Skeletal system of man

7.1Human skeleton

Support is necessary for living organisms to hold up and sustain the body against gravity and other external forces. As organisms increased in size during evolution, the need for support became greater, particularly once living organisms left water and colonised land. The skeleton in animals contributes to this support.

The human skeletal system consists of bones and cartilage. The skeleton acts as a framework that supports the soft tissues. It also allows free movement through the action of muscles across joints.

Osteology

The study of bones and cartilage.

7.1.1Structure of boneOften examined

An individual bone is made of several tissues: bone tissue, cartilage, fibrous connective tissue, blood and nerve tissue. The broad terminal parts of a long bone are the epiphyses (singular epiphysis). The middle part along the length of the bone is the diaphysis or shaft, which contains a central cavity filled with yellow bone marrow.

The outer connective tissue around the bone is the periosteum. The inner region is the endosteum. The endosteum consists of a peripheral part, the compact bone, and an inner bone mass, the spongy bone. Most of the spongy bone is present in the epiphyses. The red bone marrow is found in the spaces of the spongy bone.

7.1.2Cells associated with boneOften examined

Three types of cell are associated with bone. All are derived from osteogenic cells.

Osteoblasts

Bone forming cells. They synthesise and secrete the unmineralised ground substance (the bone matrix). Once an osteoblast is surrounded by matrix, it becomes an osteocyte.

Osteocytes

Maintain healthy bone tissue by secreting enzymes and influencing the bone mineral content. They also regulate the release of calcium from bone tissue into the blood.

Osteoclasts

Bone destroying cells. They carry out bone resorption: they break down bone and release calcium and phosphate into the blood. Their work is important for the growth and repair of bone.

7.1.3Structure of cartilageOften examined

Cartilage is not as strong as bone and is present only at particular places. It is more flexible than bone because its matrix is gel-like and contains many collagenous and elastic fibres. The cartilage matrix is covered by a dense layer of collagen fibres, the perichondrium.

Many small cavities, the lacunae, are distributed in the matrix. They contain the cartilage cells, the chondrocytes, which are the living cells of cartilage. Unlike other connective tissues, cartilage does not contain blood vessels, and the chondrocytes are supplied by diffusion. Because of this, cartilage heals very slowly.

Although the human skeleton is first made of cartilages and fibrous membranes, most of these early supports are soon replaced by bone. A few cartilages remain in adults. They are of three types.

7.2Divisions of human skeletonOften examined

The human skeletal system consists of 206 bones. They are divided primarily into two divisions: the axial skeleton and the appendicular skeleton.

7.2.1Axial skeletonOften examined

The axial skeleton includes the skeletal parts present along the central axis of the body: the skull, the vertebral column and the rib cage.

Figure 7.4 shows these parts and the bones of the skull that can be seen from the front (frontal, parietal, temporal, zygomatic, maxilla and mandible). The sections that follow take the skull, the vertebral column and the rib cage in turn.

7.2.2Head bonesOften examined

The head contains 29 bones, which are divided into four divisions: cranial bones, facial bones, ear ossicles and the hyoid bone.

The cranial bones form the cranium (brain box). There are 8 cranial bones. Two are paired, the parietal bones (left and right) and the temporal bones (left and right). Four are unpaired: the frontal bone, occipital bone, ethmoid bone and sphenoid bone.

There are fourteen facial bones, attached to the cranium to form the face. Six are paired: lacrimal, zygomatic, nasal bones, inferior nasal concha, maxilla and palatine. The unpaired bones of the face are the mandible and the vomer.

The three pairs of middle ear ossicles are the malleus, incus and stapes (pronounced stay-peez). The hyoid bone is a small single bone that lies at the base of the skull below the tongue. It does not articulate with any other bone of the head.

7.2.3Vertebral columnOften examined

The vertebral column of a human being consists of thirty-three vertebrae. They are divided into five groups.

The sacral and coccygeal vertebrae are together called the pelvic vertebrae.

