How to Use This Section
This chapter is one mechanism and one long list of counts, and the two halves are lost in completely different ways. The mechanism is lost by skipping a step; the counts are lost by writing a real number under the wrong owner. Almost every mark that goes missing in this chapter goes missing in one of those two ways.
Four habits will carry you through it.
First, treat the sliding filament theory as a walk, not as a paragraph. The signal from the central nervous system, the motor neuron, the neuromuscular junction, acetyl choline, the action potential, the release of calcium ions, the binding of calcium to a subunit of troponin, the unmasking of the active sites, the cross bridge, the pull, the new ATP that breaks the bridge, and the pumping back of the calcium. Learn it in that order and every question about it becomes a question about a place you can already find.
Second, say every swapped pair out loud with both halves in one sentence. Actin is thin and sits in the 'I' band, which is isotropic and light; myosin is thick and sits in the 'A' band, which is anisotropic and dark. HMM is the head with the short arm; LMM is the tail. During contraction the 'I' band is reduced while the 'A' band keeps its length. Red fibres have plenty of myoglobin and plenty of mitochondria and work aerobically; white fibres have very little myoglobin, few mitochondria, plenty of sarcoplasmic reticulum, and work anaerobically. Carpals are 8 and tarsals are 7. True ribs 7 pairs, false ribs 3 pairs, floating ribs 2 pairs. Each of those is a guaranteed distractor, and the only defence is having learnt both halves together.
Third, rebuild a bone count instead of recalling it. The axial skeleton is 22 plus 1 plus 6 plus 26 plus 1 plus 24, which is 80. The appendicular skeleton is 60 plus 60 plus 4 plus 2, which is 126. 80 plus 126 is 206. A recalled number fails silently; a sum tells you when it has gone wrong, because it stops adding up. Write those two sums in the margin at the start of the paper and every count question turns into a lookup.
Fourth, watch the two words that are stated per side or per pair. The ear ossicles are three in EACH middle ear, so six in all. The ribs are twelve PAIRS, so twenty-four bones. Those two rows are why a careful student adds the axial skeleton and gets 65 instead of 80.
The items below run in three tiers.
- Tier 1 - short recall. The definitions, the counts, the orders and the comparisons. Answer these aloud until none of them needs thinking about.
- Tier 2 - applied reasoning. A patient, a drug, a described bone or a set of figures is put in front of you and you have to work out what follows. Six of these are numerical items - the axial total deduced from the grand total, the carpal count deduced from the fore limb's thirty, the hand weighed against the foot, the appendicular sum, the rib arithmetic and the regions of the vertebral column - because those are the sums this chapter actually sets.
- Tier 3 - longer written answers. Full accounts written the way a written paper wants them, with the marking-scheme words in place.
One of the chapter-end exercises is answered here and nowhere else in the chapter - the match-the-column item. Every other set exercise is answered in full inside one of the thirteen teaching sections, and the last block of this section is a table telling you exactly which section and which question number holds each part. Work the exercises on paper first, then use that table to check yourself against a complete answer.
Tier 1 - Short Recall
Question 1
Q. Give the one-line test that separates locomotion from movement, and list the reasons the chapter gives for why animals move at all.
Answer. A voluntary movement which causes the animal to change its place is called LOCOMOTION. That sentence carries two tests, and a movement has to pass both.
| The test | What it asks |
|---|---|
| Is it voluntary? | an involuntary movement, such as the beating of the heart, is not locomotion |
| Does the animal change its place? | movement of the jaws, the eyelids or the tongue does not move the animal anywhere |
So all locomotion is movement, but not all movement is locomotion. Walking, running, climbing and flying are locomotion; blinking and chewing are movement only.
Animals move generally in search of FOOD, SHELTER, a MATE, a BREEDING GROUND, a BETTER CLIMATE, or to PROTECT THEMSELVES. That is a set of six and it is a one-mark answer on its own, so learn it as six items and not as "food and shelter and so on".
Question 2
Q. Name the three types of movement shown by the cells of the human body, give a place in the body where each is seen, and name the fourth kind of movement the chapter describes but keeps off that list.
Answer. Cells of the human body exhibit three main types of movements, namely AMOEBOID, CILIARY and MUSCULAR.
| Type | Where it is seen | How it is brought about |
|---|---|---|
| Amoeboid | macrophages and leucocytes in blood | pseudopodia formed by the streaming of protoplasm, with cytoskeletal elements like microfilaments involved |
| Ciliary | internal tubular organs lined by ciliated epithelium - the trachea, and the female reproductive tract | coordinated movements of cilia, which remove dust particles in the trachea and move the ova along the reproductive tract |
| Muscular | the limbs, the jaws and the tongue | the contractile property of muscles |
The fourth kind is FLAGELLAR movement, and the chapter names three places for it - the swimming of spermatozoa, maintaining the water current in the canal system of sponges, and the locomotion of protists like Euglena.
Read the stem before you answer. "Cells of the human body" wants three - amoeboid, ciliary and muscular. Flagellar movement is the standard fourth option offered, and it is not part of that set of three.
Question 3
Q. Write a data card for muscle as a tissue - its embryonic origin, the share of the body it makes up, and its properties.
| Answer. | Feature | The fact |
|---|---|---|
| Origin | a specialised tissue of MESODERMAL origin | |
| Share of the body | about 40-50 per cent of the body weight of a human adult | |
| Property 1 | excitability - it responds to a stimulus | |
| Property 2 | contractility - it can shorten and generate force | |
| Property 3 | extensibility - it can be stretched | |
| Property 4 | elasticity - it returns to its original length after being stretched |
Four properties, and contractility is only one of them. A stem asking which listed property muscle does NOT have is testing whether you know all four rather than just the obvious one. Mesodermal origin and 40-50 per cent of body weight are the other two guaranteed single-mark facts here, and the percentage is written out in words - forty to fifty per cent of the body weight of an adult, not of a child and not of the body volume.
Question 4
Q. Each muscle type carries a name from each of the three criteria of classification. Give all the names each type carries, and pick out the type that breaks the pattern.
Answer. Muscles are classified using LOCATION, APPEARANCE and the NATURE OF REGULATION of their activities. By location the three types are skeletal, visceral and cardiac, and each then earns its other names.
| Type | Location | Appearance | Regulation | What it does |
|---|---|---|---|---|
| Skeletal | closely associated with the skeletal components of the body | STRIATED | VOLUNTARY | locomotory actions and changes of body posture |
| Visceral | the inner walls of hollow visceral organs - alimentary canal, reproductive tract | SMOOTH or NONSTRIATED | INVOLUNTARY | transport of food through the digestive tract and of gametes through the genital tract |
| Cardiac | the heart | STRIATED | INVOLUNTARY - the nervous system does not control their activities directly | pumps the blood; its cells assemble in a BRANCHING pattern |
Cardiac muscle is the type that breaks the pattern - it is STRIATED and yet INVOLUNTARY. Striated does not mean voluntary, and that single line is why this comparison is set at all. The only feature skeletal and cardiac muscle share is that both are striated; they differ in location, in control, in branching and in function.
Question 5
Q. Name the connective tissue sheath, the plasma membrane, the cytoplasm and the calcium store of a skeletal muscle, and say why one muscle fibre has many nuclei.
Answer. The structural ladder runs muscle, then muscle bundle or fascicle, then muscle fibre, then myofibril, then sarcomere, and four names belong to the fibre and its wrappings.
| Name | What it is |
|---|---|
| Fascia | the common collagenous connective tissue layer holding the MUSCLE BUNDLES or FASCICLES together |
| Sarcolemma | the plasma membrane lining each muscle fibre |
| Sarcoplasm | the cytoplasm the sarcolemma encloses |
| Sarcoplasmic reticulum | the endoplasmic reticulum of the muscle fibre - the STORE HOUSE OF CALCIUM IONS |
In a muscle fibre is stored in the SARCOPLASMIC RETICULUM, and the favourite wrong answer is the sarcolemma, which is only the membrane.
