Foundations of Structure and Function & Tissue Histology Flashcards

Flashcards covering medical physics, imaging modalities, embryonic development, histology, and neuro-muscular physiology from lecture notes.

What threshold energy separates non-ionizing from ionizing electromagnetic radiation?

Radiation with energy over $10\,eV$ per photon (occurring in the UV region) is considered ionizing radiation, whereas radiation under $10\,eV$ is non-ionizing.

How do alpha ($\alpha$), gamma ($\gamma$), and beta ($\beta$) radiations differ in terms of their energy spectrum?

Alpha and gamma radiations are monoenergetic processes (emitting particles or photons with discrete energies). Beta radiation is polyenergetic, displaying a continuous spectrum of energies due to the co-emission of a neutrino/antineutrino.

What minimum photon energy is required for pair production to occur, and what happens upon positron annihilation?

Pair production requires a minimum photon energy of $1.022\,MeV$. The resulting positron collides with an electron and annihilates, producing two photons of $511\,keV$ each traveling in opposite directions.

What is the mathematical relationship between radioactive half-life ($t_{1/2}$) and mean lifetime ($\tau_0$)?

The relationship is given by $t_{1/2} = \ln(2) \tau_0$.

What radiation interaction process produces Bremsstrahlung (braking) radiation in an X-ray tube?

Bremsstrahlung radiation occurs when an accelerated electron passes close to an atomic nucleus in a dense target, experiences Coulomb forces that deviate its trajectory, and releases kinetic energy as electromagnetic photons.

What is the difference between stochastic and deterministic biological effects of ionizing radiation?

Stochastic effects (e.g., DNA mutations, cancer) have no dose threshold, and increasing dose increases the probability (not severity) of the effect. Deterministic effects (e.g., erythema) have a dose threshold, and increasing dose increases the severity of the effect.

Why are standard chest X-rays acquired in the posterior-anterior (PA) position at a distance of $150-180\,cm$?

The PA position places the heart closer to the digital detector, minimizing magnification of the cardiac silhouette, while the $150-180\,cm$ distance standardizes image geometry.

What Hounsfield Unit (HU) values correspond to air and water on a CT scan?

Air corresponds to $-1000\,HU$ and water corresponds to $0\,HU$, with dense structures like bone reaching up to $+3000\,HU$.

How do CT pitch values affect image detail and patient radiation dose?

A pitch $P = 1.0$ means contiguous beams. A pitch $P > 1.0$ leaves gaps in the x-ray helix (faster scan, lower detail, lower dose). A pitch $P < 1.0$ causes overlapping scans (higher detail, higher radiation dose).

What safety classifications apply to medical devices in an MRI environment?

Devices are classified as MR-Safe (green square, no risks), MR-Conditional (yellow triangle, safe under specified conditions), or MR-Unsafe (red circle with diagonal line, poses hazards in all MR environments).

What radiopharmaceutical is commonly used in PET scans to measure glucose metabolism?

$^{18}F\text{-FDG}$ ($^{18}F\text{-fluorodeoxyglucose}$), which accumulates in cells with high glycolytic activity via $GLUT-1$ transporters.

What equation defines the Larmor frequency of proton precession in Magnetic Resonance Imaging?

The Larmor frequency is defined by $\omega = \gamma B_0$, where $\gamma$ is the gyromagnetic ratio specific to the nucleus and $B_0$ is the static external magnetic field strength.

How do T1 and T2 relaxation processes differ in MRI?

T1 (longitudinal) relaxation is the time required for net longitudinal magnetization to recover to $63\%$ of its maximum value $M_0$. T2 (transverse) relaxation is the time required for transverse magnetization $M_{xy}$ to decay/dephase to $37\%$ of its initial value.

How is acoustic impedance ($z$) calculated, and how does impedance mismatch affect ultrasound waves at an interface?

Acoustic impedance is calculated as $z = \rho \times v$ (density times sound velocity). Similar impedances between two media result in high transmission and minimal reflection, whereas large impedance differences cause almost $100\%$ reflection.

According to the ultrasound attenuation table, what features characterize water/fluids versus bones/calcium/metal?

Water/fluids have low attenuation, appear black (anechoic), and exhibit posterior acoustic enhancement. Bones/calcium/metal have high attenuation, appear white (hyperechoic), and exhibit posterior acoustic shadowing.

What is the safety threshold for the Mechanical Index ($MI$) in diagnostic ultrasound, and what physical risk does it prevent?

The Mechanical Index must be $MI < 1.9$ to prevent acoustic cavitation, which is the formation and collapse of vapor bubbles in tissue liquid when peak negative pressure drops below vapor pressure.

How long does the process of human spermatogenesis take, and where does it occur?

Spermatogenesis takes approximately $64\,\text{days}$ and occurs continuously from puberty onwards inside the seminiferous tubules of the testes.

At what meiotic stage are primary oocytes arrested from fetal development until puberty?

Primary oocytes are arrested in the diplotene stage of prophase I within primordial follicles.

What hormonal event triggers the resumption of meiosis I and ovulation of the secondary oocyte?

A surge in Luteinizing Hormone (LH), triggered when plasma estradiol reaches a threshold level (approx. $200\,pg/mL$), causes the resumption of meiosis I and release of a secondary oocyte arrested at metaphase II.