7.2.4Rib cageOften examined

The rib cage consists of twelve pairs of ribs. The ribs articulate posteriorly with the thoracic vertebrae. Ten ribs are connected anteriorly with the sternum, either directly or through the costal cartilage. The rib cage provides support for a semi-vacuum chamber called the chest cavity.

7.2.5Appendicular skeleton

The appendicular skeleton includes the skeletal parts that are present in the appendages (arms and legs). These are the pectoral girdle, the pelvic girdle, the forelimbs and the hind limbs.

7.2.6Pectoral girdle and upper limbOften examined

The pectoral girdle consists of a pair of clavicles and a pair of scapulae. The clavicles are a pair of collar bones: one end of each curved bone articulates with the sternum, and the other end articulates with the scapula. The scapulae are the two shoulder blades.

The upper limb (forelimb) consists of the humerus, ulna, radius, carpals, metacarpals and phalanges.

7.2.7Pelvic girdle and lower limbOften examined

The pelvic girdle is made up of three units, the ileum, ischium and pubis, which together form the coxa. The two halves of the pelvic girdle are joined at the pubic symphysis. A cavity called the acetabulum is also present.

The lower limb or hind limb consists of the femur, patella, tibia, fibula, tarsals, metatarsals and phalanges.

7.3JointsOften examined

A joint or articulation is a place where two bones, or a bone and cartilage, come together. The scientific study of the structure and function of joints is called arthrology.

7.3.1Fibrous joints

When the adjacent bones are directly connected to each other by fibrous connective tissue consisting mainly of collagen, the joint is called a fibrous joint. In this joint the bones do not have a joint cavity between them. The gap between the bones may be narrow or wide.

Fibrous joint is found between:

1. Most bones of the skull, called sutures.
2. The shaft regions of the long bones in the forearm and in the leg.
3. The root of a tooth and the socket in the maxilla or mandible (the jawbones).

7.3.2Cartilaginous joints

In a cartilaginous joint the adjacent bones are united by cartilage, a tough but flexible type of connective tissue. These joints lack a joint cavity, and the bones are joined together by either hyaline cartilage or fibrocartilage. Cartilaginous joints allow little movement.

Examples of cartilaginous joints:

1. Costal cartilages that attach the ribs to the sternum.
2. Pubic symphysis and intervertebral disc.

7.3.3Synovial jointsOften examined

Synovial joints are freely moveable joints. The ends of the bones are covered with hyaline cartilage (the articular cartilage) and are held together by a surrounding, tube-like capsule of dense fibrous tissue.

The joint capsule is composed of an outer layer of ligaments and an inner lining of synovial membrane, which secretes synovial fluid.

Science, Technology and Society Connections

Name the techniques for joint replacement

Many joints of the body can be replaced by artificial joints. Joint replacement is called arthroplasty. Artificial joints are usually composed of metal, such as stainless steel or titanium alloys, in combination with modern plastics, such as high-density polythene, silastic or elastomer.

• Partial joint replacement or hemi-replacement: the bone of the articular area is removed on one side.
• Total joint replacement: both sides of the articular area are removed; the artificial articular areas are glued to the bone with a synthetic adhesive, such as methyl methacrylate.

Relate the bipedal posture of man with his skeleton and musculature

Curvatures of the vertebral column help to balance the body for bipedal stance. The intervertebral discs lend flexibility to the vertebral column and absorb vertical shock. The structure of the pelvis, in its attachment to the vertebral column, permits upright posture and locomotion on two appendages (bipedal locomotion).

Certain muscles are active posture muscles, whose primary function is to work in opposition to gravity. For example, the strong, complex muscles of the vertebral column are adapted to provide support and movement in resistance to the effect of gravity. Thus the skeleton and the muscular systems maintain the bipedal posture of man.

7.4Disorders of skeleton

Skeletal deformation may be hereditary (for example, arthritis), hormonal (for example, osteoporosis) or due to nutritional deficiency (for example, osteomalacia and rickets).

The book describes the slipped disc, spondylosis, sciatica and arthritis as common disorders of the skeleton, followed by osteoporosis.

