MCQ-Physiology-Nerves PDF

# Excitable Tissues (Nerve) ## Directions Each of the numbered items or incomplete statements in this section is followed by answers or by completions of the statement. Select the ONE lettered answer or completion that is BEST in each case. ## Questions 1. The supporting cells that form myelin she...

# Excitable Tissues (Nerve) ## Directions Each of the numbered items or incomplete statements in this section is followed by answers or by completions of the statement. Select the ONE lettered answer or completion that is BEST in each case. ## Questions 1. The supporting cells that form myelin sheaths in the peripheral nervous system are: - a) Microglia. - b) Astrocytes. - c) Schwann cells. - d) Oligodendrocytes. 2. The trigger zone where neurons generate action potentials: - a) Dendritic zone. - b) Initial segment. - c) Node of Ranvier. - d) Terminal buttons. 3. Which part of the neuron where the action potential is initiated, a part that receives input from other neurons, and a part that conducts the action potential? - a) Soma, dendrite, axon. - b) Initial segment, dendrite, axon. - c) Cell membrane, soma, dendrite. - d) Axon hillock, soma, myelin sheath. 4. Which of the following is NOT correctly paired? - a) Conduction from cell body to axon: Antidromic conduction. - b) Molecular motors: Dynein and kinesin. - c) Slow axonal transport: 0.5-10 mm/day. - d) Nerve growth factor: Retrograde transport. 5. Axoplasmic flow is a cellular process responsible for movement of proteins and polypeptides within a neuron. Which of the following statements correctly describes a property of orthograde or retrograde axonal transport? - a) The rate of retrograde fast axonal transport is ~400 mm/day - b) The rate of orthograde slow axonal transport is ~200 mm/day - c) Some viruses use retrograde transport to move from the nerve terminal to the soma. - d) Molecular motors for orthograde and retrograde transport are dynein and kinesin, respectively. 6. The electrical potential difference necessary for a single ion to be at equilibrium across a membrane is best described by the: - a) Fick's law. - b) Nernst equation. - c) Goldman equation. - d) van't Hoff equation. 7. Suppose that gated ion channels for Na+ or Ca2+ opened in the plasma membrane of a muscle cell. The membrane potential of that cell would: - a) Move toward the equilibrium potential for that ion. - b) Become less negative than the resting membrane potential. - c) Move farther away from the potassium equilibrium potential. - d) All of these. 8. Based upon the typical distribution of ions across a cell membrane, which of the following values best represents the appropriate resting membrane potential? - a) +70 mV - b) +30 mV - c) 0 mV - d) -70 mV 9. Resting membrane potential (RMP) of a nerve fiber: - a) Is mainly due to sodium influx. - b) Is mainly due to chloride influx. - c) Is mainly due to potassium influx. - d) Is due to the presence of negative charges on the inner surface in relation to its outer surface. 10. As regard resting membrane potential (RMP), all the following statements are incorrect EXCEPT: - a) It is due to K+ diffusion from outside to inside the nerve fibers. - b) It is due to diffusion of intracellular protein to outside the nerve fiber. - c) Its decrease is accompanied with decreased excitability of nerve fiber. - d) It is the potential difference across the fiber membrane with negatively charged ions inside and a positively charged ions outside. 11. Which of the following most likely forms the resting membrane potential of the cell? - a) High chloride conductance and some sodium conductance. - b) High calcium conductance and some chloride conductance. - c) High potassium conductance and some sodium conductance. - d) High potassium conductance and some chloride conductance. 12. The resting potential of a nerve membrane is primarily dependent on the concentration gradient of which of the following ions? - a) Sodium. - b) Calcium. - c) Chloride. - d) Potassium. 13. The most important diffusible ion in the establishment of the membrane potential is: - a) Cl-. - b) K+. - c) Na+. - d) Ca2+. 14. The resting membrane potential results when the tendency for ...................to diffuse out of the cell is balanced by its attraction to opposite charges inside the cell. - a) K+. - b) Na+. - c) Ca2+. - d) Negatively charged protein. 