give me mcqs from action potential graded potential and chemical and electrical synapse with answers and explainations from gyton review of physiology
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Here are 25 MCQs from Guyton & Hall Textbook of Medical Physiology covering Action Potentials, Graded/Local Potentials, and Chemical vs. Electrical Synapses.
MCQs: Action Potential, Graded Potential & Synapses
Based on Guyton & Hall Textbook of Medical Physiology
SECTION 1: ACTION POTENTIAL
Q1. The resting membrane potential of a typical large myelinated nerve fiber is approximately:
- A) -55 mV
- B) -90 mV
- C) -70 mV ✓
- D) -45 mV
Explanation: Guyton states that the resting membrane potential is -70 mV, at which point the membrane is described as "polarized." This potential is maintained primarily by the K⁺ leak channels and the Na⁺-K⁺ ATPase pump. The threshold for action potential initiation is approximately -55 mV.
Q2. During the depolarization phase of an action potential, which ion channel opens first and most dramatically?
- A) Voltage-gated K⁺ channels
- B) Voltage-gated Na⁺ channels ✓
- C) Cl⁻ channels
- D) Ca²⁺ channels
Explanation: Guyton describes that when the membrane is depolarized to threshold (~-55 mV), voltage-gated sodium channels open almost instantaneously, allowing up to a 5000-fold increase in sodium conductance. This massive Na⁺ influx causes the membrane potential to shoot to positive values (overshoot). Potassium channels open more slowly and are responsible for repolarization.
Q3. The overshoot of the action potential (positive membrane potential) occurs because:
- A) K⁺ rushes into the cell
- B) Cl⁻ rushes out of the cell
- C) Na⁺ influx exceeds the zero level due to massive sodium conductance ✓
- D) The Na⁺-K⁺ pump is activated
Explanation: According to Guyton, in large nerve fibers, the great excess of positive sodium ions moving inside causes the membrane potential to actually overshoot beyond zero and become somewhat positive. In smaller fibers and some CNS neurons, the potential merely approaches zero and does not overshoot.
Q4. Repolarization of the nerve membrane during an action potential is primarily achieved by:
- A) Inactivation of Na⁺ channels alone
- B) Activation of the Na⁺-K⁺ pump
- C) Opening of voltage-gated K⁺ channels with efflux of K⁺ ✓
- D) Influx of Cl⁻ ions
Explanation: Guyton explains that in less than 1 millisecond after sodium channels become highly permeable, sodium channels begin to close and potassium channels open more than normal. Rapid diffusion of K⁺ to the exterior reestablishes the normal negative resting membrane potential - this is repolarization.
Q5. Hyperpolarization (undershoot) after an action potential occurs because:
- A) Na⁺ channels remain open too long
- B) The Na⁺-K⁺ pump overshoots
- C) Potassium channels remain open longer than needed, allowing excess K⁺ efflux ✓
- D) Cl⁻ rushes into the cell
Explanation: Guyton states that potassium channels "may remain open longer than needed to return the membrane to its resting potential, resulting in hyperpolarization (undershoot)." Once K⁺ channels close, the membrane returns to its normal resting value.
Q6. The voltage-gated sodium channel has how many gates?
- A) One (activation gate only)
- B) Three gates
- C) Two gates - an activation gate (outside) and an inactivation gate (inside) ✓
- D) No gates; it is always open
Explanation: Guyton describes that the voltage-gated sodium channel has two gates: one near the outside called the activation gate and another near the inside called the inactivation gate. At resting potential, the activation gate is closed while the inactivation gate is open. During depolarization, both gates change state - activation gate opens rapidly, then inactivation gate closes.
Q7. During the absolute refractory period:
- A) An action potential can be elicited with a stronger-than-normal stimulus
- B) An action potential can be elicited only with a subthreshold stimulus
- C) A new action potential cannot be elicited regardless of stimulus strength ✓
- D) Hyperpolarization prevents re-excitation, but depolarization is possible
Explanation: Guyton states that during the absolute refractory period, sodium channels are inactivated and "no amount of excitatory signal applied to these channels at this point will open the inactivation gates." The membrane must return to near resting potential before channels can reopen. For large myelinated fibers, this period is about 1 millisecond, limiting transmission to ~1000 impulses/second.
Q8. During the relative refractory period, which of the following is TRUE?
