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ITEC Anatomy & Physiology practice questions

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Question 1The cell and tissues of the body

Which membrane-bound organelle contains hydrolytic enzymes that function optimally at an acidic pH?

  1. Peroxisome
  2. Lysosome Correct answer.
  3. Golgi apparatus
  4. Ribosome

Explanation

Lysosomes are membrane-bound vesicles whose interior is kept acidic by proton pumps, and this low pH is the optimum for their acid hydrolases that digest engulfed material and worn-out organelles. Ribosomes are non-membranous, and the Golgi apparatus modifies and sorts proteins rather than serving as the main acidic digestive compartment.

Question 2The cell and tissues of the body

A red blood cell is placed in a solution whose solute concentration is lower than that of the cytosol. What happens to the cell?

  1. It swells as water enters by osmosis Correct answer.
  2. It stays the same size because solutes cannot move
  3. It loses its nucleus before any change in volume
  4. It shrinks as water leaves the cell

Explanation

A solution that is hypotonic relative to the cytosol has a lower solute concentration, so water moves down its concentration gradient into the cell across the selectively permeable membrane. The resulting gain of water increases cell volume and can cause the cell to swell and eventually lyse.

Question 3The cell and tissues of the body

Which type of epithelium lines the air sacs of the lungs and is adapted for rapid gas exchange?

  1. Simple squamous epithelium Correct answer.
  2. Simple cuboidal epithelium
  3. Stratified squamous epithelium
  4. Transitional epithelium

Explanation

Simple squamous epithelium is a single layer of flattened cells, which gives the shortest diffusion distance and allows rapid exchange of oxygen and carbon dioxide across the alveolar wall. Its thinness is the key structural feature matching the function of gas exchange.

Question 4The cell and tissues of the body

Which cytoskeletal element is the primary component of microvilli and is also involved in cytokinesis?

  1. Actin microfilaments Correct answer.
  2. Microtubules
  3. Collagen fibres
  4. Intermediate filaments

Explanation

Actin microfilaments are the core of microvilli, supporting the apical surface where absorption occurs, and they form the contractile ring that pinches a dividing cell into two during cytokinesis. Microtubules and intermediate filaments serve other mechanical and transport roles.

Question 5The cell and tissues of the body

A tissue sample contains abundant collagen fibres, few cells, and very little ground substance, and it resists pulling forces in one direction. Which tissue is described?

  1. Hyaline cartilage
  2. Areolar connective tissue
  3. Adipose tissue
  4. Dense regular connective tissue Correct answer.

Explanation

Dense regular connective tissue packs parallel bundles of collagen with sparse cells and little ground substance, which gives it high tensile strength along the direction of fibre alignment, as seen in tendons and ligaments. Loose and adipose tissues have much more ground substance or stored lipid.

Question 6The cell and tissues of the body

Which cell junction forms a seal between adjacent epithelial cells that prevents leakage between them?

  1. Hemidesmosome
  2. Desmosome
  3. Tight junction Correct answer.
  4. Gap junction

Explanation

Tight junctions bring the membranes of neighbouring cells into close contact and close the intercellular space, creating a barrier that limits paracellular movement of substances. Desmosomes and hemidesmosomes provide mechanical anchoring rather than sealing.

Question 7The cell and tissues of the body

A ribosome is translating a messenger RNA molecule. Which statement correctly describes what happens next?

  1. The large ribosomal subunit synthesises lipids from the messenger RNA template
  2. Transfer RNA carries amino acids that are joined in the order specified by the messenger RNA codons Correct answer.
  3. Messenger RNA is copied into transfer RNA inside the nucleus
  4. Amino acids are added in random order until a stop signal is reached

Explanation

During translation, transfer RNA molecules pair their anticodons with complementary codons on the messenger RNA and deliver the corresponding amino acids, which the ribosome links into a polypeptide in the encoded sequence. The order of amino acids is therefore determined by the codon sequence, not by chance.

Question 8The cell and tissues of the body

Which organelle is the main site of ATP production by oxidative phosphorylation in a cell?

