral column, sternum and ribs constitute it.

Worked example: counting the axial skeleton

1. Skull = 8 cranial + 14 facial = 22 bones.
2. Add the hyoid and the ear ossicles: 22 + 1 + 6 = 29.
3. Add the vertebral column: 29 + 26 = 55.
4. Add the sternum and the ribs: 55 + 1 + 24 = 80 bones.

The skull region articulates with the superior region of the vertebral column with the help of two occipital condyles, so the human skull is dicondylic.

Vertebral column

The vertebral column is dorsally placed. It extends from the base of the skull and constitutes the main framework of the trunk. Each vertebra has a central hollow portion, the neural canal, through which the spinal cord passes. The first vertebra is the atlas, and it articulates with the occipital condyles.

The number of cervical vertebrae is seven in almost all mammals, including human beings. The vertebral column protects the spinal cord, supports the head, and serves as the point of attachment for the ribs and the musculature of the back.

Ribs and rib cage

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.

The thoracic vertebrae, ribs and sternum together form the rib cage.

What is the appendicular skeleton made of?

The bones of the limbs along with their girdles constitute the appendicular skeleton. Each limb is made of 30 bones.

The girdles

The pectoral and pelvic girdle bones help in the articulation of the upper and the lower limbs respectively with the axial skeleton. Each girdle is formed of two halves.

Worked example: counting the appendicular skeleton

1. Four limbs of 30 bones each: 4 × 30 = 120 bones.
2. Pectoral girdle: two halves, each with a clavicle and a scapula: 2 × 2 = 4 bones.
3. Pelvic girdle: two coxal bones: 2 bones.
4. Appendicular skeleton = 120 + 4 + 2 = 126 bones. With the 80 bones of the axial skeleton, the total is 126 + 80 = 206.

Figure 17.9 shows the right arm from the front: the clavicle and scapula at the shoulder, then the humerus, the radius and ulna, and the carpals, metacarpals and phalanges of the hand. Figure 17.10 shows the right leg from the front: the coxal bone, with its ilium, pubis and ischium labelled, beside the sacrum; then the femur, the patella at the knee, the tibia and fibula, and the tarsals, metatarsals and phalanges of the foot.

See Figs. 17.9 and 17.10 in your NCERT textbook

What are the types of joints?

Joints are points of contact between bones, or between bones and cartilages. They are essential for all types of movements involving the bony parts of the body. The force generated by the muscles is used to carry out movement through joints, where the joint acts as a fulcrum. Joints are classified into three major structural forms.

What are the disorders of the muscular and skeletal system?

Note: Three of the six are disorders of muscle (myasthenia gravis, muscular dystrophy, tetany) and three are disorders of bones and joints (arthritis, osteoporosis, gout). Arthritis and gout are both inflammations of joints. Gout is the one caused by uric acid crystals.

How do you answer the NCERT exercise questions?

Glossary

• Locomotion — A voluntary movement that results in a change of place or location of the animal.
• Fascicle — A muscle bundle; several fascicles held together by the fascia make up a skeletal muscle.
• Sarcolemma — The plasma membrane that lines a muscle fibre and encloses the sarcoplasm.
• Sarcoplasmic reticulum — The endoplasmic reticulum of the muscle fibre; it is the store house of calcium ions.
• Sarcomere — The portion of a myofibril between two successive Z lines; the functional unit of contraction.
• H zone — The central part of the thick filament not overlapped by thin filaments in the resting state.
• Troponin — A complex protein on tropomyosin; in the resting state one subunit masks the active sites for myosin on actin.
• Meromyosin — The monomeric protein of the thick filament, with a globular head, a short arm and a tail.
• Motor unit — A motor neuron along with the muscle fibres connected to it.
• Neuromuscular junction — The junction between a motor neuron and the sarcolemma of the muscle fibre; also called the motor-end plate.
• Cross bridge — The link formed when a myosin head binds to the exposed active sites on actin.
• Myoglobin — A red coloured oxygen-storing pigment found in muscle; its content is high in red fibres.
• Acetabulum — The cavity of the coxal bone, at the point of fusion of ilium, ischium and pubis, to which the thigh bone articulates.
• Synovial joint — A joint with a fluid-filled synovial cavity between the articulating surfaces, allowing considerable movement.

