Class 11 · Biology · Chapter 17
Locomotion and Movement
How the human body moves: the three kinds of cellular movement, the fine structure of skeletal muscle down to the actin and myosin filaments, the sliding filament mechanism of contraction, the 206 bones of the skeleton, the joints that let bones move, and the disorders that stop them. It is one of the most diagram-driven chapters in Class 11 and the base for a reliable one or two NEET questions every year.
Before you startWhat the board asks from this chapter
Class 11 Biology is a 70-mark theory paper set by the school on the CBSE pattern. Locomotion and Movement sits in Unit V, Human Physiology, which is the largest unit with about 18 marks shared among Breathing and Exchange of Gases, Body Fluids and Circulation, Excretory Products, Locomotion and Movement, Neural Control and Chemical Coordination. In a typical school final this chapter alone gives 4 to 7 marks: one or two 1-mark items (an MCQ on the number of bones, the type of joint at a named location, the storehouse of calcium, or the location of the ATPase, and an Assertion-Reason on the I band shortening or the red and white fibres), a 2-mark or 3-mark question on the types of movement, the girdles, the ribs or the differences between muscle types, and every second year a 5-mark question that pairs a labelled sarcomere with the steps of the sliding filament theory. NEET reliably takes one or two questions from the NCERT figures and numbers of this chapter.
| Question type | Marks | Usually asked |
|---|---|---|
| MCQ | 1 | Number of bones (206; 80 axial; 30 per limb; 8 cranial, 14 facial; 7 cervical; 12 pairs of ribs; 8 carpals, 7 tarsals, 14 phalanges); the joint at a given site (atlas-axis pivot, thumb saddle, knee and elbow hinge, carpals gliding, shoulder and hip ball and socket, skull sutures fibrous, vertebrae cartilaginous); the store of Ca2+ (sarcoplasmic reticulum); which band shortens (I band) and which stays constant (A band); the H zone; ATPase in the myosin head; troponin masking the active sites; acetylcholine at the neuromuscular junction; myoglobin in red fibres; hyaline cartilage joining ribs to sternum; three bones fusing to form the coxal bone; the cause of tetany (low Ca2+) and gout (uric acid crystals). |
| Assertion–Reason | 1 | During contraction the I band shortens but the A band does not because thin filaments slide over thick filaments; the muscle fibre is a syncitium because the sarcoplasm has many nuclei; the head of myosin is called the cross arm and is an ATPase; red fibres are aerobic because they have abundant myoglobin and mitochondria; white fibres fatigue quickly because they rely on anaerobic glycolysis; the skull is dicondylic because it has two occipital condyles; myasthenia gravis is an autoimmune disorder of the neuromuscular junction; osteoporosis in older women is linked to a fall in estrogen. |
| Very short answer | 2 | Distinguish movement from locomotion; name two cells showing amoeboid movement and two organs lined by ciliated epithelium; define sarcomere, motor unit, neuromuscular junction; name the ear ossicles in order; what are floating ribs; why are ribs called bicephalic; what is a dicondylic skull; name the three bones of the coxal bone; define fatigue and its cause; name the disorder from a one-line description (tetany, gout, myasthenia gravis, muscular dystrophy, osteoporosis, arthritis). |
| Short answer | 3 | The three types of movement with examples; differences between skeletal, visceral and cardiac muscle; differences between actin and myosin filaments; red versus white muscle fibres; pectoral versus pelvic girdle; the true, false and floating ribs; the vertebral formula and functions of the vertebral column; the three classes of joints with one example each; the five types of synovial joints; the structure of an actin filament or a myosin filament; the role of calcium in contraction. |
| Long answer | 5 | Labelled diagram of a sarcomere and the sliding filament theory in steps (the single most repeated 5-marker); the structure of a skeletal muscle from muscle to myofibril with the bands; the axial skeleton (skull, vertebral column, rib cage) with numbers; the appendicular skeleton (limbs and girdles) with a labelled girdle; joints with a labelled diagram of the types of synovial joints and the disorders of the muscular and skeletal systems. |
| Case-based | 4 | A passage on a sprinter versus a marathon runner (red and white fibres, fatigue, lactic acid), a passage on a patient with muscle weakness that worsens with use (myasthenia gravis versus muscular dystrophy versus tetany), a passage on the sarcomere before and after contraction with band lengths, or a passage on an elderly woman with a hip fracture (osteoporosis, estrogen, ball and socket joint, coxal bone, acetabulum). |
Hot topics, in order
- Sliding filament theory: steps of contraction and relaxation with the roles of acetylcholine, Ca2+, troponin, ATP and the myosin head
- Sarcomere: A band, I band, H zone, Z line, M line, and which of them shorten during contraction
- Structure of the thin (actin, tropomyosin, troponin) and thick (meromyosin, HMM, LMM) filaments
- The three types of muscle and the three types of movement, with examples and differences
- Axial skeleton numbers: 80 bones; 22 skull bones (8 + 14) plus hyoid and 6 ear ossicles; 26 vertebrae (7, 12, 5, 1, 1); 12 pairs of ribs (7 true, 3 vertebrochondral, 2 floating)
- Appendicular skeleton: 30 bones per limb, the pectoral girdle (clavicle, scapula, acromion, glenoid cavity) and the pelvic girdle (ilium, ischium, pubis, acetabulum, pubic symphysis)
- Joints: fibrous, cartilaginous, synovial, and the five kinds of synovial joint with their locations
- Red versus white fibres, muscle fatigue and lactic acid
- Disorders: myasthenia gravis, muscular dystrophy, tetany, arthritis, osteoporosis, gout
The whole chapter, in orderNotes
01 Introduction: movement and locomotion
Movement is one of the significant features of living beings. Animals and plants show a wide range of movements: streaming of protoplasm inside a cell, movement of cilia and flagella, the beating of the heart, the peristalsis of the gut, the swinging of a limb. Some of those movements carry the whole organism from one place to another, and NCERT reserves a separate word for that. Locomotion is the voluntary movement that results in a change of place or location; walking, running, climbing, flying and swimming are all locomotion. The rule to remember for a 1-mark question is one line: all locomotions are movements, but all movements are not locomotions. Peristalsis is a movement, not locomotion; a cheetah sprinting is both.
NCERT gives three examples that show the same structure can do both jobs. In Paramoecium the cilia sweep food into the gullet (a movement) and also propel the animal (locomotion). In Hydra the tentacles capture prey and, when the animal somersaults, act as locomotory structures. In humans the limbs change body posture and also carry us about. The methods of locomotion vary with the habitat and the demand of the situation, but locomotion is generally for search of food, shelter, mate, suitable breeding grounds, favourable climatic conditions, or to escape from enemies or predators. That list is the NCERT answer to 'why do animals move from place to place'.
The chapter then runs in a fixed order, and so should your revision: the three types of movement shown by human cells, the muscle as a tissue and the fine structure of the skeletal muscle fibre, the mechanism by which it contracts, the bones that the muscles pull on, the joints at which the pull becomes movement, and finally the diseases of muscle, bone and joint.
02 17.1 Types of Movement
Cells of the human body exhibit three main types of movement: amoeboid, ciliary and muscular. Learn each with its NCERT example, because the 2-mark and 3-mark versions of this question are asked almost every year.
Amoeboid movement. Some specialised cells in the body, such as macrophages and leucocytes in blood, show amoeboid movement. It is brought about by the formation of pseudopodia by the streaming of protoplasm, exactly as in Amoeba. Cytoskeletal elements such as microfilaments are also involved in amoeboid movement. This is how a white blood cell squeezes out of a capillary and crawls towards a site of infection.
Ciliary movement. It occurs in most of our internal tubular organs, which are lined by ciliated epithelium. NCERT names two jobs: the coordinated movement of cilia in the trachea helps to remove dust particles and some foreign substances inhaled along with the air, and the passage of the ova through the female reproductive tract (the fallopian tube) is facilitated by ciliary movement. Note that the sperm moves by a flagellum, which is the same 9 + 2 machinery on a longer scale; NCERT's syllabus line lists 'ciliary, flagellar, muscular', so mention the sperm tail if asked for flagellar movement.
