Comprehensive Examination Study Guide: Neurophysiology, Sensory Systems, and Muscle Mechanisms

Part 1: True or False Questions

1. True or False: A lack of extracellular $Ca^{2+}$ causes an increase in neurotransmitter release from presynaptic terminals.

2. True or False: Both excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs) are capable of undergoing summation.

3. True or False: An inhibitory postsynaptic potential (IPSP) can be generated by a neurotransmitter that opens ligand-gated $Cl^-$ channels.

4. True or False: A skeletal muscle stretched to the point of zero myofilament overlap produces its strongest possible active contraction when stimulated.

5. True or False: Voltage-gated ion channels in axonal membranes allow the passive diffusion of neurotransmitter molecules.

6. True or False: During skeletal muscle contraction, thick myosin filaments physically shorten in length.

7. True or False: Cone photoreceptors are responsible for detecting color vision under photopic lighting conditions.

8. True or False: Salty taste receptor cells require G-protein-coupled receptor cascades to transduce sodium signals.

9. True or False: Phasic sensory receptors adapt rapidly by diminishing or extinguishing action potential firing during sustained stimulation.

10. True or False: Sound waves are detected in the cochlea through the mechanoelectrical stimulation of hair cells.

11. True or False: Lateral inhibition reduces spatial contrast to broaden the perceived receptive field area.

12. True or False: Oligodendrocytes produce the myelin sheaths that insulate multiple axon segments within the central nervous system.

13. True or False: During the absolute refractory period, an exceptionally strong suprathreshold stimulus can elicit a second action potential.

14. True or False: Non-gated $K^+$ leak channels establish high resting membrane permeability to potassium and set the resting membrane potential.

15. True or False: Ionotropic receptors depend on intermediate heterotrimeric G-proteins to alter membrane permeability.

16. True or False: Spatial summation occurs when a single presynaptic neuron discharges action potentials in rapid succession.

17. True or False: Inhibitors of acetylcholinesterase, such as sarin nerve gas, cause continuous accumulation of acetylcholine in the synaptic cleft, leading to prolonged receptor overstimulation.

18. True or False: In the absence of light (dark current), photoreceptors maintain a partially depolarized resting membrane potential of approximately $-40\,\text{mV}$.

19. True or False: High-frequency sound waves preferentially vibrate the wide, compliant apex of the basilar membrane near the helicotrema.

20. True or False: According to the Henneman size principle, large motor units with high thresholds are recruited before small, fatigue-resistant motor units.

21. True or False: Excitation-contraction coupling in skeletal muscle requires physical conformational coupling between the dihydropyridine receptor (DHPR) and the ryanodine receptor (RyR1).

22. True or False: During an isometric contraction, muscle tension increases while overall muscle length remains unchanged.

23. True or False: Type I slow-oxidative muscle fibers contain high concentrations of myoglobin and mitochondria, making them resistant to fatigue.

24. True or False: Smooth muscle cells rely on the troponin complex to displace tropomyosin from actin filaments during contraction.

25. True or False: Single-unit (visceral) smooth muscle cells are electrically coupled via gap junctions, permitting coordinated contractions as a syncytium.

Part 2: Multiple Choice Questions

26. Which neuroglial cell type produces the myelin sheath surrounding axons in the peripheral nervous system (PNS)?

    • A) Oligodendrocytes

    • B) Astrocytes

    • C) Schwann cells

    • D) Microglial cells

27. Which functional region of a neuron contains a high concentration of voltage-gated $Na^+$ channels and acts as the integration zone where graded potentials summate to threshold?

    • A) Dendritic arbor

    • B) Axon hillock

    • C) Nissl body

    • D) Synaptic terminal

28. Conduction of an action potential that propagates by regenerating exclusively at the Nodes of Ranvier in a myelinated axon is termed:

    • A) Continuous conduction

    • B) Decremental conduction

    • C) Saltatory conduction

    • D) Electrotonic decay

29. What mechanism terminates the rapid depolarization phase of an axonal action potential at approximately $+30\,\text{mV}$?