7.4.1Slipped discOften examined

Each intervertebral disc is a cushion-like pad made of a nucleus pulposus and an annulus fibrosus.

Nucleus pulposus

An inner semifluid part that acts like a rubber ball to give the disc its elasticity and compressibility.

Annulus fibrosus

The strong outer ring of fibrocartilage, which holds the successive vertebrae together.

The discs act as shock absorbers. Severe or sudden trauma to the spine may result in herniation of one or more discs.

The herniated disc or slipped disc usually involves rupture of the annulus fibrosus followed by protrusion of the spongy nucleus pulposus. If the protrusion presses on the spinal cord or on spinal nerves, it generates severe pain or even destruction of these nervous structures. "Slipped disc" is misleading, as it is not the whole disc that slides out of position.

7.4.2Spondylosis

Spondylosis is the immobility and fusion of a vertebral joint. Cervical spondylosis results from chronic cervical degeneration, with herniation of the disc, and aging.

7.4.3Sciatica

Sciatica refers to pain, weakness, numbness or tingling in the leg. It is caused by injury to, or pressure on, the sciatic nerve.

Common causes of sciatica include:

• Slipped disc
• Pelvic injury or fracture
• Tumours

7.4.4ArthritisOften examined

Arthritis is the inflammation of joints. Typical symptoms of arthritis include pain after walking, which may later occur even at rest, creaking sounds in the joint, difficulty in getting up from a chair, and pain on walking up and down stairs. There are different types of arthritis.

Osteoarthritis

A progressive disease in which the articular cartilages gradually soften and disintegrate. It affects the knee, hip and intervertebral joints.

Rheumatoid arthritis

The result of an autoimmune disorder in which the synovial membrane becomes inflamed due to a faulty immune system.

Gouty arthritis

Results from a metabolic disorder in which an abnormal amount of uric acid is retained in the blood and sodium urate crystals are deposited in the joints. The most common joint affected is the joint of the big toe.

7.4.5Osteoporosis

Osteoporosis is a common disease characterised by reduced bone mass and an increased risk of fracture. In normal individuals, bone mass increases during skeletal growth to reach a peak between the ages of 20 and 25, but falls thereafter in both sexes.

Osteoporosis occurs because bone resorption exceeds bone deposition. The greater the bone mass before the onset of osteoporosis, the greater the tolerance for bone loss later in life. For this reason it is important for adults, especially women in their twenties and thirties, to take in adequate amounts of calcium.

7.5Bone fracturesOften examined

A fracture is the medical term for a broken bone. Fractures occur when the physical force exerted on the bone is stronger than the bone itself, so bones break when they cannot withstand a force or trauma applied to them.

Common types of fractures

Simple fracture (closed fracture)

A fracture in which the skin is intact.

Compound fracture (open fracture)

A fracture in which the bone ends penetrate the skin and form a wound.

Complicated fracture

A fracture that damages the adjacent organs.

7.5.2Bone repairOften examined

Bone is a living tissue that undergoes repair following a fracture. The repair of a simple fracture takes place in four major steps.

Science, Technology and Society Connections

Relate improper posture to bone and joint problems

Improper posture causes increased stress on joints and their supporting structures, resulting in injury, pain and early degeneration of bones and joints.

7.6Injuries to joints

Torsion or sudden impact to the side of a joint can be devastating. The book discusses two injuries: dislocation and sprain.

Dislocation of joints

A dislocated joint is a joint that slips out of place. It occurs when the ends of bones are forced from their normal positions. A severe dislocation can cause tearing of the muscles, ligaments and tendons that support the joint.

• Symptoms: swelling, intense pain and immobility of the affected joint.
• Most common causes: a blow, fall or other trauma to the joint. In some cases dislocations are caused by a disease or a defective ligament; rheumatoid arthritis can also cause joint dislocation.
• Treatment: a dislocated joint can usually only be successfully "reduced" (put back into its normal position) by a trained medical professional. Surgery may be needed to repair or tighten stretched ligaments.

