Lesson 4: Electrical Signals and Voltage- Dependent Membrane Permeability

What is the resting membrane potential (RMP)?

The RMP is a negative potential generated by neurons, typically ranging from -40 to -90 mV, created by pumping sodium ions out and potassium ions into the cell.

How is the resting membrane potential measured?

It is measured by recording the voltage inside the neuron relative to the outside using a microelectrode.

What are action potentials (APs)?

APs are all-or-none electrical signals used by neurons to transmit signals over long distances, typically down axons.

What are receptor potentials (RPs)?

RPs are graded signals generated by activating sensory neurons, reflecting the intensity of sensory stimuli.

What are synaptic potentials (SPs)?

SPs are graded potentials that occur between neurons, facilitating communication in neural circuits.

What does the amplitude of a receptor potential indicate?

The amplitude encodes the intensity of the sensory stimulus, which is converted into a digital signal proportional to the frequency of action potentials.

What happens during hyperpolarization of a neuron?

The membrane potential becomes more negative, typically due to the influx of potassium ions or the efflux of sodium ions.

What occurs during depolarization of a neuron?

The membrane potential becomes less negative or more positive, often leading to the generation of an action potential if the threshold is reached.

What is the difference between passive and active electrical responses?

Passive responses change the membrane potential proportionally to the injected current, while active responses change in fixed amplitude and can generate action potentials.

What is a subthreshold current?

A subthreshold current yields only passive conduction and does not generate an action potential.

What is a suprathreshold current?

A suprathreshold current generates an active response in the form of an action potential, maintaining amplitude over distance.

What are active transporters in neurons?

Active transporters create ion concentration gradients by moving ions against their concentration gradients.

What role do ion channels play in neuronal function?

Ion channels restrict flow across the cell membrane and allow ions to move down their concentration gradients via diffusion.

What is the Nernst equation used for?

The Nernst equation predicts the equilibrium potential for any ion based on its concentration gradient across the membrane.

How does the concentration gradient affect membrane potential?

The concentration gradient influences the membrane potential, with a higher concentration of ions inside or outside the cell affecting the potential difference.

What is electrochemical equilibrium?

Electrochemical equilibrium occurs when the electrical gradient opposes further movement of ions across the membrane, balancing the concentration gradient.

What is the equilibrium potential for potassium (K+)?

The equilibrium potential for K+ can be calculated using the Nernst equation, which predicts a value based on the concentration of K+ inside and outside the cell.

What happens to the action potential as it travels along an axon?

The action potential maintains its amplitude over distance due to regenerative mechanisms, unlike passive conduction which diminishes.

What is the significance of the axon's electrical conductivity?

Axons are poor electrical conductors compared to wires, leading to rapid dissipation of passive current flow over distance.

What is the role of the membrane's permeability in resting potential?

At rest, the membrane is more permeable to K+, leading to a continual efflux of K+ that stabilizes the resting membrane potential.

What is the relationship between the K+ concentration gradient and membrane potential?

The Nernst equation predicts a linear relationship between the membrane potential and the logarithm of the K+ concentration gradient.

What is the effect of a higher concentration of K+ inside the cell?

A higher concentration of K+ inside the cell results in an inside-negative potential across the membrane.

How does the leakiness of the axonal membrane affect signal conduction?

The leakiness of the axonal membrane prevents effective conduction of electrical signals, especially in longer axons.

What is the significance of the critical threshold in action potential generation?

The critical threshold is the membrane potential that must be reached for an action potential to occur, typically more positive than the resting potential.

What is the role of neurotransmitter release in neuronal signaling?

Neurotransmitter release is controlled by the rate of firing down the axon, influenced by the electrical signals generated in the neuron.

What is the difference between graded potentials and action potentials?

Graded potentials vary in amplitude and can be either depolarizing or hyperpolarizing, while action potentials are all-or-none events that maintain a fixed amplitude.

