The Neuron and Neural Firing - AP Psychology

1. Which statement correctly distinguishes synaptic transmission from axonal transmission?
   Answer: Axonal transmission is electrical membrane change; synaptic transmission is chemical neurotransmitter signaling across a gap. Axonal transmission involves electrical changes in membrane voltage as action potentials propagate down the axon, following the all-or-none principle with constant amplitude. In contrast, synaptic transmission is chemical, involving neurotransmitter molecules that are released from axon terminals and cross the synaptic cleft to bind with receptors on the postsynaptic neuron. This represents the fundamental electrochemical nature of neural communication - electrical signals within individual neurons and chemical signals between neurons. The electrical action potential triggers the chemical step at synapses, where neurotransmitters convert the signal back to electrical changes in the receiving neuron. During the refractory period after each action potential, the neuron temporarily cannot fire again, ensuring proper timing and directionality of both electrical transmission within the axon and chemical transmission across synapses.
2. Which event occurs at the axon terminals when an action potential arrives?
   Answer: Vesicles fuse with the presynaptic membrane and release neurotransmitters into the synaptic cleft. When an action potential arrives at axon terminals, it triggers voltage-gated calcium channels to open, allowing calcium ions to enter the terminal. This calcium influx causes synaptic vesicles containing neurotransmitters to fuse with the presynaptic membrane and release their chemical contents into the synaptic cleft. This process converts the electrical signal of the action potential into chemical communication across the synapse. The action potential that triggered this release followed the all-or-none principle, maintaining constant amplitude as it traveled down the axon. The released neurotransmitters then bind to receptors on the postsynaptic neuron, and during the refractory period, the presynaptic neuron cannot immediately fire another action potential.
3. A drug blocks receptor sites on the postsynaptic neuron; what is the most direct effect?
   Answer: Neurotransmitters cannot bind effectively, so postsynaptic influence is reduced even if neurotransmitters are released. When receptor sites on the postsynaptic neuron are blocked by a drug, neurotransmitters cannot bind effectively, even if they are released normally from the presynaptic terminal. This prevents the typical postsynaptic response (either excitatory or inhibitory) that would normally occur when neurotransmitters cross the synaptic cleft and bind to their receptors. The presynaptic neuron can still fire all-or-none action potentials and release neurotransmitters, but the chemical message cannot be received by the blocked postsynaptic neuron. This disrupts the normal electrochemical communication between neurons, as the electrical signal in the presynaptic neuron cannot be converted to electrical changes in the postsynaptic neuron. The refractory period still occurs in the presynaptic neuron after each action potential, maintaining normal timing of its firing.
4. Which statement best describes why myelin increases neural communication speed?
   Answer: Myelin increases speed by enabling saltatory conduction, allowing action potentials to jump between nodes of Ranvier. Myelin is a fatty insulation layer that wraps around many axons, with gaps called nodes of Ranvier between myelin segments. This structure enables saltatory conduction, where the action potential 'jumps' from node to node rather than traveling continuously along the entire axon membrane. This jumping pattern dramatically increases transmission speed because the action potential only needs to be regenerated at the nodes, not along every point of the axon. The myelin acts as electrical insulation, preventing current leak and maintaining signal strength between nodes. Without myelin, action potentials would propagate much more slowly through continuous conduction. During the refractory period after each action potential, the neuron temporarily cannot fire again, which helps maintain proper timing of neural signals.
5. A student says neurons communicate only electrically; which correction is most accurate?
   Answer: Neural communication is electrochemical: electrical action potentials within neurons and chemical neurotransmitters across synapses. Neural communication is fundamentally electrochemical, combining electrical transmission within individual neurons and chemical transmission between neurons. Action potentials are electrical events that follow the all-or-none principle, maintaining constant amplitude as they travel down axons to terminals. At synapses, these electrical signals trigger the release of neurotransmitters, which are chemical messengers that cross synaptic clefts to bind with receptors on postsynaptic neurons. This chemical step is essential for converting the electrical signal from one neuron into electrical changes in the next neuron. The refractory period following each action potential ensures proper timing of electrical signals, while neurotransmitter binding and receptor activation control the chemical component of synaptic transmission. This electrochemical process enables precise, directional communication throughout neural networks.