Sprain

A sprain is an injury to a ligament. The commonly injured ligaments are in the ankle, knee and wrist. The ligaments can be injured by being stretched too far from their normal position.

• The sprain should be rested.
• Sprains can usually be treated conservatively with treatments such as icing and physical therapy.
• Dressings, bandages or ace-wraps should be used to immobilise the sprain and provide support.

7.6.3First aid treatment for disorders of skeleton

Prompt and proper first aid increases the chances of a complete recovery. Usually the severity of a bone fracture or dislocation of joints depends on its cause and on the affected part. If you suspect that someone has dislocated a joint or fractured a bone, you can help by the following steps.

7.7Muscles

The specialised tissues that can undergo contraction and relaxation and provide movements of body parts or of the whole body are called muscles. The study of muscles is called myology.

Muscles also function to hold body parts in postural positions, to move body fluids and to produce heat.

• Movement of body parts or the whole body
• Holding body parts in postural positions
• Movement of body fluids
• Heat production

7.7.1Types of musclesOften examined

There are three types of muscle tissue: smooth, cardiac and skeletal.

Smooth muscles

Smooth muscles are distributed widely throughout the body and are more variable in function than the other muscle types. The smooth muscle cells are spindle shaped, with a single nucleus located in the middle of the cell. Myofilaments are not organised into sarcomeres, so smooth muscle does not have a striated appearance. Smooth muscle cells contain noncontractile intermediate filaments. Smooth muscles are involuntary in function. They are found in the digestive, reproductive and urinary tracts, blood vessels and so on.

Cardiac muscles

Cardiac muscles are found only in the heart. They branch extensively. Cardiac muscles are striated like skeletal muscle, but each cell usually contains one nucleus located near the centre. Adjacent cells join together to form branching fibres by specialised cell-to-cell attachments called intercalated discs, which have gap-like junctions that allow action potentials to pass from cell to cell.

Skeletal muscles

Skeletal muscles are attached to bone and are responsible for movements of body parts and whole body movements (locomotion).

7.8Structure of skeletal musclesOften examined

Skeletal muscles or striated muscles show alternate light and dark regions under the microscope. Skeletal muscles are composed of muscle fibres or muscle cells. Bundles of muscle fibres are enclosed by collagen fibres and connective tissue. At the ends of the muscle, the collagen and connective tissue form tendons, which attach the muscle to skeletal elements.

Each skeletal muscle fibre is a single cylindrical cell, enclosed by a plasma membrane called the sarcolemma, and it has several nuclei. The sarcolemma of the muscle fibre penetrates deep into the cell to form a hollow elongated tube, the transverse tubule (T-tubule). The cytoplasm of the muscle fibre is called sarcoplasm, which contains the sarcoplasmic reticulum.

Within the muscle fibres are numerous thin myofibrils which show characteristic cross striations. The myofibrils are 1 to 2 µm in diameter, run in parallel fashion and extend the entire length of the cell. Each myofibril is composed of two types of myofilaments: thin myofilaments and thick myofilaments.

Connective tissue wrappings of a muscle

Externally the muscle is covered in a connective tissue wrapping called the epimysium. Each skeletal muscle consists of hundreds to thousands of muscle fibres (muscle cells). The muscle is divided into discrete bundles of muscle cells called fascicles. Each fascicle is surrounded by the perimysium. Each fibre within the fascicle is covered by a layer of connective tissue called the endomysium.

7.8.1Ultra-structure of skeletal musclesOften examined

Under a light microscope only the striated nature of the myofibrils can be observed. It is seen as a regular alternation of light and dark bands, called the I bands and A bands respectively, traversed by thin, dark lines. Electron microscope studies show clearly that the bands are due to the regular arrangement of thin and thick myofilaments.

Traversing the middle of each I band is a dark line called the Z line. The section of the myofibril between two Z lines is called a sarcomere, which is the contractile unit. From the Z line the thin myofilaments extend in both directions, while the thick myofilaments are found in the centre of the sarcomere.