  1. Nucleolus
  2. Golgi apparatus
  3. Smooth endoplasmic reticulum
  4. Mitochondrion Correct answer.

Explanation

The inner mitochondrial membrane houses the electron transport chain and ATP synthase, which together carry out oxidative phosphorylation and generate most of the cell's ATP. The other listed organelles have different primary functions.

Question 9The cell and tissues of the body

In the cell cycle, during which phase does DNA replication occur?

  1. G1 phase
  2. S phase Correct answer.
  3. G2 phase
  4. M phase

Explanation

DNA replication takes place during the S phase of interphase, so that each chromosome is duplicated before mitosis. G1 and G2 are gap phases of growth and preparation, and M phase is when the duplicated chromosomes are separated and the cell divides.

Question 10The cell and tissues of the body

Which statement best describes the role of the sodium-potassium pump in maintaining the resting membrane potential of a neuron?

  1. It transports chloride ions into the cell to make the interior more negative
  2. It allows both ions to diffuse freely so that the membrane potential disappears
  3. It moves sodium and potassium down their concentration gradients without using energy
  4. It expels three sodium ions and admits two potassium ions per ATP hydrolysed, contributing to a negative interior Correct answer.

Explanation

The sodium-potassium pump is an active transporter that uses ATP to move three sodium ions out and two potassium ions in against their gradients. Because more positive charge leaves than enters, the pump contributes to the negative resting membrane potential in addition to establishing the ion gradients.

Question 11The cell and tissues of the body

Which feature distinguishes cardiac muscle from skeletal muscle?

  1. Cardiac muscle cells are branched and joined by intercalated discs containing gap junctions Correct answer.
  2. Cardiac muscle cells contain no mitochondria
  3. Cardiac muscle cells are multinucleated and voluntary
  4. Cardiac muscle cells lack striations entirely

Explanation

Cardiac muscle cells branch and meet at intercalated discs, which contain desmosomes for mechanical coupling and gap junctions that let electrical activity spread so the heart contracts as a coordinated unit. Skeletal muscle fibres are instead long, unbranched and multinucleated.

Question 12The cell and tissues of the body

A cell that secretes large amounts of protein hormone would be expected to contain especially abundant amounts of which organelle arrangement?

  1. Smooth endoplasmic reticulum and peroxisomes
  2. Mitochondria and microtubules only
  3. Lysosomes and nucleoli
  4. Rough endoplasmic reticulum and Golgi apparatus Correct answer.

Explanation

Proteins destined for secretion are synthesised on the rough endoplasmic reticulum and then processed, sorted and packaged by the Golgi apparatus before release. Cells specialised for protein secretion therefore show an expanded rough endoplasmic reticulum and prominent Golgi stacks.

Question 13The cell and tissues of the body

Which tissue type is avascular, has cells called chondrocytes in lacunae, and resists compression?

  1. Compact bone
  2. Smooth muscle
  3. Cartilage Correct answer.
  4. Dense irregular connective tissue

Explanation

Cartilage is avascular and its chondrocytes sit in small cavities called lacunae within a firm matrix that resists compression while allowing some flexibility. Because it lacks blood vessels, nutrients reach the cells mainly by diffusion from surrounding tissue.

Question 14The cell and tissues of the body

During which stage of mitosis do chromosomes align along the equator of the cell?

  1. Metaphase Correct answer.
  2. Telophase
  3. Anaphase
  4. Prophase

Explanation

In metaphase the chromosomes line up along the metaphase plate at the cell equator, held by spindle fibres attached to their kinetochores. This alignment allows sister chromatids to be pulled to opposite poles in the next stage.

Question 15The cell and tissues of the body

A cell is placed in a hypertonic solution. Which change is expected, and why?

  1. The cell swells because solute enters faster than water leaves
  2. The cell bursts because the membrane becomes freely permeable
  3. The cell volume is unchanged because the membrane is impermeable to water
  4. The cell shrinks because water leaves by osmosis Correct answer.

Explanation

In a hypertonic solution the surrounding fluid has a higher solute concentration than the cytosol, so water moves out of the cell by osmosis and the cell shrinks. The membrane is permeable to water, which is why the osmotic gradient drives the volume change.

