Summary and Exam Tips

The Chapter in One Page

Movement is one of the significant features of living beings. Cells of the human body exhibit three main types of movement - AMOEBOID, CILIARY and MUSCULAR. Amoeboid movement is shown by some specialised cells like macrophages and leucocytes in blood; it is effected by PSEUDOPODIA formed by the streaming of protoplasm, as in Amoeba, and cytoskeletal elements like MICROFILAMENTS are also involved. Ciliary movement occurs in most of our internal tubular organs which are lined by ciliated epithelium; the coordinated movements of cilia in the trachea help in removing dust particles and some of the foreign substances inhaled along with the atmospheric air, and the passage of ova through the female reproductive tract is also facilitated by ciliary movement. Muscular movement moves our limbs, jaws and tongue. Locomotion is a voluntary movement that causes a change of place, and movements which result in a change of place or location are called locomotion - so all locomotion is movement, but not all movement is locomotion. Locomotion requires a perfect coordinated activity of the MUSCULAR, SKELETAL and NEURAL systems. Flagellar movement, which the chapter names separately, helps in the swimming of spermatozoa, in maintaining the water current in the canal system of sponges, and in the locomotion of protists like Euglena.

Muscle is a specialised tissue of MESODERMAL origin, and it contributes about 40-50 per cent of the body weight of a human adult. Muscle has four properties - EXCITABILITY, CONTRACTILITY, EXTENSIBILITY and ELASTICITY. Muscles are classified using three criteria - LOCATION, APPEARANCE and the NATURE OF REGULATION of their activities. Based on location, three types are identified - SKELETAL, VISCERAL and CARDIAC. SKELETAL muscles are closely associated with the skeletal components of the body; they show a striped appearance under the microscope and are therefore called STRIATED muscles; their activities are under voluntary control, so they are also called VOLUNTARY muscles; and they are primarily involved in locomotory actions and changes of body posture. VISCERAL muscles are located in the inner walls of visceral organs of the body such as the alimentary canal and the reproductive tract; they do not exhibit any striation and are therefore SMOOTH or NONSTRIATED muscles; their activities are not under voluntary control, so they are INVOLUNTARY muscles; and they assist in the transport 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; by appearance they are STRIATED, and yet they are INVOLUNTARY in nature because the nervous system does not control their activities directly. Cardiac muscle is the one type that is striated and involuntary at the same time.

Each organised skeletal muscle in our body is made of a number of MUSCLE BUNDLES or FASCICLES held together by a common collagenous connective tissue layer called FASCIA. Each muscle bundle contains a number of MUSCLE FIBRES, and each muscle fibre is lined by the plasma membrane called SARCOLEMMA enclosing the SARCOPLASM. A muscle fibre is a SYNCITIUM, as the sarcoplasm contains many nuclei. The endoplasmic reticulum of the muscle fibre is called the SARCOPLASMIC RETICULUM, and it is the store house 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, caused by the distribution pattern of two important proteins - ACTIN and MYOSIN. The LIGHT bands contain ACTIN and are called ISOTROPIC or 'I' BANDS, whereas the DARK bands contain MYOSIN and are called ANISOTROPIC or 'A' BANDS. Actin filaments are thinner and are commonly called THIN FILAMENTS; myosin filaments are thicker and are called THICK FILAMENTS. In the centre of each 'I' band is an elastic fibre called the 'Z' LINE which bisects it, and 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 myofibrils. The portion of the myofibril between two successive 'Z' lines is considered as the functional unit of contraction and is called a SARCOMERE. In a resting state, the edges of the 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 free - this central part of the thick filament, not overlapped by thin filaments, is called the 'H' ZONE.

Each actin (thin) filament is made of two 'F' (filamentous) ACTINS helically wound to each other, and 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, 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 filaments. Each myosin (thick) filament is also a polymerised protein, and many monomeric proteins called MEROMYOSINS constitute one thick filament. Each meromyosin has two important parts - a globular head with a short arm, called the HEAVY MEROMYOSIN (HMM), and a tail, called the LIGHT MEROMYOSIN (LMM). The HMM component, that is the head and the short arm, projects outwards at regular distance and angle from each other from the surface of a 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.