Common errors and misconceptions

• Misconception: All movements are locomotion. Correct: All locomotions are movements, but all movements are not locomotions. Locomotion involves a change of place.
• Misconception: Cardiac muscle is voluntary because it is striated. Correct: Cardiac muscle is striated but involuntary. The nervous system does not control its activities directly.
• Misconception: The A band shortens during contraction. Correct: The I bands get reduced, whereas the A bands retain their length.
• Misconception: The I band contains myosin. Correct: The light I band contains actin. The dark A band contains myosin.
• Misconception: Calcium binds to tropomyosin to expose the active sites. Correct: Calcium binds with a subunit of troponin, which removes the masking of the active sites for myosin.
• Misconception: ATP is needed only to form the cross bridge. Correct: The energy of ATP hydrolysis is used to form the cross bridge, and a new ATP must bind for the cross bridge to be broken.
• Misconception: The sacral and coccygeal regions are each counted as several separate vertebrae in the total of 26. Correct: The sacral and coccygeal regions are each fused and count as 1 unit, so the vertebral column is formed by 26 units: 7 cervical, 12 thoracic, 5 lumbar, 1 fused sacral and 1 fused coccygeal.
• Misconception: The false ribs are the floating ribs. Correct: The 8th, 9th and 10th pairs are the vertebrochondral (false) ribs. The 11th and 12th pairs are the floating ribs.

Exam-style questions with model answers

Q1. State the sliding filament theory of muscle contraction. [1 mark]

1. The sliding filament theory states that contraction of a muscle fibre takes place by the sliding of the thin (actin) filaments over the thick (myosin) filaments.

Q2. Distinguish between red and white muscle fibres. [2 marks]

1. Red fibres have a high myoglobin content and plenty of mitochondria, and use stored oxygen for ATP production, so they are called aerobic muscles.
2. White fibres have very little myoglobin and few mitochondria but a high amount of sarcoplasmic reticulum, and depend on the anaerobic process for energy.

Q3. What is a sarcomere? Name its bands and lines. [2 marks]

1. A sarcomere is the portion of a myofibril between two successive Z lines; it is the functional unit of contraction.
2. It has a central A band of thick myosin filaments, with the M line and the H zone in its middle, and half an I band of thin actin filaments on either side, ending at the Z lines.

Q4. How do you distinguish between a skeletal muscle and a cardiac muscle? [3 marks]

1. Location: skeletal muscles are closely associated with the skeletal components of the body; cardiac muscles are the muscles of the heart.
2. Control: skeletal muscles are under the voluntary control of the nervous system; cardiac muscles are involuntary, as the nervous system does not control their activities directly.
3. Appearance: both are striated, but cardiac muscle cells assemble in a branching pattern.

Q5. Describe the structure of an actin (thin) filament. [3 marks]

1. Each actin filament is made of two F (filamentous) actins helically wound to each other. Each F actin is a polymer of monomeric G (globular) actins.
2. Two filaments of another protein, tropomyosin, run close to the F actins throughout their length.
3. A complex protein, troponin, is distributed at regular intervals on the tropomyosin. In the resting state a subunit of troponin masks the active binding sites for myosin on the actin filaments.

Q6. Classify the ribs of the human body. [3 marks]

1. True ribs: the first seven pairs, attached dorsally to the thoracic vertebrae and ventrally to the sternum with the help of hyaline cartilage.
2. Vertebrochondral (false) ribs: the 8th, 9th and 10th pairs, which do not articulate directly with the sternum but join the seventh rib with the help of hyaline cartilage.
3. Floating ribs: the last two pairs, the 11th and 12th, which are not connected ventrally.

