MCAT Biological and Biochemical Foundations of Living Systems Flashcards: 3a Action Potentials Synaptic Transmission

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This deck focuses on 3a Action Potentials Synaptic Transmission, giving you a quick way to review the definitions, rules, and examples that matter most for MCAT Biological and Biochemical Foundations of Living Systems.

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Work through these flashcards in short sessions. Try to answer each prompt before flipping the card, then revisit any cards you miss until the explanation feels automatic.

Flashcard 1: What type of receptor is a G protein-coupled receptor (metabotropic receptor)?

Answer: A receptor that signals via G proteins and second messengers. Ligand binding activates G proteins, modulating intracellular pathways and effectors for slower, modulatory postsynaptic effects.

Flashcard 2: What type of receptor is a ligand-gated ion channel (ionotropic receptor)?

Answer: A receptor that directly opens an ion channel upon binding. Neurotransmitter binding induces a conformational change that directly gates ion flow, producing rapid postsynaptic responses.

Flashcard 3: What triggers neurotransmitter release from a presynaptic terminal?

Answer: $\text{Ca}^{2+}$ influx through voltage-gated $\text{Ca}^{2+}$ channels. Arrival of an action potential depolarizes the terminal, opening these channels and allowing calcium entry to trigger vesicle fusion and exocytosis.

Flashcard 4: What ion has the highest intracellular concentration and drives repolarization in neurons?

Answer: $\text{K}^+$. High intracellular potassium concentration establishes a gradient that, when channels open, allows efflux, restoring the membrane potential to resting levels.

Flashcard 5: What ion has the highest extracellular concentration and drives depolarization in neurons?

Answer: $\text{Na}^+$. High extracellular sodium concentration creates a gradient that, upon channel opening, drives influx, causing rapid depolarization during action potentials.

Flashcard 6: What is the approximate threshold membrane potential to trigger an action potential?

Answer: Approximately $-55\ \text{mV}$. At this potential, voltage-gated sodium channels begin to open, leading to rapid depolarization if the stimulus is sufficient to initiate an action potential.

Flashcard 7: What is temporal summation in synaptic integration?

Answer: Summation of repeated inputs over time at one synapse. Repeated subthreshold stimuli from the same presynaptic neuron accumulate, potentially reaching threshold if occurring within a short time frame.

Flashcard 8: Which neurotransmitter is classically degraded in the synaptic cleft by acetylcholinesterase?

Answer: Acetylcholine. Acetylcholinesterase rapidly hydrolyzes acetylcholine into choline and acetate, terminating its signaling at cholinergic synapses.

Flashcard 9: What are the three main mechanisms that terminate neurotransmitter signaling in a synapse?

Answer: Reuptake, enzymatic degradation, and diffusion away. These processes clear neurotransmitters from the cleft, preventing prolonged receptor activation and allowing synapse reset.

Flashcard 10: What is spatial summation in synaptic integration?

Answer: Summation of inputs from multiple synapses at once. Simultaneous subthreshold inputs from different presynaptic neurons integrate at the postsynaptic cell, possibly summating to threshold.

Flashcard 11: What is the relative refractory period?

Answer: Stronger stimulus needed; membrane is hyperpolarized. Hyperpolarization during this period raises the threshold, requiring a larger depolarizing stimulus to initiate another action potential.

Flashcard 12: What is the role of the nodes of Ranvier in myelinated axons?

Answer: Sites of high voltage-gated channel density for regeneration. These unmyelinated gaps concentrate voltage-gated sodium channels, allowing action potential regeneration and saltatory conduction.

Flashcard 13: What property of axons prevents action potentials from traveling backward?

Answer: Refractory period behind the depolarization wave. The absolute refractory period ensures unidirectional propagation by preventing re-excitation in recently depolarized segments.

Flashcard 14: What does the all-or-none principle state for action potentials?

Answer: Once threshold is reached, amplitude is constant. Action potentials are regenerative events where, once initiated, voltage-gated channels ensure a fixed amplitude and duration independent of stimulus intensity.

Flashcard 15: Which change increases conduction velocity: myelination or demyelination?

Answer: Myelination. Myelin sheath insulates the axon, promoting saltatory conduction and reducing capacitance for quicker signal propagation.

Flashcard 16: What is the absolute refractory period?

Answer: No new action potential; $\text{Na}^+$ channels are inactivated. During this phase, sodium channels remain inactivated, preventing depolarization regardless of stimulus strength until they reset.

Flashcard 17: What causes the undershoot (afterhyperpolarization) after an action potential?

Answer: Delayed closing of voltage-gated $\text{K}^+$ channels. Continued potassium efflux after sodium channels close hyperpolarizes the membrane below resting potential before returning to baseline.

Flashcard 18: What are the ion movements during the falling phase of a neuronal action potential?

Answer: Voltage-gated $\text{K}^+$ channels open; $\text{K}^+$ efflux. At peak depolarization, sodium channels inactivate while potassium channels open, permitting potassium outflow that repolarizes the membrane.

Flashcard 19: What are the ion movements during the rising phase of a neuronal action potential?

Answer: Voltage-gated $\text{Na}^+$ channels open; $\text{Na}^+$ influx. Depolarization to threshold activates these channels, allowing rapid sodium entry that further depolarizes the membrane in a positive feedback loop.

Flashcard 20: What is the primary function of the $\text{Na}^+/\text{K}^+$ ATPase in neurons?

Answer: Maintains $\text{Na}^+$ out and $\text{K}^+$ in using ATP. The pump actively transports three sodium ions out and two potassium ions in per ATP molecule, establishing ion gradients essential for resting potential and action potentials.

Flashcard 21: Which change increases action potential conduction velocity: increased axon diameter or decreased diameter?

Answer: Increased axon diameter. Larger diameter reduces internal resistance to current flow, enabling faster passive spread of depolarization along the axon.