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. Muscle contraction is initiated by a signal sent by the CENTRAL NERVOUS SYSTEM 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. A neural signal reaching this junction releases a neurotransmitter, ACETYL CHOLINE, which generates an ACTION POTENTIAL in the sarcolemma. This spreads through the muscle fibre and causes the release of calcium ions into the sarcoplasm. The increased $\mathrm{Ca^{2+}}$ 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. Utilising the energy from ATP hydrolysis, the myosin head 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, and the 'Z' lines attached to these actins are also pulled inwards, causing a SHORTENING OF THE SARCOMERE, that is contraction. The myosin, releasing the 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 head and the cycle of cross bridge formation and breakage is repeated, causing further sliding. The process continues till the $\mathrm{Ca^{2+}}$ 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. During the shortening of the muscle, the 'I' bands get REDUCED whereas the 'A' bands RETAIN THEIR LENGTH. The reaction time of the fibres can vary in different muscles, and repeated activation of the muscles can lead to the accumulation of LACTIC ACID due to the ANAEROBIC BREAKDOWN OF GLYCOGEN in them, causing FATIGUE.

Muscle contains a red coloured oxygen storing pigment called MYOGLOBIN. Muscles which have a high quantity of myoglobin give a reddish appearance and are called RED FIBRES; these muscles also contain plenty of mitochondria which can utilise the large amount of oxygen stored in them for ATP production, and are therefore also called AEROBIC MUSCLES. Some of the muscles possess a very less quantity of myoglobin and therefore appear pale or whitish - these are the WHITE FIBRES; the number of mitochondria is also low in them, but the amount of SARCOPLASMIC RETICULUM is HIGH, and they depend on ANAEROBIC PROCESS for energy. That sarcoplasmic reticulum row is the only feature of which white fibres have MORE than red fibres.

The SKELETAL SYSTEM consists of a framework of bones and a few cartilages. 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 this system is made up of 206 BONES and a few cartilages, and it is 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. The SKULL is composed of two sets of bones - CRANIAL and FACIAL - that totals to 22 BONES. Cranial bones are 8 in number and they form the hard protective outer covering, the CRANIUM, for the brain. The facial region is made up of 14 skeletal elements which form the front part of the skull. A single U-shaped bone called HYOID is present at the base of the buccal cavity. Each middle ear contains three tiny bones - MALLEUS, INCUS and STAPES - collectively called the EAR OSSICLES. The skull region articulates with the superior region of the vertebral column with the help of two OCCIPITAL CONDYLES, that is, it is DICONDYLIC. Our vertebral column 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 and it articulates with the occipital condyles. The vertebral column is differentiated into CERVICAL (7), THORACIC (12), LUMBAR (5), SACRAL (1 fused) and COCCYGEAL (1 fused) regions starting from the skull. 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 musculature of the back. 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. First seven pairs of ribs are called TRUE RIBS; dorsally they are attached to the thoracic vertebrae and ventrally connected to the sternum with the help of HYALINE CARTILAGE. The 8th, 9th and 10th pairs of ribs do not articulate directly with the sternum but join the seventh rib with the help of hyaline cartilage - these are called VERTEBROCHONDRAL or FALSE RIBS. Last 2 pairs (11th and 12th) of ribs are not connected ventrally and are therefore called FLOATING RIBS. Thoracic vertebrae, ribs and sternum together form the RIB CAGE.

The bones of the limbs along with their girdles constitute the APPENDICULAR SKELETON. Each limb is made of 30 BONES. The bones of the hand (fore limb) are HUMERUS, RADIUS and ULNA, CARPALS (wrist bones - 8 in number), METACARPALS (palm bones - 5 in number) and PHALANGES (digits - 14 in number). The bones of the legs (hind limb) are FEMUR (thigh bone - the longest bone), TIBIA and FIBULA, TARSALS (ankle bones - 7 in number), METATARSALS (5 in number) and PHALANGES (digits - 14 in number). A cup shaped bone called PATELLA covers the knee ventrally. PECTORAL and PELVIC GIRDLE bones help in the articulation of the upper and lower limbs respectively with the axial skeleton. Each girdle is formed of two halves. Each half of the pectoral girdle consists of a CLAVICLE and a SCAPULA. Scapula is a large triangular flat bone situated in the dorsal part of the thorax between the SECOND and the SEVENTH RIBS; it has a slightly elevated ridge called the SPINE which projects as a flat, expanded process called the ACROMION; the CLAVICLE articulates with this 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 commonly called the COLLAR BONE. PELVIC GIRDLE 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 the above bones is a cavity called ACETABULUM to which the thigh bone articulates. The two halves of the pelvic girdle meet ventrally to form the PUBIC SYMPHYSIS containing FIBROUS CARTILAGE.