A muscle fibre is a SYNCITIUM, because the sarcoplasm contains MANY NUCLEI. One fibre, many nuclei - that is exactly what the word means here.
Question 6
Q. Complete the chain for each of the two contractile proteins - protein, filament, band, the other name of that band, and how the band looks.
Answer. The striated appearance of a myofibril is due to the distribution pattern of two proteins, ACTIN and MYOSIN, and each protein owns one complete chain.
| Protein | Filament | Band | Other name of the band | Appearance |
|---|---|---|---|---|
| ACTIN | THIN | 'I' band | ISOTROPIC | LIGHT |
| MYOSIN | THICK | 'A' band | ANISOTROPIC | DARK |
Actin filaments are THINNER than the myosin filaments, hence they are called thin and thick filaments respectively - thin means narrower, not shorter and not weaker.
The memory hook is the first letter. 'A' band goes with Anisotropic and with the A of dArk; 'I' band goes with Isotropic and with the I of lIght. Every question in this part of the chapter is one row of that table with one entry rubbed out, and the commonest wrong option calls the 'A' band isotropic.
Question 7
Q. Place the 'Z' line, the 'M' line and the 'H' zone inside the sarcomere, and say what lies in each.
Answer. The portion of the myofibril between two successive 'Z' lines is the SARCOMERE, the functional unit of contraction.
| Landmark | Where it lies | What it is or holds |
|---|---|---|
| 'Z' line | in the centre of each 'I' band, which it BISECTS | an elastic fibre; the THIN filaments are firmly attached to it |
| 'M' line | in the middle of the 'A' band | a thin fibrous membrane holding the THICK filaments together |
| 'H' zone | the central part of the thick filaments | thick filaments ONLY - the part of the thick filament NOT overlapped by thin filaments |
The 'Z' line and the 'M' line are the pair that gets swapped: the 'Z' line bisects the LIGHT band, the 'M' line lies in the middle of the DARK band.
The 'H' zone is the region students get wrong. In the resting state the edges of the thin filaments partially overlap the free ends of the thick filaments, leaving the central part of the thick filaments free, and that free central part is the 'H' zone. It is not a region of overlap, and it does not represent both filaments.
Question 8
Q. Count the parts of one thin filament - how many 'F' actins, how many tropomyosin filaments, and what troponin sits on.
Answer. 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.
| Part | How many | Where it sits |
|---|---|---|
| 'F' actin | 2, helically wound to each other | the backbone of the filament |
| 'G' actin | many | the globular monomer that polymerises into one 'F' actin |
| Tropomyosin | 2 filaments | running close to the 'F' actins throughout their length |
| Troponin | at regular intervals | on the TROPOMYOSIN, not on the actin |
Three proteins build a thin filament - actin, tropomyosin and troponin. In the resting state a SUBUNIT OF TROPONIN MASKS the active binding sites for myosin on the actin filaments, which is the fact the whole contraction mechanism turns on.
'F' is filamentous and 'G' is globular - do not swap them, and do not put troponin directly on the actin.
Question 9
Q. Describe one meromyosin - its two parts, the name of the portion that projects out, and the two kinds of site its head carries.
Answer. Each myosin (thick) filament is a polymerised protein, and many monomeric proteins called MEROMYOSINS constitute one thick filament.
| Part of the meromyosin | Its name |
|---|---|
| a GLOBULAR HEAD with a SHORT ARM | HEAVY MEROMYOSIN (HMM) |
| the TAIL | LIGHT MEROMYOSIN (LMM) |
Say it the right way round: HMM is the head with the short arm, LMM is the tail. The letters give you no help, so learn the two rows as a table.
The HMM component projects outwards at 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 it carries two different kinds of site:
- BINDING SITES FOR ATP
- ACTIVE SITES FOR ACTIN
That head is the only part of either filament with enzyme activity - actin has none.
Question 10
Q. State the sliding filament theory in the chapter's own words, and then say plainly what does NOT happen during contraction.
Answer. The mechanism of muscle contraction is best explained by the SLIDING FILAMENT THEORY, which states that the contraction of a muscle fibre takes place by the SLIDING OF THE THIN FILAMENTS OVER THE THICK FILAMENTS.
Three parts have to appear in that sentence - the name of the theory, the thin filaments, and their sliding over the thick filaments.
What does NOT happen is that nothing shortens by contracting. Neither the actin nor the myosin filament gets shorter. The filaments only SLIDE, and the muscle shortens because the region of overlap between the two sets of filaments increases, pulling the two 'Z' lines closer together.
The second trap is the pairing. The thin filament is ACTIN and the thick filament is MYOSIN, and it is the THIN filament that does the sliding - it slides towards the centre of the sarcomere.
Question 11
Q. Name in order the events that run from the signal leaving the central nervous system to the unmasking of the active sites on actin.
Answer. This is a sequence and it is marked as a sequence, so give it numbered and in order.
- 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 signal reaches the NEUROMUSCULAR JUNCTION, also called the MOTOR-END PLATE - the junction between a motor neuron and the sarcolemma of the muscle fibre.
- A neurotransmitter, ACETYL CHOLINE, is released there.
- Acetyl choline generates an ACTION POTENTIAL in the sarcolemma.
- The action potential spreads through the muscle fibre and causes the release of CALCIUM IONS into the sarcoplasm.
- The rise in leads to the binding of calcium with a SUBUNIT OF TROPONIN on the actin filaments.
- That binding REMOVES THE MASKING of the active sites for myosin.
Everything after step 7 is the cross bridge cycle, and everything before it is the delivery of the signal. The step most often dropped is step 4 - students go straight from acetyl choline to calcium and lose the action potential.
Question 12
Q. Say what happens to the 'I' band, the 'A' band, the 'H' zone, the sarcomere and the filaments themselves during contraction, and give the reason for the 'A' band's behaviour.
| Answer. | Region | During contraction | Why |
|---|---|---|---|
| 'I' band | gets REDUCED | it is the stretch of thin filament not yet overlapped by myosin, and the overlap grows | |
| 'A' band | RETAINS ITS LENGTH | it measures the thick filament, and the thick filament does not change length | |
| 'H' zone | NARROWS | the thin filaments slide further in over the thick ones and fill it | |
| Sarcomere, 'Z' line to 'Z' line | SHORTENS | the 'Z' lines are pulled inwards towards the centre of the 'A' band | |
| The filaments themselves | KEEP THEIR LENGTH | they only slide - nothing shortens by contracting |
During the shortening of the muscle the 'I' bands get REDUCED whereas the 'A' bands RETAIN THEIR LENGTH. That is the sentence the marking scheme wants, and the wrong option always says the 'A' band shortens.
The 'I' band and the 'H' zone shrink for the same reason - both are regions where the two sets of filaments do not yet overlap, and both are filled in as the overlap grows.
Question 13
Q. Write the five-row comparison of red and white muscle fibres, and mark the one row that runs the other way.
Answer. Muscle contains a red coloured OXYGEN STORING pigment called MYOGLOBIN, and how much of it a muscle holds decides everything else.
| Feature | Red fibres | White fibres |
|---|---|---|
| Myoglobin content | HIGH | VERY LESS |
| Colour | reddish appearance | pale or whitish |
| Number of mitochondria | PLENTY | FEW |
| Amount of sarcoplasmic reticulum | LOW | HIGH |
| Process depended on for energy | AEROBIC - hence also called aerobic muscles | ANAEROBIC |
The sarcoplasmic reticulum row is the one that runs the other way, and it is the row students forget precisely because it is the only feature in which white fibres have MORE of something than red fibres do.
Four of the five rows run together - more myoglobin, more colour, more mitochondria, the aerobic route. The colour row is really the myoglobin row stated twice, because the pigment is red, so a muscle full of it looks reddish and a muscle nearly without it looks pale.
