Easy USMLE Neurophysiology Practice Questions

Concept Explanation

Neurophysiology is the study of the functional properties of neurons, glia, and neural networks within the nervous system. At its core, this field focuses on how electrical impulses, known as action potentials, are generated and transmitted across synapses to facilitate communication between different parts of the body. Understanding the resting membrane potential, which typically sits around $-70 \text{ mV}$ in a standard neuron, is fundamental for any student engaging with USMLE Prep. This potential is maintained by the selective permeability of the cell membrane and the activity of the sodium-potassium pump ($Na^+/K^+ \text{-ATPase}$), which moves three sodium ions out of the cell for every two potassium ions it brings in.

Key concepts in neurophysiology include the electrochemical gradients that drive ion movement. According to the Nernst equation, the equilibrium potential for a specific ion is determined by its concentration gradient across the membrane. For example, because potassium ($K^+$) concentration is much higher inside the cell, its chemical gradient pushes it outward, while the electrical gradient (the negative interior) pulls it back in. When these forces balance, the ion is at equilibrium. During an action potential, voltage-gated sodium channels open rapidly, leading to depolarization. This is followed by the opening of voltage-gated potassium channels and the inactivation of sodium channels, resulting in repolarization and a brief period of hyperpolarization.

Synaptic transmission adds another layer of complexity. When an action potential reaches the axon terminal, it triggers the opening of voltage-gated calcium channels. The influx of $Ca^{2+}$ causes neurotransmitter vesicles to fuse with the presynaptic membrane and release their contents into the synaptic cleft. These neurotransmitters then bind to receptors on the postsynaptic cell, causing either Excitatory Postsynaptic Potentials (EPSPs) or Inhibitory Postsynaptic Potentials (IPSPs). These concepts are frequently tested in USMLE Neuroanatomy Practice Questions and physiology modules alike.

Solved Examples

1. Calculating Equilibrium Potential: A researcher measures the extracellular concentration of an ion at $145 \text{ mM}$ and the intracellular concentration at $15 \text{ mM}$. Using a simplified Nernst equation at body temperature, calculate the equilibrium potential ($E_{ion}$).
   1. Identify the formula: E_{ion} = \frac{61}{z} \log_{10} \left( \frac{[ion]_{out}}{[ion]_{in}} ight).
   2. Assume the ion is Sodium ($z = +1$).
   3. Plug in the values: E_{Na} = 61 \log_{10} \left( \frac{145}{15} ight).
   4. Calculate the log: $\log_{10}(9.67) \approx 0.985$.
   5. Final result: $61 \times 0.985 \approx +60 \text{ mV}$.
2. Understanding Refractory Periods: Explain why a second action potential cannot be triggered immediately after the first one, even with a very strong stimulus.
   1. During the absolute refractory period, voltage-gated $Na^+$ channels are in an inactivated state.
   2. The inactivation gate (h-gate) closes at the peak of depolarization and will not reopen until the membrane repolarizes.
   3. Because these channels cannot be reopened, no amount of stimulus can trigger a new depolarization.
   4. This ensures the one-way propagation of signals down the axon.
3. Synaptic Delay: Determine the primary cause of the $0.5 \text{ to } 1.0 \text{ ms}$ delay observed at chemical synapses.
   1. The delay is not caused by the travel of the electrical signal down the axon.
   2. It is primarily due to the time required for calcium influx to trigger vesicle docking and neurotransmitter release.
   3. Additional time is spent on the diffusion of the neurotransmitter across the synaptic cleft and the binding to postsynaptic receptors.

Practice Questions

1. Which ion has the highest permeability in a resting neuron, contributing most significantly to the resting membrane potential?
2. An experimental drug blocks voltage-gated calcium channels in the presynaptic terminal. What is the most likely effect on synaptic transmission?
3. If the extracellular potassium concentration is acutely increased (hyperkalemia), what happens to the resting membrane potential of a neuron?