Muscular movement. The movement of our limbs, jaws, tongue and so on requires muscular movement. The contractile property of muscles is used effectively for locomotion and other movements by human beings and by the majority of multicellular organisms. The point examiners look for: locomotion requires a perfect coordinated activity of the muscular, skeletal and neural systems. The muscle generates the force, the bone is the lever and the nerve gives the command. The rest of the chapter takes those three in turn, muscle first.
03 17.2 Muscle: origin, properties and the three types
Muscle is a specialised tissue of mesodermal origin. About 40 to 50 per cent of the body weight of an adult human is contributed by muscles. Muscles have four special properties, and a 2-mark question can ask you to list them: excitability (they respond to a stimulus), contractility (they shorten forcefully), extensibility (they can be stretched) and elasticity (they return to their original length).
Muscles are classified on the basis of their location into three types: skeletal, visceral and cardiac.
Skeletal muscles are closely associated with the skeletal components of the body. They have a striped appearance under the microscope and hence are called striated muscles. As their activities are under the voluntary control of the nervous system, they are known as voluntary muscles. They are primarily involved in locomotory actions and changes of body postures.
Visceral muscles are located in the inner walls of hollow visceral organs of the body like the alimentary canal and the reproductive tract. They do not exhibit striations and are called smooth muscles (non-striated muscle). Their activities are not under the voluntary control of the nervous system, so they are involuntary muscles. They assist, for example, in the transportation of food through the digestive tract and of gametes through the genital tract.
Cardiac muscles are the muscles of the heart. Many cardiac muscle cells assemble in a branching pattern to form a cardiac muscle. Based on appearance cardiac muscles are striated; based on the nature of regulation they are involuntary, as the nervous system does not control their activities directly. This combination, striated yet involuntary, is the single most asked point about muscle types.
The table the examiner wants for a 3-mark 'distinguish' question: location (attached to bones / walls of hollow organs / heart), striations (present / absent / present), control (voluntary / involuntary / involuntary), nuclei (many, peripheral, the fibre is a syncitium / single, central / single, central) and shape (long cylindrical unbranched / spindle-shaped / branched with intercalated discs). The first three rows are NCERT; the last two are standard additions that earn the third mark.
04 17.2 Structure of a skeletal muscle: from muscle to sarcomere
NCERT builds the skeletal muscle from the outside in, and the 5-mark diagram question follows the same ladder. Each muscle is made of many muscle bundles (fascicles) held together by a common collagenous connective tissue layer called fascia. Each muscle bundle contains a number of muscle fibres. Each muscle fibre is lined by a plasma membrane called sarcolemma enclosing the sarcoplasm. The muscle fibre is a syncitium because the sarcoplasm contains many nuclei. The endoplasmic reticulum of the fibre, called the sarcoplasmic reticulum, is the storehouse of calcium ions. A characteristic feature of the muscle fibre is the presence of a large number of parallelly arranged filaments in the sarcoplasm called myofilaments or myofibrils.
Each myofibril has alternate dark and light bands on it. The banding is due to the distribution of two important proteins, actin and myosin. The light band contains actin and is called the I band or isotropic band; the dark band is called the A band or anisotropic band and contains myosin. Both proteins are arranged as rod-like structures, parallel to each other and also to the longitudinal axis of the myofibril. Actin filaments are thinner than myosin filaments, hence they are commonly called thin and thick filaments respectively.
In the centre of each I band is an elastic fibre called the Z line, which bisects it. The thin filaments are firmly attached to the Z line. The thick filaments in the A band are also held together in the middle of this band by a thin fibrous membrane called the M line. The A and I bands are arranged alternately throughout the length of the myofibril. The portion of the myofibril between two successive Z lines is the functional unit of contraction and is called a sarcomere.
In the resting state, the edges of thin filaments on either side of the thick filaments partially overlap the free ends of the thick filaments, leaving the central part of the thick filaments uncovered. This central part of the thick filament, not overlapped by thin filaments, is called the H zone. So the A band contains the whole thick filament plus the overlapping ends of the thin filaments; the H zone is the part of the A band with thick filaments only; the I band has thin filaments only.
05 17.2.1 Structure of Contractile Proteins
The thin and thick filaments are each built from more than one protein, and the exam asks for both structures, usually with a diagram.
Actin (thin) filament. 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. Two filaments of another protein, tropomyosin, also run close to the F actins throughout their length. 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. So the thin filament has three proteins: actin (two F actins), tropomyosin (two strands) and troponin (at intervals). This is the NCERT fill-in-the-blank: the thin filament contains 2 F actins and two other proteins namely tropomyosin and troponin.
Myosin (thick) filament. Each myosin filament is also a polymerised protein. Many monomeric proteins called meromyosins constitute one thick filament. Each meromyosin has two important parts, a globular head with a short arm, and a tail. The head with its short arm is called the heavy meromyosin (HMM) and the tail is the light meromyosin (LMM). The HMM component, that is the head and short arm, projects outwards at a regular distance and angle from each other from the surface of the polymerised myosin filament and is known as the cross arm. The globular head is an active ATPase enzyme and has binding sites for ATP and active sites for actin.
Two facts from this paragraph are asked as 1-markers nearly every year: the ATPase of muscle sits in the head of myosin, and the protein that masks the myosin-binding sites on actin at rest is troponin (not tropomyosin; tropomyosin lies along the groove, troponin does the masking, in NCERT wording). For a 3-mark 'differences between actin and myosin filaments' question use: thin versus thick, I band (extending into A band) versus A band only, attached to Z line versus held by M line, made of F actin plus tropomyosin plus troponin versus meromyosin units, no enzyme activity versus ATPase in the head, and the actin filament slides while the myosin filament stays in place.
06 17.2.2 Mechanism of Muscle Contraction: the sliding filament theory
The 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. Write the steps in this order; each is a marking point in the 5-mark answer.
1. Signal. Muscle contraction is initiated by a signal sent by the central nervous system (CNS) via a motor neuron. 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.
2. Neurotransmitter and action potential. A neural signal reaching this junction releases a neurotransmitter, acetylcholine, which generates an action potential in the sarcolemma. The action potential spreads through the muscle fibre and causes the release of calcium ions from the sarcoplasmic reticulum into the sarcoplasm.
3. Unmasking. The increase in Ca2+ level leads to the binding of calcium with a subunit of troponin on the actin filaments and thereby removes the masking of the active sites for myosin.
4. Cross bridge and power stroke. 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 the A band. The Z line attached to these actins is also pulled inwards, thereby causing a shortening of the sarcomere, that is, contraction. During shortening the I bands get reduced, whereas the A bands retain their length. The H zone also narrows, because the thin filaments now cover more of the thick filament.
5. Detachment and repetition. The myosin, releasing ADP and Pi, 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.
6. Relaxation. The process continues till the Ca2+ ions are pumped back to the sarcoplasmic cisternae, resulting in the masking of the actin filaments. This causes the return of the Z lines to their original position, that is, relaxation. Relaxation also needs ATP: one ATP to break the cross bridge and ATP to run the calcium pump, which is why a muscle deprived of ATP after death stays stiff (rigor mortis; a useful one-line extra, not NCERT).
The reaction time of the fibres can vary in different muscles, which prepares the ground for the red and white fibres of the next section.
07 17.2.2 Muscle fatigue, red fibres and white fibres
The last paragraph of the muscle section holds three facts that appear as case-study passages about athletes.
Fatigue. Repeated activation of the muscles can lead to the accumulation of lactic acid due to the anaerobic breakdown of glycogen in them, causing fatigue. So the NCERT definition of fatigue is not 'tiredness'; it is the fall in the ability of a muscle to contract because lactic acid has built up when oxygen supply could not keep up with demand.
Myoglobin. Muscle contains a red-coloured, oxygen-storing pigment called myoglobin. It is related to haemoglobin but sits inside the muscle fibre and holds a reserve of O2 for the mitochondria.
Red fibres. Muscles that have a high content of myoglobin appear reddish and are called red fibres. They also contain plenty of mitochondria, which use the large amount of O2 stored in them for ATP production. These muscles are therefore called aerobic muscles. They contract slowly, sustain contraction for long periods and are the muscles of posture and endurance; a marathon runner and the back muscles that keep you upright depend on them.
White fibres. Muscles that possess very little myoglobin are pale or whitish and are called white fibres. The number of mitochondria is also few in them, but the amount of sarcoplasmic reticulum is high. They depend on anaerobic processes for energy. They contract fast and powerfully but fatigue quickly because lactic acid accumulates; the sprinter's thigh and the eye muscles are built of them.