Question 14
Q. Give every count that belongs to the head, and give the word the occipital condyles earn for the human skull.
| Answer. | What is counted | Number | The detail that is asked with it |
|---|---|---|---|
| Cranial bones | 8 | they form the CRANIUM, the hard protective outer covering for the brain | |
| Facial bones | 14 | they form the front part of the skull | |
| Skull, in total | 22 | two sets of bones - cranial and facial | |
| Hyoid | 1 | a single U-SHAPED bone at the base of the BUCCAL CAVITY | |
| Ear ossicles | 3 in EACH middle ear | MALLEUS, INCUS and STAPES - so 6 in all | |
| Occipital condyles | 2 | the skull articulates with the superior region of the vertebral column through them |
Because there are TWO occipital condyles, the human skull is described as DICONDYLIC. The word comes from the two condyles, not from the two sets of skull bones.
Read which count the stem is asking for. 8 is cranial, 14 is facial, 22 is the whole skull - a stem saying "the human cranium is made of _ bones" wants 8, and a stem saying "each middle ear" wants three.
Question 15
Q. Write out the vertebral column data card - how many units, where it lies, where it starts, its five regions with their counts, its first vertebra, and its three functions.
Answer. 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.
| Region, in order from the skull | Number |
|---|---|
| Cervical | 7 |
| Thoracic | 12 |
| Lumbar | 5 |
| Sacral | 1 (fused) |
| Coccygeal | 1 (fused) |
| Total | 26 |
The FIRST vertebra is the ATLAS, and it articulates with the OCCIPITAL CONDYLES of the skull - that is how the head sits on the neck.
The three functions, asked as a set of three:
- It PROTECTS the SPINAL CORD.
- It SUPPORTS the HEAD.
- It serves as the POINT OF ATTACHMENT for the RIBS and for the MUSCULATURE OF THE BACK.
The number of cervical vertebrae is SEVEN in almost all mammals, human beings included - a giraffe has seven as well, because the neck lengthens by making each vertebra longer, not by adding more of them.
Question 16
Q. Describe the sternum and the general plan of a rib, and say why a rib is called bicephalic.
Answer. The STERNUM is a FLAT BONE on the VENTRAL MIDLINE of the THORAX. Ventral means the front of the body - it is the bone you can feel down the centre of your chest.
There are 12 PAIRS OF RIBS, and every one of them follows the same plan:
- Each rib is a THIN FLAT BONE.
- It is 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.
Bicephalic means two-headed, and the two heads are at the DORSAL end, where the rib meets the vertebral column - not at the sternal end. An option that puts the two surfaces on the ventral end is the standard distractor, and so is the figure 11 pairs in place of twelve.
The THORACIC VERTEBRAE, the RIBS and the STERNUM together form the RIB CAGE - the rib cage is not just ribs.
Question 17
Q. Name the three kinds of rib with the pairs in each, and say exactly how each kind is fixed at the front.
Answer. All twelve pairs are attached the same way at the back and differently at the front, and that is what splits them into three kinds.
| Kind of rib | Pairs | Which pairs | How it is fixed at the FRONT |
|---|---|---|---|
| TRUE ribs | 7 pairs | the 1st to the 7th | connected to the STERNUM with the help of HYALINE CARTILAGE |
| VERTEBROCHONDRAL or FALSE ribs | 3 pairs | the 8th, 9th and 10th | they do NOT articulate directly with the sternum; they JOIN THE SEVENTH RIB with the help of HYALINE CARTILAGE |
| FLOATING ribs | 2 pairs | the 11th and the 12th | NOT CONNECTED VENTRALLY at all |
Say the pairing in the order the table gives it: true 7, false 3, floating 2. 7 plus 3 plus 2 is 12, so if you can recall only two of the three kinds the sum recovers the third.
All three kinds are attached dorsally to the THORACIC VERTEBRAE - the back end is the same for every rib. The seventh rib is the hinge of the whole block, because the true ribs end at it and the false ribs join it; an option saying the false ribs join the eighth rib, or reach the sternum directly, is wrong.
Question 18
Q. Put the bones of a fore limb and a hind limb side by side with their counts, and name the bone that appears in one list and not in the other.
Answer. Each limb is made of 30 BONES, and the two lists run parallel bone for bone.
| Fore limb (the hand) | Count | Hind limb (the leg) | Count |
|---|---|---|---|
| Humerus - upper arm | 1 | Femur - thigh bone, THE LONGEST BONE | 1 |
| Radius and ulna - forearm | 2 | Tibia and fibula - shank | 2 |
| Carpals - wrist bones | 8 | Tarsals - ankle bones | 7 |
| Metacarpals - palm bones | 5 | Metatarsals - sole bones | 5 |
| Phalanges - digits | 14 | Phalanges - digits | 14 |
| - | - | PATELLA - the knee cap | 1 |
| Total | 30 | Total | 30 |
The bone that appears in one list and not the other is the PATELLA, a cup shaped bone covering the knee VENTRALLY - the knee cap.
The two lists differ in exactly two places and the differences cancel. The ankle has one bone fewer than the wrist - tarsals 7 against carpals 8 - and the leg has one bone the arm does not have, the patella. 1 plus 2 plus 8 plus 5 plus 14 is 30, and 1 plus 2 plus 7 plus 5 plus 14 is 29, plus the patella is 30.
Question 19
Q. Say what each half of each girdle is made of, then name the socket of each girdle with the bone head and the joint it gives.
Answer. Pectoral and pelvic girdle bones help in the ARTICULATION of the UPPER and the LOWER limbs respectively with the AXIAL SKELETON, and each girdle is formed of TWO HALVES.
| PECTORAL girdle | PELVIC girdle | |
|---|---|---|
| What each half is | a CLAVICLE and a SCAPULA - two bones | one COXAL BONE, itself the fusion of the ILIUM, the ISCHIUM and the PUBIS |
| Bones in the whole girdle | 4 | 2 |
| The socket | the GLENOID CAVITY, a depression below the acromion | the ACETABULUM, a cavity at the point of fusion of the three bones |
| The head it receives | the head of the HUMERUS | the THIGH BONE - the head of the FEMUR |
| The joint formed | the SHOULDER joint | the HIP joint |
| How the halves meet in front | they do NOT meet ventrally | they MEET VENTRALLY at the PUBIC SYMPHYSIS, which contains FIBROUS CARTILAGE |
Learn the sockets as two rows and not as six loose words: glenoid cavity - humerus - shoulder, and acetabulum - femur - hip. The pelvic girdle is the one with the fused bone, and the pubic symphysis contains FIBROUS cartilage - hyaline cartilage belongs to the ribs.
Question 20
Q. Give the definition of a joint, the part it plays when a muscle contracts, and the three structural types with the movement each allows.
Answer. Joints are POINTS OF CONTACT between bones, or between bones and cartilages. They are essential for all types of movements involving the bony parts of the body, locomotion included.
Force generated by the muscles is used to carry out movement through joints, where THE JOINT ACTS AS A FULCRUM. Think of the arm as a lever - the muscle supplies the force, the bone is the rigid bar, and the joint is the fixed point the bar turns about. The standard distractors for that word are "lever", "effort" and "load".
| Structural type | How it is held | Degree of movement | The chapter's example |
|---|---|---|---|
| FIBROUS | dense fibrous connective tissue in the form of SUTURES | NO movement | the flat skull bones, which fuse end-to-end to form the cranium |
| CARTILAGINOUS | the bones are joined with the help of CARTILAGES | LIMITED movement | the joint between ADJACENT VERTEBRAE; also the pubic symphysis |
| SYNOVIAL | a FLUID FILLED SYNOVIAL CAVITY between the articulating surfaces | CONSIDERABLE movement | the joints that help in locomotion and many other movements |
Read the ladder as none, limited, considerable. A fibrous joint is held by fibrous connective tissue and does not move; a cartilaginous joint is held by cartilage and moves a little. The word "fibrous" appearing inside "fibrous cartilage" is what makes students call the pubic symphysis a fibrous joint - it is cartilaginous.