In certain regions of the sarcomere, thin and thick myofilaments overlap. Transverse sections in these regions show that six thin myofilaments surround each thick myofilament. This arrangement of thin and thick myofilaments results in a number of other bands being recognisable in the sarcomere.

Thick and thin myofilaments

Thick myofilaments are 16 nm in diameter and are composed of only myosin protein.

Thin filaments are 7 to 8 nm in diameter and are composed of three proteins. Two intertwisted beaded chains of actin form the core of the filament. Two strands of tropomyosin spiral about the actin core and help stiffen it. In a relaxed muscle fibre, they block the myosin binding sites on actin so that the myosin heads cannot bind to the thin filaments.

Troponin is a three-polypeptide complex found at regular intervals on the thin myofilaments: troponin I (TnI), troponin T (TnT) and troponin C (TnC). These proteins are crucial for regulating muscle contraction in both skeletal and cardiac muscles. TnI and TnT are primarily found in cardiac muscle, with small amounts of TnT also present in skeletal muscle. TnC is found in both cardiac and skeletal muscle.

Both troponin and tropomyosin help control the myosin-actin interactions involved in contraction.

Science titbit: the molecules of the filaments

Each myosin molecule consists of six polypeptides, arranged so that each myosin molecule has a tail and two globular heads. Each thick filament contains about 300 myosin molecules bundled together, with their tails forming the central part of the thick filament and their heads facing outward and in opposite directions at each end.

The kidney-shaped polypeptide subunits of actin, called globular actin or G actin, bear the active sites to which the myosin heads attach during contraction. G actin monomers are polymerised into long actin filaments called fibrous, or F actin. The backbone of each thin filament appears to be formed by two intertwined actin filaments that look like a twisted double strand of pearls.

7.9Muscle contraction: sliding filament modelOften examined

The sliding filament theory of contraction states that during contraction the thin myofilaments slide past the thick ones so that they overlap to a greater degree. In a relaxed muscle fibre, the thick and thin myofilaments overlap only at the ends of the A band.

When muscle fibres are stimulated by the nervous system, the myosin heads attach to the myosin binding sites on actin in the thin myofilaments, and the sliding begins. These links are called cross bridges. They are formed and broken several times during a contraction, acting like tiny ratchets to generate tension and propel the thin myofilaments toward the centre of the sarcomere.

As this event occurs simultaneously in sarcomeres throughout the cell, the muscle cell shortens.

7.9Control of cross bridgesOften examined

Muscle contraction is initiated by a nerve impulse arriving at the neuromuscular junction. The nerve impulse is carried through the sarcolemma to the T tubule and then to the sarcoplasmic reticulum (SR). The calcium gates of the SR open, releasing calcium into the cytosol.

When the muscle is at rest, the tropomyosin is disposed in such a way that it covers the sites on the actin chain where the heads of myosin become attached. When calcium ions bind with the troponin molecules, they cause them to move slightly. This has the effect of displacing the tropomyosin and exposing the binding sites for the myosin head. Once the myosin head has become attached to the actin filament, ATP is hydrolysed and the cross bridges are broken down. The formation and breakdown of cross bridges occur again and again during the sliding of the filament.

7.10Antagonistic arrangement of skeletal musclesOften examined

Bones are attached to bones through connective tissue called ligament. When a muscle contracts, one end normally remains stationary and the other end is drawn towards it. The end that remains stationary is called the origin and the one that moves is called the insertion. Both are points of attachment to bones. Every muscle has its own origin and insertion. The belly is the thick part between the origin and insertion, which contracts.

Normally the bone of insertion is pulled when the muscle contracts and is drawn towards the origin, one bone moving on the other at the joint.

Flexor muscle

When it contracts, it bends the bone at the joint.

Extensor muscle

When it contracts, it straightens the bone at the joint.

Antagonistic arrangement

For the movement of a bone in two directions, muscles work in pairs. When flexors contract, the extensors relax, and vice versa. Such an arrangement of muscles is called antagonistic arrangement.

