Membrane Potentials and Conduction of Electrical Impulses

What are ion channels?

Ion channels are membrane-spanning proteins that allow specific ions (Na⁺, K⁺, Ca²⁺, Cl⁻) to cross the cell membrane.

What determines ion channel selectivity?

Ion channels are selective based on ion type, size, and charge.

What is the function of ion channels?

Ion channels control membrane potential and generate electrical signals.

Why are ion channels important?

They are crucial for neuronal firing, muscle contraction, and cardiac rhythms.

What are leak (ungated) ion channels?

Leak ion channels are always open and help set the resting membrane potential.

What are voltage-gated ion channels?

Voltage-gated ion channels open and close in response to voltage changes.

What are ligand-gated ion channels?

Ligand-gated ion channels open when a chemical (e.g., neurotransmitter) binds to them.

What are mechanically-gated ion channels?

Mechanically-gated ion channels open in response to stretch or pressure (e.g., touch receptors).

What is the chemical driving force?

The chemical driving force moves molecules from higher to lower concentration, down the concentration gradient.

How do molecules move according to the chemical driving force?

Molecules naturally move from areas of high concentration to areas of low concentration.

What is the electrical driving force?

The electrical driving force affects charged molecules (ions) and moves them toward regions with the opposite charge.

How do ions behave due to the electrical driving force?

Ions (e.g., Na⁺, K⁺, Cl⁻) move toward areas with the opposite charge because opposite charges attract.

What is membrane permeability?

Membrane permeability is a measure of how easily an ion can cross the cell membrane.

What factors affect membrane permeability?

1. Whether there are channels for that ion

2. Whether those channels are open

3. The number and type of channels present

What is diffusion potential?

Diffusion potential is the voltage generated by ion movement across a semipermeable membrane.

When does diffusion potential occur?

It occurs only if the membrane is permeable to the ion.

How is diffusion potential measured?

It is measured in millivolts (mV).

What happens during cerebral ischemia?

During cerebral ischemia, lack of oxygen and glucose leads to ATP depletion, disrupting ion gradients, especially for K⁺ and Na⁺.

How does ATP depletion affect the Na⁺/K⁺ ATPase pump?

The Na⁺/K⁺ ATPase fails, causing K⁺ to leak out and Na⁺ to leak in.

What happens to the membrane during ischemia?

The membrane becomes less negative (depolarized).

How are diffusion potentials affected during brain ischemia?

Diffusion potentials are altered because ion gradients collapse.

What is membrane potential (Vm)?

Membrane potential (also known as transmembrane potential) refers to the electrical potential difference across the membrane of a cell, measured in voltage.

What causes membrane potential?

It is caused by unequal ion distribution across the membrane.

Where is membrane potential present?

Membrane potential is present in all living cells, not just neurons.

What factors affect membrane potential?

Membrane potential varies depending on ion gradients and permeability.

What is the general term for voltage across a membrane?

Membrane potential is the general term for any voltage across the membrane.

What does the Na⁺/K⁺ ATPase pump do?

The Na⁺/K⁺ ATPase pump moves 3 Na⁺ out and 2 K⁺ in per ATP hydrolyzed, maintaining ion concentration gradients essential for electrical excitability.

What is the role of K⁺ leak channels in membrane potential?

K⁺ leak channels allow continuous K⁺ efflux, helping to set a negative resting potential (~ -70mV in neurons).

How does Na⁺ influx affect the resting membrane potential?

Na⁺ influx is restricted at rest, keeping the inside of the cell negative.

What is the Nernst potential (equilibrium potential)?

The Nernst potential is the membrane voltage at which there is no net movement of a specific ion across the membrane.

What balances at the Nernst potential?

The chemical driving force (concentration gradient) is exactly balanced by the electrical driving force (voltage).

When does the Nernst potential occur?

The Nernst potential occurs when the chemical driving force is perfectly balanced by the electrical driving force.

What is the clinical significance of Nernst potential?

Nernst potential is important in understanding how ion concentration changes, like in conditions such as hyperkalemia.

What happens in hyperkalemia?