15. Concerning RMP of a cell, which of the following is CORRECT? - a) Is dependent on the permeability of the cell membrane to K+ being greater than the permeability to Na+. - b) Is the difference in electrical potential between the inside & outside membrane surface, under resting condition. - c) Is the result of the unequal distribution of ions on both sides of the membrane is mainly due to selective permeability of the membrane for Na+ & K+, & to the presence of Na+-K+ pump. - d) All of the above are true. 16. Which of the following is actively transported out of the neurons? - a) K+. - b) HPO4. - c) Cl. - d) Na+. 17. About Na+-K+ pump, all the following is correct EXCEPT: - a) Keeps intracellular Nat low. - b) Is explained by facilitated diffusion. - c) Requires high energy phosphate bonds. - d) Contributes for the creation of resting membrane potential. 18. The sodium-potassium pump transport: - a) Three sodium ions into the cell & two potassium out of the cell. - b) Two sodium ions into the cell & three potassium out of the cell. - c) Three sodium ions out of the cell & two potassium into the cell. - d) Two sodium ions out of the cell & three potassium into the cell. 19. Na+-K+ pump: - a) Couples Na+ and K+ with ratio 3:2. - b) Is a passive process needs no energy. - c) Helps intracellular accumulation of Na+. - d) Requires energy which is taken by GTP. 20. The sodium pump of an excitable membrane: - a) Pumps sodium ions to the outside in the resting state. - b) Pumps sodium ions to the outside during depolarization. - c) Pumps potassium ions to the inside during depolarization. - d) Pumps potassium ions to the outside during depolarization. 21. Inactivation of the sodium-potassium pump will cause: - a) An increase in the intracellular volume. - b) Hyperpolarization of the membrane potential. - c) An increase in the flow of sodium out of the cell. - d) An increase in the intracellular potassium concentration. 22. The resting cell membrane is more permeable to __________ than to __________. Although __________ contribute little to the resting membrane potential, they play a key role in generating electrical signals in excitable tissues. - a) K+, Na+, Na+. - b) K+, Na+, K+. - c) Na+, K+, Na+. - d) K+, Na+, Ca++. 23. The equilibrium potential for potassium, as determined by the Nernst equation, differs from the resting potential of the neuron. Which of the following best accounts for this difference? - a) The membrane is selectively permeable only to the potassium ion. - b) An active sodium-potassium pump makes an important contribution to the regulation of the resting potential. - c) The Nernst equation basically considers only the relative distribution of potassium ions across the membrane. - d) The resting potential is basically dependent upon the concentration of sodium but not potassium ions across the membrane. 24. If the resting membrane potential (RMP) of a nerve cell falls (becomes less negative), there is a net: - a) Gain of K+ ions into the cell. - b) Gain of Na+ ions into the cell. - c) Loss of protein ions from the cell. - d) Loss of chloride ions from the cell. 25. If the permeability of the plasma membrane to K+ increases, the resting membrane potential difference __________, This is called __________. - a) Decreases, depolarization. - b) Decreases, hyperpolarization. - c) Increases, depolarization. - d) Increases, hyperpolarization. 26. The resting membrane potential is characterized by which of the following? - a) Passive fluxes of Na+ and K+ are balanced by an active pump that derives energy from enzymatic hydrolysis of ATP. - b) A membrane is depolarized when the differences between the charges across the membrane are increased. - c) As the inside of the cell is made more negative with respect to the outside, the cell becomes depolarized. - d) In a cell whose membrane possesses only K+ channels, the membrane potential cannot be determined. 27. Leak ion channels: - a) Open in response to small voltage changes. - b) Open when a chemical signal binds to its receptor. - c) Allow substances to move into the cell but not out. - d) Are responsible for the ion permeability of the resting plasma membrane. 28. Any shift from resting membrane potential toward a more positive value is called: - a) Repolarization. - b) Depolarization. - c) Membrane potential. - d) Hyperpolarization. 