Science, Technology and Society Connections

Justify why the muscles pull but do not push

Bones act as the levers, while joints perform as living fulcrums. A muscle, attached to bones by tendons and other connective tissue, exerts force by converting chemical energy into tension and contraction. When a muscle contracts, it shortens, in many cases pulling a bone like a lever across its hinge. Muscles move and by their motions we move. We are capable of performing a wide variety of actions, but despite this, muscle itself moves only by becoming shorter. They shorten and then they rest. In other words, a muscle can pull but it cannot push.

7.10Movement in knee jointOften examined

The knee or tibio-femoral joint is located between the femur and the tibia. It is a complex hinge joint that permits limited rolling and gliding movements in addition to flexion and extension.

Flexion

Flexion is carried out by the flexor muscles. These are the hamstring muscles present at the back of the upper part of the leg (thigh). The major hamstring muscle is biceps femoris. It has two origins, one from the pelvic girdle and the other from the top of the femur. At its insertion the tendon divides into two portions to attach at the upper part of the tibia and fibula. A fusiform muscle located in the posterior compartment of the thigh is called semitendinosus. One of the hamstring muscles located in the posterior (back) compartment of the thigh is called semimembranosus.

Extension

Extension is carried out by the extensor muscles, which are present in the front of the thigh. The main extensor muscles are the quadriceps femoris. They originate at the ilium and femur, come together in a tendon surrounding the patella (kneecap), and insert at the tibia. These extend the leg at the knee joint and are important for standing, walking and almost all activities involving the legs.

7.11Muscle disorders

There are many problems related to muscle that are generally called muscle disorders. The book discusses muscle fatigue, cramp and tetany.

Muscle fatigue

When the muscles lose the ability to contract, the physiological state of the muscles is called muscle fatigue. Other factors that contribute to muscle fatigue are the accumulation of lactic acid and ionic imbalance. The cause of extreme fatigue is lactic acid, which causes the muscle pH to drop and the muscle to ache.

Cramp

Cramp is also known as tetanic contraction of the entire muscle. It lasts for just a few seconds to several hours, causing the muscle to become taut (tightly drawn) and painful. It is most common in the thigh and hip muscles. It usually occurs at night or after exercise. It reflects low blood sugar level, electrolyte depletion, dehydration and irritability of the spinal cord and neurons.

Tetany

In tetany the body shakes from continuous muscle contraction and convulsion occurs due to calcium imbalance. It results in excitability of neurons and results in loss of sensation. If untreated, the symptoms progress to spasm of the larynx, paralysis and ultimately death.

Science, Technology and Society Connections

Reason out the rigor mortis

When death occurs, ATP is no longer made. It is a short-lived chemical and so it runs out fairly quickly. This causes the muscles to lock into position, as cross-bridges that formed between actin and myosin filaments before death can no longer be broken. The skeletal muscles undergo a partial contraction that causes the joints to become fixed. This condition, rigor mortis (rigidity of death), happens in all body muscles. It appears about four hours after death and lasts about 34 hours. After this time, muscle proteins are destroyed by enzymes within the cells and so rigor mortis disappears.

STEAM Activity 7.1

1. Identification of the bones of the pelvic girdle, pectoral girdle, arms and legs by using a model of the human skeleton.
2. Comparison of the structure of skeletal, smooth and cardiac muscles with the help of prepared slides.

For the first activity, use Figure 7.4 and the tables of the bones of the upper and lower limbs to name each bone on the model. For the second, compare the slides with Table 7.2: striations, number of nuclei and cell shape.

Key terms

Osteology

The study of bones and cartilage.

Epiphysis

The terminal broad part of a long bone; it holds most of the spongy bone.

Diaphysis (shaft)

The middle part along the length of a long bone, with a central cavity filled with yellow bone marrow.

Periosteum

The outer connective tissue around the bone.

Endosteum

The inner region of the bone, made of compact bone and spongy bone.

Osteoblasts

Bone forming cells that synthesise and secrete the unmineralised ground substance; once surrounded by matrix they become osteocytes.