For the 3-mark 'differentiate red and white muscle fibres' question, four rows are enough: myoglobin (high / low), mitochondria (many / few), sarcoplasmic reticulum (less / more), mode of respiration and speed of fatigue (aerobic, slow to fatigue / anaerobic, fatigues quickly). Add colour and the example in a fifth row if you want to be safe.
08 17.3 Skeletal System: the axial skeleton
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, and cartilage has a slightly pliable matrix due to chondroitin salts. In human beings the system is made up of 206 bones and a few cartilages, grouped into two principal divisions: the axial skeleton (80 bones) along the main axis of the body, and the appendicular skeleton (126 bones) of the limbs and girdles.
Skull. The skull is composed of two sets of bones: cranial (8) and facial (14). The cranial bones form the hard protective outer covering, the cranium, for the brain. The 14 facial skeletal elements form the front part of the skull. A single U-shaped bone, the hyoid, at the base of the buccal cavity is also included in the skull. Each middle ear contains three tiny bones, the ear ossicles: malleus, incus and stapes, in that order from the ear drum inwards; the stapes is the smallest bone of the body. Counting: 8 + 14 + 1 + 6 = 29 bones in the skull region. The skull articulates with the superior region of the vertebral column with the help of two occipital condyles, so the human skull is dicondylic.
Vertebral column. It is formed by 26 serially arranged units called vertebrae and 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, which articulates with the occipital condyles; the second is the axis. The vertebral column is differentiated into cervical (7), thoracic (12), lumbar (5), sacral (1 fused) and coccygeal (1 fused) regions, from the skull downwards: 7 + 12 + 5 + 1 + 1 = 26. The number of cervical vertebrae is seven in almost all mammals, including human beings; a giraffe has seven, a mouse has seven. Functions: it protects the spinal cord, supports the head and serves as the point of attachment for the ribs and the musculature of the back.
Sternum and ribs. The sternum is a flat bone on the ventral midline of the thorax. There are 12 pairs of 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. The first seven pairs are true ribs: dorsally attached to the thoracic vertebrae and ventrally connected to the sternum with the help of hyaline cartilage. The 8th, 9th and 10th pairs do not articulate directly with the sternum but join the seventh rib with the help of hyaline cartilage; these are the vertebrochondral (false) ribs. The last two pairs, 11th and 12th, are not connected ventrally and are the floating ribs. The thoracic vertebrae, the ribs and the sternum together form the rib cage. Axial count: 29 (skull region) + 26 (vertebrae) + 1 (sternum) + 24 (ribs) = 80.
09 17.3 Skeletal System: the appendicular skeleton (limbs and girdles)
The appendicular skeleton consists of the bones of the limbs along with their girdles. Each limb is made of 30 bones.
Forelimb (30). Humerus (upper arm, 1), radius and ulna (forearm, 2), carpals (wrist bones, 8), metacarpals (palm bones, 5) and phalanges (digits, 14): 1 + 2 + 8 + 5 + 14 = 30. The thumb has two phalanges and each of the other four fingers has three, which gives 14.
Hind limb (30). Femur (thigh bone, the longest bone of the body, 1), tibia and fibula (2), tarsals (ankle bones, 7), metatarsals (5) and phalanges (14): 1 + 2 + 7 + 5 + 14 = 29, plus the patella, a cup-shaped bone that covers the knee ventrally (the knee cap), makes 30. Four limbs give 120 bones.
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.
Pectoral girdle. Each half consists of a clavicle and a scapula. The scapula is a large triangular flat bone situated in the dorsal part of the thorax between the second and the seventh ribs. The dorsal, flat, triangular body of the scapula has a slightly elevated ridge called the spine, which projects as a flat, expanded process called the acromion. The clavicle articulates with the acromion. Below the acromion is a depression called the glenoid cavity, which articulates with the head of the humerus to form the shoulder joint. Each clavicle is a long slender bone with two curvatures, commonly called the collar bone. Four bones in all.
Pelvic girdle. It consists of two coxal bones. Each coxal bone is formed by the fusion of three bones: ilium, ischium and pubis. At the point of fusion of these three bones is a cavity called the acetabulum, to which the thigh bone (head of femur) articulates to form the hip joint. The two halves of the pelvic girdle meet ventrally to form the pubic symphysis, which contains fibrous cartilage. Two bones in all. Appendicular count: 120 + 4 + 2 = 126, and 80 + 126 = 206.
For the 3-mark 'pectoral versus pelvic girdle' question use these rows: bones (clavicle + scapula per half / one coxal bone per half made of ilium, ischium, pubis), the socket (glenoid cavity, shallow / acetabulum, deep), which limb (forelimb via humerus / hind limb via femur), attachment to the axial skeleton (loosely attached, allows a wide range of arm movement / firmly attached to the sacrum, weight-bearing) and the midline union (the two halves do not meet / meet at the pubic symphysis).
10 17.4 Joints
Joints are essential for all types of movements involving the bony parts of the body. They are points of contact between bones, or between bones and cartilages. The force generated by the muscles is used to carry out movement through joints, where the joint acts as a fulcrum; the bone is the lever and the muscle applies the effort. Joints are classified into three structural forms depending on the degree of movement they allow.
Fibrous joints do not allow any movement. The flat skull bones fuse end to end with the help of dense fibrous connective tissue in the form of sutures to form the cranium. So the answer to 'type of joint between cranial bones' is fibrous joint (suture).
Cartilaginous joints are those in which the bones are joined together with the help of cartilages. The joint between the adjacent vertebrae in the vertebral column is of this type (the intervertebral disc) and it permits limited movement. The pubic symphysis, with its fibrous cartilage, is also a cartilaginous joint.
Synovial joints are characterised by the presence of a fluid-filled synovial cavity between the articulating surfaces of the two bones. Such an arrangement allows considerable movement; these joints help in locomotion and many other movements. The synovial fluid lubricates the joint and the articular cartilage on the bone ends reduces friction. NCERT names five kinds, and the location of each is a guaranteed 1-mark question:
- Ball and socket joint: between the humerus and the pectoral girdle (shoulder); also the femur and the acetabulum (hip). Movement in all planes.
- Hinge joint: the knee; also the elbow and the joints between phalanges. Movement in one plane.
- Pivot joint: between the atlas and the axis. Rotation, as when you shake your head to say no.
- Gliding joint: between the carpals (and between tarsals). Sliding movement.
- Saddle joint: between the carpal and the metacarpal of the thumb. Movement in two planes, which is what lets the thumb oppose the fingers.
Put the three classes in one line for a quick answer: fibrous, immovable, skull sutures; cartilaginous, slightly movable, between vertebrae; synovial, freely movable, knee and shoulder.
11 17.5 Disorders of Muscular and Skeletal System
NCERT gives six disorders in one-line definitions, and the exam asks for them exactly that way, usually 'name the disorder' from a description, or 'define any four' for 2 to 4 marks.
Myasthenia gravis. An autoimmune disorder affecting the neuromuscular junction, leading to fatigue, weakening and paralysis of skeletal muscle. Antibodies block the acetylcholine receptors on the sarcolemma, so the nerve signal fails to start the action potential; the drooping eyelids and weakness that worsens with use in a case passage point here.
Muscular dystrophy. Progressive degeneration of skeletal muscle, mostly due to a genetic disorder. The muscle fibres themselves waste away; the neuromuscular junction is normal. This is the distinction from myasthenia gravis that examiners test.
Tetany. Rapid spasms (wild contractions) in muscle due to low Ca2+ in body fluid. Low calcium makes the nerve and muscle membranes over-excitable; the link to the parathyroid gland (parathormone raises blood calcium) is the cross-chapter question with Chemical Coordination.
Arthritis. Inflammation of joints. Osteoarthritis (wear of articular cartilage with age) and rheumatoid arthritis (autoimmune) are the two common forms; NCERT's definition is the two-word one.
Osteoporosis. An age-related disorder characterised by decreased bone mass and increased chances of fractures. Decreased levels of estrogen is a common cause, which is why it is frequent in women after menopause; the passage about an elderly woman with a hip fracture is asking for this word.
Gout. Inflammation of joints due to the accumulation of uric acid crystals. It most often strikes the joint of the big toe, and the reason it is a joint disorder rather than a bone disorder is that the crystals deposit in the synovial cavity.
