MCAT Biological and Biochemical Foundations of Living Systems Flashcards: 3a Neuron Structure Signal Propagation

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This deck focuses on 3a Neuron Structure Signal Propagation, 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: Which glial cell type myelinates axons in the central nervous system?

Answer: Oligodendrocyte. Oligodendrocytes form myelin sheaths around multiple CNS axons, facilitating efficient signal transmission.

Flashcard 2: Which ion channel opening most directly mediates the repolarization phase of an action potential?

Answer: Voltage-gated K$^+$ channels opening. Potassium efflux restores the membrane potential to resting levels following sodium channel inactivation.

Flashcard 3: What is the absolute refractory period of an action potential?

Answer: Time when no new action potential can occur due to Na$^+$ inactivation. Inactivated sodium channels prevent depolarization, ensuring unidirectional propagation and a recovery period.

Flashcard 4: What is the relative refractory period of an action potential?

Answer: Time when a stronger stimulus is needed due to ongoing K$^+$ efflux. Hyperpolarization from potassium currents raises the threshold, requiring greater stimulation for another action potential.

Flashcard 5: What does the all-or-none principle state for neuronal action potentials?

Answer: Once threshold is reached, action potential amplitude is constant. This principle ensures that suprathreshold stimuli produce full-strength action potentials, maintaining signal reliability.

Flashcard 6: Identify the direction of action potential propagation in a normal axon and the key reason.

Answer: Away from soma; refractory period prevents backward propagation. Orthodromic propagation occurs as the refractory state behind the action potential blocks retrograde travel.

Flashcard 7: What is saltatory conduction?

Answer: Action potentials appear to jump node-to-node in myelinated axons. In myelinated fibers, depolarization skips insulated segments, regenerating at nodes for faster conduction.

Flashcard 8: Which receptor type is G protein-coupled and typically mediates slower, modulatory synaptic effects?

Answer: Metabotropic receptor. Ligand binding activates G proteins, leading to second messenger cascades that modulate cellular activity over longer timescales.

Flashcard 9: Which receptor type is a ligand-gated ion channel that mediates fast synaptic transmission?

Answer: Ionotropic receptor. Direct binding of ligand opens the channel, allowing rapid ion flow for immediate postsynaptic response.

Flashcard 10: What event directly triggers neurotransmitter vesicle fusion at a chemical synapse?

Answer: Ca$^{2+}$ influx through presynaptic voltage-gated Ca$^{2+}$ channels. Action potential arrival opens calcium channels, elevating intracellular calcium to promote exocytosis of synaptic vesicles.

Flashcard 11: Which change most directly increases axonal conduction velocity: increased diameter or decreased diameter?

Answer: Increased diameter. Larger axons have lower internal resistance, allowing faster passive current spread and thus quicker conduction.

Flashcard 12: What is the primary function of dendrites in a typical multipolar neuron?

Answer: Receive synaptic input and conduct graded potentials to the soma. Dendrites integrate incoming signals from other neurons, allowing graded potentials to propagate toward the cell body for potential summation and action potential initiation.

Flashcard 13: What is the primary function of the axon hillock (initial segment) in neurons?

Answer: Site of action potential initiation via voltage-gated Na$^+$ channels. The axon hillock sums graded potentials and, upon reaching threshold, triggers action potentials through the opening of voltage-gated sodium channels.

Flashcard 14: What is the main role of the myelin sheath in action potential propagation?

Answer: Increase conduction velocity by increasing membrane resistance. Myelin insulates axons, reducing capacitance and increasing resistance, which enables faster saltatory conduction of action potentials.

Flashcard 15: What are nodes of Ranvier?

Answer: Gaps in myelin with high densities of voltage-gated Na$^+$ channels. These unmyelinated segments concentrate sodium channels, allowing action potentials to regenerate and 'jump' during saltatory conduction.

Flashcard 16: Which glial cell type myelinates axons in the peripheral nervous system?

Answer: Schwann cell. Schwann cells wrap around individual PNS axons to create myelin, supporting rapid nerve impulse conduction.

Flashcard 17: What is the typical resting membrane potential of a neuron?

Answer: Approximately $-70\ \text{mV}$. This value reflects the balance of ion gradients and permeabilities, primarily influenced by potassium efflux at rest.

Flashcard 18: What is the definition of the equilibrium potential for an ion (Nernst potential)?

Answer: Membrane potential where net flux of that ion is $0$. It represents the voltage at which the electrical driving force equals the chemical gradient, resulting in no net ion movement.

Flashcard 19: What is the role of the Na$^+$/K$^+$ ATPase in maintaining neuronal gradients?

Answer: Pumps $3\ \text{Na}^+$ out and $2\ \text{K}^+$ in per ATP. This electrogenic pump maintains sodium and potassium gradients essential for resting potential and action potential generation.

Flashcard 20: Which ion has the greatest resting membrane permeability in most neurons?

Answer: K$^+$. High potassium permeability allows its equilibrium potential to dominate the resting membrane potential via leak channels.

Flashcard 21: What is an excitatory postsynaptic potential (EPSP)?

Answer: A graded depolarization that increases action potential probability. EPSPs result from neurotransmitter-induced cation influx, bringing the membrane closer to threshold for firing an action potential.

Flashcard 22: What is an inhibitory postsynaptic potential (IPSP)?

Answer: A graded hyperpolarization that decreases action potential probability. IPSPs arise from anion influx or cation efflux, moving the membrane potential away from threshold to inhibit neuronal firing.

Flashcard 23: What is meant by temporal summation in synaptic integration?

Answer: Summation of inputs from one synapse arriving close in time. Repeated stimulation from a single presynaptic neuron can accumulate EPSPs or IPSPs over time to reach threshold.

Flashcard 24: What is meant by spatial summation in synaptic integration?

Answer: Summation of inputs from multiple synapses arriving together. Simultaneous inputs from different synapses integrate at the postsynaptic neuron to determine if threshold is met.

Flashcard 25: Which ion channel opening most directly mediates the rapid upstroke of an action potential?

Answer: Voltage-gated Na$^+$ channels opening. Sodium influx upon depolarization rapidly depolarizes the membrane to peak potential during the action potential.