Question 21
Q. Name the six disorders of this chapter with the one word that identifies each, and split them into muscle disorders and skeletal or joint disorders.
| Answer. | Disorder | What it is | The giveaway word |
|---|---|---|---|
| Myasthenia gravis | an AUTO IMMUNE disorder affecting the NEUROMUSCULAR JUNCTION, leading to fatigue, weakening and paralysis of skeletal muscle | auto immune | |
| Muscular dystrophy | progressive DEGENERATION of skeletal muscle, mostly due to a GENETIC disorder | genetic | |
| Tetany | rapid SPASMS - wild contractions - in muscle, due to LOW CALCIUM in body fluid | low calcium | |
| Arthritis | INFLAMMATION OF JOINTS | joints, with no cause named | |
| Osteoporosis | an AGE-RELATED disorder characterised by DECREASED BONE MASS and INCREASED CHANCES OF FRACTURES; decreased levels of ESTROGEN is a common cause | age and bone mass | |
| Gout | INFLAMMATION OF JOINTS due to the accumulation of URIC ACID CRYSTALS | uric acid crystals |
The first three are MUSCLE disorders - the muscle, or the junction that drives it, is at fault. The last three are SKELETAL and JOINT disorders - the bone or the joint is at fault.
Two pairs get confused every year. Myasthenia gravis is auto immune and attacks the neuromuscular junction; muscular dystrophy is genetic and degenerates the muscle itself. Arthritis is inflammation of joints with no cause named; gout is inflammation of joints WITH a named cause, the uric acid crystals.
Tier 2 - Applied Reasoning
Question 22
Q. The chapter gives the human skeleton as 206 bones, and the appendicular skeleton works out to 126 bones. Deduce the axial figure from those two, then check your answer by adding the axial skeleton up part by part.
Answer. Deduce it first. 206 minus 126 is 80, so the axial skeleton has 80 bones - which is exactly the figure the chapter states.
Now check it by building the axial skeleton from its four parts - the SKULL, the VERTEBRAL COLUMN, the STERNUM and the RIBS.
| Part of the axial skeleton | Count |
|---|---|
| Skull, that is 8 cranial plus 14 facial | 22 |
| Hyoid | 1 |
| Ear ossicles, 3 in each middle ear | 6 |
| Vertebral column | 26 |
| Sternum | 1 |
| Ribs, 12 pairs | 24 |
| Total | 80 |
22 plus 1 plus 6 plus 26 plus 1 plus 24 is 80. The deduced answer and the built answer agree, which is the whole point of doing it both ways.
Two rows are where the sum goes wrong. The ear ossicles are 3 in EACH middle ear and there are two middle ears, so 6. The ribs are 12 PAIRS, so 24 bones. A student who enters 3 and 12 gets 65 and thinks he has misremembered the 80 - he has not, he has forgotten to double.
Question 23
Q. A fore limb has 30 bones altogether. You are given the humerus as 1, the radius and ulna as 2, the metacarpals as 5 and the phalanges as 14. Work out the number of carpals, and name the count it is most often confused with.
Answer. Add what you are given, then subtract from the total.
| Bone | Count |
|---|---|
| Humerus | 1 |
| Radius and ulna | 2 |
| Metacarpals | 5 |
| Phalanges | 14 |
| Given so far | 22 |
| Fore limb total | 30 |
| Carpals, by subtraction | 30 minus 22 = 8 |
So the carpals are 8 in number, and the carpals are the WRIST bones.
The count it is confused with is the TARSALS, which are 7 - the ankle bones of the hind limb. The wrist has one more than the ankle: carpals 8, tarsals 7. The second confusion is with the metacarpals, which are 5 and are the PALM bones; a student who writes 8 for the palm and 5 for the wrist has the pair upside down.
Question 24
Q. Count the bones of one hand from the wrist downwards and the bones of one foot from the ankle downwards. Which has more, and by how many?
Answer. Take the three rows below the forearm in one limb and below the shank in the other.
| The hand, wrist downwards | Count | The foot, ankle downwards | Count |
|---|---|---|---|
| Carpals | 8 | Tarsals | 7 |
| Metacarpals | 5 | Metatarsals | 5 |
| Phalanges | 14 | Phalanges | 14 |
| Total | 27 | Total | 26 |
8 plus 5 plus 14 is 27, and 7 plus 5 plus 14 is 26.
The hand has ONE more bone than the foot, and the extra bone is a carpal. That is the only place the two lists differ below the elbow and the knee, because the metacarpals and the metatarsals are BOTH 5 and the phalanges are 14 in each limb.
Notice what this does not contradict. Each limb still comes to 30 bones, because the hind limb makes up its one missing tarsal with the PATELLA, which lies at the knee and not in the foot at all.
Question 25
Q. Two students work out the appendicular skeleton. One gets 120 and the other gets 126. Do the sum yourself and say what the first student has left out.
Answer. The appendicular skeleton is the bones of the limbs ALONG WITH THEIR GIRDLES, and that last phrase is the whole question.
| Part of the appendicular skeleton | Count |
|---|---|
| Fore limbs, 30 each | 60 |
| Hind limbs, 30 each | 60 |
| Pectoral girdle - 2 clavicles and 2 scapulae | 4 |
| Pelvic girdle - 2 coxal bones | 2 |
| Total | 126 |
60 plus 60 plus 4 plus 2 is 126.
The student who got 120 has counted the four limbs and left out both girdles. Four limbs at 30 each is 120, and the two girdles add 6 bones between them - 4 from the pectoral and 2 from the pelvic.
The girdle rows are the ones to be careful with. Both girdles are formed of two halves, but a pectoral half is TWO bones - a clavicle and a scapula - while a pelvic half is ONE bone, the coxal bone, because the ilium, the ischium and the pubis have already fused into it. Fusion removes bones from the count, so the pelvic girdle contributes 2 and not 6.
Question 26
Q. Work out how many individual rib bones a person has, and how many of those bones do not reach the sternum.
Answer. Start from the pairs and double them. There are 12 PAIRS of ribs, so 12 times 2 = 24 rib BONES.
| Kind of rib | Pairs | Individual bones | Does it reach the sternum? |
|---|---|---|---|
| True ribs | 7 | 14 | YES - connected to the sternum with HYALINE CARTILAGE |
| False (vertebrochondral) ribs | 3 | 6 | NO - they join the SEVENTH RIB instead |
| Floating ribs | 2 | 4 | NO - not connected ventrally at all |
| Total | 12 | 24 | - |
The bones that do not reach the sternum are 6 plus 4, that is 10. The ones that do reach it are 14, and 14 plus 10 is 24, so the sum closes.
Watch the units in the stem. A question asking for "the number of ribs in man" and offering both 12 and 24 is testing whether you read the word PAIRS. 12 is the number of pairs; 24 is the number of bones, and the axial sum uses 24.
Question 27
Q. Add the five regions of the vertebral column, then work out how many vertebrae lie above the lumbar region and how many lie below the thoracic region.
| Answer. | Region, in order from the skull | Count | Running total |
|---|---|---|---|
| Cervical | 7 | 7 | |
| Thoracic | 12 | 19 | |
| Lumbar | 5 | 24 | |
| Sacral (fused) | 1 | 25 | |
| Coccygeal (fused) | 1 | 26 |
7 plus 12 plus 5 plus 1 plus 1 is 26, the figure the chapter gives for the number of vertebrae.
- Above the lumbar region lie the cervical and the thoracic vertebrae: 7 plus 12 = 19.
- Below the thoracic region lie the lumbar, sacral and coccygeal: 5 plus 1 plus 1 = 7.
- Check: 19 plus 7 is 26, and the thoracic region is counted once on each side of the two questions, which is why 19 and 7 do not simply add to 26 without noticing that the thoracic block sits inside the first figure and not the second.