4. Describe the role of the myelin sheath in increasing the conduction velocity of an action potential.
5. A patient presents with muscle weakness. Electrophysiological studies show a decrease in the amplitude of the end-plate potential. Which autoimmune condition targeting postsynaptic acetylcholine receptors is most likely?
6. What is the difference between temporal summation and spatial summation in a postsynaptic neuron?
7. Which neurotransmitter is the primary excitatory signal in the central nervous system?
8. During the undershoot (hyperpolarization) phase of an action potential, the membrane potential moves closer to the equilibrium potential of which ion?
9. How does the length constant ($\lambda$) of an axon affect the spread of local currents?
10. What happens to the velocity of nerve conduction if the internal resistance ($R_i$) of the axon is decreased by increasing the axon diameter?

Answers & Explanations

1. Potassium ($K^+$): In a resting state, the membrane is much more permeable to potassium than to sodium due to the presence of leak channels. This allows $K^+$ to move down its concentration gradient out of the cell, bringing the resting potential close to the $K^+$ equilibrium potential of roughly $-90 \text{ mV}$.
2. Inhibition of Neurotransmitter Release: Calcium influx is the essential trigger for exocytosis. Without calcium entering the presynaptic terminal, vesicles cannot fuse with the membrane, and the chemical signal cannot be passed to the next cell.
3. Depolarization: Increasing extracellular $K^+$ reduces the concentration gradient. Since less potassium leaves the cell, the interior becomes less negative (more positive), bringing the neuron closer to the firing threshold.
4. Saltatory Conduction: Myelin acts as an insulator, preventing charge leakage. Action potentials "jump" between the nodes of Ranvier, where voltage-gated channels are concentrated, significantly speeding up transmission compared to unmyelinated fibers.
5. Myasthenia Gravis: This condition involves antibodies against the nicotinic acetylcholine receptors at the neuromuscular junction. This reduces the number of functional receptors, leading to smaller end-plate potentials that may fail to reach the threshold for muscle contraction.
6. Summation Types: Temporal summation occurs when a single presynaptic neuron fires rapidly in succession. Spatial summation occurs when multiple different presynaptic neurons fire simultaneously at different locations on the same postsynaptic neuron.
7. Glutamate: Glutamate is the major excitatory neurotransmitter. It acts on receptors like NMDA and AMPA to allow cation influx, leading to depolarization. You can find more on this in our USMLE Physiology Practice Questions with Answers.
8. Potassium ($K^+$): After the sodium channels inactivate, the delayed rectifier potassium channels remain open. Since the membrane is now almost exclusively permeable to $K^+$, the potential dips toward $-90 \text{ mV}$ before returning to rest.
9. Signal Distance: A larger length constant means the electrical signal can travel further along the membrane before dissipating. This is increased by high membrane resistance (myelination) and low internal resistance (large diameter).
10. Increased Velocity: Decreasing internal resistance allows local currents to flow more easily and quickly to the next segment of the membrane, reaching the threshold for the next action potential faster.

1. Which mechanism is responsible for the absolute refractory period?

• AClosure of the potassium leak channels
• BInactivation of voltage-gated sodium channels
• COpening of chloride channels
• DActivation of the sodium-potassium pump

Frequently Asked Questions

What is the difference between an action potential and a graded potential?

Action potentials are all-or-nothing, self-propagating electrical signals that do not lose strength over distance. Graded potentials vary in size depending on the stimulus strength and dissipate as they move away from the source.

How does Tetrodotoxin (TTX) affect the nervous system?

Tetrodotoxin, found in pufferfish, selectively blocks voltage-gated sodium channels. This prevents the generation of action potentials, leading to paralysis and potentially respiratory failure.

What defines the threshold potential?

The threshold is the critical membrane voltage, usually around $-55 \text{ mV}$, at which the inward sodium current exceeds the outward potassium current. Once this point is reached, an action potential is guaranteed to fire.

Why is the sodium-potassium pump considered electrogenic?

The pump is electrogenic because it creates a net charge imbalance across the membrane. By pumping out three positive sodium ions for every two positive potassium ions it brings in, it contributes a small negative charge to the cell interior.

What is the role of the Nodes of Ranvier?

Nodes of Ranvier are unmyelinated gaps in the myelin sheath where high densities of voltage-gated sodium channels are located. They allow the action potential to be regenerated at intervals, supporting saltatory conduction.

How do local anesthetics like lidocaine work?

Lidocaine works by binding to and blocking the intracellular portion of voltage-gated sodium channels. This prevents the initiation and conduction of pain signals along sensory nerves.