NEUR101 - Neurobiology & Action Potentials Flashcards

Question 1

What is the typical resting membrane potential (RMP) of a neuron?

Accepted Answer

The resting membrane potential (RMP) is the electrical potential difference across the neuronal membrane at rest, typically around -70 mV (inside negative relative to outside).

Question 2

What are the primary factors that establish and maintain the resting membrane potential?

Accepted Answer

The RMP is primarily established by the differential permeability of the membrane to ions (mainly K+) and the concentration gradients of ions (Na+, K+, Cl-). It is maintained by the active transport of the Na+/K+ ATPase pump.

Question 3

Describe the ion distribution across the neuronal membrane at rest for Na+, K+, and Cl-.

Accepted Answer

At rest: 
- **Na+**: High concentration outside the cell, low inside.
- **K+**: High concentration inside the cell, low outside.
- **Cl-**: High concentration outside the cell, low inside.
- Large anionic proteins are mostly trapped inside the cell.

Question 4

What is the function of the Na+/K+ ATPase pump?

Accepted Answer

The Na+/K+ ATPase pump actively transports 3 Na+ ions out of the cell and 2 K+ ions into the cell, consuming ATP. This maintains the concentration gradients for Na+ and K+ and contributes to the negative resting membrane potential.

Question 5

What does the Nernst equation calculate?

Accepted Answer

The Nernst equation calculates the equilibrium potential ($E_x$) for a single ion, which is the membrane potential at which there is no net movement of that ion across the membrane, given its concentration gradient.

Question 6

State the Nernst equation and define its variables.

Accepted Answer

The Nernst equation is: $E_x = rac{RT}{zF} \ln rac{[X]_{out}}{[X]_{in}}$
Where:
- $E_x$: Equilibrium potential for ion X
- R: Gas constant (8.314 J/(mol·K))
- T: Absolute temperature (in Kelvin)
- z: Valence (charge) of the ion
- F: Faraday's constant (96,485 C/mol)
- $[X]_{out}$: Extracellular concentration of ion X
- $[X]_{in}$: Intracellular concentration of ion X

Question 7

Why is the resting membrane potential close to the equilibrium potential for K+?

Accepted Answer

The neuronal membrane is significantly more permeable to K+ at rest compared to Na+ or other ions, primarily due to the presence of numerous K+ leak channels. Therefore, the RMP is largely dictated by the K+ concentration gradient.

Question 8

What does the Goldman-Hodgkin-Katz (GHK) equation calculate, and why is it more comprehensive than the Nernst equation?

Accepted Answer

The GHK equation calculates the actual resting membrane potential ($V_m$) by considering the permeability and concentration gradients of multiple ions (typically Na+, K+, and Cl-). It is more comprehensive because it accounts for the relative permeabilities of all major contributing ions, unlike the Nernst equation which is for a single ion.

Question 9

Provide a simplified form of the GHK equation for Na+, K+, and Cl- ions.

Accepted Answer

A simplified form of the GHK equation is: 
$V_m = rac{RT}{F} \ln rac{P_K[K^+]_{out} + P_{Na}[Na^+]_{out} + P_{Cl}[Cl^-]_{in}}{P_K[K^+]_{in} + P_{Na}[Na^+]_{in} + P_{Cl}[Cl^-]_{out}}$
Where $P_X$ represents the membrane permeability for ion X. Note the inversion of Cl- concentrations due to its negative charge.

Question 10

What is an action potential?

Accepted Answer

An action potential is a rapid, transient, all-or-none electrical signal propagated along the axon of a neuron, caused by a rapid change in membrane potential due to the sequential opening and closing of voltage-gated ion channels.

Question 11

What is the threshold potential for an action potential?

Accepted Answer

The threshold potential is the critical level of depolarization (typically around -55 mV) that must be reached for an action potential to be initiated. At this point, a sufficient number of voltage-gated Na+ channels open to trigger the regenerative depolarization.