The sacral and the coccygeal regions each count as ONE fused unit, and that is why 26 vertebrae is a smaller number than the vertebrae you might expect to find in a spine. The 12 thoracic vertebrae match the 12 pairs of ribs, one pair to each - that is not a coincidence and it is a useful cross-check on both counts.
Question 28
Q. Match Column I with Column II. Column I: (a) Smooth muscle, (b) Tropomyosin, (c) Red muscle, (d) Skull. Column II: (i) Myoglobin, (ii) Thin filament, (iii) Sutures, (iv) Involuntary. This is one of the chapter-end exercises.
Answer. The matched pairs are (a)-(iv), (b)-(ii), (c)-(i) and (d)-(iii).
| Column I | Column II | Why they go together |
|---|---|---|
| (a) Smooth muscle | (iv) Involuntary | Smooth or nonstriated muscle is the visceral muscle, found in the inner walls of hollow visceral organs such as the alimentary canal and the reproductive tract. It is NOT under voluntary control - it is involuntary. You cannot decide to push food along your gut, and that is the whole point of the pairing. |
| (b) Tropomyosin | (ii) Thin filament | The thin filament is built of two 'F' actins helically wound together, with TWO FILAMENTS OF TROPOMYOSIN running close to the 'F' actins throughout their length, and troponin distributed at regular intervals on the tropomyosin. Tropomyosin belongs to the thin filament and to nothing else - the thick filament is meromyosin and has no such associated protein. |
| (c) Red muscle | (i) Myoglobin | Myoglobin is a red coloured OXYGEN STORING pigment, and its content is HIGH in some muscles, which gives them a reddish appearance - those are the red fibres. The pigment is the cause and the colour is the effect, so myoglobin is the one word that defines a red muscle. |
| (d) Skull | (iii) Sutures | 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. Sutures are fibrous joints and allow NO movement, and they are found nowhere else in this chapter. |
Two of the four pairs settle themselves by elimination, which is worth practising. Sutures are the only joint in Column II, and the skull is the only bony structure in Column I, so that pair fixes itself. Thin filament is the only structure of a myofibril, and tropomyosin is the only protein in Column I, so that pair fixes itself too. That leaves smooth muscle and red muscle against involuntary and myoglobin, and myoglobin is a pigment, which can only belong to the muscle named by its colour.
One warning about (a). Cardiac muscle is involuntary as well, so "involuntary" is not unique to smooth muscle in the chapter as a whole - but cardiac muscle is not in Column I, and smooth muscle has no other partner available in Column II. Match the columns you are given, not the chapter you remember.
Question 29
Q. A drug is found that blocks the ATPase activity of the myosin head. Predict what would happen to cross bridge formation, to the sliding of the filaments, and to the shortening of the sarcomere.
Answer. ATP is used twice in the cycle and for two different jobs, so start by separating them.
| The ATP event | What it does |
|---|---|
| ATP HYDROLYSIS by the globular head | supplies the energy with which the myosin head binds the exposed active sites on actin to FORM A CROSS BRIDGE |
| The binding of a NEW ATP | BREAKS the cross bridge |
Blocking the ATPase blocks the hydrolysis, which is the first of those two events.
- No cross bridge can form, because it is the energy from ATP hydrolysis that lets the myosin head bind the actin.
- With no cross bridge there is no pull, so the thin filaments cannot be pulled towards the centre of the 'A' band and no sliding takes place.
- With no sliding the 'Z' lines are not drawn inwards, so the sarcomere does not shorten - and shortening of the sarcomere is contraction. The muscle therefore cannot contract.
- The bands would stay as they are - the 'I' band would not be reduced and the 'H' zone would not narrow, because both of those changes are caused by the sliding.
The step that survives is worth noticing. The signal would still arrive, acetyl choline would still be released, the action potential would still spread and calcium would still be released and still unmask the active sites. Everything up to the unmasking is intact; the failure is at the cross bridge. That is why this question is a good test of whether you know the sequence rather than the headline.
Question 30
Q. Calcium ions are prevented from being pumped back into the sarcoplasmic cisternae after a contraction. What state would the muscle be left in, and which disorder of this chapter is the opposite fault?
Answer. Relaxation is a calcium event, not a separate mechanism. The process continues till the ions are PUMPED BACK to the sarcoplasmic cisternae, resulting in the MASKING of the actin filaments, and this causes the return of the 'Z' lines to their original position - that is, relaxation.
So if the calcium cannot be pumped back:
- The calcium level in the sarcoplasm stays high.
- Calcium stays bound to the subunit of troponin, so the active sites on actin stay UNMASKED.
- Cross bridges keep forming and breaking, and the filaments keep sliding.
- The 'Z' lines never return to their original position, so the sarcomere stays shortened.
- The muscle is left CONTRACTED and cannot relax.
The opposite fault is TETANY - rapid spasms, that is wild contractions, in muscle, due to LOW CALCIUM in body fluid.
Keep the two calcium compartments apart, because that is where this looks like a contradiction and is not. Inside the fibre, a rise in calcium in the sarcoplasm STARTS contraction and the pumping of it back into the sarcoplasmic reticulum ENDS the contraction. Outside, in the body fluid, it is a FALL in the calcium level that makes the muscle fire uncontrollably and produces tetany. Two different fluids, two different directions.
Question 31
Q. Two patients. The first tires within minutes, grows weak and is finally paralysed, and the fault is at the junction where the nerve meets the muscle, which the body's own immune system is attacking. In the second the muscle tissue itself is wasting away steadily, and the condition runs in the family. Name each disorder, and give the two questions that separate them.
Answer. The first patient has MYASTHENIA GRAVIS - an AUTO IMMUNE disorder affecting the NEUROMUSCULAR JUNCTION, leading to fatigue, weakening and paralysis of skeletal muscle.
The second patient has MUSCULAR DYSTROPHY - progressive DEGENERATION of skeletal muscle, mostly due to a GENETIC disorder.
Two questions separate them every time.
| The question | Myasthenia gravis | Muscular dystrophy |
|---|---|---|
| What causes it? | AUTO IMMUNE - the body's own immune system | GENETIC - it runs in families |
| What does it damage? | the NEUROMUSCULAR JUNCTION - the muscle fibre itself is not being destroyed, the signal simply stops getting across | the SKELETAL MUSCLE ITSELF, which it degenerates progressively |
Ask the cause and then ask the target, and the pair comes apart. The commonest single mistake from this part of the chapter is swapping "auto immune" and "genetic" between the two, and the second commonest is thinking that myasthenia gravis destroys muscle tissue. It does not - it blocks the junction.
Question 32
Q. A rib is attached at the back to a thoracic vertebra, but at the front it does not reach the sternum - it joins the seventh rib instead. Which pairs could it be, what is that kind of rib called, and what joins it to the seventh rib?
Answer. It must be one of the 8th, 9th or 10th pairs.
- The kind of rib: VERTEBROCHONDRAL ribs, commonly called FALSE ribs. There are 3 such pairs - the 8th, 9th and 10th.
- What joins it to the seventh rib: HYALINE CARTILAGE. They do not articulate directly with the sternum but join the seventh rib with the help of hyaline cartilage.
The name explains itself once you split it. "Vertebro" is the vertebral column at the back and "chondral" is the cartilage at the front - attached to bone behind, to cartilage in front.
Two ribs are ruled out by the description. It cannot be one of the first 7 pairs, because those are the TRUE ribs and they reach the sternum themselves, with hyaline cartilage. It cannot be the 11th or 12th pair, because those are the FLOATING ribs and they are NOT CONNECTED VENTRALLY AT ALL - they join nothing at the front, not even the seventh rib.
Question 33
Q. A bone is described as large, triangular and flat, lying in the dorsal part of the thorax between the second and the seventh ribs. Name it, name the ridge on it and the process that ridge becomes, and name the depression below that process together with the joint it makes.