6. If sodium channels open rapidly, what phase of the action potential is occurring?
   Answer: Depolarization, as positive ions enter and the membrane potential rises toward and beyond threshold. When sodium channels open rapidly, the neuron is experiencing depolarization, the rising phase of the action potential where positive sodium ions rush into the cell. This causes the membrane potential to become less negative and rise toward and beyond threshold. Once threshold is reached, the all-or-none action potential is fully triggered and propagates down the axon with constant amplitude. The electrical signal travels to axon terminals where it triggers neurotransmitter release for electrochemical communication across synapses. Following the action potential, repolarization occurs as sodium channels close and potassium channels open, and then the refractory period begins during which the neuron cannot immediately fire another action potential. This depolarization phase is crucial for initiating the electrical signal that enables neural communication.
7. A drug inhibits reuptake transporters; what change occurs in the synapse?
   Answer: Neurotransmitters remain in the synaptic cleft longer, increasing the duration of their effects on receptors. When reuptake transporters are inhibited, neurotransmitters remain in the synaptic cleft for a longer duration because they cannot be efficiently removed by the presynaptic neuron. This prolonged presence increases the time that neurotransmitters can bind to postsynaptic receptors, potentially amplifying and extending their effects. The action potential that originally triggered neurotransmitter release followed the all-or-none principle, maintaining constant amplitude as it traveled down the axon. Normal neural communication involves electrochemical signaling - electrical action potentials within neurons and chemical neurotransmitter signaling across synapses. During the refractory period following the action potential, the presynaptic neuron cannot immediately fire again, but the extended presence of neurotransmitters in the cleft can continue to influence the postsynaptic neuron.
8. Which statement best describes neural transmission along an axon?
   Answer: It is electrochemical overall, but the action potential along the axon is an electrical change in membrane voltage. Neural transmission along an axon is fundamentally electrical, involving rapid changes in membrane voltage as the action potential propagates. However, overall neural communication is electrochemical because it combines electrical transmission within neurons (action potentials) with chemical transmission between neurons (neurotransmitters at synapses). The action potential follows the all-or-none principle, maintaining constant amplitude regardless of stimulus strength as it travels down the axon. When the electrical signal reaches axon terminals, it triggers neurotransmitter release into synaptic clefts. These chemical messengers then bind to receptors on the next neuron, potentially triggering another electrical action potential. The refractory period following each action potential temporarily prevents immediate refiring, ensuring proper signal timing and direction.
9. A neuron's message reaches the end of the axon; what must happen for the next neuron to be affected?
   Answer: Neurotransmitters must be released into the synaptic cleft and bind to receptors on the postsynaptic neuron. When an action potential reaches the end of an axon, neurotransmitters must be released from vesicles into the synaptic cleft and then bind to receptor sites on the postsynaptic neuron for communication to continue. This conversion from electrical to chemical signaling is essential in electrochemical neural communication. The action potential that traveled down the axon followed the all-or-none principle, maintaining constant amplitude regardless of the original stimulus strength. Once neurotransmitters bind to postsynaptic receptors, they can influence whether the receiving neuron reaches threshold to fire its own action potential. This chemical step across the synapse is necessary because electrical signals cannot jump directly across the synaptic cleft. After any resulting action potential in the postsynaptic neuron, the refractory period will temporarily prevent immediate refiring, maintaining proper timing in neural networks.
10. A student labels the neuron part that releases neurotransmitters into the synapse; which part is correct?
    Answer: Terminal buttons (axon terminals), which release neurotransmitters from vesicles into the synaptic cleft. Terminal buttons (also called axon terminals) are the endpoints of the axon that contain vesicles filled with neurotransmitters. When an action potential reaches these terminals, it triggers voltage-gated calcium channels to open, causing vesicles to fuse with the presynaptic membrane and release their chemical contents into the synaptic cleft. This process converts the electrical signal of the action potential into chemical communication across the synapse. The neurotransmitters then diffuse across the gap and bind to specific receptor sites on the postsynaptic neuron. During the refractory period, the presynaptic neuron cannot immediately fire another action potential, helping ensure proper signal timing and unidirectional flow of information.