Question 16The cell and tissues of the body

Which statement correctly describes the relationship between transcription and translation in a human cell?

  1. Both processes occur inside the mitochondrion only
  2. Transcription occurs in the nucleus and translation occurs on ribosomes in the cytosol Correct answer.
  3. Both processes occur simultaneously on the same messenger RNA in the cytosol
  4. Translation produces messenger RNA, which is then transcribed into protein

Explanation

In eukaryotic cells transcription takes place in the nucleus, where DNA is used as a template to make messenger RNA, and the messenger RNA is then exported to the cytosol for translation on ribosomes. The two processes are therefore separated in space and time.

Question 17The cell and tissues of the body

A mutation changes a single DNA base but the resulting polypeptide has the same amino acid sequence. Which explanation best accounts for this?

  1. Transfer RNA can substitute any amino acid for another
  2. The genetic code is degenerate, so different codons can specify the same amino acid Correct answer.
  3. The mutation occurred in an intron that was removed before translation
  4. The ribosome proofreads and reverses every mutation

Explanation

The genetic code is degenerate, meaning that most amino acids are encoded by more than one codon, so a base change can produce a synonymous codon and leave the protein sequence unchanged. Such a change is called a silent mutation.

Question 18The cell and tissues of the body

An epithelial sheet must withstand strong mechanical stretching without tearing, and its cells must remain electrically and chemically coupled. Which junction combination is most appropriate?

  1. Hemidesmosomes and tight junctions only
  2. Desmosomes and gap junctions Correct answer.
  3. Gap junctions only
  4. Tight junctions and gap junctions only

Explanation

Desmosomes anchor intermediate filaments of adjacent cells to one another and provide the mechanical strength needed to resist stretching, while gap junctions allow ions and small molecules to pass so that cells remain coupled. Using both together gives strength plus communication.

Question 19The cell and tissues of the body

A tissue is described as having a mineralised matrix, osteocytes housed in lacunae, and a rich blood supply entering through canals. Which tissue is it, and what is the main function of its matrix?

  1. Hyaline cartilage, providing flexible support
  2. Reticular connective tissue, providing a framework for lymphoid organs
  3. Areolar connective tissue, providing a diffusion medium
  4. Compact bone, providing rigidity and protection Correct answer.

Explanation

Compact bone has a mineralised matrix containing collagen and calcium salts, osteocytes in lacunae, and a blood supply carried through central canals. This combination gives bone its rigidity, load-bearing strength and protective role, unlike the unmineralised matrices of cartilage and loose connective tissues.

Question 20The cell and tissues of the body

A drug blocks the electron transport chain in a cell that is actively secreting protein. Which prediction is most consistent with cell biology principles?

  1. The Golgi apparatus begins to synthesise DNA to compensate
  2. Protein synthesis increases because the ribosome no longer needs energy
  3. Osmosis stops immediately because water transport requires ATP
  4. ATP production falls, and secretion of the protein is impaired because transport and packaging require energy Correct answer.

Explanation

Oxidative phosphorylation supplies most of the ATP used by the cell, and steps such as vesicular transport from the endoplasmic reticulum to the Golgi and exocytosis depend on ATP. Blocking the electron transport chain therefore reduces ATP and impairs the secretory pathway.

Question 21The skeletal system

Which type of cartilage forms the embryonic model that is later replaced by bone during endochondral ossification?

  1. Fibrocartilage
  2. Hyaline cartilage Correct answer.
  3. Reticular cartilage
  4. Elastic cartilage

Explanation

Endochondral ossification begins with a hyaline cartilage template that is gradually resorbed and replaced by osseous tissue. Hyaline cartilage is the most abundant skeletal cartilage and provides the smooth, low-friction surface that persists as articular cartilage. The other cartilage types are not used as ossification templates.

Question 22The skeletal system

The shaft of a long bone, which contains the medullary cavity, is correctly termed the:

  1. Diaphysis Correct answer.
  2. Epiphysis
  3. Metaphysis
  4. Periosteum

Explanation

The diaphysis is the tubular shaft of a long bone and encloses the medullary cavity that houses marrow in adults. The epiphyses are the expanded ends, and the metaphysis is the region where the diaphysis meets the epiphysis, containing the epiphyseal plate in growing bone. The periosteum is the outer connective tissue covering rather than a region of the shaft.