Answer. The bone is the SCAPULA, one half of the pectoral girdle, whose other bone is the clavicle.
| Feature | The chapter's words |
|---|---|
| Shape and size | a large TRIANGULAR FLAT bone |
| Position | in the DORSAL part of the thorax - on the back |
| Extent | between the SECOND and the SEVENTH ribs |
| The ridge | the dorsal, flat, triangular body has a slightly elevated ridge called the SPINE |
| What the ridge becomes | it projects as a flat, expanded process called the ACROMION |
| What meets the acromion | the CLAVICLE articulates with it |
| The depression below the acromion | the GLENOID CAVITY |
| The joint it makes | it articulates with the HEAD OF THE HUMERUS to form the SHOULDER JOINT |
Read the chain in order: body, then spine, then acromion where the clavicle comes in, then the glenoid cavity below it where the arm comes in.
Three traps live in this description. The rib numbers are the SECOND and the SEVENTH - the false versions offer the first and the sixth, or the third and the eighth. The shoulder joint is glenoid cavity plus the head of the humerus, not acromion plus clavicle. And the socket of the other girdle, the ACETABULUM, takes the head of the FEMUR - an option that gives the scapula an acetabulum has swapped the two girdles.
Tier 3 - Longer Written Answers
Question 34
Q. Describe the structure of a skeletal muscle in full, working from the whole muscle down to the sarcomere, and label every region of the sarcomere with what it holds.
Answer. Work down the ladder in order - muscle, muscle bundle, muscle fibre, myofibril, sarcomere.
1. The whole muscle. 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.
2. The muscle fibre.
- Each muscle fibre is lined by the plasma membrane called the SARCOLEMMA, enclosing the SARCOPLASM.
- A muscle fibre is a SYNCITIUM, as the sarcoplasm contains MANY NUCLEI.
- The endoplasmic reticulum of the muscle fibre, the SARCOPLASMIC RETICULUM, is the STORE HOUSE OF CALCIUM IONS - in a muscle fibre is stored there.
3. The myofibrils. A characteristic feature of the muscle fibre is a large number of parallelly arranged filaments in the sarcoplasm called MYOFILAMENTS or MYOFIBRILS. Each myofibril has alternate dark and light bands on it, and the striated appearance is due to the distribution pattern of two proteins, ACTIN and MYOSIN. Both proteins are arranged as rod-like structures, parallel to each other and also to the longitudinal axis of the myofibrils.
4. The sarcomere. The portion of the myofibril between two successive 'Z' lines is considered the FUNCTIONAL UNIT OF CONTRACTION and is called a SARCOMERE.
| Region of the sarcomere | Where it lies | What it holds |
|---|---|---|
| 'Z' line | in the centre of each 'I' band, which it BISECTS | an elastic fibre; the thin filaments are firmly attached to it |
| 'I' band, the LIGHT or ISOTROPIC band | on either side of the 'Z' line | THIN filaments - ACTIN - only |
| 'A' band, the DARK or ANISOTROPIC band | between the two half 'I' bands | the whole length of the THICK filaments - MYOSIN - with the overlapping ends of the thin filaments at its two edges |
| 'H' zone | in the middle of the 'A' band | the central part of the thick filaments, NOT overlapped by thin filaments - thick filaments only |
| 'M' line | in the middle of the 'A' band, inside the 'H' zone | a thin fibrous membrane holding the thick filaments together |
To picture one sarcomere, start at a 'Z' line and walk to the next. A 'Z' line, then a half 'I' band of thin filaments alone, then the whole 'A' band with the 'H' zone and the 'M' line at its centre, then another half 'I' band, then the next 'Z' line. The 'A' and 'I' bands are arranged alternately throughout the length of the myofibril, and in the resting state the edges of the thin filaments partially overlap the free ends of the thick filaments, which is what leaves the 'H' zone free.
Question 35
Q. Give a full account of the sliding filament theory - the statement, the steps in order, and what changes in the sarcomere.
Answer. The statement. The mechanism of muscle contraction is best explained by the SLIDING FILAMENT THEORY, which states that the contraction of a muscle fibre takes place by the SLIDING OF THE THIN FILAMENTS OVER THE THICK FILAMENTS.
The steps, in order.
- 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 action potential spreads through the muscle fibre and causes the release of CALCIUM IONS into the sarcoplasm.
- The increase in 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. 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 myosin head, and the cycle of cross bridge formation and breakage is repeated, causing further sliding.
- The process continues till the 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.
Steps 6 to 9 are one cycle and that cycle repeats. A single cross bridge pulls the actin only a short way; the muscle shortens as far as it does because the cycle of cross bridge formation and breakage is repeated again and again, causing further sliding each time.
What changes in the sarcomere.
| Region | During contraction |
|---|---|
| 'I' band | gets REDUCED |
| 'A' band | RETAINS ITS LENGTH |
| 'H' zone | NARROWS |
| Sarcomere | SHORTENS |
| The filaments themselves | KEEP THEIR LENGTH - they only slide |
Two points carry most of the marks. ATP is used twice and for two different jobs - hydrolysis gives the energy to FORM the cross bridge, and the binding of a NEW ATP BREAKS it. Calcium moves in two directions - released into the sarcoplasm to begin contraction, pumped back into the sarcoplasmic cisternae to end it.
Question 36
Q. Describe both contractile proteins in full - what each filament is built from, what runs along it, and which of the two carries enzyme activity.
Answer. The thin filament, built of ACTIN.
- Each actin (thin) 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.
The thick filament, built of MYOSIN.
- Each myosin (thick) filament is also a polymerised protein. Many monomeric proteins called MEROMYOSINS constitute one thick filament.
- Each meromyosin has two parts - a GLOBULAR HEAD WITH A SHORT ARM, the HEAVY MEROMYOSIN (HMM), and a TAIL, the LIGHT MEROMYOSIN (LMM).
- The HMM component projects outwards at 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, carrying BINDING SITES FOR ATP and ACTIVE SITES FOR ACTIN.
The two side by side.
| ACTIN | MYOSIN | |
|---|---|---|
| Filament it forms | the THIN filament | the THICK filament |
| Band it occupies | the 'I' band - light, isotropic | the 'A' band - dark, anisotropic |
| Built from | two 'F' actins helically wound, each a polymer of 'G' actins | many monomeric MEROMYOSINS |
| Associated proteins | TROPOMYOSIN and TROPONIN | none is named |
| Enzyme activity | none | the globular head is an active ATPase |
| Attachment | firmly attached to the 'Z' line | held in the middle of the 'A' band by the 'M' line |
| Role in contraction | it is the filament that SLIDES | its head is the part that PULLS |
Read the "built from" row twice - actin is built of actins, myosin is built of meromyosins. The enzyme row is the one an exam turns into a wrong option, by putting ATPase activity on actin. Actin has none.
Question 37
Q. Give a full account of the axial skeleton - its parts, every count in it, and the sum that makes 80.
Answer. The skeletal system consists of a framework of BONES and a few CARTILAGES, both specialised connective tissues. Bone has a very HARD matrix due to CALCIUM SALTS; 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 the AXIAL and the APPENDICULAR skeleton.
The AXIAL skeleton comprises 80 BONES distributed along the MAIN AXIS of the body, and it has four parts - the SKULL, the VERTEBRAL COLUMN, the STERNUM and the RIBS.
1. The skull. Two sets of bones, cranial and facial, totalling 22 BONES. Cranial bones are 8 in number and 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. Counted with the head are a single U-SHAPED HYOID bone at the base of the buccal cavity, and three EAR OSSICLES - malleus, incus and stapes - in EACH middle ear. The skull articulates with the superior region of the vertebral column through TWO OCCIPITAL CONDYLES, and is therefore DICONDYLIC.