11. At rest, a neuron is about $-70\text{ mV}$; what is this state called?
    Answer: Resting potential, a stable negative internal charge maintained before the neuron reaches threshold to fire. Resting potential refers to the stable electrical state of a neuron when it is not actively firing, typically around -70 mV inside relative to outside. This negative charge is maintained by the sodium-potassium pump and selective membrane permeability that keeps more negative ions inside the cell. The neuron remains in this state until sufficient excitatory input brings it to threshold (around -55 mV), at which point an all-or-none action potential is triggered. Once fired, the action potential travels down the axon as an electrical signal that maintains constant amplitude regardless of stimulus strength. Neurotransmitters are then released at synapses to communicate with other neurons, and during the refractory period, the neuron cannot immediately fire again.
12. A neuron's outgoing message travels away from the soma toward other cells; which structure carries it?
    Answer: Axon, a long fiber that conducts an all-or-none action potential away from the cell body toward terminals. The axon is a long fiber that carries action potentials away from the cell body (soma) toward the terminal buttons. When the neuron reaches threshold, an all-or-none action potential is generated at the axon hillock and travels down the axon as an electrical change in membrane voltage. This electrical signal maintains constant amplitude regardless of the original stimulus strength. At the axon terminals, the electrical signal triggers the release of neurotransmitters into the synaptic cleft. These chemical messengers then cross the gap to bind with receptors on the next neuron. After firing, the neuron enters a refractory period during which it cannot immediately fire another action potential.
13. After neurotransmitters bind postsynaptic receptors, which synaptic step most directly helps clear them from the synaptic cleft?
    Answer: Reuptake, where transporter proteins in the presynaptic membrane take neurotransmitters back into the sending neuron. After neurotransmitters bind to receptors on the postsynaptic neuron, they must be cleared from the synaptic cleft to terminate the signal and prepare for the next transmission. Reuptake is the primary mechanism where specialized transporter proteins in the presynaptic membrane actively pump neurotransmitters back into the sending neuron for recycling or breakdown. This process is crucial for proper neural function - many antidepressants work by blocking reuptake transporters. Enzymes can also break down neurotransmitters in the cleft. The all-or-none principle refers to action potential firing, not neurotransmitter removal, and myelin insulates axons rather than absorbing neurotransmitters.
14. Which description correctly matches a neuron part with its primary function?
    Answer: Dendrites: receive messages; axon: transmits action potentials; terminal buttons: release neurotransmitters. This description correctly identifies the primary functions of key neuron structures in electrochemical communication. Dendrites are specialized to receive incoming chemical signals (neurotransmitters) from other neurons and convert them to electrical changes. The axon transmits all-or-none action potentials as electrical signals that maintain constant amplitude as they propagate toward the terminals. Terminal buttons release neurotransmitters from vesicles into synaptic clefts when action potentials arrive, converting electrical signals back to chemical communication. This represents the fundamental electrochemical nature of neural transmission - electrical within neurons, chemical between neurons. During the refractory period following each action potential, the neuron cannot immediately fire again, ensuring proper timing and directionality of neural communication throughout these structures.
15. A student confuses dendrites and axons; which correction is accurate?
    Answer: Dendrites mainly receive input; axons mainly transmit action potentials away from the cell body to terminals. Dendrites are the branching structures that primarily receive input from other neurons through neurotransmitter binding at receptor sites, while axons are the long projections that transmit action potentials away from the cell body toward terminals. This represents the fundamental input-output organization of neurons in electrochemical communication. When dendrites receive neurotransmitter signals, they generate electrical changes that travel toward the cell body for integration. If threshold is reached, an all-or-none action potential is generated and travels down the axon with constant amplitude to the terminals. There, the electrical signal triggers neurotransmitter release for chemical communication with the next neuron. During the refractory period following each action potential, the neuron cannot immediately fire again, ensuring proper timing and directionality from dendritic input to axonal output.