- A) No action potential can be generated under any condition
- B) An action potential can be generated, but requires a greater-than-normal stimulus ✓
- C) The membrane potential is more positive than resting
- D) Na⁺ channels are maximally open
Explanation: Guyton explains that during the relative refractory period (2-4 milliseconds during the hyperpolarization phase), "the neuron can develop another action potential, but a greater stimulus is required compared to the stimulus needed to elicit an action potential in a resting neuron." This is because the membrane is hyperpolarized.
Q9. The "All-or-Nothing" principle of the action potential means:
- A) The action potential amplitude varies with stimulus strength
- B) Once threshold is reached, the full action potential propagates; below threshold, it does not propagate at all ✓
- C) Action potentials can be graded in size
- D) The action potential travels only in one direction
Explanation: Guyton defines the all-or-nothing principle: "Once an action potential has been elicited at any point on the membrane of a normal fiber, the depolarization process travels over the entire membrane if conditions are right, but it does not travel at all if conditions are not right." This is distinct from graded potentials, which vary in amplitude with stimulus intensity.
Q10. The maximum frequency of impulse transmission in large myelinated nerve fibers is approximately:
- A) 100 impulses/second
- B) 500 impulses/second
- C) 1000 impulses/second ✓
- D) 5000 impulses/second
Explanation: Guyton calculates this directly: since the absolute refractory period for large myelinated nerve fibers is about 1 millisecond, "one can readily calculate that such a fiber can transmit a maximum of about 1000 impulses per second."
SECTION 2: GRADED (LOCAL/ACUTE SUBTHRESHOLD) POTENTIALS
Q11. Acute local potentials (subthreshold potentials) differ from action potentials in that they:
- A) Always propagate along the full length of the nerve fiber
- B) Follow the all-or-nothing principle
- C) Are generated only by chemical stimuli
- D) Are graded in amplitude and do not propagate; they fade out near the stimulus site ✓
Explanation: Guyton describes that a weak stimulus causes a "local potential change at the membrane," which is called an acute local potential. These are subthreshold responses - they are graded (vary with stimulus strength), do not follow the all-or-nothing principle, and do not propagate. Only when the local potential rises to threshold does an action potential fire.
Q12. At which point on a stimulated neuron is the threshold for action potential generation LOWEST (most easily excited)?
- A) The soma (cell body)
- B) The dendrites
- C) The initial segment of the axon (axon hillock) ✓
- D) The presynaptic terminal
Explanation: Guyton explains that "the action potential does not begin adjacent to the excitatory synapses. Instead, it begins in the initial segment of the axon where the axon leaves the neuronal soma." The soma has relatively few voltage-gated Na⁺ channels. The initial segment has a low threshold because of the high concentration of voltage-gated Na⁺ channels there.
Q13. The excitatory postsynaptic potential (EPSP) is an example of a graded potential. When a single presynaptic terminal fires, the resulting EPSP in a typical anterior motor neuron is:
- A) Always sufficient to trigger an action potential
- B) About -65 mV change (resting potential)
- C) Too small to trigger an action potential alone; approximately 40-80 terminals must discharge simultaneously ✓
- D) Exactly at threshold level
Explanation: Guyton states that "discharge of a single presynaptic terminal does not increase the neuronal potential from -65 mV all the way up to -45 mV. An increase of this magnitude requires simultaneous discharge of many terminals - about 40 to 80 for the usual anterior motor neuron." This demonstrates the graded, summable nature of postsynaptic potentials.
Q14. Temporal summation of postsynaptic potentials occurs because:
- A) Multiple synaptic terminals fire simultaneously
- B) Successive discharges of the same terminal produce potentials that overlap and add up, since the changed postsynaptic potential lasts up to 15 milliseconds ✓
- C) The axon hillock amplifies each signal
- D) Na⁺-K⁺ pump activity is suppressed
Explanation: Guyton explains temporal summation: "the changed postsynaptic potential lasts up to 15 milliseconds after the synaptic membrane channels have already closed. Therefore, a second opening of the same channels can increase the postsynaptic potential further." This is distinct from spatial summation, where multiple synapses fire simultaneously.