29. Which of the following statements is true regarding plasma membrane Na+ and K+ leak channels? - a) Na+ passively moves through leak channels to exit cell. - b) Na+ actively moves through leak channels to exit cell. - c) K+ passively moves through leak channels to exit cell. - d) K+ actively moves through leak channels to exit cell. 30. Which of the following best describes the status of voltage-gated sodium channels at the resting membrane potential? - a) Activation gates and inactivation gates are open. - b) Activation gates and inactivation gates are closed. - c) Activation gates are open and inactivation gates are closed. - d) Activation gates are closed and inactivation gates are open. 31. Which of the following statements best characterizes a basic function of voltage gated sodium channels? - a) They are rapidly activated by tetrodotoxin. - b) They are opened when the membrane is hyperpolarized. - c) They display a high conductance in the resting membrane. - d) They open rapidly following depolarization of the membrane. 32. The first step in nerve impulse initiation is: - a) Decreased Na* permeability. - b) Membrane depolarization. - c) Increased K+ permeability. - d) Membrane hyperpolarization. 33. The membrane potential will depolarize by the greatest amount if the membrane permeability increases for: - a) Sodium. - b) Potassium. - c) Sodium & potassium. - d) Potassium & chloride. 34. The ascending limb of spike of nerve fiber action potential is caused by: - a) Calcium influx. - b) Rapid sodium influx. - c) Rapid potassium efflux. - d) Closure of activation gates of voltage gated sodium channels. 35. During action potential, the rapid depolarization of the membrane is due to: - a) Increased sodium permeability. - b) Increased calcium permeability. - c) Increased chloride permeability. - d) Increased potassium permeability. 36. During the upstroke of the nerve action potential: - a) There is net inward current and the cell interior becomes less negative. - b) There is net inward current and the cell interior becomes more negative. - c) There is net outward current and the cell interior becomes less negative. - d) There is net outward current and the cell interior becomes more negative. 37. Which of the following is primarily responsible for the rising phase of the action potential? - a) Gap junctions. - b) Sodium-potassium pump. - c) Voltage-gated sodium channel. - d) Voltage-gated potassium channel. 38. When a nerve fiber is stimulated, depolarization phase stops at +30 mV. This is because: - a) The activation gates and inactivation gates close at the same time. - b) The inactivation gates rapidly close to prevent further entry of Na+ ions. - c) Closing of the inactivation gates is faster than the opening of activation gates. - d) Activation of the Na+-K+ pump that prevent accumulation of Na+ ions inside the nerve membrane. 39. The action potential of nerve: - a) Is initiated by Na+ efflux. - b) Is terminated by K+ efflux. - c) Declines in amplitude as it moves along the axon. - d) Is not associated with any movement of Na+ or K+ across the membrane. 40. After the occurrence of an action potential, there is a repolarization of the membrane. Which of the following is the principal explanation for this event? - a) Sodium channels have been opened. - b) Potassium channels have been opened. - c) Potassium channels have been inactivated. - d) The membrane becomes impermeable to all ions. 41. Which of the following ionic changes is CORRECTLY matched with a component of the action potential? - a) A decrease in extracellular Ca2+: Repolarization. - b) Opening of voltage-gated Na+ channels: Depolarization. - c) Opening of voltage-gated K+ channels: After-hyperpolarization. - d) Rapid closure of voltage-gated Na+ channels: Resting membrane potential. 42. Preventing the inactivation of the sodium channels will decrease: - a) The relative refractory period of nerve cells. - b) The upstroke velocity of nerve cell action potentials. - c) The downstroke velocity of nerve cell action potentials. - d) The magnitude of the overshoot in nerve cell action potentials. 43. What would be the result of removing the inactivation gate from the Na+ channels of a nerve axon? - a) Na+ conductance would decrease. - b) The membrane would hyperpolarize. - c) Action potentials could not be elicited. - d) The rate of repolarization during the action potential would be slowed. 