Osteocytes

Cells that maintain healthy bone tissue by secreting enzymes and influencing bone mineral content, and that regulate the release of calcium from bone to blood.

Osteoclasts

Bone destroying cells that carry out bone resorption, releasing calcium and phosphate into the blood.

Lacunae

Small cavities in the cartilage matrix that contain the chondrocytes.

Chondrocytes

The living cells of cartilage.

Perichondrium

The dense layer of collagen fibres that covers the cartilage matrix.

Hyaline cartilage

Cartilage found at the ends of long bones and in the nose, larynx and trachea.

Fibrocartilage

Cartilage with wide rows of thick collagenous fibres, found in the discs between the vertebrae and in the cartilage of the knee.

Axial skeleton

The skeletal parts along the central axis of the body: skull, vertebral column and rib cage.

Appendicular skeleton

The skeletal parts present in the appendages: pectoral girdle, pelvic girdle, forelimbs and hind limbs.

Cranial bones

The eight bones that form the cranium (brain box): parietal, temporal, frontal, occipital, ethmoid and sphenoid.

Hyoid bone

A small single bone at the base of the skull below the tongue that does not articulate with any other bone of the head.

Atlas and axis

The first and second cervical vertebrae.

Sacrum and coccyx

The sacrum is the five fused sacral vertebrae; the coccyx is the four fused coccygeal vertebrae. Together they are the pelvic vertebrae.

True, false and floating ribs

Seven pairs of ribs attach directly to the sternum (true); the 8th, 9th and 10th attach by a common costal cartilage (false); the 11th and 12th do not attach to the sternum (floating).

Pectoral girdle

A pair of clavicles (collar bones) and a pair of scapulae (shoulder blades).

Pelvic girdle

Ilium, ischium and pubis, which together form the coxa; the two halves join at the pubic symphysis.

Acetabulum

The cavity of the pelvic girdle into which the head of the femur fits.

Joint (articulation)

A place where two bones, or a bone and cartilage, come together.

Fibrous joint

A joint in which bones are directly connected by fibrous connective tissue; immoveable.

Cartilaginous joint

A joint in which bones are united by hyaline cartilage or fibrocartilage; slightly moveable.

Synovial joint

A freely moveable joint whose bone ends are covered with hyaline cartilage and held by a capsule that secretes synovial fluid.

Arthroplasty

Joint replacement with artificial joints.

Nucleus pulposus

The inner semifluid part of an intervertebral disc that gives it elasticity and compressibility.

Annulus fibrosus

The strong outer ring of fibrocartilage of an intervertebral disc that holds successive vertebrae together.

Slipped (herniated) disc

Rupture of the annulus fibrosus followed by protrusion of the nucleus pulposus, which may press on the spinal cord or spinal nerves.

Spondylosis

The immobility and fusion of a vertebral joint.

Sciatica

Pain, weakness, numbness or tingling in the leg caused by injury to or pressure on the sciatic nerve.

Arthritis

The inflammation of joints; types are osteoarthritis, rheumatoid arthritis and gouty arthritis.

Osteoporosis

A disease characterised by reduced bone mass and an increased risk of fracture, because bone resorption exceeds bone deposition.

Simple (closed) fracture

A fracture in which the skin is intact.

Compound (open) fracture

A fracture in which the bone ends penetrate the skin and form a wound.

Complicated fracture

A fracture that damages the adjacent organs.

Haematoma

A mass of clotted blood that forms at a fracture site.

Callus

A porous mass of bone and cartilage that surrounds a break and holds the bone ends together; the bony callus is its later hard form of spongy bone.

Dislocation

A joint that slips out of place when the ends of bones are forced from their normal positions.

Sprain

An injury to a ligament caused by stretching it too far from its normal position.

Intercalated discs

Specialised cell-to-cell attachments of cardiac muscle with gap-like junctions that let action potentials pass from cell to cell.

Sarcolemma

The plasma membrane of a muscle fibre.

Transverse tubule (T-tubule)

A hollow elongated tube formed where the sarcolemma penetrates deep into the muscle fibre.