Question 23The skeletal system

Which cell type is responsible for resorbing bone matrix during remodeling and calcium mobilization?

  1. Osteoprogenitor cell
  2. Osteoblast
  3. Osteoclast Correct answer.
  4. Osteocyte

Explanation

Osteoclasts are large multinucleated cells derived from the monocyte lineage that secrete acid and proteolytic enzymes to dissolve mineral and degrade matrix. Osteoblasts build new matrix, while osteocytes are mature cells housed in lacunae that maintain matrix and sense mechanical load. Osteoprogenitor cells are the mesenchymal precursors that give rise to osteoblasts.

Question 24The skeletal system

The concentric rings of mineralized matrix that surround a central canal in compact bone are called:

  1. Canaliculi
  2. Lamellae Correct answer.
  3. Lacunae
  4. Trabeculae

Explanation

Lamellae are the concentric layers of collagen-rich mineralized matrix that make up an osteon around a central (Haversian) canal. Lacunae are the small cavities that house osteocytes, and canaliculi are the narrow channels connecting them. Trabeculae are the branching struts of spongy bone, not the rings of compact bone.

Question 25The skeletal system

Which bone of the axial skeleton does not articulate directly with any other bone?

  1. Mandible
  2. Vomer
  3. Sternum
  4. Hyoid Correct answer.

Explanation

The hyoid is a U-shaped bone in the anterior neck that is suspended by muscles and ligaments and forms no direct synovial or cartilaginous joint with another bone. The vomer is part of the nasal septum and articulates with several skull bones, and the mandible articulates with the temporal bone at the temporomandibular joint. The sternum articulates with the clavicles and costal cartilages.

Question 26The skeletal system

The joint between the two pubic bones, in which bones are united by fibrocartilage, is classified as a:

  1. Symphysis Correct answer.
  2. Suture
  3. Gomphosis
  4. Synovial joint

Explanation

A symphysis is a cartilaginous joint in which fibrocartilage binds the opposed bone surfaces, allowing slight movement; the pubic symphysis and intervertebral joints are examples. Sutures and gomphoses are fibrous joints, and synovial joints possess a fluid-filled cavity.

Question 27The skeletal system

Which region of the vertebral column contains the largest number of individual vertebrae in a normal adult?

  1. Cervical
  2. Sacral
  3. Lumbar
  4. Thoracic Correct answer.

Explanation

The thoracic region contains twelve vertebrae, more than any other region of the column. The cervical region has seven, the lumbar region has five, and the sacrum is formed from five fused vertebrae counted as a single bone.

Question 28The skeletal system

In a healthy adult, the epiphyseal plate of a long bone has been replaced by which structure?

  1. A secondary ossification center of cartilage
  2. A synovial membrane
  3. An epiphyseal line of fused bone Correct answer.
  4. A layer of hyaline articular cartilage

Explanation

Longitudinal growth ends when the cartilage of the epiphyseal plate is fully replaced by bone, leaving a thin bony epiphyseal line. The plate exists only during growth, and its closure is a marker of skeletal maturity. Articular cartilage persists on the joint surface but does not occupy the former plate site.

Question 29The skeletal system

A deficiency of vitamin D in a child leads to defective mineralization of osteoid, producing which condition?

  1. Osteomalacia
  2. Rickets Correct answer.
  3. Achondroplasia
  4. Osteoporosis

Explanation

Vitamin D is required for intestinal calcium and phosphate absorption, and its deficiency in growing children impairs mineralization of the epiphyseal cartilage and osteoid, producing rickets with bowed limbs. The same defective mineralization in adults, after plate closure, is termed osteomalacia. Osteoporosis is a loss of bone mass rather than a mineralization defect, and achondroplasia is a genetic growth disorder.

Question 30The skeletal system

Parathyroid hormone raises blood calcium partly by acting on bone. Which cellular response best explains this effect?