16. Which neuron structure most directly releases neurotransmitters into the synaptic cleft to signal the next neuron?
    Answer: Terminal buttons (axon terminals), which contain vesicles that release neurotransmitters into the synaptic cleft. Terminal buttons (also called axon terminals) are the specialized structures at the end of the axon that release neurotransmitters into the synaptic cleft. These terminals contain synaptic vesicles filled with neurotransmitters. When an action potential arrives, calcium channels open, triggering vesicles to fuse with the membrane and release their contents. This electrochemical process converts the electrical signal into a chemical one that can cross the synaptic gap. Dendrites receive signals but don't release neurotransmitters, the cell body integrates signals but doesn't directly release into synapses, and myelin insulates axons rather than releasing chemicals. The all-or-none action potential ensures consistent neurotransmitter release, followed by a refractory period.
17. A neurotransmitter binds but is quickly cleared from the synapse by enzymes; what is the outcome?
    Answer: The neurotransmitter's effect ends sooner because it is broken down, reducing continued receptor activation. When neurotransmitters are quickly broken down by enzymes in the synaptic cleft, their effect on postsynaptic receptors ends sooner, reducing the duration of receptor activation and postsynaptic response. This enzymatic breakdown is one mechanism for terminating synaptic transmission and clearing the cleft for subsequent signals. The original action potential that triggered neurotransmitter release followed the all-or-none principle as it traveled down the axon. Normal electrochemical communication involves electrical action potentials within neurons and chemical neurotransmitter signaling across synapses. Quick enzymatic breakdown ensures that each synaptic signal is brief and discrete, preventing prolonged activation that could interfere with subsequent neural messages. The refractory period in the presynaptic neuron still occurs after the action potential, maintaining proper timing for future electrical signals even as the chemical signal is rapidly terminated.
18. Which scenario best illustrates the all-or-none nature of action potentials?
    Answer: Once threshold is reached, the neuron fires a full spike; below threshold, it does not fire an action potential. The all-or-none nature of action potentials means that once threshold is reached, the neuron fires a complete action potential of fixed amplitude, or if threshold is not reached, no action potential occurs at all. There is no partial firing or variable-sized action potentials based on stimulus strength. This electrical signal travels down the axon with constant amplitude to the terminals, where it triggers a standard amount of neurotransmitter release. The electrochemical communication process is binary at the level of individual neurons - either an action potential occurs and neurotransmitters are released, or it doesn't happen. During the refractory period following any action potential, the neuron cannot immediately fire again regardless of stimulus intensity. Stimulus intensity is instead encoded by firing frequency and the number of neurons activated, not by varying action potential size.
19. Multiple sclerosis damages myelin; which neural function is most directly impaired by this damage?
    Answer: Faster action potential conduction along the axon, because myelin normally insulates and speeds signal transmission. Myelin is a fatty insulation that wraps around axons in segments, dramatically increasing the speed of action potential conduction through saltatory conduction - the signal "jumps" between gaps in myelin called nodes of Ranvier. Multiple sclerosis damages this myelin sheath, causing slower and less reliable signal transmission along axons. This can lead to various neurological symptoms depending on which neurons are affected. Myelin doesn't synthesize neurotransmitters (that occurs in the cell body and terminals), doesn't bridge synaptic gaps (which remain chemical), and doesn't control action potential amplitude (which follows the all-or-none principle). The refractory period ensures proper signal spacing regardless of myelin.
20. In a reflex pathway, which component ensures the message is carried quickly along the axon?
    Answer: Myelin sheath, which insulates the axon and allows faster conduction by enabling action potentials to jump nodes. The myelin sheath acts as electrical insulation around axons, enabling saltatory conduction where action potentials jump rapidly between nodes of Ranvier rather than traveling continuously along the membrane. This jumping pattern dramatically increases the speed of neural transmission, which is crucial for quick reflex responses. The action potential maintains constant amplitude due to the all-or-none principle as it propagates to the axon terminals. Fast transmission ensures that the electrochemical signal - electrical within the neuron and chemical across synapses - reaches its target quickly. At the terminals, neurotransmitter release occurs when the action potential arrives, and during the subsequent refractory period, the neuron cannot immediately fire another action potential. This rapid, efficient transmission is essential for protective reflexes that require immediate responses.

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