Neuronal Membranes, Action Potentials, and Ion Dynamics

What is the cell membrane primarily made of?

Phospholipids, glycolipids, cholesterol, and proteins.

What does amphipathic mean in relation to phospholipids?

Phospholipids have both a hydrophilic (water-loving) portion and a hydrophobic (water-avoiding) portion.

What functions can membrane proteins serve?

Channels, transporters, receptors, enzymes, identity markers, and anchors.

What is diffusion?

The movement of molecules from an area of higher concentration to an area of lower concentration.

What drives the movement of molecules during diffusion?

The concentration gradient.

What is selective permeability in relation to the cell membrane?

The ability of the membrane to allow some substances to cross more easily than others.

Which types of molecules can cross the cell membrane relatively easily?

Small, nonpolar, lipid-soluble molecules.

What types of molecules cannot easily cross the cell membrane?

Large proteins, polar/water-soluble molecules, and ions.

What are the two competing forces acting on ions like K⁺?

Concentration gradient (wants to leave the cell) and electrostatic pressure (wants to enter the cell).

What is the equilibrium potential?

The voltage at which the concentration force and electrical force on an ion are balanced.

What is the typical range for resting membrane potential (Vm) in neurons?

−50 to −80 mV.

What three components contribute to resting membrane potential?

Concentration gradients, selective permeability, and electrostatic pressure.

Why is the inside of a neuron negatively charged?

The difference in charge is concentrated at the membrane surface, and there are negatively charged proteins inside the cell.

Why is K⁺ particularly important for resting membrane potential?

Neurons have more K⁺ leakage channels than Na⁺ channels, making the membrane more permeable to K⁺.

What are leakage channels?

Channels that are always open, allowing ions to move according to their electrochemical gradients.

What does the Nernst equation calculate?

The equilibrium potential for one particular ion.

What is the general form of the Nernst equation for a monovalent cation at body temperature?

Eion = 61.5 × log₁₀([ion]o/[ion]i).

What does [ion]o and [ion]i represent in the Nernst equation?

[ion]o is the concentration outside the cell, and [ion]i is the concentration inside the cell.

What is the significance of the resting membrane potential?

It reflects the electrical potential difference across the membrane when the neuron is at rest.

What happens at the equilibrium potential?

There is no net movement of that ion across the membrane, even though ions may still move.

Why does the resting membrane potential not equal the equilibrium potential (Eₖ)?

Because the neuron is also somewhat permeable to other ions, particularly Na⁺.

What is the role of ion channels in relation to membrane potential?

Ion channels allow specific ions to cross the membrane, influencing the membrane potential.

How does the concentration gradient affect K⁺ movement?

K⁺ wants to move out of the cell due to a higher concentration inside.

What is the relationship between K⁺ permeability and resting membrane potential?

Higher K⁺ permeability makes the resting membrane potential closer to Eₖ.

What are the implications of the electrostatic pressure on K⁺?

The negative charge inside the neuron attracts K⁺ back into the cell.

What is the importance of understanding the forces acting on ions for neurophysiology?

It is crucial for understanding resting membrane potential and neuronal signaling.

What does the term 'net movement' refer to in the context of ion movement across the membrane?

The overall movement of ions when opposing forces are balanced.

What is the Nernst equation used for?

To calculate the equilibrium potential for a specific ion.

What is the equilibrium potential (Eₖ) for K⁺ when [K⁺] outside is 5 and [K⁺] inside is 100?

Approximately -80 mV.

What happens to the resting membrane potential when extracellular K⁺ increases?

The resting membrane potential becomes less negative.

Why does increasing extracellular K⁺ reduce the concentration gradient for K⁺?

It reduces the driving force for K⁺ to leave the cell.

Why doesn't the Nernst equation perfectly predict resting membrane potential?

It considers only one ion, while neurons are permeable to multiple ions.

What does the Goldman-Hodgkin-Katz (GHK) equation account for?

It accounts for multiple ions and their relative permeabilities.

How is the GHK equation different from the Nernst equation?

GHK estimates membrane potential considering multiple ions, while Nernst estimates equilibrium potential for one ion.

How much more permeable is the membrane to K⁺ compared to Na⁺?

Approximately 100 times more permeable.

What is the role of the Na⁺/K⁺ pump?

To maintain ion concentration gradients by pumping 3 Na⁺ out and 2 K⁺ in.

How does the Na⁺/K⁺ pump affect the membrane potential?

It makes the inside slightly more negative by moving more positive charges out than in.

What is the Weaver mouse mutation an example of?