  1. Increased osteoclast activity and bone resorption Correct answer.
  2. Reduced renal calcium reabsorption
  3. Increased osteoblast activity and matrix deposition
  4. Increased osteoid calcification without resorption

Explanation

Parathyroid hormone stimulates osteoclast-mediated resorption, directly or indirectly via osteoblast signaling, releasing calcium and phosphate into blood. It also increases renal calcium reabsorption and activates vitamin D. The net effect is to defend plasma calcium at the expense of bone mineral.

Question 31The skeletal system

Which statement correctly describes the arrangement of spongy bone?

  1. It consists of trabeculae oriented along lines of mechanical stress Correct answer.
  2. It lacks osteocytes because it has no lacunae
  3. It is organized into cylindrical osteons with central canals
  4. It forms the outer shell of every long bone shaft

Explanation

Spongy bone is composed of a network of trabeculae that align along the directions of habitual stress, giving strength with low mass. It lacks the tightly packed osteons of compact bone, though it still contains osteocytes in lacunae nourished by canaliculi. Compact bone, not spongy bone, forms the dense outer shell of the diaphysis.

Question 32The skeletal system

A vertebra with a prominent spinous process that projects inferiorly and overlaps the next vertebra, plus facets for rib articulation, belongs to which region?

  1. Thoracic Correct answer.
  2. Coccygeal
  3. Lumbar
  4. Cervical

Explanation

Thoracic vertebrae are identified by costal facets for rib articulation and long spinous processes that slope downward, shingling over the vertebra below. Cervical vertebrae have small bodies and often bifid spinous processes with transverse foramina, and lumbar vertebrae are large-bodied with blunt, horizontally projecting spinous processes.

Question 33The skeletal system

Which structural feature distinguishes a true rib from a floating rib?

  1. A true rib attaches directly to the sternum by its own costal cartilage Correct answer.
  2. A true rib has no costal cartilage at all
  3. A true rib articulates only with the lumbar vertebrae
  4. A true rib lacks a head and tubercle

Explanation

True ribs are the first seven pairs, each attaching directly to the sternum through its own costal cartilage. Floating ribs are the last two pairs, whose cartilages do not reach the sternum. All ribs bear a head, and false ribs may join the sternum indirectly or not at all.

Question 34The skeletal system

During intramembranous ossification, bone develops directly from which tissue?

  1. Skeletal muscle
  2. Dense regular connective tissue
  3. Mesenchymal connective tissue Correct answer.
  4. Hyaline cartilage

Explanation

Intramembranous ossification forms bone directly within sheets of mesenchymal tissue, as occurs in the flat bones of the skull and the clavicle. Mesenchymal cells condense and differentiate into osteoblasts that deposit osteoid without a cartilage intermediate. Endochondral ossification, by contrast, uses a hyaline cartilage model.

Question 35The skeletal system

The hormone calcitonin influences blood calcium by which mechanism?

  1. Inhibiting osteoclast activity and promoting calcium deposition Correct answer.
  2. Stimulating osteoclast resorption
  3. Increasing intestinal calcium absorption
  4. Reducing renal calcium excretion

Explanation

Calcitonin, secreted by thyroid parafollicular cells, lowers blood calcium by inhibiting osteoclast resorption and favoring mineral deposition in bone. It also increases renal calcium excretion. Its effect is modest in adults but pharmacologically useful in hypercalcemia.

Question 36The skeletal system

Which joint is a hinge-type synovial joint that permits flexion and extension only?

  1. The hip joint
  2. The atlantoaxial joint
  3. The shoulder joint
  4. The elbow joint Correct answer.

Explanation

The humeroulnar part of the elbow is a hinge (ginglymus) joint whose motion is essentially confined to flexion and extension in one plane. The shoulder and hip are ball-and-socket joints allowing movement in multiple planes, and the atlantoaxial joint is a pivot joint permitting rotation.

Question 37The skeletal system

In the pelvic girdle, which bone bears the acetabulum and articulates with the sacrum at the sacroiliac joint?