44. In excitable cells, repolarization is most closely associated with which of the following events: - a) Na+ influx. - b) Na+ efflux. - c) K+ influx. - d) K+ efflux. 45. The after-hyperpolarization phase of the action potential is due to: - a) Net Na+ efflux to the exterior. - b) Inactivation of the Na+ channels. - c) Increased excitability of the membranes. - d) Slow return of the K+ channels to the closed state. 46. The membrane potential becomes more negative during: - a) Depolarization. - b) Repolarization. - c) After depolarization. - d) After hyperpolarization. 47. Tetrodotoxin blocks nerve impulse transmission by: - a) Anticholinergic action. - b) Depleting acetylcholine. - c) Blocking Na+ channels. - d) Blocking Ca2+ channels. 48. A newly developed local anesthetic blocks Na+ channels in nerves. Which of the following effects on the action potential would it be expected to produce? - a) Shorten the absolute refractory period. - b) Decrease the Na+ equilibrium potential. - c) Abolish the hyperpolarizing after potential. - d) Decrease the rate of rise of the upstroke of the action potential. 49. Although the equilibrium potential of Na+ is +60 mV, the membrane potential does not reach this value during the overshoot because: - a) The membrane is impermeable to Na+. - b) Na+ channels undergo rapid inactivation. - c) The concentration gradient of Na+ is reversed. - d) K+ efflux commences immediately before the upstroke. 50. Which phase of the action potential is caused by inactivation of Na+ channels in a nerve axon? - a) Upstroke. - b) Absolute refractory period. - c) Downstroke. - d) Relative refractory period. 51. The absolute refractory period (ARP) of nerve fiber: - a) Is when the fiber is more excitable. - b) Cover all the phases of action potential. - c) Is when a stronger stimulus is required to excite the fiber. - d) Occurs during depolarization and the first part of the repolarization phase. 52. During the relative refractory period (RRP) of nerve: - a) Threshold stimuli can excite the nerve. - b) Some of Na+ channels are still in the activation state. - c) Some of Na+ channels are still in the inactivation state. - d) Inactivation of K+ channels occur by opening of the K+ gates. 53. A stronger than normal stimulus can cause excitation of nerve or muscle during the: - a) Absolute refractory period (ARP). - b) Relative refractory period (RRP). - c) Spike potential. - d) Overshoot. 54. Increasing the refractory period would have what effect on action potentials? - a) Decrease the amplitude of action potentials. - b) Decrease the frequency of action potentials. - c) Increase the amplitude of action potentials. - d) Increase the frequency of action potentials. 55. A traveling nerve impulse does not depolarize the area immediately behind it, because: - a) It is hyperpolarized. - b) It is not self-propagating. - c) The conduction is always orthodromic. - d) The area immediately behind is refractory. 56. Regarding excitability of a nerve: - a) Is zero during RRP. - b) Is high during repolarization. - c) Is maximum at the firing level. - d) Can respond to a second stimulus during ARP. 57. An excitable cell has an RMP of -70 mV and a firing level of – 55 mV. This cell would be most excitable when its membrane potential is: - a) - 30 mV - b) - 55 mV - c) - 70 mV - d) - 90 mV 58. An excitable cell has an RMP of -70 mV and a firing level of - 55 mV. This cell would be unexcitable when its membrane potential is: - a) - 30 mV - b) - 55 mV - c) - 70 mV - d) - 90 mV 59. Saltatory (Jumping) conduction of action potential: - a) Occurs in unmyelinated nerve fibers. - b) Is slower than the point-to-point conduction. - c) Consumes less energy than point to point conduction. - d) Is inversely proportional to the diameter of the nerve fiber. 60. The speed at which a myelinated axon conducts an action potential is directly related to: - a) The amount of axonal branching. - b) The diameter of the dendrites. - c) The diameter of the axon. - d) The length of the axon. 61. The velocity of conduction of action potentials along a nerve will be increased by: - a) Myelinating the nerve. - b) Lengthening the nerve fiber. - c) Stimulating the Na+-K+ pump. - d) Decreasing the diameter of the nerve. 62. The propagation velocity of an action potential would be increased by: - a) An increase in the rate of depolarization during the upstroke of the action potential. - b) An increase in the diameter of the axon. - c) Both. - d) Neither. 