2. The vertebral column. 26 SERIALLY ARRANGED VERTEBRAE, DORSALLY placed, extending from the base of the skull, forming the main framework of the trunk. Each vertebra has a NEURAL CANAL through which the SPINAL CORD passes. The first vertebra is the ATLAS. Cervical 7, thoracic 12, lumbar 5, sacral 1 fused, coccygeal 1 fused.
3. The sternum. A FLAT BONE on the VENTRAL MIDLINE of the THORAX.
4. The ribs. 12 PAIRS, each a thin flat bone, BICEPHALIC because it has two articulation surfaces on its dorsal end. True ribs 7 pairs, false or vertebrochondral 3 pairs, floating 2 pairs. The thoracic vertebrae, the ribs and the sternum together form the RIB CAGE.
The sum.
| Part | Count |
|---|---|
| Skull, 8 cranial plus 14 facial | 22 |
| Hyoid | 1 |
| Ear ossicles, 3 in each middle ear | 6 |
| Vertebral column | 26 |
| Sternum | 1 |
| Ribs, 12 pairs | 24 |
| Total | 80 |
22 plus 1 plus 6 plus 26 plus 1 plus 24 is 80, which is the figure the chapter states. The two rows that break the sum are the ear ossicles and the ribs, because both are stated per side or per pair and have to be doubled.
Question 38
Q. Give a full account of the appendicular skeleton - both limbs bone by bone, both girdles, and the sum that makes 126.
Answer. The bones of the limbs along with their girdles constitute the APPENDICULAR SKELETON. Each limb is made of 30 BONES.
The fore limb, the hand. HUMERUS, the bone of the upper arm; RADIUS and ULNA, the two bones of the forearm; CARPALS, the wrist bones - 8 in number; METACARPALS, the palm bones - 5 in number; PHALANGES, the digits - 14 in number. 1 plus 2 plus 8 plus 5 plus 14 is 30.
The hind limb, the leg. FEMUR, the thigh bone and THE LONGEST BONE in the body; TIBIA and FIBULA, the two bones of the shank; TARSALS, the ankle bones - 7 in number; METATARSALS - 5 in number; PHALANGES, the digits - 14 in number; and a cup shaped bone called the PATELLA, which covers the knee VENTRALLY - the knee cap. 1 plus 2 plus 7 plus 5 plus 14 is 29, plus the patella is 30.
The pectoral girdle. Each half consists of a CLAVICLE and a SCAPULA, so 4 bones in all. The scapula is a large triangular flat bone in the DORSAL part of the thorax between the SECOND and the SEVENTH ribs, carrying a ridge called the SPINE, which projects as the ACROMION where the clavicle articulates, and below it the GLENOID CAVITY, which takes 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.
The pelvic girdle. Two COXAL BONES, so 2 bones in all. Each coxal bone is formed by the FUSION of the ILIUM, the ISCHIUM and the PUBIS. At the point of fusion is a cavity called the ACETABULUM, to which the THIGH BONE articulates - the hip joint. The two halves meet VENTRALLY at the PUBIC SYMPHYSIS, which contains FIBROUS CARTILAGE.
The sum.
| Part | Count |
|---|---|
| Fore limbs, 30 each | 60 |
| Hind limbs, 30 each | 60 |
| Pectoral girdle - 2 clavicles and 2 scapulae | 4 |
| Pelvic girdle - 2 coxal bones | 2 |
| Total | 126 |
60 plus 60 plus 4 plus 2 is 126, and 80 plus 126 is 206, the total the chapter gives. The pelvic girdle contributes 2 and not 6, because the three named bones have already FUSED into one coxal bone on each side - fusion removes bones from the count, exactly as it does in the sacral and coccygeal regions of the vertebral column.
Question 39
Q. Describe the two girdles in full and set out every point on which they differ.
Answer. Pectoral and pelvic girdle bones help in the ARTICULATION of the UPPER and the LOWER limbs respectively with the AXIAL SKELETON, and each girdle is formed of TWO HALVES. A girdle is a joining device - the limb bones cannot hang off the vertebral column by themselves, which is why the girdles are counted with the limbs in the appendicular skeleton.
The pectoral girdle, half by half. 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.
- Its dorsal, flat, triangular body 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.
The pelvic girdle, half by half. Each half is a single COXAL BONE.
- Each coxal bone is formed by the FUSION OF THREE BONES - the ILIUM, the ISCHIUM and the PUBIS.
- At the point of fusion is a cavity called the ACETABULUM, to which the THIGH BONE articulates.
- The two halves meet VENTRALLY to form the PUBIC SYMPHYSIS, containing FIBROUS CARTILAGE.
The differences, point by point.
| Point of difference | PECTORAL girdle | PELVIC girdle |
|---|---|---|
| Which limb it attaches | the UPPER limbs | the LOWER limbs |
| What each half is made of | a CLAVICLE and a SCAPULA - two bones | one COXAL BONE, the fusion of ilium, ischium and pubis |
| Total bones in the girdle | 4 | 2 |
| The socket | the GLENOID CAVITY, below the acromion | the ACETABULUM, at the point of fusion |
| The bone head received | the head of the HUMERUS | the head of the FEMUR |
| The joint formed | the SHOULDER joint | the HIP joint |
| Union of the two halves in front | they do NOT meet ventrally | they MEET at the PUBIC SYMPHYSIS, with FIBROUS CARTILAGE |
The last row explains the two joints. The pectoral girdle has no ventral union between its halves, which is part of why the shoulder moves so freely; the pelvic girdle is closed in front at the pubic symphysis, which makes the hip a firm weight-bearing ring.
Question 40
Q. Give a full account of joints - what a joint is, the part it plays, the three structural types, and the five synovial joints with their sites.
Answer. Joints are POINTS OF CONTACT between bones, or between bones and cartilages. They are essential for all types of movements involving the bony parts of the body, and locomotory movements are no exception. Force generated by the muscles is used to carry out movement through joints, where THE JOINT ACTS AS A FULCRUM. The movability at these joints varies depending on different factors, which is what the classification captures.
The three structural forms.
| Type | How the bones are held | Movement | Example |
|---|---|---|---|
| FIBROUS | dense fibrous connective tissues in the form of SUTURES | NONE | the flat skull bones, which fuse end-to-end to form the CRANIUM |
| CARTILAGINOUS | the bones are joined with the help of CARTILAGES | LIMITED | the joint between ADJACENT VERTEBRAE; also the PUBIC SYMPHYSIS |
| SYNOVIAL | a FLUID FILLED SYNOVIAL CAVITY between the articulating surfaces of the two bones | CONSIDERABLE | the joints that help in locomotion and many other movements |
The five synovial joints.
| Synovial joint | Lies between | What it allows |
|---|---|---|
| Ball and socket | the HUMERUS and the PECTORAL GIRDLE; also the FEMUR and the ACETABULUM of the pelvic girdle | movement in all planes |
| Hinge | the KNEE JOINT; also between the PHALANGES | movement in one plane only, like a door |
| Pivot | the ATLAS and the AXIS | one bone turning about another - it is what lets you shake your head |
| Gliding | between the CARPALS | flat surfaces sliding over each other |
| Saddle | between the CARPAL AND METACARPAL OF THE THUMB | it is what makes the human thumb opposable |
Three of these are where the marks go missing. The joints between the PHALANGES are HINGE joints, not gliding - gliding is between the carpals. The joints between the CRANIAL BONES are FIBROUS sutures and allow no movement at all. The joint between the two PUBIC BONES is CARTILAGINOUS - the pubic symphysis with its fibrous cartilage - and not fibrous, even though the word "fibrous" appears inside "fibrous cartilage". Read the word that comes after "fibrous" before you answer.
Question 41
Q. Give a full account of the six disorders of the muscular and skeletal system, and separate the two pairs that are most often confused.
Answer. The chapter names six disorders, three of muscle and three of the skeleton and joints.
The three muscle disorders.