What is the membrane potential when the membrane is permeable to sodium with [Na+] outside at 10 mM and [Na+] inside at 1 mM?

+58 mV

What is the membrane potential when the membrane is permeable to chloride with [Cl-] inside at 10 mM and [Cl-] outside at 1 mM?

+58 mV

What influences ion movement across the membrane?

The membrane potential

How does the electrical potential affect ion flux?

It can determine the direction and magnitude of ion movement.

What happens to K+ ions when the membrane potential (Vm) is set at the equilibrium potential for K+ (EK)?

There is no net flux of K+.

What occurs if Vm is made more negative than EK?

K+ will move against its concentration gradient from outside to inside.

What is the voltage needed to counter a tenfold difference in concentration for K+?

-58 mV

What is the resting membrane potential (RMP) range for a neuronal cell?

-60 mV to -70 mV

What creates the resting membrane potential?

Separation of charge across the cell membrane.

What maintains the separation of charges across the membrane?

The lipid bilayer and the Na-K pump.

What are electrotonic potentials?

Passive responses of the membrane that do not lead to action potential generation.

What happens during depolarization?

The membrane potential becomes less negative or more positive.

What happens during hyperpolarization?

The membrane potential becomes more negative than the resting membrane potential.

What is the relationship between current pulse size and electrotonic potential change?

The size of the change in potential is proportional to the size of the current pulse.

What defines the direction of current flow?

The net movement of positive charge.

What occurs when the intracellular electrode is made negative relative to the extracellular electrode?

Reversal of membrane potential, making it more negative.

How are ions distributed across the cell membrane?

Unevenly, with Na+ and Cl- concentrated outside and K+ concentrated inside.

What does A- represent in the context of ionic distribution?

Various proteins/amino acids concentrated inside the cell that cannot cross the membrane.

How do ionic concentrations in vertebrate nerve cells compare to those in the giant squid axon?

Ionic concentrations in vertebrate nerve cells are typically much lower.

What ion channels are predominantly open at rest in neurons?

K+ channels

What is the typical resting membrane potential (RMP) of glial cells?

About -75 mV

What drives K+ ions to diffuse out of the cell?

The concentration gradient of K+ ions

What happens to the charge distribution when K+ ions diffuse out of the cell?

A net positive charge accumulates on the outside of the cell membrane.

What two forces act on ions in a neuron?

Chemical driving force and electrical driving force

What is the equilibrium potential for K+ (EK)?

The point where the outward movement of K+ equals the inward movement due to electrical potential difference.

What happens to K+ equilibrium when Na+ starts to move into the cell?

K+ is no longer in equilibrium, leading to a net driving force on K+ out of the cell.

What is the role of Na+ in neuronal depolarization?

Na+ influx depolarizes the cell slightly from the equilibrium potential for K+.

What is the relationship between membrane potential (Vm) and ionic conductance during an action potential (AP)?

Ionic conductance increases greatly during an AP.

What did Hodgkin and Katz discover regarding Na+ concentration and action potentials?

The amplitude of the AP is reduced when the external concentration of sodium is lowered.

What initiates an action potential in a neuron?

When the neuronal membrane potential becomes more positive than the threshold.

What changes occur in membrane permeability during an action potential?

A rapid rise in sodium permeability followed by a slower rise in potassium permeability.

What is the primary electrical signal generated by neurons?

The action potential (AP).

What is the formula for ion flux across a membrane?

Ion flux = (electrical driving force + chemical driving force) x membrane conductance.

What is the significance of voltage clamp studies in understanding ionic permeability?

They provide evidence for how changes in membrane potential affect ionic permeability.

What happens to K+ driving force when Vm depolarizes?

The negative driving force on K+ into the cell is reduced, creating a net driving force out.

What is the effect of having multiple permeable ions on membrane potential?

It contributes to the driving forces acting on all permeable ions.

What is the typical behavior of K+ channels at rest?

They are typically open and not voltage-gated.

What is the primary factor that stabilizes the membrane potential?