63. The all or none law states that a threshold stimulus produces an impulse which: - a) Does not have refractory period. - b) Is weaker than that produced by a stronger stimulus. - c) Is propagated only partially along the cell membrane. - d) Is not different from that produced by a stronger stimulus. 64. With stimuli of increasing strength, which of these is capable of a graded response? - a) Motor unit. - b) Neuron axon. - c) Whole cardiac muscle. - d) Whole skeletal muscle. 65. A subthreshold stimulus: - a) Produces an afterpotential. - b) Produces a graded potential. - c) Causes an all-or-none response. - d) Produces more action potentials than a submaximal stimulus. 66. An action potential has the following characteristics: - a) No response to threshold stimulus. - b) Slight response to threshold stimulus. - c) Maximal response to threshold stimulus. - d) Increased response to increased stimulus. 67. Concerning local response (graded potential), the following statements are true, EXCEPT: - a) It can be summated. - b) It does not obey all or non law. - c) It is accompanied by decreased excitability. - d) Magnitude and duration vary with the strength of the stimulus. 68. Which of these is NOT a characteristic of postsynaptic potentials (graded potentials)? - a) They are graded in amplitude. - b) They are all-or-none in amplitude. - c) They are produced in dendrites and cell bodies. - d) They are produced by chemically-gated channels. 69. Which of these is NOT a characteristic of action potentials? - a) They are all-or-none in amplitude. - b) They are conducted without decrement. - c) They are produced by voltage- gated channels. - d) They are produced in dendrites and cell bodies. 70. A subthreshold graded potential would result in: - a) An action potential. - b) No action potential. - c) A subthreshold action potential. - d) A reversal in the direction of propagation of the action potential. 71. Which one of the following statements about electrotonic potentials is INCORRECT? - a) They are graded responses. - b) They are local (non-propagated) responses. - c) They are produced by a threshold stimulus. - d) They may be depolarizing or hyperpolarizing. 72. Which of the following changes in electrical potential require voltage-sensitive channels? - a) Action potentials. - b) Synaptic potentials. - c) Receptor potentials. - d) Electrotonic potentials. 73. To which of the following does the term all-or-none response most closely relate? - a) The action potential. - b) The resting potential. - c) The end plate potential. - d) The generator potential. 74. As the strength of a depolarizing stimulus to an axon is increased, - a) The duration of action potentials increases. - b) The amplitude of action potentials increases. - c) The speed with which action potentials are conducted increases. - d) The frequency with which action potentials are produced increases. 75. The conduction of action potentials in a myelinated nerve fiber is: - a) Saltatory. - b) Without decrement. - c) Faster than in an unmyelinated fiber. - d) All of these. 76. Concerning excitable tissues, all the following is true EXCEPT: - a) Become more excitable when partially depolarized. - b) Usually respond to stimuli that decrease their membrane Na+ permeability. - c) They spend energy during rest to maintain their membranes at a polarized state. - d) Become more permeable to Na+ ions when membrane potential decreases to critical level. 77. Choose the INCORRECT statement below: - a) Action potentials are of equal magnitude at the beginning and at the end of an axon. - b) Conduction velocity of action potentials is slower in myelinated than in unmyelinated axons. - c) Conduction velocity of action potentials is faster in myelinated than in unmyelinated axons. - d) In saltatory conduction of action potentials, the action potential seems to jump functionally from node to node. 78. Membrane excitability will be increased by the greatest amount by: - a) Increasing extracellular Na+. - b) Increasing extracellular K+. - c) Decreasing extracellular Cl-. - d) Decreasing extracellular Ca2+. 