JOINTS are points of contact between bones, or between bones and cartilages. Force generated by muscles is used to carry out movement through joints, where the joint acts as a FULCRUM. Joints have been classified into three major structural forms - FIBROUS, CARTILAGINOUS and SYNOVIAL. FIBROUS JOINTS do not allow any movement; the flat skull bones fuse end to end with the help of dense fibrous connective tissues in the form of SUTURES to form the cranium. In CARTILAGINOUS JOINTS the bones involved are joined together with the help of cartilages, and they permit limited movement - the joint between the adjacent vertebrae in the vertebral column is of this pattern. SYNOVIAL JOINTS are characterised by the presence of a fluid filled SYNOVIAL CAVITY between the articulating surfaces of the two bones, and this arrangement allows considerable movement, so these joints help in locomotion. The synovial joints named are the BALL AND SOCKET JOINT (between the humerus and the pectoral girdle), the HINGE JOINT (knee joint), the PIVOT JOINT (between the atlas and the axis), the GLIDING JOINT (between the carpals) and the SADDLE JOINT (between the carpal and metacarpal of the thumb).

Finally, the disorders. MYASTHENIA GRAVIS is an auto immune disorder affecting the neuromuscular junction, leading to fatigue, weakening and paralysis of skeletal muscle. MUSCULAR DYSTROPHY is a progressive degeneration of skeletal muscle mostly due to a genetic disorder. TETANY is rapid spasms - wild contractions - in muscle due to low calcium in body fluid. ARTHRITIS is inflammation of joints. OSTEOPOROSIS is an age-related disorder characterised by decreased bone mass and increased chances of fractures, and decreased levels of estrogen is a common cause. GOUT is inflammation of joints due to accumulation of uric acid crystals.

The Bone-Count Sheet

Every count in the chapter, in one place, arranged so that it adds up. The chapter states 206, states 80, and states 30 per limb - but it never adds anything up, so the two sums below are the work the chapter leaves to you, and they are what makes a forgotten count recoverable instead of lost.

The axial skeleton - it must come to 80

Check it: 22 + 1 + 6 + 26 + 1 + 24 = 80. The two rows that break the sum are the ear ossicles and the ribs, because both are given per side or per pair. Three ossicles in EACH middle ear is 6. Twelve PAIRS of ribs is 24. Enter 3 and 12 instead and you land on 65 and start doubting a count that was right.

The appendicular skeleton - it must come to 126

Check it: 60 + 60 + 4 + 2 = 126. The pelvic girdle contributes 2 bones, not 6 - the ilium, ischium and pubis are FUSED into one coxal bone, and it is the coxal bone that gets counted. Fusion removes bones from the count, exactly as it does in the vertebral column, where the sacral and coccygeal regions count as 1 fused unit each.

And the grand total

80 + 126 = 206. The three figures form a closed triangle and any one of them can be got from the other two. 206 - 80 = 126. The chapter's own sentence is that the system is made up of 206 bones AND A FEW CARTILAGES - the cartilages are not bones and are not inside the 206, so the hyaline cartilage joining the true ribs to the sternum and the fibrous cartilage of the pubic symphysis are not counted.

Now every individual count, in one list

Read that list for its collisions, because the paper builds its wrong options out of them. 8, 14 and 22 all belong to the skull. 22 and 26 are the skull and the vertebral column, in that order, and they are the pair most often swapped. 7 appears three times - 7 cervical vertebrae, 7 pairs of true ribs, 7 tarsals. 8 appears twice - 8 cranial bones and 8 carpals. 12 appears twice - 12 thoracic vertebrae and 12 pairs of ribs, and that is no coincidence, since each pair of ribs attaches to a thoracic vertebra. 5 appears three times - 5 lumbar vertebrae, 5 metacarpals, 5 metatarsals. 14 appears twice - 14 facial bones and 14 phalanges per limb. And the wrist against the ankle is the swap that costs most - carpals 8, tarsals 7.

Write the two sums in the margin before you answer a single skeleton question, and every count item becomes a lookup instead of a memory test. Recall fails silently under pressure; a sum tells you when it has gone wrong, because it stops adding to 80, or to 126, or to 206.

The Sarcomere Sheet

One line per region - what it is, what it contains, and what happens to it during contraction. This is the single most-asked figure in the chapter, and almost every item on it is one row of the table below read in one direction or the other.