Group them for revision: two of muscle (myasthenia gravis at the junction, muscular dystrophy in the fibre), one of the calcium balance (tetany), two of the joint (arthritis, gout) and one of the bone (osteoporosis). The exemplar MCQ that asks which is not a bone disorder puts atherosclerosis in the list; it is a disorder of arteries.
VocabularyKey terms
Keep this on one pageFacts and values to remember
| Name | Expression | Notes |
|---|---|---|
| Types of movement with NCERT examples | Amoeboid: macrophages and leucocytes (pseudopodia, microfilaments). Ciliary: trachea (removes dust), female reproductive tract (moves ova). Muscular: limbs, jaws, tongue (needs muscular + skeletal + neural systems) | Flagellar (sperm tail) is in the syllabus line though not in the NCERT paragraph. |
| Muscle: origin, share and properties | Mesodermal origin; 40 to 50 per cent of adult body weight; excitability, contractility, extensibility, elasticity | Exemplar MCQ on the percentage; 2-mark list of properties. |
| Three types of muscle | Skeletal: striated, voluntary, attached to bones, locomotion and posture. Visceral: non-striated (smooth), involuntary, walls of hollow organs. Cardiac: striated, involuntary, branching cells of the heart | Cardiac = striated AND involuntary is the most tested combination. |
| Hierarchy of a skeletal muscle | Muscle → muscle bundles (fascicles, held by fascia) → muscle fibres (sarcolemma, sarcoplasm, many nuclei = syncitium, sarcoplasmic reticulum stores Ca2+) → myofibrils → thick (myosin) and thin (actin) filaments | Give this ladder as the opening of any 5-mark structure answer. |
| Bands of the sarcomere | I band: light, isotropic, actin only, bisected by Z line. A band: dark, anisotropic, myosin plus overlapping actin, M line at centre. H zone: central A band with myosin only. Sarcomere: Z line to Z line | On contraction I band and H zone shorten, A band constant, filaments do not change length. |
| Thin filament | 2 F actin (helically wound; each a polymer of G actin) + 2 strands of tropomyosin + troponin at regular intervals | A subunit of troponin masks the active sites for myosin in the resting state. |
| Thick filament | Polymer of meromyosins; HMM = globular head + short arm = cross arm; LMM = tail; head = ATPase with ATP-binding site and actin-binding site | ATPase of muscle is located in the head of myosin (exemplar MCQ). |
| Steps of contraction | CNS → motor neuron → acetylcholine at neuromuscular junction → action potential in sarcolemma → Ca2+ from sarcoplasmic reticulum → Ca2+ binds troponin, active sites exposed → myosin head + ATP hydrolysis → cross bridge → actin pulled to A band centre → Z lines in → sarcomere shortens; ADP + Pi released, new ATP breaks the bridge, cycle repeats; Ca2+ pumped back → relaxation | Write as numbered steps; each is a marking point. |
| Red versus white fibres | Red: high myoglobin, many mitochondria, less sarcoplasmic reticulum, aerobic, slow to fatigue. White: low myoglobin, few mitochondria, more sarcoplasmic reticulum, anaerobic, fatigue fast | Fatigue = lactic acid from anaerobic breakdown of glycogen. |
| Bone counts of the axial skeleton (80) | Skull 22 = 8 cranial + 14 facial; hyoid 1; ear ossicles 6 (malleus, incus, stapes × 2); vertebrae 26 = C7 + T12 + L5 + S1 + Co1; sternum 1; ribs 24 (12 pairs) | Skull is dicondylic (two occipital condyles); atlas is the first vertebra. |
| Ribs | 12 pairs, bicephalic; 1 to 7 true (hyaline cartilage to sternum); 8, 9, 10 vertebrochondral or false (join 7th rib); 11, 12 floating; thoracic vertebrae + ribs + sternum = rib cage | NCERT true/false: 11 pairs is FALSE. |
| Bone counts of the appendicular skeleton (126) | Each limb 30: forelimb = humerus 1, radius + ulna 2, carpals 8, metacarpals 5, phalanges 14; hind limb = femur 1, tibia + fibula 2, tarsals 7, metatarsals 5, phalanges 14, patella 1. Pectoral girdle 4 (2 clavicle + 2 scapula); pelvic girdle 2 coxal bones | Femur is the longest bone; stapes the smallest. |
| Girdles | Pectoral: clavicle (collar bone, two curvatures) + scapula (triangular, between 2nd and 7th ribs, spine → acromion, glenoid cavity for humerus). Pelvic: coxal bone = ilium + ischium + pubis, acetabulum for femur, two halves meet at pubic symphysis (fibrous cartilage) | 3-mark comparison question. |
| Joints | Fibrous (immovable): skull sutures. Cartilaginous (limited): between vertebrae, pubic symphysis. Synovial (free, synovial cavity): ball and socket (shoulder, hip), hinge (knee, elbow, phalanges), pivot (atlas–axis), gliding (carpals), saddle (carpal–metacarpal of thumb) | The joint is the fulcrum. |
| Disorders | Myasthenia gravis: autoimmune, neuromuscular junction, weakness and paralysis. Muscular dystrophy: genetic, progressive degeneration of skeletal muscle. Tetany: low Ca2+, spasms. Arthritis: inflammation of joints. Osteoporosis: low bone mass, low estrogen, fractures. Gout: uric acid crystals in joints | One line each is a full answer. |
Practise with a pencilDiagrams that earn marks
Structure of a sarcomere (NCERT Fig. 17.3)
Draw a horizontal strip with two vertical Z lines at the ends. Between them draw the thick myosin filaments as a set of heavy horizontal lines in the middle third with short cross arms projecting outwards, and the thin actin filaments as lighter lines starting from each Z line and overlapping the ends of the thick filaments. Bracket and label the A band (the full length of the thick filaments), the I band (thin filaments only, half on each side of the Z line, so the I band spans two neighbouring sarcomeres), the H zone (the central part with thick filaments only) and the M line (a vertical line through the centre of the H zone). Label the Z line, the thin (actin) filament, the thick (myosin) filament and mark the whole Z-to-Z length as the sarcomere. Eight labels: Z line, A band, I band, H zone, M line, actin, myosin, sarcomere.
Sarcomere before and after contraction (sliding filament)
Draw two sarcomeres one below the other, the upper labelled relaxed and the lower labelled contracted, with the same filaments drawn at the same length in both. In the lower one bring the Z lines closer, make the thin filaments overlap the thick filaments further, and draw the H zone as a narrower gap and the I bands as shorter regions. Under the diagram write which bands changed: the I band and the H zone shorten, the A band stays the same, the sarcomere shortens, and the filaments themselves do not change length. Adding arrows on the actin pointing towards the centre of the A band shows the direction of sliding, which examiners reward.
Skeletal muscle organisation: muscle to myofibril (NCERT Fig. 17.1)
Draw the muscle as a spindle with tendons at both ends, then a magnified cut section showing several muscle bundles (fascicles) wrapped in fascia, then one muscle fibre drawn as a long cylinder with the sarcolemma as its outline and several nuclei at the periphery, then a single myofibril pulled out of it showing alternating dark and light bands. Labels: tendon, fascia, muscle bundle (fascicle), muscle fibre, sarcolemma, sarcoplasm, nuclei, sarcoplasmic reticulum, myofibril, A band, I band, Z line.
Actin (thin) filament (NCERT Fig. 17.4a)
Draw two strings of small beads twisted around each other; each bead is a G actin and each string is an F actin. Along the groove between them draw two thin lines for the tropomyosin strands, and at regular intervals along the tropomyosin draw a small three-lobed blob for troponin. Label: G actin, F actin (two, helically wound), tropomyosin, troponin, and mark one active site for myosin on the actin covered by a troponin subunit in the resting state.
Myosin monomer (meromyosin) and thick filament (NCERT Fig. 17.4b)
Draw one meromyosin as a long tail (light meromyosin, LMM) ending in a short arm that carries a globular head (together the heavy meromyosin, HMM, or cross arm). On the head mark an ATP-binding site and an actin-binding site and write ATPase next to it. Beside it draw a thick filament as a bundle of tails with heads projecting outwards at regular intervals and angles on both halves, bare in the middle where the M line will be. Labels: head, short arm, tail, HMM, LMM, cross arm, ATP-binding site, actin-binding site.