79. If the membrane potential of a neuron becomes more negative than it was at rest, then the neuron is ________________. In this state, the neuron is __________________ excitable. - a) Depolarized; more. - b) Hyperpolarized; more. - c) Depolarized; less. - d) Hyperpolarized; less. 80. Which of the following statements correctly explains how a change in concentration of an ion inside or outside of the neuron would change its resting membrane potential? - a) A decrease in ECF Ca2+ concentration would stabilize the membrane and reduce its excitability. - b) A decrease in the ECF Na+ concentration would reduce the size of the resting membrane potential. - c) An increase in the ECF K+ concentration would move the resting membrane potential from a normal value of 100 mV to -70 mV. - d) A decrease in the ECF K+ concentration increases the gradient for K+ to leak out of the neuron, making the cell more hyperpolarized. 81. How would hypokalemia affect neuronal excitability? - a) Increased excitability due to a decreased resting membrane potential. - b) Increased excitability due to an increased resting membrane potential. - c) Decreased excitability due to a decreased resting membrane potential. - d) Decreased excitability due to an increased resting membrane potential. 82. Excitability of nerve fibers is increased in all the following conditions EXCEPT: - a) During catelectrotonic state. - b) When extracellular Ca++ is decreased. - c) When extracellular fluid K+ is decreased. - d) With any condition that increase the membrane permeability for Na+. 83. Excitability of nerve fibers is decreased in all the following conditions EXCEPT: - a) During anelectronic state. - b) When extracellular fluid K+ is decreased. - c) During the local excitatory state (local response). - d) With any conditions that decrease membrane permeability to Na+. 84. Alkalosis can cause tetany. What causes increased excitability of nerves and muscle membranes that can lead to continuous contraction of skeletal muscle fibers? - a) Depolarization of the nerve and muscle membranes. - b) Activation of sodium channels at more negative membrane potentials. - c) Decreased release of inhibitory neurotransmitter from nerve terminals. - d) Spontaneous release of calcium from the sarcoplasmic reticulum (SR). 85. In periodic familial paralysis: - a) Nerve is depolarized. - b) Nerve is hyperpolarized. - c) Nerve excitability is increased. - d) Acetylcholine is rapidly inactivated by choline esterase. 86. Hypokalemia would be expected to result in: - a) No change in RMP. - b) A more negative RMP. - c) Increased neuronal excitability. - d) A decrease in firing level of neurons. 87. Which of the following changes in extracellular ion concentrations would expect to hyperpolarize nerve membrane? - a) Increased K+ concentration. - b) Decreased K+ concentration. - c) Increased Na+ concentration. - d) Decreased Cl concentration. 88. Which of the following is most likely to cause the muscle weakness in periodic hyperkalemic paralysis? - a) Hyperpolarization of muscle cells. - b) Inactivation of sodium channels in muscle cells. - c) Decreased potassium conductance in muscle cells. - d) Increased release of neurotransmitters from a-motoneurons. 89. Which of the following is the mechanism of action behind heart arrhythmias caused by hyperkalemia? - a) Increased K+ hyperpolarizes the cell. - b) Increased K+ prolongs action potential duration. - c) Increased K+ increases heart rate via funny current channels. - d) Increased K+ depolarizes the cell bringing excitable nerves closer to action potential. 90. About electrotonic potentials, what is the WRONG statement? - a) A strong catelectrotonus can cause nerve block. - b) Catelectrotonus may be followed by the local response. - c) Catelectrotonus is accompanied by increased excitability of the nerve. - d) It results from stimulation of the nerve by a subthreshold galvanic current. 91. Regarding nerve fibers: - a) A fibers are most sensitive to local anesthetics. - b) B fibers are not affected by hypoxia. - c) C fibers are affected by local anesthetics more than A fibers. - d) Postganglionic fibers belong to groups B. 92. Which of the following is a property of C fibers? - a) Are somatic motor nerves. - b) Are preganglionic autonomic fibers. - c) Have the largest diameter of any nerve fiber type. - d) Have the slowest conduction velocity of any nerve fiber type. 93. Which one of the following nerves has the slowest conduction velocity? - a) Aa fibers. - b) Aß fibers. - c) B fibers. - d) C fibers. 94. The nerve fiber type most susceptible to conduction block by pressure is: - a) Type A. - b) Type B. - c) Type C. - d) All the above. 