Q15. A "facilitated" neuron is one in which:
- A) An action potential has already been generated
- B) Inhibitory postsynaptic potentials (IPSPs) dominate
- C) The summated postsynaptic potential is excitatory but has not yet reached threshold for firing ✓
- D) The neuron is in its absolute refractory period
Explanation: Guyton defines facilitation directly: "When the summated postsynaptic potential is excitatory but has not risen high enough to reach the threshold for firing by the postsynaptic neuron, the neuron is said to be facilitated." A facilitated neuron is primed to fire with even a small additional input.
SECTION 3: CHEMICAL VS. ELECTRICAL SYNAPSES
Q16. Which of the following is the KEY structural difference between chemical and electrical synapses?
- A) Chemical synapses have a larger synaptic cleft
- B) Electrical synapses use gap junction channels connecting pre- and postsynaptic cells directly; chemical synapses use neurotransmitters across a cleft ✓
- C) Chemical synapses are faster than electrical synapses
- D) Electrical synapses require Ca²⁺ for signal transmission
Explanation: Guyton's Figure 46.5 illustrates the structural difference clearly. Chemical synapses (Panel A) have a presynaptic terminal with synaptic vesicles and a synaptic cleft, using neurotransmitters. Electrical synapses (Panel B) have gap junction channels that directly connect the pre- and postsynaptic cells, allowing ionic current to flow directly.
Q17. One-way (unidirectional) conduction is a property of:
- A) Electrical synapses only
- B) Both chemical and electrical synapses equally
- C) Neither - all synapses are bidirectional
- D) Chemical synapses only ✓
Explanation: Guyton explicitly states: "Chemical synapses always transmit signals in one direction - from the presynaptic neuron to the postsynaptic neuron. This one-way conduction at chemical synapses is different from conduction through electrical synapses, which often transmit signals in either direction." Bidirectional transmission in electrical synapses helps coordinate large groups of neurons.
Q18. Electrical synapses are particularly useful for which of the following functions?
- A) Precise, targeted inhibitory signaling
- B) Slow neuromodulatory effects
- C) Coordinating simultaneous firing of large groups of neurons and detecting coincident subthreshold depolarizations ✓
- D) One-way signal transmission to effector organs
Explanation: Guyton states electrical synapses are "useful in detecting the coincidence of simultaneous subthreshold depolarizations within a group of interconnected neurons; this enables increased neuronal sensitivity and promotes synchronous firing." He also notes hypothalamic hormone-secreting neurons use electrical synapses to fire simultaneously for burst hormone secretion.
Q19. When an action potential reaches a presynaptic terminal at a chemical synapse, what directly triggers neurotransmitter vesicle release?
- A) Change in K⁺ permeability
- B) Opening of voltage-gated Na⁺ channels
- C) Binding of neurotransmitter to autoreceptors
- D) Depolarization of the presynaptic terminal membrane, which causes vesicles to empty into the cleft ✓
Explanation: Guyton states: "When an action potential spreads over a presynaptic terminal, depolarization of its membrane causes a small number of vesicles to empty into the cleft. The released transmitter in turn binds to receptors on the postsynaptic membrane." (Note: in fuller detail from other Guyton sections, depolarization triggers voltage-gated Ca²⁺ channels, which drive vesicle fusion.)
Q20. The excitatory postsynaptic potential (EPSP) is primarily produced by:
- A) Opening of K⁺ channels causing K⁺ efflux
- B) Opening of Cl⁻ channels causing Cl⁻ influx
- C) Activation of the Na⁺-K⁺ pump
- D) Opening of Na⁺ channels causing Na⁺ influx, making the membrane potential less negative ✓
Explanation: Guyton explains that excitatory transmitters act on membrane receptors to "increase the membrane's permeability to Na⁺." Sodium ions diffuse rapidly inside due to the large concentration gradient and electrical negativity inside, increasing membrane potential from -65 mV toward -45 mV. This less-negative value is the EPSP.
Q21. The inhibitory postsynaptic potential (IPSP) is primarily caused by:
- A) Na⁺ influx into the postsynaptic cell
- B) Depolarization of the postsynaptic membrane
- C) Opening of Cl⁻ channels (Cl⁻ influx) or K⁺ channels (K⁺ efflux), making the interior more negative ✓
- D) Activation of excitatory receptors
Explanation: Guyton states that inhibitory synapses "mainly open chloride channels, allowing for easier passage of chloride ions" into the cell, carrying negative charges inward and increasing membrane negativity. Increased K⁺ conductance (K⁺ efflux) also contributes. In Fig. 46.11C, the inhibited neuron shows a more negative intraneuronal potential (-70 mV vs. resting -65 mV).