    • A) Activation of delayed rectifier $K^+$ channels

    • B) Closure of voltage-gated $Na^+$ inactivation gates

    • C) Opening of voltage-gated $Ca^{2+}$ channels

    • D) Direct inhibition of the $Na^+/K^+$ ATPase pump

30. The relative refractory period occurs primarily due to:

    • A) Inactivated voltage-gated $Na^+$ channels

    • B) Delayed closure of voltage-gated $K^+$ channels permitting continued hyperpolarizing outward current

    • C) Complete depletion of intracellular $K^+$

    • D) Reversal of the chloride equilibrium potential

31. In chemical synapses, what event directly triggers the fusion and exocytosis of synaptic vesicles containing neurotransmitters?

    • A) Influx of $Na^+$ through ligand-gated channels

    • B) Efflux of $K^+$ through delayed rectifiers

    • C) Influx of $Ca^{2+}$ through voltage-gated calcium channels

    • D) Activation of the $Na^+/K^+$ ATPase pump

32. Which characteristic distinguishes excitatory postsynaptic potentials (EPSPs) from action potentials?

    • A) EPSPs are all-or-none events

    • B) EPSPs exhibit absolute refractory periods

    • C) EPSPs are graded responses that decay over distance

    • D) EPSPs propagate over meters without decrement

33. The nicotinic acetylcholine receptor (nAChR) is categorized as:

    • A) A metabotropic G-protein-coupled receptor

    • B) A pentameric ligand-gated ion channel conducting inward $Na^+$ current

    • C) A voltage-gated calcium channel

    • D) An intracellular nuclear steroid receptor

34. Activation of muscarinic acetylcholine receptors (mAChR) in cardiac pacemaker cells slows heart rate by:

    • A) Opening inwardly rectifying $K^+$ channels via dissociated G-protein $\beta\gamma$ complexes

    • B) Stimulating phospholipase C to elevate cytosolic $Ca^{2+}$

    • C) Direct phosphorylation of myosin light chains

    • D) Inhibiting acetylcholinesterase

35. What enzyme anchored within the synaptic cleft terminates cholinergic signaling by cleaving acetylcholine into acetate and choline?

    • A) Choline acetyltransferase

    • B) Monoamine oxidase

    • C) Acetylcholinesterase

    • D) Myosin light-chain kinase

36. Which cutaneous mechanoreceptor adapts rapidly (phasic) and is specialized for detecting high-frequency mechanical vibration?

    • A) Merkel disc

    • B) Ruffini ending

    • C) Pacinian corpuscle

    • D) Hair follicle receptor

37. A body region exhibiting high tactile acuity and a very small two-point discrimination threshold is characterized by:

    • A) Large receptive fields and small cortical representation in the somatosensory cortex

    • B) Small receptive fields and large cortical representation in the somatosensory cortex

    • C) Low density of sensory receptors

    • D) Complete absence of lateral inhibition

38. Transduction of sour tastants by taste receptor cells is initiated primarily by:

    • A) Sodium entry through amiloride-sensitive epithelial sodium channels (ENaC)

    • B) Protons ($H^+$) entering through proton channels or blocking potassium leak channels

    • C) Binding of sugars to T1R2 and T1R3 heterodimeric GPCRs

    • D) Binding of denatonium to monomeric T2R GPCRs

39. During olfactory signal transduction, activation of $G_{\text{olf}}$ stimulates which enzyme to produce cyclic AMP ($\text{cAMP}$)?

    • A) Phospholipase C

    • B) Adenylyl cyclase type III

    • C) Phosphodiesterase 6

    • D) Guanylyl cyclase

40. Sound pressure amplification across the middle ear is achieved primarily through:

    • A) Fluid movement within the semicircular canals

    • B) The surface area ratio between the large tympanic membrane and the small stapes footplate

    • C) Electrical resonance of the cochlear duct

    • D) Opening of the auditory tube during swallowing

41. According to the Place Theory of pitch discrimination:

    • A) High-frequency sounds preferentially stimulate the compliant cochlear apex

    • B) High-frequency sounds preferentially stimulate the narrow, stiff cochlear base

    • C) Low-frequency sounds bypass the basilar membrane entirely

    • D) Pitch is coded solely by the amplitude of hair cell stereociliary bending

42. When light strikes a rod photoreceptor, what biochemical sequence occurs?

    • A) Transducin activates PDE6, degrading $\text{cGMP}$, closing CNG cation channels, and hyperpolarizing the cell

    • B) Guanylyl cyclase is activated, opening CNG channels and depolarizing the cell

    • C) Retinal converts from all-trans to 11-cis, stimulating glutamate release

    • D) Voltage-gated $Ca^{2+}$ channels open, releasing inhibitory GABA

43. Which retinal cells generate propagated action potentials and form the axons of the optic nerve (cranial nerve II)?

    • A) Photoreceptors (rods and cones)