  1. Femur
  2. Ischium alone
  3. Os coxae Correct answer.
  4. Sacrum

Explanation

Each os coxae is a fused hip bone formed from the ilium, ischium, and pubis, and it contains the acetabulum and articulates with the sacrum. The femur is the thigh bone that fits into the acetabulum, and the sacrum is part of the axial skeleton. The ischium is only one component of the hip bone.

Question 38The skeletal system

A fracture in which the broken bone pierces the overlying skin is best described as:

  1. Comminuted fracture
  2. Open (compound) fracture Correct answer.
  3. Greenstick fracture
  4. Closed (simple) fracture

Explanation

An open or compound fracture is one in which the bone ends communicate with the external environment through a break in the skin, raising the risk of infection. A closed fracture leaves the skin intact, a greenstick fracture is an incomplete break in a pliable bone, and a comminuted fracture involves multiple fragments.

Question 39The skeletal system

Which sequence correctly describes the stages of bone fracture repair?

  1. Hematoma formation, fibrocartilaginous callus, bony callus, remodeling Correct answer.
  2. Fibrocartilaginous callus, remodeling, hematoma formation, bony callus
  3. Remodeling, bony callus, hematoma formation, fibrocartilaginous callus
  4. Bony callus, hematoma formation, remodeling, fibrocartilaginous callus

Explanation

Repair begins with a hematoma that organizes into a soft callus, which is converted to a fibrocartilaginous callus and then replaced by a bony callus. Finally, excess callus is removed and the bone is remodeled along lines of stress. The other sequences place remodeling or callus formation before the initial hematoma, which is impossible.

Question 40The skeletal system

A long bone's growth in length occurs at which site, and by which process?

  1. At the periosteum, by appositional growth
  2. At the medullary cavity, by endosteal deposition
  3. At the articular surface, by intramembranous ossification
  4. At the epiphyseal plate, by interstitial growth of cartilage Correct answer.

Explanation

Lengthwise growth depends on interstitial growth of cartilage within the epiphyseal plate, where chondrocytes divide and are progressively replaced by bone on the diaphyseal side. Appositional growth at the periosteum increases bone thickness, not length. Intramembranous ossification is not responsible for elongation of long bones.

Question 41The skeletal system

Why does spongy bone tolerate compressive load with far less mass than an equal volume of compact bone?

  1. Its matrix lacks collagen and is therefore lighter
  2. Its lacunae are filled with marrow rather than osteocytes
  3. Its osteocytes are absent, eliminating cellular mass
  4. Its trabeculae are arranged along stress lines, concentrating material where force is greatest Correct answer.

Explanation

Trabeculae form a lattice aligned with habitual loading directions, so material is placed where stress is highest while unloaded space is minimized. This architecture yields high strength-to-weight ratio, as seen in the femoral neck. The matrix still contains collagen, and osteocytes occupy lacunae in spongy bone as in compact bone.

Question 42The skeletal system

Removal of the parathyroid glands would most directly produce which set of changes?

  1. Hypercalcemia with increased bone resorption
  2. Hypercalcemia with decreased neuromuscular excitability
  3. Hypocalcemia with increased neuromuscular excitability Correct answer.
  4. No change in calcium because calcitonin compensates fully

Explanation

Loss of parathyroid hormone reduces osteoclastic resorption, renal calcium reabsorption, and calcitriol activation, causing hypocalcemia. Low plasma calcium raises neuromuscular excitability, producing tetany. Calcitonin cannot fully compensate because its calcium-lowering action would worsen, not correct, the deficit.

Question 43The skeletal system

A patient's radiograph shows reduced bone density with preserved mineral composition and thinning of trabeculae. Which process best accounts for these findings?

  1. Failure of the epiphyseal plate to close
  2. Impaired mineralization of osteoid
  3. An imbalance in which resorption exceeds formation Correct answer.
  4. Excessive deposition of osteoid without resorption

Explanation

Osteoporosis reflects a remodeling imbalance in which osteoclastic resorption outpaces osteoblastic formation, reducing bone mass while the remaining matrix is normally mineralized. Trabeculae thin and become more widely spaced, weakening the bone. Impaired mineralization would instead indicate osteomalacia.