Human skeleton: axial and appendicular (NCERT Fig. 17.6)
Draw a front view of the skeleton in outline. Label the skull (cranium and facial bones), the vertebral column, the sternum, the ribs, and then, for the forelimb, the clavicle, scapula, humerus, radius, ulna, carpals, metacarpals and phalanges, and for the hind limb the coxal bone (pelvic girdle), femur, patella, tibia, fibula, tarsals, metatarsals and phalanges. Use a bracket or two colours to mark the axial skeleton (skull, vertebral column, sternum and ribs) and the appendicular skeleton (girdles and limbs). Get the side right: radius on the thumb side, tibia the larger inner bone of the leg.
Vertebral column and rib cage (NCERT Fig. 17.7 and 17.8)
Draw the vertebral column as a stack of blocks with a gentle S curve and bracket it into cervical (7, top, mark the atlas and axis), thoracic (12), lumbar (5), sacral (1, fused) and coccyx (1, fused). Next to it draw the rib cage from the front: the sternum as a flat vertical bone, the first seven pairs of ribs curving from the thoracic vertebrae to the sternum via a strip of hyaline cartilage, the 8th to 10th pairs joining the cartilage of the seventh rib, and the 11th and 12th pairs ending free. Labels: sternum, true ribs, vertebrochondral (false) ribs, floating ribs, hyaline cartilage, thoracic vertebrae.
Pectoral girdle with the upper arm (NCERT Fig. 17.9)
Draw the scapula as a flat triangle with a raised ridge (the spine) across its back ending in a flat projection (the acromion); draw the clavicle as an S-shaped slender bone meeting the acromion. Below the acromion draw the shallow glenoid cavity with the rounded head of the humerus fitting into it, and continue the humerus downwards. Labels: scapula, spine of scapula, acromion, clavicle (collar bone), glenoid cavity, head of humerus, humerus, shoulder joint (ball and socket).
Pelvic girdle (NCERT Fig. 17.10)
Draw the two coxal bones as a basin seen from the front, each bone with a broad upper wing (ilium), a lower back part (ischium) and a lower front part (pubis) meeting at a deep cup on the outer side (the acetabulum). Show the two pubic bones meeting at the midline as the pubic symphysis and the sacrum wedged between the two ilia at the back. Labels: ilium, ischium, pubis, acetabulum, pubic symphysis, sacrum, coxal bone, and note that the head of the femur fits into the acetabulum.
Types of synovial joints (NCERT Fig. 17.11)
Draw five small sketches in a row and name the location under each: a ball and socket (rounded head in a cup, shoulder or hip), a hinge (two bones meeting like a door hinge, knee or elbow), a pivot (a peg turning inside a ring, atlas and axis), a gliding (two flat surfaces sliding, between carpals) and a saddle (two saddle-shaped surfaces at right angles, carpal and metacarpal of thumb). Draw one of them in section showing the synovial cavity with synovial fluid, the articular cartilage on the bone ends and the joint capsule, since that section is what defines a synovial joint.
TextbookNCERT questions the board keeps asking
1 Draw the diagram of a sarcomere of skeletal muscle showing different regions.3 m
Draw a myofibril segment between two Z lines and label: Z line (at both ends; the sarcomere lies between two successive Z lines), thin (actin) filaments attached to each Z line, thick (myosin) filaments in the middle with cross arms, the A band (dark, anisotropic; the full length of the thick filaments), the I band (light, isotropic; thin filaments only, bisected by the Z line), the H zone (the central part of the A band with thick filaments only), and the M line (a thin membrane at the centre of the A band holding the thick filaments). Write below it: sarcomere = the functional unit of contraction; on contraction the I band and H zone shorten while the A band stays constant.
2 Define sliding filament theory of muscle contraction.2 m
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. The myosin heads form cross bridges with actin using energy from ATP hydrolysis and pull the thin filaments towards the centre of the A band. The Z lines attached to the thin filaments are drawn inwards, so the sarcomere shortens; the I bands and H zone become shorter while the A band keeps its length, and neither filament changes its own length.
3 Describe the important steps in muscle contraction.5 m
1. A signal from the central nervous system travels down a motor neuron to the neuromuscular junction (motor end plate); a motor neuron with its muscle fibres is a motor unit.
2. The neurotransmitter acetylcholine is released at the junction and generates an action potential in the sarcolemma.
3. The action potential spreads through the fibre and causes the sarcoplasmic reticulum to release Ca2+ into the sarcoplasm.
4. Ca2+ binds to a subunit of troponin on the actin filament, removing the masking of the active sites for myosin.
5. Using energy from ATP hydrolysis the myosin head binds to the exposed active site on actin to form a cross bridge.
6. The cross bridge pulls the actin filament towards the centre of the A band; the Z line is pulled inwards and the sarcomere shortens (I band and H zone shorten, A band unchanged).
7. Myosin releases ADP and Pi and returns to its relaxed state; a new ATP binds and the cross bridge breaks; the ATP is hydrolysed again and the cycle of cross bridge formation and breakage repeats, causing further sliding.
8. When the stimulus stops, Ca2+ is pumped back into the sarcoplasmic cisternae, troponin masks the active sites again, the Z lines return to their original positions and the muscle relaxes.
4 Write true or false. If false change the statement so that it is true. (a) Actin is present in thin filament. (b) H-zone of striated muscle fibre represents both thick and thin filaments. (c) 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.5 m
(a) True. The thin filament is made of two F actins with tropomyosin and troponin.
(b) False. The H zone of a striated muscle fibre represents only thick (myosin) filaments; it is the central part of the A band not overlapped by thin filaments.
(c) True. 80 axial + 126 appendicular = 206.
(d) False. There are 12 pairs of ribs in man (7 true, 3 vertebrochondral, 2 floating).
(e) True. The sternum is a flat bone on the ventral midline of the thorax.
5(a) Write the difference between actin and myosin.3 m
Actin filament: thin filament; found in the I band and extends into the A band; attached to the Z line; made of two helically wound F actins (polymers of G actin) with two tropomyosin strands and troponin; has active sites for myosin, masked by troponin at rest; no enzyme activity; it is the filament that slides during contraction.
Myosin filament: thick filament; confined to the A band; held at the centre by the M line; a polymer of meromyosin units each with a head (HMM, cross arm) and tail (LMM); the head is an ATPase with binding sites for ATP and actin; it stays in place and pulls the actin filaments by cross bridges.
5(b) Write the difference between red and white muscles.3 m
Red muscle fibres: high myoglobin content, so reddish; plenty of mitochondria; less sarcoplasmic reticulum; obtain ATP aerobically using the O2 stored by myoglobin; contract slowly and can sustain contraction for long without fatigue (aerobic muscles); example: postural muscles of the back.
White muscle fibres: very little myoglobin, so pale; few mitochondria; high amount of sarcoplasmic reticulum; depend on anaerobic processes for energy; contract rapidly and powerfully but fatigue quickly as lactic acid accumulates; example: eye muscles, sprinting muscles.
5(c) Write the difference between pectoral and pelvic girdle.3 m
Pectoral girdle: each half made of a clavicle and a scapula; the socket is the shallow glenoid cavity below the acromion of the scapula; articulates with the head of the humerus (forelimb); the two halves do not meet ventrally; loosely attached to the axial skeleton, giving the arm a wide range of movement; not weight-bearing.
Pelvic girdle: each half is a single coxal bone formed by the fusion of ilium, ischium and pubis; the socket is the deep acetabulum at the point of fusion; articulates with the head of the femur (hind limb); the two halves meet ventrally at the pubic symphysis (fibrous cartilage); firmly attached to the sacrum; bears the weight of the body.
6 Match Column I with Column II: (a) Smooth muscle, (b) Tropomyosin, (c) Red muscle, (d) Skull; with (i) Myoglobin, (ii) Thin filament, (iii) Sutures, (iv) Involuntary.2 m
(a) Smooth muscle – (iv) Involuntary
(b) Tropomyosin – (ii) Thin filament
(c) Red muscle – (i) Myoglobin
(d) Skull – (iii) Sutures
7 What are the different types of movements exhibited by the cells of human body?3 m
Three types. Amoeboid movement: by pseudopodia formed through streaming of protoplasm, with the help of microfilaments; shown by macrophages and leucocytes in blood. Ciliary movement: by the coordinated beating of cilia of the ciliated epithelium lining most internal tubular organs; it removes dust particles and foreign substances from the trachea and moves the ova through the female reproductive tract. Muscular movement: by the contraction of muscles; moves the limbs, jaws, tongue and so on, and is used for locomotion, which needs coordinated activity of the muscular, skeletal and neural systems. (Flagellar movement of the sperm tail is also listed in the syllabus.)