    • B) Bipolar cells

    • C) Retinal ganglion cells

    • D) Horizontal cells

44. What microscopic structural boundary defines the functional repeating unit of a sarcomere?

    • A) From one M line to the adjacent M line

    • B) From one Z disc to the adjacent Z disc

    • C) The span of the entire A band

    • D) The margins of the H zone

45. During the cross-bridge cycle of skeletal muscle contraction, the power stroke is triggered by:

    • A) The binding of an ATP molecule to the myosin head

    • B) The release of inorganic phosphate ($P_i$) from the cocked myosin head

    • C) The hydrolysis of ATP into ADP and $P_i$

    • D) Dephosphorylation of troponin by phosphatase

46. Rigor mortis occurs after death because:

    • A) Excessive ATP concentrations lock myosin heads in the cocked configuration

    • B) Depletion of cellular ATP prevents cross-bridge detachment from actin

    • C) Calcium pumps hyper-sequester calcium into terminal cisternae

    • D) Tropomyosin permanently detaches from thin filaments

47. Complete, fused tetanus in skeletal muscle results when:

    • A) The stimulation frequency is high enough that sarcoplasmic $Ca^{2+}$ remains persistently elevated, preventing relaxation

    • B) Muscle fibers deplete all intracellular glycogen stores

    • C) Individual twitches are separated by long recovery intervals

    • D) Motor units cycle asynchronously at sub-threshold voltages

48. In skeletal muscle fibers, calcium ions are stored and sequestered in the terminal cisternae bound to:

    • A) Calmodulin

    • B) Myomesin

    • C) Calsequestrin

    • D) Alpha-actinin

49. In smooth muscle, excitation induces contraction when intracellular $Ca^{2+}$ binds to:

    • A) Troponin C

    • B) CalmodulAin, which subsequently activates Myosin Light-Chain Kinase (MLCK)

    • C) Ryanodine receptors directly

    • D) Dihydropyridine receptors on the sarcolemma

50. The "latch state" in vascular and visceral smooth muscle allows the tissue to:

    • A) Rapidly relax in response to sympathetic bursts

    • B) Maintain prolonged tonic force with minimal ATP consumption

    • C) Undergo rapid twitch contractions without calcium entry

    • D) Prevent phosphorylation of myosin regulatory light chains

Part 3: Answer Key and Explanations

1. False – A lack of extracellular $Ca^{2+}$ prevents calcium influx through presynaptic voltage-gated channels, decreasing or abolishing neurotransmitter exocytosis.

2. True – Both EPSPs and IPSPs are graded electrotonic potentials lacking refractory periods, allowing mathematical summation.

3. True – Opening ligand-gated $Cl^-$ channels drives $Cl^-$ influx, hyperpolarizing the postsynaptic membrane (producing an IPSP).

4. False – Overstretching eliminates filament overlap between actin and myosin heads, preventing cross-bridge formation and dropping tension to zero.

5. False – Axonal ion channels permit only the passive movement of specific inorganic ions (e.g., $Na^+$, $K^+$), never large organic molecules like neurotransmitters.

6. False – In the sliding filament mechanism, thin and thick filaments slide past one another; neither filament type physically shortens.

7. True – Cones provide photopic, high-acuity color vision, utilizing distinct opsin proteins tuned to blue, green, and red wavelengths.

8. False – Salty taste is mediated by direct influx of $Na^+$ via epithelial sodium channels (ENaC), requiring no GPCR.

9. True – Phasic receptors adapt rapidly to sustained stimuli, signaling changes or dynamic movement.

10. True – Acoustic displacement of basilar membrane shears stereocilia on hair cells, opening mechanically gated cation channels.

11. False – Lateral inhibition suppresses parallel neighboring inputs to sharpen boundaries and enhance spatial contrast.

12. True – Oligodendrocytes in the CNS extend processes to myelinate segments of up to $30\text{--}50$ separate axons.

13. False – During the absolute refractory period, voltage-gated $Na^+$ channels are completely inactivated; no stimulus can fire an action potential.

14. True – Non-gated $K^+$ leak channels establish high resting permeability to $K^+$, maintaining the negative resting membrane potential ($\sim -70\,\text{mV}$).