1. MYASTHENIA GRAVIS - an AUTO IMMUNE disorder affecting the NEUROMUSCULAR JUNCTION, leading to fatigue, weakening and paralysis of skeletal muscle. The fault is at the place where the motor neuron meets the muscle fibre; the muscle tissue itself is not being destroyed, the signal simply stops getting across.
2. MUSCULAR DYSTROPHY - progressive DEGENERATION of skeletal muscle, mostly due to a GENETIC disorder. Here the muscle tissue itself is destroyed, and destroyed progressively.
3. TETANY - rapid SPASMS - wild contractions - in muscle, due to LOW CALCIUM in body fluid. This follows from the contraction mechanism: calcium controls whether the active sites on actin are unmasked, so the calcium level of the body fluid controls how easily a muscle fires, and when it falls the muscle fires uncontrollably.
The three skeletal and joint disorders.
4. ARTHRITIS - INFLAMMATION OF JOINTS. That is the whole definition, and no cause is named.
5. 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 seen so much more often in women after menopause.
6. GOUT - INFLAMMATION OF JOINTS due to the accumulation of URIC ACID CRYSTALS.
The two confused pairs.
| The pair | How to separate them |
|---|---|
| Myasthenia gravis and muscular dystrophy | Ask the cause and then the target. Myasthenia gravis is AUTO IMMUNE and its target is the NEUROMUSCULAR JUNCTION. Muscular dystrophy is GENETIC and its target is the SKELETAL MUSCLE ITSELF, degenerated progressively. |
| Arthritis and gout | Both are inflammation of joints, and the difference is one clause. If only "inflammation of joints" is offered, the answer is ARTHRITIS. If URIC ACID CRYSTALS appear anywhere in the stem, the answer is GOUT. |
Osteoporosis is not a joint disorder at all - it is a loss of BONE MASS, it is AGE-RELATED, and decreased estrogen is a common cause. For tetany, remember that the trigger is LOW calcium, not high - low calcium makes the muscle fire more, not less.
Where Every Chapter-End Exercise Is Answered
This chapter has TEN exercises at the end. Counted properly they come to TWENTY-SIX parts, because exercise 4, the true-or-false item, has five parts a to e, exercise 5, the write-the-difference item, has three parts a to c, exercise 9, the name-the-joint item, has six parts a to f, and exercise 10, the fill-in-the-blanks item, has six parts a to f. Every other exercise is a single part. That is the largest exercise set in the book.
TWENTY-FIVE of those twenty-six parts are answered in full inside the thirteen teaching sections of this chapter. The remaining one - exercise 6, the match-the-column item - is answered in this section and nowhere else.
The parts of the four multi-part exercises are deliberately split across different sections, because each part belongs with the topic it tests. Exercise 4(a) sits with the contractile proteins while 4(c) sits with the bone count; exercise 10(d) sits with the sarcomere while 10(e) sits with the ribs. So do not go looking for all five parts of exercise 4, or all six parts of exercise 10, in one place. Exercise 9 is the one exception - all six of its parts are joint-naming items, so all six sit together in the section on joints.
Attempt each exercise on paper first, then turn to the section named. The answer there is written out in full, with the wording a marking scheme is looking for.
| Exercise | The question, in short | Section | Answered as |
|---|---|---|---|
| 1 | draw the diagram of a sarcomere | Section 3 - The Skeletal Muscle and the Sarcomere | Question 12 |
| 2 | define the sliding filament theory | Section 5 - The Sliding Filament Theory of Muscle Contraction | Question 1 |
| 3 | describe the important steps in muscle contraction | Section 5 - The Sliding Filament Theory of Muscle Contraction | Question 2 |
| 4 (a) | true or false - actin is present in thin filament | Section 4 - The Contractile Proteins | Question 11 |
| 4 (b) | true or false - H-zone represents both filaments | Section 3 - The Skeletal Muscle and the Sarcomere | Question 13 |
| 4 (c) | true or false - human skeleton has 206 bones | Section 11 - The Bone Count - Adding the Skeleton Up | Question 3 |
| 4 (d) | true or false - there are 11 pairs of ribs in man | Section 8 - The Axial Skeleton - Sternum and Ribs | Question 3 |
| 4 (e) | true or false - sternum is on the ventral side | Section 8 - The Axial Skeleton - Sternum and Ribs | Question 2 |
| 5 (a) | write the difference between actin and myosin | Section 4 - The Contractile Proteins | Question 12 |
| 5 (b) | write the difference between red and white muscles | Section 6 - Red and White Muscle Fibres | Question 7 |
| 5 (c) | write the difference between the two girdles | Section 10 - The Girdles - Pectoral and Pelvic | Question 11 |
| 6 | match Column I with Column II | Section 14 - Solved Examples | Question 28 |
| 7 | types of movement shown by the cells of the body | Section 1 - Types of Movement | Question 3 |
| 8 | distinguish skeletal muscle from cardiac muscle | Section 2 - Muscle - Types and Properties | Question 10 |
| 9 (a) | name the joint between the atlas and the axis | Section 12 - Joints | Question 8 |
| 9 (b) | name the joint at the carpal and metacarpal of thumb | Section 12 - Joints | Question 9 |
| 9 (c) | name the joint between the phalanges | Section 12 - Joints | Question 10 |
| 9 (d) | name the joint between the femur and acetabulum | Section 12 - Joints | Question 11 |
| 9 (e) | name the joint between the cranial bones | Section 12 - Joints | Question 12 |
| 9 (f) | name the joint between the pubic bones | Section 12 - Joints | Question 13 |
| 10 (a) | fill in - cervical vertebrae in mammals | Section 7 - The Axial Skeleton - Skull and Vertebral Column | Question 7 |
| 10 (b) | fill in - phalanges in each limb | Section 9 - The Appendicular Skeleton - The Limb Bones | Question 5 |
| 10 (c) | fill in - the two other proteins of the thin filament | Section 4 - The Contractile Proteins | Question 13 |
| 10 (d) | fill in - where calcium is stored in a muscle fibre | Section 3 - The Skeletal Muscle and the Sarcomere | Question 14 |
| 10 (e) | fill in - which pairs of ribs are floating ribs | Section 8 - The Axial Skeleton - Sternum and Ribs | Question 4 |
| 10 (f) | fill in - the human cranium is made of how many bones | Section 7 - The Axial Skeleton - Skull and Vertebral Column | Question 6 |
Read the table as a revision plan, because it tells you where the marks sit.
Eight of the twenty-six parts - exercises 1, 2, 3, 4(a), 4(b), 5(a), 6 in part, and 10(c) and 10(d) - come out of the muscle at the level of the filament, that is the sarcomere, the contractile proteins and the sliding filament theory. Learn the sarcomere diagram, the make-up of the two filaments and the ten steps of contraction, and roughly a third of the exercise set is answered before you start.
Nine parts - 4(c), 4(d), 4(e), 5(c), 10(a), 10(b), 10(e), 10(f) and part of 6 - are pure skeletal counts and positions. Cranial 8, cervical 7, phalanges 14 per limb, ribs 12 pairs with the 11th and 12th floating, the sternum on the ventral midline, the skeleton at 206, and the two girdles. Each is worth one mark and takes one line, and there is no partial credit for nearly remembering a number.
Six parts are exercise 9 alone, and all six are answered by one table - the five synovial joints with their sites, plus the fibrous sutures of the cranium and the cartilaginous pubic symphysis. Learn that table once and six of the twenty-six parts are finished.
The remaining three - exercises 7, 8 and 5(b) - sit at the two ends of the chapter, at the three types of movement shown by human cells on one side and the muscle types and the red and white fibres on the other. Those are the parts students skip because they come before the mechanism and after it, and they carry three parts between them.
One last thing worth knowing about the exercise set: it never asks about the DISORDERS at all, and it never asks about the bone count except through the true-or-false in 4(c). Do not read that as permission to skip them. The six disorders and the arithmetic of the skeleton carry a large share of the objective questions on this chapter, which is why they have full teaching sections and a good many items in the tiers above.