8 How do you distinguish between a skeletal muscle and a cardiac muscle?3 m
Skeletal muscle: attached to the bones of the skeleton; striated; voluntary, controlled by the somatic nervous system; fibres long, cylindrical and unbranched; multinucleate (syncitium) with peripheral nuclei; no intercalated discs; fatigues on repeated activation; used for locomotion and posture.
Cardiac muscle: found only in the wall of the heart; striated; involuntary, not under direct control of the nervous system; cells short and branched, joined in a branching pattern by intercalated discs; usually one central nucleus per cell; contracts rhythmically throughout life without fatigue.
9 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.3 m
(a) Atlas/axis: pivot joint (synovial).
(b) Carpal/metacarpal of thumb: saddle joint (synovial).
(c) Between phalanges: hinge joint (synovial).
(d) Femur/acetabulum: ball and socket joint (synovial).
(e) Between cranial bones: fibrous joint (suture); immovable.
(f) Between pubic bones in the pelvic girdle: cartilaginous joint (pubic symphysis, with fibrous cartilage).
10 Fill in the blank spaces: (a) All mammals (except a few) have ____ cervical vertebrae. (b) The number of phalanges in each limb of human is ____. (c) Thin filament of myofibril contains 2 F actins and two other proteins namely ____ and ____. (d) In a muscle fibre Ca2+ is stored in ____. (e) ____ and ____ pairs of ribs are called floating ribs. (f) The human cranium is made of ____ bones.3 m
(a) seven
(b) 14
(c) tropomyosin and troponin
(d) sarcoplasmic reticulum
(e) 11th and 12th
(f) 8
SA What is the difference between the matrix of bones and cartilage?2 m
Both are specialised connective tissues. The matrix of bone is very hard because it contains calcium salts (mainly calcium phosphate), which makes bone rigid and able to bear weight and protect organs. The matrix of cartilage is slightly pliable because it contains chondroitin salts, so cartilage can bend and cushion, as in the ear pinna, the tip of the nose, the intervertebral discs and the costal cartilages joining the ribs to the sternum.
LA Explain the role of calcium ions in muscle contraction. How does a fall in calcium level produce tetany?5 m
Role of Ca2+: the action potential arriving through the sarcolemma causes the sarcoplasmic reticulum to release Ca2+ into the sarcoplasm. Ca2+ binds to a subunit of troponin on the thin filament; this changes the position of the troponin–tropomyosin complex and unmasks the active sites for myosin on actin. Myosin heads can now form cross bridges, hydrolyse ATP and pull the actin filaments, so the sarcomere shortens. When Ca2+ is pumped back into the sarcoplasmic cisternae, troponin masks the sites again and the muscle relaxes. Without Ca2+ the cross bridges cannot form at all, however much ATP is present.
Tetany: when the Ca2+ level in the body fluid falls (for example when the parathyroid glands secrete too little parathormone), the membranes of nerve and muscle cells become over-excitable and fire spontaneously, producing rapid spasms or wild, sustained contractions of the muscles. NCERT defines tetany as rapid spasms (wild contractions) in muscle due to low Ca2+ in body fluid.
Previous years and frequently askedBoard questions with model answers
MCQ The functional unit of contraction in a skeletal muscle fibre is the- Myofibril
- Sarcomere
- A band
- Motor unit
Frequently asked1 m
MCQ During muscle contraction which of the following does NOT change in length?- I band
- H zone
- A band
- Sarcomere
Frequently asked1 m
MCQ In a muscle fibre calcium ions are stored in the- Sarcolemma
- Mitochondria
- Sarcoplasmic reticulum
- Troponin
Frequently asked1 m
MCQ The ATPase activity needed for muscle contraction is located in- Actin
- Troponin
- Tropomyosin
- Myosin head
Frequently asked1 m
MCQ The type of joint between the atlas and the axis is a- Hinge joint
- Pivot joint
- Saddle joint
- Gliding joint
Frequently asked1 m
MCQ Which of the following is a striated but involuntary muscle?- Muscle of the biceps
- Muscle of the wall of the intestine
- Muscle of the heart
- Muscle of the urinary bladder wall
Frequently asked1 m
MCQ The total number of bones in one human forelimb (excluding the girdle) is- 26
- 28
- 30
- 32
Frequently asked1 m
MCQ Which pairs of ribs are called floating ribs?- 1st to 7th
- 8th to 10th
- 11th and 12th
- 9th to 12th
Frequently asked1 m
MCQ The three bones that fuse to form the coxal bone are- Ilium, ischium and sacrum
- Ilium, ischium and pubis
- Ischium, pubis and coccyx
- Ilium, pubis and acetabulum
Frequently asked1 m
MCQ Gout is a disorder caused by- Low Ca2+ in body fluid
- Accumulation of uric acid crystals in joints
- Decreased estrogen level
- Antibodies against the neuromuscular junction
Frequently asked1 m
MCQ Which of the following statements about the thin filament is correct?- It is made of meromyosin units
- It is confined to the A band
- Its active sites for myosin are masked by troponin in the resting state
- It is held at the centre by the M line
Frequently asked1 m
Assertion–Reason Assertion (A): During contraction of a skeletal muscle the I band shortens but the A band does not. Reason (R): Thin filaments slide over the thick filaments and neither filament changes its own length.- Both A and R are true and R is the correct explanation of A
- Both A and R are true but R is not the correct explanation of A
- A is true but R is false
- A is false but R is true
Frequently asked1 m
Assertion–Reason Assertion (A): A skeletal muscle fibre is described as a syncitium. Reason (R): The sarcoplasm of a muscle fibre contains many nuclei.- Both A and R are true and R is the correct explanation of A
- Both A and R are true but R is not the correct explanation of A
- A is true but R is false
- A is false but R is true
Frequently asked1 m
Assertion–Reason Assertion (A): White muscle fibres fatigue quickly during vigorous exercise. Reason (R): White fibres have abundant myoglobin and many mitochondria.- Both A and R are true and R is the correct explanation of A
- Both A and R are true but R is not the correct explanation of A
- A is true but R is false
- A is false but R is true
Frequently asked1 m
Assertion–Reason Assertion (A): Myasthenia gravis leads to weakening and paralysis of skeletal muscle. Reason (R): It is an autoimmune disorder that affects the neuromuscular junction.- Both A and R are true and R is the correct explanation of A
- Both A and R are true but R is not the correct explanation of A
- A is true but R is false
- A is false but R is true
Frequently asked1 m
Assertion–Reason Assertion (A): The human skull is called dicondylic. Reason (R): The skull articulates with the first vertebra, the atlas, by means of two occipital condyles.- Both A and R are true and R is the correct explanation of A
- Both A and R are true but R is not the correct explanation of A
- A is true but R is false
- A is false but R is true
Frequently asked1 m
Very short answer Distinguish between movement and locomotion with one example of each.Frequently asked2 m
Movement is any change in the position of a part of the body or of the whole organism; it includes protoplasmic streaming, beating of cilia, peristalsis and the heartbeat. Locomotion is a voluntary movement that results in a change of place or location of the whole organism, such as walking, swimming or flying. All locomotions are movements but all movements are not locomotions: the movement of food along the gut by peristalsis is a movement but not locomotion; a bird flying is both.
Very short answer What is a motor unit? Name the neurotransmitter released at the neuromuscular junction.Frequently asked2 m
A motor unit is a motor neuron together with all the muscle fibres connected to it. The junction between the motor neuron and the sarcolemma of the muscle fibre is the neuromuscular junction or motor end plate, where the neurotransmitter acetylcholine is released to generate an action potential in the sarcolemma.
Very short answer Name the ear ossicles in order from the ear drum inwards. Which of them is the smallest bone of the human body?Frequently asked2 m
Malleus, incus and stapes, in that order from the tympanic membrane to the oval window of the inner ear. Each middle ear has three, giving six in all, and they are counted with the skull in the axial skeleton. The stapes is the smallest bone of the body.