What three sequential phases characterize gamete interaction during fertilization?

Phase 1: Penetration of the corona radiata; Phase 2: Penetration of the zona pellucida via ZP3 ligand-receptor binding and acrosome reaction; Phase 3: Fusion of sperm and oocyte plasma membranes, triggering cortical granule release and the zona reaction to prevent polyspermy.

When does human blastocyst implantation occur relative to fertilization and the menstrual cycle?

Implantation begins on days $5-6$ post-fertilization, corresponding to days $20-23$ of a standard $28\text{-day}$ menstrual cycle.

What embryonic germ layers are formed during gastrulation, and from which initial cell layer do they derive?

Gastrulation generates the endoderm, mesoderm, and ectoderm. All three germ layers derive from epiblast cells invaginating through the primitive streak and node.

What primary structures develop from the paraxial, intermediate, and lateral plate mesoderm?

Paraxial mesoderm forms somites (sclerotome, myotome, dermatome); intermediate mesoderm forms the urogenital system; lateral plate mesoderm splits into somatic/parietal (dermis, limb skeletons) and splanchnic/visceral (gut smooth muscle, heart) layers.

What molecular signaling centers and factors regulate proximodistal, dorsoventral, and anteroposterior limb patterning?

Proximodistal growth is regulated by the Apical Ectodermal Ridge (AER) via $FGF4$ and $FGF8$; dorsoventral patterning is directed by $WNT-7$ in dorsal ectoderm; anteroposterior patterning is directed by the Zone of Polarizing Activity (ZPA) via $SHH$.

What anatomical segments contain the cell bodies of preganglionic sympathetic versus parasympathetic neurons?

Sympathetic preganglionic cell bodies are located in the intermediolateral cell columns of spinal segments $T1-L2$. Parasympathetic preganglionic cell bodies reside in brainstem nuclei (cranial nerves III, VII, IX, X) and sacral spinal cord segments $S2-S4$.

What neurotransmitters and receptors are utilized by preganglionic and postganglionic fibers in the sympathetic and parasympathetic systems?

All preganglionic fibers release Acetylcholine (Ach) acting on nicotinic ($N_2$) receptors. Parasympathetic postganglionic fibers release Ach onto muscarinic receptors. Most sympathetic postganglionic fibers release Noradrenaline/Adrenaline onto adrenergic receptors ($\alpha$, $\beta$), except sweat glands/piloerector muscles which receive cholinergic postganglionic innervation.

What type of collagen is characteristic of cartilage matrix versus bone matrix?

Cartilage matrix is characterized by type II collagen, whereas bone matrix is predominantly composed of type I collagen ($90-95\%$ of organic matrix).

What structural components define an osteon (Haversian system) in mature compact bone?

An osteon consists of concentric lamellae of calcified bone matrix surrounding a central Haversian canal that houses blood vessels and nerves, with osteocytes residing in lacunae connected by canaliculi.

What glial cell types are responsible for myelinating axons in the CNS and PNS, respectively?

Oligodendrocytes myelinate multiple axon segments in the Central Nervous System, whereas Schwann cells myelinate a single axon segment per cell in the Peripheral Nervous System.

What structural features define the sarcomere in skeletal muscle?

A sarcomere is the segment between two Z discs containing a central dark A band (thick filaments overlapping thin filaments), a central H zone (thick filament tails only) with an M line, and outer light I bands (thin filaments only) bisected by the Z discs.

How does calcium trigger cross-bridge cycling in skeletal muscle versus smooth muscle?

In skeletal muscle, $Ca^{2+}$ binds troponin C, moving tropomyosin to expose actin binding sites for myosin. In smooth muscle (lacking troponin), $Ca^{2+}$ binds calmodulin to activate Myosin Light Chain Kinase (MLCK), which phosphorylates myosin light chains to enable actin binding.

How do Type I, Type IIA, and Type IIB skeletal muscle fibers differ in contraction speed and metabolism?

Type I (red) fibers are slow-twitch, highly oxidative, fatigue-resistant with abundant myoglobin/mitochondria. Type IIA (intermediate) fibers are fast-twitch, oxidative-glycolytic, fatigue-resistant. Type IIB (white) fibers are fast-twitch, highly glycolytic, and fatigue rapidly.

What mathematical expression defines the membrane time constant ($\tau$) and length constant ($\lambda$)?

The time constant is $\tau = R_m C_m$ (membrane resistance times capacitance). The length constant is $\lambda = \sqrt{\frac{R_m}{R_i}}$ (square root of membrane resistance over internal axoplasmic resistance).

What gating mechanism explains the absolute refractory period of a voltage-gated sodium channel?

The absolute refractory period occurs while the slow inactivation gate of the voltage-gated $Na^+$ channel remains closed following depolarization, preventing any new action potential regardless of stimulus strength until the channel resets to its resting state.

What pathology results from the presynaptic cleaving of SNARE proteins by Botulinum toxin?

Botulinum toxin light-chain endopeptidase cleaves v-SNAREs or t-SNAREs at cholinergic presynaptic terminals, blocking Acetylcholine exocytosis and causing flaccid muscle paralysis and autonomic dysfunction.