Question 12

Describe the depolarization phase of an action potential.

Accepted Answer

The depolarization phase is characterized by a rapid influx of Na+ ions into the cell through voltage-gated Na+ channels, causing the membrane potential to quickly become positive (e.g., from -70 mV to +30 mV).

Question 13

Describe the repolarization phase of an action potential.

Accepted Answer

The repolarization phase involves the inactivation of voltage-gated Na+ channels and the efflux of K+ ions out of the cell through voltage-gated K+ channels, causing the membrane potential to return towards negative values.

Question 14

What is the hyperpolarization (undershoot) phase of an action potential?

Accepted Answer

The hyperpolarization (undershoot) phase is a brief period where the membrane potential becomes more negative than the resting potential (e.g., -80 mV) due to the slow closing of voltage-gated K+ channels, leading to excess K+ efflux before returning to RMP.

Question 15

What is the absolute refractory period?

Accepted Answer

The absolute refractory period is a period during which a second action potential cannot be initiated, regardless of the strength of the stimulus. This is because voltage-gated Na+ channels are either already open or are in an inactivated state and cannot be reopened.

Question 16

What is the relative refractory period?

Accepted Answer

The relative refractory period is a period during which a second action potential can be initiated, but only by a stronger-than-normal stimulus. This occurs because some voltage-gated Na+ channels have recovered from inactivation, but K+ channels are still open, making the membrane hyperpolarized and requiring more depolarization to reach threshold.

Question 17

Briefly describe the process of synaptic transmission.

Accepted Answer

Synaptic transmission is the process by which neurons communicate. It involves the release of neurotransmitters from the presynaptic neuron's axon terminal, their diffusion across the synaptic cleft, and binding to receptors on the postsynaptic neuron, leading to a change in its membrane potential.

Question 18

What is an excitatory neurotransmitter, and give an example.

Accepted Answer

An excitatory neurotransmitter causes depolarization of the postsynaptic membrane, increasing the likelihood of an action potential. Examples include Glutamate (in the CNS) and Acetylcholine (at the neuromuscular junction).

Question 19

What is an inhibitory neurotransmitter, and give an example.

Accepted Answer

An inhibitory neurotransmitter causes hyperpolarization or stabilization of the postsynaptic membrane, decreasing the likelihood of an action potential. Examples include GABA (Gamma-aminobutyric acid) and Glycine.

Question 20

Distinguish between ionotropic and metabotropic receptors.

Accepted Answer

**Ionotropic receptors** are ligand-gated ion channels that open directly upon neurotransmitter binding, causing rapid changes in membrane potential. 
**Metabotropic receptors** are G-protein coupled receptors that, upon neurotransmitter binding, initiate a slower, more complex signaling cascade involving second messengers, indirectly affecting ion channels or gene expression.

Question 21

What is spatial summation in synaptic integration?

Accepted Answer

Spatial summation is the integration of multiple postsynaptic potentials (PSPs) occurring at different synaptic locations on a neuron at approximately the same time. If the combined effect reaches threshold, an action potential is fired.

Question 22

What is temporal summation in synaptic integration?

Accepted Answer

Temporal summation is the integration of multiple postsynaptic potentials (PSPs) occurring at the same synapse in rapid succession. If the successive potentials arrive before the previous ones have fully decayed, their effects can summate to reach threshold.

Question 23

What is the role of voltage-gated Na+ channels in an action potential?

Accepted Answer

Voltage-gated Na+ channels are responsible for the rapid depolarization phase of the action potential. They open quickly in response to threshold depolarization, allowing a massive influx of Na+ ions, and then inactivate shortly after.

Question 24

What is the role of voltage-gated K+ channels in an action potential?

Accepted Answer

Voltage-gated K+ channels are responsible for the repolarization and hyperpolarization phases of the action potential. They open more slowly than Na+ channels in response to depolarization, allowing K+ efflux, which helps restore the negative membrane potential.

NEUR101 - Neurobiology & Action Potentials

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