Very short answer Why are ribs called bicephalic? What is the rib cage made of?Frequently asked2 m
Each rib is a thin flat bone with two articulation surfaces (two heads) on its dorsal end by which it attaches to the vertebral column, hence it is called bicephalic. The rib cage is formed by the 12 thoracic vertebrae, the 12 pairs of ribs and the sternum together.
Very short answer What is muscle fatigue? State its cause.Frequently asked2 m
Fatigue is the reduced ability of a muscle to contract after repeated activation. It is caused by the accumulation of lactic acid in the muscle, formed by the anaerobic breakdown of glycogen when the supply of oxygen cannot keep up with the demand of the working muscle.
Very short answer Name the disorder in each case: (a) inflammation of joints due to uric acid crystals, (b) rapid spasms of muscle due to low calcium, (c) decreased bone mass in old age, (d) progressive degeneration of skeletal muscle of genetic origin.Frequently asked2 m
(a) Gout
(b) Tetany
(c) Osteoporosis
(d) Muscular dystrophy
Short answer Describe the structure of a thin filament and a thick filament of a myofibril.Frequently asked3 m
Thin (actin) filament: made of two F (filamentous) actins helically wound around each other; each F actin is a polymer of monomeric G (globular) actin. Two strands of tropomyosin run close to the F actins along their length, and 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.
Thick (myosin) filament: a polymer of many monomeric proteins called meromyosins. Each meromyosin has a globular head with a short arm (the heavy meromyosin, HMM) and a tail (the light meromyosin, LMM). The heads with their short arms project outwards from the surface of the filament at regular distances and angles and are called cross arms. The globular head is an active ATPase enzyme and carries binding sites for ATP and active sites for actin.
Short answer Give the vertebral formula of humans. State three functions of the vertebral column.Frequently asked3 m
The vertebral column has 26 vertebrae: cervical 7, thoracic 12, lumbar 5, sacral 1 (formed by fusion of 5) and coccygeal 1 (formed by fusion of 4), from the base of the skull downwards. The first vertebra, the atlas, articulates with the occipital condyles of the skull.
Functions: (i) each vertebra has a neural canal through which the spinal cord passes, so the column protects the spinal cord; (ii) it supports the head and forms the main framework of the trunk; (iii) it serves as the point of attachment for the ribs and the musculature of the back.
Short answer Classify joints on the basis of the degree of movement they allow, with one example of each.Frequently asked3 m
Joints are points of contact between bones, or between bones and cartilages, and are of three structural forms.
(i) Fibrous joints allow no movement: the flat bones of the skull are fused end to end by dense fibrous connective tissue as sutures to form the cranium.
(ii) Cartilaginous joints join bones with cartilage and permit limited movement: the joint between adjacent vertebrae (intervertebral disc), and the pubic symphysis.
(iii) Synovial joints have a fluid-filled synovial cavity between the articulating surfaces and allow considerable movement: ball and socket (shoulder), hinge (knee), pivot (atlas–axis), gliding (carpals) and saddle (thumb).
Short answer Describe the pectoral girdle of humans.Frequently asked3 m
The pectoral girdle attaches the forelimb to the axial skeleton. It has two halves, each made of a clavicle and a scapula. The scapula is a large, flat, triangular bone lying in the dorsal part of the thorax between the second and seventh ribs. Its dorsal surface bears a raised ridge, the spine, which ends in a flat expanded process, the acromion; the clavicle articulates with the acromion. Below the acromion is a depression, the glenoid cavity, which receives the head of the humerus to form the shoulder joint, a ball and socket joint. The clavicle or collar bone is a long slender bone with two curvatures that links the scapula to the sternum. Because the girdle is loosely attached to the axial skeleton, the arm has a wide range of movement.
Short answer Distinguish between the three types of muscles found in the human body.Frequently asked3 m
Skeletal muscle: attached to bones; striated; voluntary; long cylindrical multinucleate fibres; used for locomotion and posture.
Visceral (smooth) muscle: in the walls of hollow organs such as the alimentary canal and the reproductive tract; non-striated; involuntary; spindle-shaped uninucleate cells; moves food through the gut and gametes through the genital tract.
Cardiac muscle: in the heart wall only; striated; involuntary; short branched cells joined in a branching pattern, usually one nucleus; contracts rhythmically without fatigue.
Long answer Draw a labelled diagram of a sarcomere and explain the sliding filament theory of muscle contraction.Frequently asked5 m
Diagram: a segment of myofibril between two Z lines showing thin actin filaments attached to the Z lines and thick myosin filaments in the middle with cross arms; label Z line, A band, I band, H zone, M line, actin, myosin and the sarcomere (Z to Z).
Sliding filament theory: contraction takes place by the sliding of the thin filaments over the thick filaments. (1) A signal from the CNS reaches the neuromuscular junction through a motor neuron and acetylcholine is released, generating an action potential in the sarcolemma. (2) The action potential spreads through the fibre and Ca2+ is released from the sarcoplasmic reticulum into the sarcoplasm. (3) Ca2+ binds to a subunit of troponin and removes the masking of the active sites for myosin on actin. (4) The myosin head, using energy from ATP hydrolysis, binds to the exposed site and forms a cross bridge, which pulls the actin filament towards the centre of the A band; the Z lines move inwards and the sarcomere shortens, the I band and H zone shortening while the A band keeps its length. (5) Myosin releases ADP and Pi, a new ATP binds, the cross bridge breaks, ATP is hydrolysed again and the cycle repeats, giving further sliding. (6) When Ca2+ is pumped back into the sarcoplasmic cisternae, troponin masks actin again, the Z lines return to their original positions and the muscle relaxes.
Long answer Describe the axial skeleton of humans, giving the number of bones in each part.Frequently asked5 m
The axial skeleton has 80 bones lying along the main axis of the body: the skull, vertebral column, sternum and ribs.
Skull: 8 cranial bones forming the cranium that protects the brain, and 14 facial bones forming the front of the skull, 22 in all; the single U-shaped hyoid bone at the base of the buccal cavity is included with the skull, and each middle ear has three ear ossicles (malleus, incus, stapes), 6 in all. The skull articulates with the vertebral column by two occipital condyles and is therefore dicondylic.
Vertebral column: 26 vertebrae placed dorsally, each with a neural canal for the spinal cord: cervical 7 (the first is the atlas, which meets the occipital condyles), thoracic 12, lumbar 5, sacral 1 (fused) and coccygeal 1 (fused). It protects the spinal cord, supports the head and gives attachment to the ribs and the back muscles.
Sternum: a flat bone on the ventral midline of the thorax, 1.
Ribs: 12 pairs of thin flat bicephalic bones attached dorsally to the thoracic vertebrae. Pairs 1 to 7 (true ribs) reach the sternum through hyaline cartilage; pairs 8, 9 and 10 (vertebrochondral or false ribs) join the seventh rib by cartilage; pairs 11 and 12 (floating ribs) are free ventrally. Thoracic vertebrae, ribs and sternum together form the rib cage. Total: 22 + 1 + 6 + 26 + 1 + 24 = 80.
Long answer Describe the appendicular skeleton of humans with a labelled diagram of the pelvic girdle.Frequently asked5 m
The appendicular skeleton (126 bones) is made of the limbs and the girdles that attach them to the axial skeleton. Each limb has 30 bones. Forelimb: humerus, radius and ulna, 8 carpals (wrist), 5 metacarpals (palm) and 14 phalanges (digits). Hind limb: femur (the longest bone), tibia and fibula, 7 tarsals (ankle), 5 metatarsals, 14 phalanges and the cup-shaped patella covering the knee ventrally.
Pectoral girdle: each half is a clavicle (collar bone, with two curvatures) and a scapula (a flat triangular bone between the 2nd and 7th ribs whose spine ends in the acromion; below it the glenoid cavity receives the head of the humerus at the shoulder joint).
Pelvic girdle: two coxal bones, each formed by the fusion of ilium, ischium and pubis; at the point of fusion is the acetabulum, which receives the head of the femur at the hip joint; the two halves meet ventrally at the pubic symphysis, which contains fibrous cartilage.