15. False – Ionotropic receptors contain intrinsic ligand-gated ion channels; metabotropic receptors utilize G-proteins.

16. False – Rapid repeated firing of a single neuron produces temporal summation; spatial summation involves multiple convergent presynaptic inputs.

17. True – Inhibiting AChE blocks ACh hydrolysis, causing receptor overstimulation, spastic paralysis, and depolarizing blockade.

18. True – Unbound $\text{cGMP}$ keeps CNG channels open, sustaining the depolarizing inward "dark current" at $\sim -40\,\text{mV}$.

19. False – High frequencies vibrate the stiff, narrow base near the oval window; low frequencies vibrate the compliant apex.

20. False – The Henneman size principle dictates that small, excitable, fatigue-resistant motor units are recruited first.

21. True – Skeletal muscle E-C coupling relies on a direct mechanical interaction between sarcolemmal DHPR and sarcoplasmic RyR1.

22. True – In isometric contractions, cross-bridges cycle to produce tension without gross shortening of the muscle length.

23. True – Type I slow oxidative fibers are rich in myoglobin, capillaries, and mitochondria, granting high fatigue resistance.

24. False – Smooth muscle lacks troponin; regulation occurs through calcium-calmodulin activation of MLCK.

25. True – Single-unit smooth muscle cells are united by gap junctions, enabling action potentials to sweep across the tissue sheet.

26. C – Schwann cells produce myelin in the PNS, wrapping around one segment of an axon.

27. B – The axon hillock has a high density of voltage-gated $Na^+$ channels and serves as the integration site for summation.

28. C – Saltatory conduction describes the jumping of action potentials from node to node in myelinated axons.

29. B – At $\sim +30\,\text{mV}$, voltage-gated $Na^+$ inactivation gates close, halting $Na^+$ entry.

30. B – Delayed rectifier $K^+$ channels remain open, hyperpolarizing the membrane and counteracting depolarizing stimuli.

31. C – Calcium influx through presynaptic voltage-gated $Ca^{2+}$ channels activates synaptotagmin to drive vesicle fusion.

32. C – EPSPs are graded local electrotonic potentials that vary in amplitude and decay across distance.

33. B – The nAChR is an ionotropic pentameric receptor conducting predominantly inward $Na^+$ current upon ACh binding.

34. A – Muscarinic activation in pacemaker cells releases G-protein $\beta\gamma$ subunits that open inwardly rectifying $K^+$ channels, hyperpolarizing the cell.

35. C – Acetylcholinesterase hydrolyzes acetylcholine into choline and acetate within milliseconds.

36. C – Pacinian corpuscles are encapsulated, rapidly adapting mechanoreceptors responding to high-frequency vibration.

37. B – Small receptive fields coupled with disproportionately large somatosensory cortical representation yield high tactile acuity.

38. B – Acidic protons ($H^+$) pass directly through proton channels (OTOP1) or block $K^+$ leak channels to depolarize the taste cell.

39. B – $G_{\text{olf}}$ stimulates adenylyl cyclase type III, elevating $\text{cAMP}$ to open CNG cation channels.

40. B – Impedance matching and pressure amplification ($\sim 17\text{--}20\text{-fold}$) rely on the area ratio between the large tympanic membrane and small stapes footplate.

41. B – High-frequency sounds resonate with the narrow, stiff base; low-frequency sounds resonate with the wide, flexible apex.

42. A – Metarhodopsin II activates transducin, which stimulates PDE6 to hydrolyze $\text{cGMP}$, closing CNG channels and causing hyperpolarization.

43. C – Retinal ganglion cell axons converge at the optic disc to form the optic nerve and fire genuine action potentials.

44. B – A sarcomere is defined as the contractile segment spanning between two adjacent Z discs.

45. B – Dissociation of inorganic phosphate ($P_i$) from the energized myosin head triggers the mechanical power stroke.

46. B – ATP binding is required for myosin cross-bridge detachment; its absence causes persistent rigor complex locking.

47. A – High stimulation frequencies prevent SERCA pumps from clearing sarcoplasmic $Ca^{2+}$, locking troponin in an active configuration.

48. C – Calsequestrin reversibly binds and stores high concentrations of calcium within SR terminal cisternae.

49. B – Four $Ca^{2+}$ ions bind to calmodulin; the complex activates MLCK to phosphorylate myosin heads.

50. B – Dephosphorylation of actin-bound myosin heads by MLCP creates latch-bridges, holding tension with low ATP expenditure.