Question 44The skeletal system

Which feature of the pelvic girdle most reliably distinguishes a female pelvis from a male pelvis?

  1. A more vertically oriented ilium
  2. A longer, more curved sacrum
  3. A wider, more circular pelvic inlet Correct answer.
  4. A narrower subpubic angle

Explanation

The female pelvis typically has a wider, more circular inlet and a broader subpubic angle to accommodate childbirth. Male pelves tend to be narrower with a sharper subpubic angle and a longer sacrum. These differences are statistical tendencies rather than absolute criteria.

Question 45The muscular system

Which property is shared by skeletal muscle, cardiac muscle, and smooth muscle, and is defined as the ability to shorten forcibly when adequately stimulated?

  1. Elasticity
  2. Contractility Correct answer.
  3. Excitability
  4. Extensibility

Explanation

Contractility is the special functional characteristic of muscle tissue that allows it to generate tension and shorten. Extensibility, elasticity, and excitability are separate properties that muscle also possesses but that describe different behaviours. Only contractility specifically refers to forcible shortening after stimulation.

Question 46The muscular system

A muscle fibre is described as having striations visible under the microscope. Which ultrastructural arrangement produces this banded appearance?

  1. Radial arrays of microtubules anchoring the sarcolemma
  2. Uniformly distributed glycogen granules between mitochondria
  3. Alternating dark A bands and light I bands created by overlapping thick and thin myofilaments Correct answer.
  4. Randomly oriented collagen fibres in the surrounding epimysium

Explanation

Striations arise because myosin thick filaments and actin thin filaments occupy predictable positions within each sarcomere. The overlap creates alternating dark A bands and lighter I bands, with Z discs marking sarcomere boundaries. This regular repeating geometry is what appears banded in skeletal and cardiac muscle.

Question 47The muscular system

During the cross-bridge cycle in a skeletal muscle fibre, which event immediately follows the binding of ATP to the myosin head?

  1. Detachment of the myosin head from actin Correct answer.
  2. Power stroke and filament sliding
  3. Binding of tropomyosin to the myosin head
  4. Calcium release from the sarcoplasmic reticulum

Explanation

ATP binding to the myosin head lowers its affinity for actin and causes cross-bridge detachment. Hydrolysis of that ATP then re-cocks the head into its high-energy position. Calcium release and the power stroke occur at other points in the sequence, so detachment is the immediate consequence of ATP binding.

Question 48The muscular system

A toxin prevents acetylcholine from being broken down in the synaptic cleft of a neuromuscular junction. Which immediate consequence is expected at that muscle fibre?

  1. Blockade of calcium release from the sarcoplasmic reticulum
  2. Prolonged depolarisation and sustained contraction Correct answer.
  3. Immediate flaccid paralysis with no endplate potential
  4. Failure of the action potential to reach the sarcolemma

Explanation

Acetylcholinesterase normally terminates transmission by hydrolysing acetylcholine. If it is inhibited, acetylcholine remains bound to nicotinic receptors, keeping ligand-gated channels open and the endplate depolarised. The result is prolonged stimulation and sustained contraction rather than loss of excitation.

Question 49The muscular system

Which protein complex directly senses the rise in cytosolic calcium during excitation-contraction coupling in skeletal muscle?

  1. Titin
  2. Tropomyosin
  3. Dystrophin
  4. Troponin C Correct answer.

Explanation

Troponin C is the calcium-binding subunit of the troponin complex on the thin filament. When calcium binds, the complex shifts tropomyosin away from myosin-binding sites on actin. This exposes the sites and permits cross-bridge formation.

Question 50The muscular system

A motor unit is defined by which anatomical relationship?

  1. A single muscle fibre and its associated sensory receptor
  2. One somatic motor neuron and all the muscle fibres it innervates Correct answer.
  3. One sarcomere and the T tubules surrounding it
  4. All the motor neurons that supply a single muscle belly

Explanation

The motor unit is the functional contractile unit of neural control: a single alpha motor neuron plus every muscle fibre it supplies. Fibres of one motor unit are dispersed through the muscle but contract together. Recruitment of additional motor units is a principal means of grading force.