The balance between concentration gradients and electrostatic forces.

What occurs when the outward movement of K+ is balanced by the inward movement of Na+?

A new resting potential is reached.

What is the purpose of the voltage clamp method in neuroscience?

To understand ionic permeability at any membrane potential by clamping the membrane potential at a desired level.

Who developed the voltage clamp technique?

Kenneth Cole.

What does the Goldman Equation predict?

The membrane potential (Vm) when more than one ion is crossing the membrane.

What factors does the Goldman Equation take into account?

The concentration gradient and the relative permeability of several ions.

How does the Goldman Equation differ from the Nernst equation?

The Goldman Equation considers permeability to multiple ions, while the Nernst equation does not.

What happens to the Goldman Equation if the membrane is only permeable to K+?

All terms involving Na+ and Cl- drop out, reducing it to the Nernst equation.

What occurs during hyperpolarization of the membrane?

A brief capacitive current or redistribution of charge across the membrane.

What did Hodgkin and Huxley use to study permeability changes during action potentials?

The voltage clamp technique.

What was the hypothesis tested by Hodgkin and Huxley regarding Na+ and K+ permeability?

That membrane potential-sensitive Na+ and K+ permeability changes are necessary and sufficient to produce an action potential.

What happens to ionic currents when the membrane potential is changed?

Ionic currents flow across the membrane when the potential is changed.

What is observed during depolarization of the membrane?

A rapid rising inward current followed by a delayed outward current.

What indicates that membrane permeability is voltage-dependent?

The observation that depolarization elicits ionic currents.

What happens at negative membrane potentials below resting potential?

No appreciable current flows.

What did Hodgkin and Huxley notice about current flow at more positive potentials?

Current flows and reverses its polarity.

What are the two types of currents produced by membrane depolarizations?

Inward current and outward current.

What did Hodgkin and Huxley determine about membrane current behavior?

It behaves differently depending on the membrane potential.

What is the significance of the early current observed at +52 mV?

No current flows at this potential, indicating it is carried by Na+ entry into the axon.

What is the concentration gradient for Na+ in squid neurons?

[Na+]o is 440 mM and [Na+]i is 50 mM.

What does the absence of early current at ENa suggest?

That the early current is carried by the entry of Na+ into the axon.

What is the role of capacitive currents during membrane potential changes?

They represent a brief redistribution of charge across the membrane.

What is the relationship between membrane potential and ionic permeability?

Ionic permeability is voltage-sensitive and changes with membrane potential.

What did Hodgkin and Huxley conclude about ionic permeability during action potentials?

It is influenced by changes in membrane potential.

What is the effect of hyperpolarization on ion flow across the membrane?

There is very little ion flow across the membrane after hyperpolarization.

What is the significance of observing ionic currents during depolarization?

It establishes that the membrane permeability of axons is voltage-dependent.

What does the term 'voltage-sensitive' refer to in the context of ionic permeability?

It refers to how ionic currents change based on the membrane potential.

What happens to the early current when external sodium is removed?

The early current reverses in polarity, becoming an outward current.

What effect does increasing sodium permeability have when external sodium is absent?

Sodium should flow outward, as the electrochemical gradient is reversed.

What occurs to the inward current when external sodium is restored?

The inward current returns to normal.

What ion is responsible for the outward current when external sodium is removed?

The outward current is due to the flux of potassium ions (K+).

What evidence supports that K+ is responsible for the outward current?

The amount of K+ efflux from the neuron correlates with the magnitude of the late outward current.

What does the transient inward current and sustained outward current suggest?

There are two different mechanisms of ionic permeability.

Which drug blocks Na+ current?

Tetrodotoxin (TTX).

Which drug blocks K+ current?

Tetraethylammonium (TEA).

What is the relationship between permeability and membrane conductance?

They are closely related but not identical; for this context, they are considered synonymous.

What does Ohm's Law state?

Voltage (V) is equal to the product of current (I) and resistance (R).