95. Which nerve fiber type is most susceptible to conduction block by local anesthetics? - a) Type A. - b) Type B. - c) Type C. - d) All the above. 96. Which part of a neuron has the highest concentration of Na+ channels per square micrometer of cell membrane? - a) Dendrites - b) Axon hillock. - c) Node of Ranvier. - d) Axonal membrane under myelin. 97. Excitability, in neurophysiology, is defined as: - a) Presence of a resting membrane potential. - b) Presence of voltage gated ion channels in a tissue. - c) Response to a threshold stimulus with a propagated action potential. - d) Use of more than 30% of ATP synthesized for powering the Na+-K+ ATPase. 98. From the strength-duration curve, all the following is true EXCEPT: - a) The utilization time is a common measure of excitability. - b) Stimuli of extremely short durations fail to excite the verve. - c) Within limits, the stronger the current the shorter the duration required to excite. - d) The threshold galvanic current which can excite the nerve is called the rheobase. 99. From the strength-duration curve, all the following statements is true EXCEPT: - a) Chronaxie is directly proportional to nerve excitability. - b) Rheobase is the threshold galvanic current which can excite the nerve. - c) The stronger the current the shorter the duration required to excite, within limit - d) Chronaxie is the time needed by current of twice the rheobase intensity to excite the nerve. 100. Chronaxie is: - a) The strength of threshold galvanic current. - b) Used to measure the conductivity of nerve fibers. - c) Longer in smooth muscles than in skeletal muscles. - d) Shorter in cardiac muscles than in skeletal muscles. 101. Chronaxie: - a) Is directly proportional to excitability. - b) Is the time taken by a rheobase stimulus to excite a nerve fiber. - c) Is the minimum duration of any stimulus to excite a nerve fiber. - d) Is the time taken by a double rheobase stimulus to excite a nerve fiber. 102. Chronaxie is: - a) Twice the rheobase. - b) The utilization time. - c) Used to determine the degree of nerve excitability. - d) The time taken for the rheobase to stimulate the nerve fiber. 103. The minimum stimulus strength that produces action potential in nerve or muscle is: - a) Rheobase. - b) Chronaxie. - c) Twice rheobase. - d) Twice chronaxie. 104. The correct temporal sequence for events at the neuromuscular junction (NMJ) is: - a) Release of ACh; action potential in the motor nerve; action potential in the muscle. - b) Release of ACh; action potential in the muscle end plate; action potential in the muscle. - c) Uptake of Ca2+ into the motor end plate; action potential in the motor end plate; action potential in the muscle. - d) Uptake of Ca2+ into the presynaptic terminal; release of acetylcholine (ACh); depolarization of the muscle end plate. 105. In the NMJ, all the following is true EXCEPT: - a) Lack of Ca2+ diminishes the release of acetyl choline. - b) There is a high concentration of the cholinesterase enzyme. - c) The nerve ending contains many vesicles and mitochondria. - d) The acetyl choline receptors are similar to those in smooth muscle. 106. Which of the following statements about synaptic transmission at the NMJ is TRUE? - a) It is enhanced by high levels of cholinesterase. - b) It is depressed by abnormally low levels of magnesium. - c) It is produced by the release of acetylcholine from the motor neuron. - d) It is caused by an influx of potassium ions through the muscle membrane. 107. In the absence of extracellular Ca2+, an action potential in a motor neuron failed to initiate contraction of the innervated skeletal muscle because of: - a) Stimulation of ACh degradation. - b) Inhibition of ACh release from the motor neuron. - c) Inhibition of Ca2+ influx into the innervated muscle. - d) Inhibition of ACh binding to receptors on the muscle end plate. 108. Receptors that bind to ACh at the postsynaptic membrane are: - a) Passive channels. - b) Voltage-gated channels. - c) Chemically gated channels. - d) Mechanically gated channels. 109. Neuromuscular transmission (NMT): - a) Can be facilitated by curare. - b) Is caused by release of acetylcholine from the muscle side of MEP. - c) Is blocked by curare because it competes with Na+ influx at the MEP. - d) Shows a permeability change to Na+ and K+ at the receptor side of the neuromuscular junction. 110. At the muscle