Two rows carry the sheet. The 'I' band shortens and the 'A' band does not, and the reason is worth carrying rather than the fact: the 'A' band is the length of the thick filament, and no filament changes length. The 'I' band and the 'H' zone are the only two regions that shrink, and they shrink for the same reason - both are regions where the two sets of filaments do not yet overlap, and the overlap is what grows.

Say the two lines the right way round once and they will not swap again. The 'Z' line bisects the LIGHT 'I' band. The 'M' line sits in the middle of the DARK 'A' band. Z with I, M with A.

The Contraction Sequence

Ten steps, in order, one line each. This is asked as a sequence and marked as a sequence, so learn it as a numbered list and never as a paragraph. The two ATP events are marked, because that is where the marks hide.

1. The signal. The CNS sends a signal via a MOTOR NEURON - a motor neuron together with the muscle fibres connected to it is a MOTOR UNIT.
2. The junction. The signal reaches the NEUROMUSCULAR JUNCTION, also called the MOTOR-END PLATE - the junction between the motor neuron and the sarcolemma of the fibre.
3. The transmitter. ACETYL CHOLINE is released and generates an ACTION POTENTIAL in the sarcolemma.
4. The calcium out. The action potential spreads through the muscle fibre and causes the release of $\mathrm{Ca^{2+}}$ into the sarcoplasm.
5. The unmasking. Calcium binds a SUBUNIT OF TROPONIN on the actin filaments and REMOVES THE MASKING of the active sites for myosin.
6. The cross bridge - FIRST ATP EVENT. Using the energy from ATP HYDROLYSIS, the myosin head binds the exposed active sites on actin to form a CROSS BRIDGE.
7. The pull. The cross bridge pulls the attached actin filaments towards the CENTRE OF THE 'A' BAND; the 'Z' lines attached to those actins are pulled inwards, SHORTENING THE SARCOMERE - that is contraction.
8. The release - SECOND ATP EVENT. The myosin releases the ADP and Pi and goes back to its relaxed state; a NEW ATP BINDS and the CROSS BRIDGE IS BROKEN.
9. The repeat. The ATP is hydrolysed again by the myosin head, and the cycle of cross bridge formation and breakage repeats, causing further sliding.
10. The calcium back. The process continues till the $\mathrm{Ca^{2+}}$ ions are PUMPED BACK to the sarcoplasmic cisternae, MASKING the actin filaments again; the 'Z' lines return to their original position - that is RELAXATION.

Steps 6 to 9 are one cycle and the cycle repeats. A single cross bridge pulls the actin only a short way; the muscle shortens as much as it does because the cycle runs again and again.

The two ATP events do two different jobs, and a question will name one and offer the other. ATP HYDROLYSIS supplies the energy for the head to BIND actin and FORM the cross bridge. A NEW ATP BINDING is what BREAKS the cross bridge. What breaks the bridge is a new ATP, not calcium, and not the release of ADP.

The calcium runs in two directions and both are examinable. Released INTO the sarcoplasm to start contraction. Pumped BACK into the sarcoplasmic cisternae to end it.