Q22. Approximately what percentage of presynaptic terminals on a typical anterior motor neuron are located on dendrites?
- A) 5-20%
- B) 50%
- C) 80-95% ✓
- D) 100%
Explanation: Guyton states that the motor neuron may have as many as 10,000 to 200,000 presynaptic terminals (synaptic knobs), with "about 80% to 95% of them on the dendrites and only 5% to 20% on the soma." This arrangement gives the neuron a massive computational surface for integrating incoming signals.
Q23. Which neurotransmitter is listed under Class I (alone, not grouped with amines) in Guyton's classification of small-molecule, rapidly acting transmitters?
- A) Dopamine
- B) Serotonin
- C) Glutamate
- D) Acetylcholine ✓
Explanation: Guyton's Table 46.1 classifies small-molecule transmitters as: Class I - Acetylcholine (alone); Class II - Amines (norepinephrine, epinephrine, dopamine, serotonin, melatonin, histamine); Class III - Amino Acids (GABA, glycine, glutamate, aspartate); Class IV - ATP, arachidonic acid, nitric oxide, carbon monoxide.
Q24. Regarding conduction of action potentials along a myelinated nerve fiber, which statement is CORRECT?
- A) The action potential travels only in one direction (toward the axon terminal)
- B) Myelin sheaths speed up conduction by allowing continuous conduction
- C) The safety factor for propagation must be less than 1 for the impulse to continue
- D) The action potential travels in all directions away from the stimulus (both directions along the fiber) ✓
Explanation: Guyton states: "An excitable membrane has no single direction of propagation, but the action potential travels in all directions away from the stimulus - even along all branches of a nerve fiber - until the entire membrane has become depolarized." The "safety factor" must be greater than 1 for continued propagation (not less than 1).
Q25. After many action potentials, the Na⁺-K⁺ pump is needed to restore ionic gradients. Which of the following is TRUE about this process?
- A) It is a passive process requiring no energy
- B) A single action potential causes a measurable drop in ionic gradients
- C) Pump activity is stimulated when extracellular Na⁺ accumulates
- D) Pump activity is stimulated when excess intracellular Na⁺ accumulates, and the process requires ATP ✓
Explanation: Guyton explains that each action potential slightly reduces Na⁺/K⁺ concentration differences. The Na⁺-K⁺ pump is stimulated "approximately in proportion to the third power" of intracellular Na⁺ accumulation. The process is active (requires ATP) and is evidenced by increased heat production as impulse frequency rises. A single action potential is so minor that "100,000 to 50 million impulses can be transmitted before the concentration differences reach the point that action potential conduction ceases."
Quick Reference Summary Table
| Topic | Key Guyton Fact |
|---|---|
| Resting membrane potential | -70 mV |
| Threshold | ~-55 mV |
| Depolarization ion | Na⁺ influx (5000x conductance increase) |
| Repolarization ion | K⁺ efflux |
| Absolute refractory period | ~1 ms (max 1000 impulses/sec) |
| Relative refractory period | 2-4 ms (hyperpolarization phase) |
| EPSP triggers AP at | Axon initial segment (not soma) |
| Terminals to generate EPSP to threshold | 40-80 simultaneous |
| Synapse terminals on dendrites | 80-95% |
| Chemical synapse direction | One-way only |
| Electrical synapse direction | Often bidirectional |
| Electrical synapse structure | Gap junction channels |
| Graded potential | Summable, non-propagating, below threshold |
All questions and explanations sourced directly from Guyton and Hall Textbook of Medical Physiology (9780443111013), Chapters 5 and 46.
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Neurophysiology MCQ StudyGuide
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What's inside the document:
- Cover page - title banner in navy/teal, source citation, and "How to Use" instructions
- 3 color-coded sections:
- Section 1 - Action Potential (Q1-Q10)
- Section 2 - Graded/Local Potentials (Q11-Q15)
- Section 3 - Chemical vs. Electrical Synapses (Q16-Q25)
- Each question includes:
- Question stem in bold navy
- 4 options with the correct answer highlighted green with a ✓
- A blue-tinted answer + explanation block citing Guyton directly
- Thin horizontal rule separator
- Quick Reference Table at the end - 15 key Guyton facts (resting potential, threshold, refractory periods, synapse types, etc.) in an alternating-row striped table
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