Q7. Describe the important steps in muscle contraction. [5 marks]

1. A signal from the CNS reaches the neuromuscular junction through a motor neuron and releases the neurotransmitter acetylcholine, which generates an action potential in the sarcolemma.
2. The action potential spreads through the muscle fibre and causes the release of calcium ions into the sarcoplasm. Calcium binds with a subunit of troponin on the actin filaments and removes the masking of the active sites for myosin.
3. Using the energy from ATP hydrolysis, the myosin head binds to the exposed active sites on actin to form a cross bridge.
4. This pulls the actin filaments towards the centre of the A band. The Z lines are pulled inwards and the sarcomere shortens: the I bands get reduced and the A bands retain their length.
5. The myosin releases ADP and Pi and goes back to its relaxed state. A new ATP binds and the cross bridge is broken. The cycle repeats until Ca²⁺ is pumped back to the sarcoplasmic cisternae, which masks the actin filaments again and brings about relaxation.

Q8. Describe the three structural types of joints, with an example of each, and name the kinds of synovial joints. [5 marks]

1. Joints are points of contact between bones, or between bones and cartilages. They are classified as fibrous, cartilaginous and synovial.
2. Fibrous joints do not allow any movement. The flat skull bones fuse end-to-end by dense fibrous connective tissue in the form of sutures to form the cranium.
3. Cartilaginous joints have bones joined together with the help of cartilages and permit limited movement, as between adjacent vertebrae.
4. Synovial joints have a fluid-filled synovial cavity between the articulating surfaces and allow considerable movement, so they help in locomotion.
5. Kinds of synovial joints: ball and socket (between humerus and pectoral girdle), hinge (knee), pivot (between atlas and axis), gliding (between the carpals) and saddle (between the carpal and metacarpal of the thumb).

Key takeaways

• All locomotions are movements, but all movements are not locomotions; human cells show amoeboid, ciliary and muscular movement.
• Skeletal muscle is striated and voluntary, visceral muscle is smooth and involuntary, and cardiac muscle is striated, branched and involuntary.
• A sarcomere lies between two Z lines, with an A band of thick myosin filaments in the centre and half an I band of thin actin filaments on each side.
• Calcium released from the sarcoplasmic reticulum binds troponin and unmasks the active sites on actin, so that myosin heads can form cross bridges.
• During contraction the thin filaments slide over the thick filaments: the I bands get reduced and the A bands retain their length.
• The human skeleton has 206 bones: 80 in the axial skeleton and 126 in the appendicular skeleton.
• The vertebral column has 26 vertebrae, and there are 12 pairs of ribs: 7 true, 3 vertebrochondral and 2 floating.
• Joints are fibrous, cartilaginous or synovial; only synovial joints, with a fluid-filled cavity, allow considerable movement.

Test yourself

Which cells of the human body show amoeboid movement?

Some specialised cells, like macrophages and leucocytes in blood, show amoeboid movement by means of pseudopodia.

How much of the body weight of a human adult is contributed by muscles?

About 40 to 50 per cent of the body weight of a human adult is contributed by muscles.

Where are calcium ions stored in a muscle fibre?

Calcium ions are stored in the sarcoplasmic reticulum, which is the endoplasmic reticulum of the muscle fibre.

Which protein is found in the I band?

The light I band, or isotropic band, contains actin, which forms the thin filaments.

Which neurotransmitter is released at the neuromuscular junction?

Acetylcholine is released at the neuromuscular junction, and it generates an action potential in the sarcolemma.

How many bones does each limb have?

Each limb is made of 30 bones, so the four limbs together have 120 bones.

What kind of joint is found between the atlas and the axis?

The joint between the atlas and the axis is a pivot joint, which is a type of synovial joint.

What causes tetany?

Tetany, which is rapid spasms or wild contractions in muscle, is caused by low Ca²⁺ in the body fluid.