The Mistakes That Cost Marks in This Chapter

1. Swapping the 'I' and the 'A' bands. This is the commonest single error in the chapter. The 'I' band is the LIGHT band, it is ISOTROPIC, and it contains ACTIN - the thin filament. The 'A' band is the DARK band, it is ANISOTROPIC, and it contains MYOSIN - the thick filament. The letters give you no help, so fix it by the pair that does: I is light and thin, A is dark and thick.
2. Saying the 'H' zone contains both filaments. It does not. The 'H' zone is the CENTRAL PART OF THE THICK FILAMENT that is NOT overlapped by the thin filaments, so it contains THICK FILAMENTS ONLY. It is the opposite of a region of overlap - it is exactly the part the thin filaments have not reached.
3. Saying the 'A' band shortens during contraction. The 'I' bands get REDUCED whereas the 'A' bands RETAIN THEIR LENGTH. Carry the reason and the fact comes with it: the 'A' band measures the thick filament, and the thick filament does not change length. The 'H' zone narrows too, and that is the harder version of the same question.
4. Swapping HMM and LMM. HEAVY MEROMYOSIN is the GLOBULAR HEAD WITH A SHORT ARM, and it is the part that projects outwards as the CROSS ARM. LIGHT MEROMYOSIN is the TAIL. Learn it as a table, because the words heavy and light will not tell you which is which.
5. Thinking the filaments themselves shorten. Nothing shortens by contracting. The thin filaments SLIDE over the thick filaments towards the centre of the 'A' band, and both sets keep their own length. What shortens is the distance between two successive 'Z' lines, and it shortens because the region of overlap increases. The name of the theory - the SLIDING filament theory - is the answer to this one.
6. Calling cardiac muscle voluntary. Cardiac muscle is STRIATED and yet INVOLUNTARY, because the nervous system does not control its activities directly. Striated does not mean voluntary - cardiac muscle is the one type that breaks that pattern, and that is exactly why it is the type most often asked.
7. Swapping carpals and tarsals. CARPALS are the WRIST bones and they number 8. TARSALS are the ANKLE bones and they number 7. Both counts also collide with other rows - 8 is also the cranial bones, 7 is also the cervical vertebrae and the pairs of true ribs - so say wrist 8, ankle 7 as one phrase.
8. Saying 11 pairs of ribs. There are 12 PAIRS of ribs, which is 24 individual bones. The 11th and 12th pairs are the FLOATING ribs - and that is where the 11 comes from, so a student who half-remembers the floating pairs reports 11 as the total. Twelve pairs, split 7 true, 3 false, 2 floating, and 7 + 3 + 2 = 12 is the check.
9. Calling the joint between the phalanges a gliding joint. The joints between the PHALANGES are HINGE joints. The GLIDING joint is between the CARPALS. The saddle joint, the third of the hand's joints, lies between the carpal and the metacarpal of the thumb - three different joints in the same hand, and each has its own address.
10. Calling the cranial sutures cartilaginous. The joints between the flat skull bones are FIBROUS - dense fibrous connective tissue in the form of SUTURES - and they allow NO movement at all. Keep them apart from the two cartilaginous joints the chapter names - between adjacent vertebrae, and the PUBIC SYMPHYSIS between the two pubic bones. The pubic symphysis is CARTILAGINOUS even though the cartilage in it is called fibrous cartilage - read the word AFTER "fibrous" before you answer.
11. Forgetting that white fibres have MORE sarcoplasmic reticulum. Four rows of the red-against-white table run the same way - red fibres have more myoglobin, a redder colour, more mitochondria and the aerobic route. The sarcoplasmic reticulum row runs the other way: the amount is LOW in red fibres and HIGH in white fibres. The standard wrong option gives white fibres less of everything, and it is wrong on exactly this one row.

Writing the Chapter-End Exercises Well

Class 11 has no board paper, but the chapter-end exercises and your school tests are still written answers, marked by a person reading for particular words. This chapter's exercise set has an unusually mixed shape - alongside the ordinary short-answer questions it carries a true-or-false exercise with five parts, a "write the difference" exercise with three parts, a "name the type of joint" exercise with six parts, a fill-in-the-blanks exercise with six parts, and a "draw the diagram" item. Each shape has its own way of being written, and getting the format right is worth as much as knowing the fact.

For a true-or-false item, write the verdict first and then the corrected statement in full. Open with the single word - True or False - so the marker sees your decision before anything else, and then, if the verdict is False, write out the corrected version of the WHOLE statement, not just the word that was wrong. "False. The number of phalanges in each limb is fourteen, not twelve." "False. The 'H' zone of a striated muscle fibre represents the central part of the thick filaments that is not overlapped by thin filaments, so it contains thick filaments only." That earns the verdict mark and the correction mark together. A bare "False" earns half of what was on offer, and a correction that only names the wrong word leaves the marker to write your answer for you. When the verdict is True, stop there or add one confirming clause - do not hedge a true statement, because a hedge reads as uncertainty and can cost a mark you had already earned.

For a fill-in-the-blank item, restate the whole sentence. Do not write the missing word on its own beside the letter of the part. Copy the sentence out and put the word in. "All mammals except a few have SEVEN cervical vertebrae." "The number of floating ribs in humans is TWO pairs." "Thin filament of myofibril contains 2 'F' actins and two other proteins, namely TROPOMYOSIN and TROPONIN." Two practical reasons. The first is that a restated sentence proves you understood the sentence rather than pattern-matched a word, which is what the marker is checking. The second is that a restated sentence gives you room to write both names where the chapter gives two - vertebrochondral or false ribs, myofilaments or myofibrils, neuromuscular junction or motor-end plate - and a marking scheme that accepts either will always accept both.