It illustrates how ion-channel selectivity affects neuronal function.

What is the function of astrocytes regarding extracellular K⁺?

They help buffer and regulate extracellular K⁺ concentrations.

What are the three characteristics of an action potential?

Very short duration (~1 ms), all-or-none, and encoded by frequency, not amplitude.

What is depolarization?

The membrane becomes more positive relative to resting potential.

What is hyperpolarization?

The membrane becomes more negative relative to resting potential.

What is the threshold in the context of action potentials?

The membrane potential that must be reached to initiate an action potential.

What happens if a stimulus is below threshold?

No action potential is initiated.

What occurs when voltage-gated Na⁺ channels open?

Na⁺ enters the cell, causing further depolarization.

What is a positive feedback loop in the context of action potentials?

The process where depolarization causes more Na⁺ channels to open, leading to further depolarization.

What is the role of Na⁺ leakage channels?

They allow Na⁺ to gradually enter the neuron, potentially disrupting resting membrane potential.

How does the Na⁺/K⁺ pump contribute to the energy use of the brain?

It is estimated to use about 70% of the brain's ATP to maintain ion gradients.

What is the relationship between stimulus intensity and action potential frequency?

A stronger stimulus results in more frequent action potentials, not larger ones.

What happens during the depolarization phase of an action potential?

The membrane potential rapidly becomes more positive due to Na⁺ influx.

What is the effect of extracellular K⁺ on neuronal excitability?

High extracellular K⁺ can disrupt normal neuronal impulses.

What is the importance of maintaining extracellular K⁺ levels?

It is essential for proper neuronal function and impulse generation.

What initiates depolarization in a neuron?

Opening of Na⁺ channels allowing Na⁺ to enter the cell.

What happens to Na⁺ channels after they open?

They undergo inactivation, decreasing Na⁺ conductance.

How do voltage-gated K⁺ channels respond to depolarization?

They open in response to depolarization but are slower to activate than Na⁺ channels.

What is the result of K⁺ efflux during repolarization?

The membrane potential becomes more negative.

What is the hyperpolarizing afterpotential?

It occurs when K⁺ channels remain open longer, causing the membrane to become more negative than resting potential.

What is the sequence of events during an action potential?

Resting → Depolarization to threshold → Na⁺ channels open → Rapid depolarization → Peak → K⁺ channels open → Repolarization → Hyperpolarization → Return to resting potential.

What is the role of tetrodotoxin (TTX) in neuronal function?

TTX selectively blocks Na⁺ permeability changes, affecting action potential generation.

What does the Goldman equation (GHK) calculate?

The membrane potential considering multiple ions and their permeabilities.

Why is the resting membrane potential closer to Eₖ than Eₙₐ?

Because the membrane is more permeable to K⁺ than Na⁺ at rest.

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

To maintain Na⁺ and K⁺ concentration gradients by moving 3 Na⁺ out and 2 K⁺ in using ATP.

What happens to the membrane potential during depolarization?

The membrane potential becomes more positive due to Na⁺ influx.

What is the typical resting potential of a neuron?

Between −50 to −80 mV.

What occurs during the peak of an action potential?

Na⁺ channels begin to inactivate and K⁺ channels start to open.

What is the effect of a stronger stimulus on action potential frequency?

It increases the frequency of action potentials, not their size.

What is the significance of K⁺ leakage channels?

They are always open, contributing to the resting membrane potential.

What is the difference between activation and inactivation of ion channels?

Activation refers to the channel opening, while inactivation refers to the channel stopping conduction despite membrane depolarization.

What is the relationship between concentration gradient and electrical force for K⁺?

The concentration gradient drives K⁺ out, while the electrical force drives K⁺ in.

What happens when the threshold is reached in a neuron?

Na⁺ channels open, leading to a rapid influx of Na⁺ and depolarization.

What is the duration of an action potential?

Approximately 1 millisecond.

What does the all-or-none principle state?

A stronger stimulus does not produce a larger action potential; it affects the frequency instead.

Why do K⁺ channels contribute to the undershoot?

Because they remain open longer than necessary, allowing more K⁺ to leave the cell.

What is the importance of astrocytes in neuronal function?

They help maintain the ionic environment necessary for neuronal function.

How does the membrane potential return to resting potential after an action potential?

K⁺ channels close, and the Na⁺/K⁺ pump restores the ion gradients.

What is the typical relative permeability of K⁺ compared to Na⁺?

K⁺ permeability is approximately 100 times greater than Na⁺ permeability.