Sarcoplasm and sarcoplasmic reticulum

The cytoplasm of the muscle fibre, and the reticulum it contains whose gates release calcium.

Myofibril

A thin, cross-striated structure 1 to 2 µm in diameter running the length of a muscle fibre, made of thin and thick myofilaments.

Sarcomere

The section of a myofibril between two Z lines; the contractile unit.

I band, A band and H zone

I band: thin filaments only (isotropic). A band: the full length of the thick filaments (anisotropic). H zone: the lighter centre of the A band with no overlap in a relaxed sarcomere.

Z line and M line

The Z line crosses the middle of the I band; the M line bisects the H zone and joins adjacent myosin filaments.

Actin and myosin

Actin (with tropomyosin and troponin) makes the thin myofilaments; myosin alone makes the thick myofilaments.

Tropomyosin and troponin

Tropomyosin covers the myosin binding sites on actin in a relaxed fibre; the troponin complex (TnI, TnT, TnC) helps control the myosin-actin interaction, TnC binding calcium ions.

Sliding filament theory

During contraction the thin myofilaments slide past the thick ones so that they overlap to a greater degree.

Cross bridges

Links formed between myosin heads and actin that form and break repeatedly like tiny ratchets during contraction.

Origin and insertion

The stationary attachment of a muscle and the attachment that moves when the muscle contracts.

Flexor and extensor

A flexor bends a bone at the joint when it contracts; an extensor straightens it.

Antagonistic arrangement

Muscles working in pairs so that when flexors contract the extensors relax, and vice versa.

Muscle fatigue

The physiological state in which muscles lose the ability to contract.

Cramp and tetany

Cramp is tetanic contraction of an entire muscle; tetany is continuous contraction and convulsion due to calcium imbalance.

Rigor mortis

Rigidity of death: muscles lock because ATP is no longer made and cross bridges cannot be broken.

FBISE 2026 questions

Questions from the FBISE HSSC-II 2026 (1st Annual) papers that belong to this unit, answered the way an examiner expects.

Exam focus

1. Explain the sliding filament model of muscle contraction with a labelled diagram of the relaxed, partly contracted and fully contracted sarcomere. (long)
   The 7-mark Section C question of Set B (Q.3 OR); Set D MCQ 5 asks the same model. Book long question 10.
   Go to answer
2. State what happens to the I band, H zone, A band and Z lines when a sarcomere contracts. (short)
   Asked in Set B (MCQ 4, Q.2 (iii) OR) and Set D (Q.2 (iii) b). The A band staying constant is the repeated point.
   Go to answer
3. Identify the bands and lines of a sarcomere (Z line, I band, A band, H zone, M line) from a diagram and draw it labelled. (short)
   Set D MCQ 15 shows an unlabelled sarcomere and asks for a band; book short question 19 asks to draw and label it.
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4. Describe the role of calcium ions, troponin and tropomyosin in controlling the cross bridges. (short)
   Set D Q.2 (iii) a asks the role of calcium; Set B MCQ 14 asks which proteins make the thin filament.
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5. Describe osteoporosis (cause and symptoms) and compare it with arthritis. (short)
   Both sets: Set B Q.2 (vii) compares arthritis and osteoporosis and Set D Q.2 (vii) asks causes and symptoms.
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6. What is a sprain? Give its symptoms and management, and distinguish it from dislocation. (short)
   Set D Q.2 (v) OR; book long question 7.
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7. Differentiate between rheumatoid arthritis and gouty arthritis. (short)
   Arthritis was compared in Set B Q.2 (vii); the book asks this difference as short question 20 (h).
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8. Explain the structure of bone and of cartilage with labelled diagrams. (long)
   No bone or cartilage question in 2026, but both are book long questions 1 and 2 and are standard draw-and-label items.
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9. Differentiate between tetany and tetanus, and describe fatigue, cramp and tetany. (short)
   Not asked in 2026; book short question 20 (m) and long question 13, a typical 3-mark comparison.
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