For a "name the joint" item, give the joint type AND the bones it lies between. The type on its own is half an answer. "A pivot joint, between the atlas and the axis." "A gliding joint, between the carpals." "A saddle joint, between the carpal and the metacarpal of the thumb." "A ball and socket joint, between the humerus and the pectoral girdle." "A hinge joint, between the phalanges." "A fibrous joint - the sutures between the flat bones of the skull." The extra clause takes six words, and it does two jobs: it shows the marker the type was not a guess, and it protects you if your type name is slightly off, because a correct address beside an imperfect name usually still scores.

For a "draw the diagram" item, label every region even if the drawing is rough - the labels carry the marks. You are not marked on the quality of the line. For a sarcomere, draw the two 'Z' lines first and label them, then the 'A' band in the middle with the 'M' line down its centre and the 'H' zone around that, then the two half 'I' bands one at each end, and mark which filament runs where - the thin actin filaments attached to the 'Z' lines and running inwards, the thick myosin filaments spanning the whole 'A' band. Every one of those six labels - 'Z' line, 'I' band, 'A' band, 'M' line, 'H' zone, sarcomere - is a separate mark. An unlabelled diagram in this chapter is worth almost nothing, and a rough diagram with all six labels beats a beautiful one with three.

Three habits that pay across the whole exercise set. When an exercise asks for a count, give the number with what it counts and with its unit of counting - 12 PAIRS of ribs, not 12 ribs; 3 ossicles in EACH middle ear, not 3 in all; 30 bones per LIMB - because the pair-or-single distinction is exactly what the marker is watching. When an exercise asks you to describe a process, write it in the order the events happen, one event per line, because the sequence is marked as well as the content - the ten steps of contraction from the signal in the motor neuron to the calcium pumped back. And when an exercise asks you to write the difference between two things, draw a two-column table and give the same features in the same order in both columns - actin against myosin, red fibres against white fibres, pectoral girdle against pelvic girdle - because a marker awards a difference only when both halves of it are on the page.

The Night Before - What to Revise, in Order

Read in this order and stop when the list runs out. Nothing new goes in tonight.

1. The bone-count sheet, and the two sums first. Twenty minutes, and the most valuable twenty in the chapter. Write out 22 + 1 + 6 + 26 + 1 + 24 = 80 and 60 + 60 + 4 + 2 = 126 from memory, check that 80 + 126 = 206, and only then read the list of individual counts. Double-check the two rows that get halved - ossicles 3 in each middle ear gives 6, ribs 12 pairs gives 24.

2. The contraction sequence, walked out loud from the top. Fifteen minutes. Motor neuron, neuromuscular junction, acetyl choline, action potential, calcium out, troponin unmasked, cross bridge formed, actin pulled in, new ATP breaks the bridge, calcium pumped back. Then walk it a second time saying only where the ATP is used and which way the calcium is going at each step. If you can do both walks without stopping, every muscle question in the paper is already answered.

3. The sarcomere sheet. Ten minutes. Say each region, then what it contains, then what happens to it during contraction - 'I' band, 'A' band, 'Z' line, 'M' line, 'H' zone, sarcomere. Then say the two sentences that settle the whole figure: the 'I' band shortens and the 'A' band does not, and the 'H' zone is thick filaments only.

4. The eleven mistakes above. Ten minutes. These are the marks you are most likely to lose while knowing the material perfectly well, which makes them the cheapest to save. Spend longest on the 'I' and 'A' bands, HMM against LMM, and carpals 8 against tarsals 7.

5. The two comparison tables. Ten minutes. Red against white fibres, five rows - myoglobin, colour, mitochondria, sarcoplasmic reticulum, aerobic or anaerobic - with the sarcoplasmic reticulum row the right way round. Pectoral against pelvic girdle - clavicle and scapula against a single coxal bone; acromion and glenoid cavity against acetabulum and pubic symphysis.

6. The joints and the disorders. Five minutes each. Fibrous, cartilaginous and synovial; then the five synovial joints with the bones each lies between. Then myasthenia gravis, muscular dystrophy, tetany, arthritis, osteoporosis and gout, one line each, with the giveaway word attached - auto immune, genetic, low calcium, joints, bone mass, uric acid crystals.

If you have ten minutes and no more, write the two bone sums and walk the ten steps of contraction once. This chapter rewards exact counts and a correctly ordered mechanism over everything else, and a student who can rebuild 80 and 126 and recite the sequence will out-score one who has read the whole chapter through again.