Diagram: the two coxal bones as a basin from the front; label ilium, ischium, pubis, acetabulum, pubic symphysis and the sacrum between the ilia. Count: 4 × 30 + 4 + 2 = 126; with the 80 axial bones the skeleton has 206.
Long answer Write short notes on the disorders of the muscular and skeletal systems mentioned in NCERT.Frequently asked5 m
Myasthenia gravis: an autoimmune disorder affecting the neuromuscular junction, in which antibodies block acetylcholine receptors; it leads to fatigue, weakening and paralysis of skeletal muscle.
Muscular dystrophy: progressive degeneration of skeletal muscle, mostly due to a genetic disorder; the fibres waste away and are replaced by fat and connective tissue.
Tetany: rapid spasms (wild contractions) in muscle due to low Ca2+ in the body fluid, as in parathormone deficiency.
Arthritis: inflammation of joints, with pain, swelling and stiffness; osteoarthritis and rheumatoid arthritis are the common forms.
Osteoporosis: an age-related disorder characterised by decreased bone mass and increased chances of fractures; decreased levels of estrogen is a common cause, so it is frequent in women after menopause.
Gout: inflammation of joints due to accumulation of uric acid crystals, often in the joint of the big toe.
Case-based Two athletes train for a school meet: Priya runs the 100 m sprint and Amit runs the 5 km race. Priya's coach notes that her leg muscles produce very powerful, rapid contractions but tire within seconds, while Amit can keep a steady pace for half an hour. A biopsy would show that the muscles of the two athletes differ in the amount of a red pigment, in the number of mitochondria and in the amount of sarcoplasmic reticulum.
(i) Name the red pigment and state its function.
(ii) Which type of muscle fibre predominates in Priya's sprint muscles and in Amit's endurance muscles?
(iii) Explain why Priya's muscles tire within seconds.
(iv) Which athlete's fibres have more sarcoplasmic reticulum, and why is that useful?Frequently asked4 m
(i) Myoglobin, an oxygen-storing pigment in muscle that holds a reserve of O2 for the mitochondria.
(ii) Priya: white fibres (little myoglobin, few mitochondria, anaerobic). Amit: red fibres (much myoglobin, many mitochondria, aerobic).
(iii) White fibres depend on the anaerobic breakdown of glycogen for ATP; repeated activation makes lactic acid accumulate in them quickly, causing fatigue.
(iv) Priya's white fibres have more sarcoplasmic reticulum. A larger store of Ca2+ can be released and pumped back faster, allowing the rapid, powerful contractions a sprint needs.
Case-based A 68-year-old woman slips in her bathroom and fractures the neck of her femur. The doctor tells the family that her bones have lost mass over the years since her menopause, that the joint involved is the one where the head of the femur fits into a cup of the hip bone, and that this cup is formed where three bones fused during childhood.
(i) Name the disorder that made the fracture likely and give its common cause.
(ii) Name the cup and the three bones that fuse to form it.
(iii) What type of joint is the hip joint? Name one other joint of the same type.
(iv) Name the girdle involved and state how its two halves meet in the midline.Frequently asked4 m
(i) Osteoporosis: an age-related disorder with decreased bone mass and increased chances of fractures; decreased estrogen level after menopause is a common cause.
(ii) The acetabulum; it lies at the point of fusion of the ilium, ischium and pubis, which together form the coxal bone.
(iii) A ball and socket synovial joint; the shoulder joint between the head of the humerus and the glenoid cavity is another.
(iv) The pelvic girdle, made of two coxal bones; the two halves meet ventrally at the pubic symphysis, which contains fibrous cartilage.
Case-based An electron micrograph of a resting sarcomere shows a dark band 1.6 μm long with a paler central region 0.4 μm wide, flanked on each side by a light band, the Z lines being 2.4 μm apart. After the muscle is stimulated the Z lines are found 2.0 μm apart.
(i) Name the dark band, the paler central region and the light band.
(ii) What is the length of the dark band after contraction? Justify.
(iii) What happens to the paler central region and the light bands on contraction?
(iv) Which ion and which protein on the thin filament made the contraction possible?Frequently asked4 m
(i) The dark band is the A band (anisotropic); the paler central region is the H zone; the light band is the I band (isotropic).
(ii) Still 1.6 μm. The A band is the length of the thick myosin filaments, and the filaments do not shorten; the thin filaments slide over them.
(iii) Both shorten. The thin filaments slide 0.2 μm inwards from each side, so the H zone (thick filaments only) narrows from 0.4 μm to zero and each I band (thin filaments only) shortens from 0.4 μm to 0.2 μm; the two I bands together lose 0.4 μm, which matches the Z lines moving from 2.4 to 2.0 μm.
(iv) Ca2+ released from the sarcoplasmic reticulum binds to a subunit of troponin, unmasking the active sites on actin so that myosin heads can form cross bridges.
Test yourselfPractice quiz
Marks lost every yearCommon mistakes
- Writing that the A band shortens during contraction. The A band equals the length of the thick filaments and stays constant; the I band and the H zone shorten. This single line decides a mark in nearly every paper.
- Saying that the filaments shorten. In the sliding filament theory neither actin nor myosin changes length; the sarcomere shortens because the overlap increases. Use the word 'slide' in the answer.
- Confusing troponin with tropomyosin. In NCERT wording it is a subunit of troponin that masks the active sites on actin at rest and that binds Ca2+; tropomyosin is the strand it sits on.
- Calling cardiac muscle non-striated. Cardiac muscle is striated and involuntary; only visceral (smooth) muscle is non-striated. Similarly, the muscles of the alimentary canal are smooth, not striated.
- Getting the bone counts wrong: 12 pairs of ribs (not 11), 8 cranial and 14 facial bones, 7 cervical vertebrae, 8 carpals but 7 tarsals, 14 phalanges per limb, 30 bones per limb, 26 vertebrae (not 33; the sacral and coccygeal vertebrae are counted as one each because they are fused).
- Mixing up the sockets and the girdles: the glenoid cavity is on the scapula (pectoral) for the humerus, the acetabulum is on the coxal bone (pelvic) for the femur. The acromion is a process of the scapula, not a cavity.
- Naming the joint without its class. Exercise 9 wants 'fibrous joint' for cranial bones and 'cartilaginous joint' for the pubic symphysis; writing 'gliding' or 'hinge' for those is wrong because they are not synovial.
- Attributing red fibres to anaerobic respiration or giving white fibres the higher myoglobin. Red = myoglobin rich, mitochondria rich, aerobic, slow to fatigue; white = the reverse but with more sarcoplasmic reticulum.
- Writing lactic acid for gout or uric acid for fatigue. Gout is uric acid crystals in joints; fatigue is lactic acid from anaerobic breakdown of glycogen in muscle. And tetany is not tetanus.
- Drawing the sarcomere without the H zone and M line, or placing the Z line in the A band. The Z line bisects the I band and bounds the sarcomere; the M line and H zone sit in the centre of the A band.
How to prepareStrategy for this chapter
This chapter is two lists and one mechanism, so split your time that way. Spend the first sitting on the muscle half: draw the sarcomere from memory until you can label Z line, A band, I band, H zone and M line without looking, then write the sliding filament theory as eight numbered steps and check each against NCERT. Draw the thin filament (two F actins, tropomyosin, troponin) and the meromyosin unit (head, short arm, tail, ATPase) once each. Those three drawings plus the steps are the 5-mark question in almost every school final and the source of the standard NEET question.
Spend the second sitting on the skeleton. Memorise the numbers as sums, not as isolated facts: 80 axial = 22 + 1 + 6 + 26 + 1 + 24, and 126 appendicular = 4 × 30 + 4 + 2, with the 30 of each limb broken into its five parts. Then learn the six sites of exercise 9 with the type and class of joint at each, and the six disorders as one-line definitions with the key word in each.
For the 3-mark 'differentiate' questions (skeletal, visceral and cardiac muscle; actin and myosin; red and white fibres; pectoral and pelvic girdle) prepare a table of four or five rows each; the examiner marks rows, not paragraphs. In the paper, draw every diagram in pencil with ruled label lines and give each diagram a title. If a question says 'describe', write in steps or bullet points so that the marking points stand out. Ten minutes of the night before the exam should go to reciting the bone counts and the joint sites; the rest of this chapter rewards understanding the mechanism, which does not need cramming once the drawings are in your hand.