In hyperkalemia, the extracellular potassium concentration ([K⁺]out) increases from ~4 mEq/L to 6–7 mEq/L, which makes the Nernst potential (E_K⁺) less negative (e.g., from -94 mV to -75 mV).

What is the normal extracellular potassium concentration and its Nernst potential?

Normal [K⁺]out is ~4 mEq/L, and the Nernst potential (E_K⁺) is -94 mV.

What is the resting membrane potential (RMP)?

The resting membrane potential is the steady voltage across the cell membrane when the cell is not actively sending a signal.

What is the resting membrane potential in most neurons?

In most neurons, the resting membrane potential is around -70 mV.

What does the resting membrane potential indicate about the cell?

It means the inside of the cell is more negative than the outside.

What is the main driver of the resting membrane potential (RMP)?

K⁺ leak channels are the main driver, as K⁺ diffuses out of the cell through ungated (leak) channels, leaving behind negative charge and making the inside more negative.

How does the Na⁺/K⁺ ATPase pump contribute to the resting membrane potential?

The Na⁺/K⁺ ATPase pump moves 3 Na⁺ out and 2 K⁺ in, creating and maintaining the K⁺ and Na⁺ gradients. It is electrogenic and contributes about -4 mV to the RMP.

What role do anionic proteins inside the cell play in the resting membrane potential?

Anionic proteins inside the cell are large, negatively charged proteins that do not cross the membrane, helping to maintain the negative internal charge.

What happens to the resting membrane potential (RMP) in hyperkalemia?

In hyperkalemia, [K⁺]outside increases, causing the RMP to become less negative. This makes it easier to reach threshold, leading to muscle twitching and arrhythmias.

What happens to the resting membrane potential (RMP) in hypokalemia?

In hypokalemia, [K⁺]outside decreases, causing the RMP to become more negative. This makes it harder to excite the cell, leading to muscle weakness and paralysis.

What does conductance refer to?

Conductance refers to the probability of a channel being open and carrying electrical current across the membrane (C = 1/R).

What happens when a channel is open in terms of ion flow?

When a channel is open, ions can flow down their electrochemical gradient.

What happens to gated sodium and potassium channels during the resting state?

During the resting state, all gated sodium and potassium channels are closed (deactivated).

How do gated channels differ from leakage channels?

Gated channels only open once an action potential has been triggered, while leakage channels are always open.

What are voltage-gated channels?

Voltage-gated channels open and close depending on the voltage difference across the cell membrane.

How many gates do voltage-gated sodium and potassium channels have?

Voltage-gated sodium channels have two gates (gate m and gate h), while the potassium channel only has one gate (gate n).

What is an action potential?

An action potential is the systematic change in the electrical potential between the inside and outside of a neuron, where the reversal of polarity creates a signal that travels down the neuron.

What happens when a neuron fires?

When the neuron fires, it means the action potential has occurred and the signal is traveling down the neuron.

What is the role of action potentials in the body?

Action potentials are the basic mechanisms for the transmission of information in the nervous system and all types of muscle cells.

What is the resting membrane potential (RMP) in neurons?

The resting membrane potential (RMP) is when the neuron is not actively sending a signal, typically around -70 mV in neurons.

What happens during the triggering event of an action potential step 1?

A triggering event occurs that depolarizes the cell body. The signal comes from other cells connecting to the neuron, causing positively charged ions to flow into the cell body.

What happens to the cell during depolarization?

Depolarization makes the cell less polar.

How do voltage-gated sodium channels contribute to depolarization?

Voltage-gated sodium channels at the part of the axon closest to the cell body activate, allowing sodium to enter the neuron.

What effect does sodium entry have on the membrane potential?

The entry of sodium causes the local membrane potential to quickly become positive, producing the depolarization phase of the action potential.

What happens during repolarization in an action potential?

During repolarization, the inactivation gates (h) of the sodium channels close, stopping the inward rush of positive ions.

What happens to potassium channels during repolarization?

Potassium channels open during repolarization, allowing potassium to exit the cell.

Why does potassium exit the cell during repolarization?

There is much more potassium inside the cell than outside, so when the potassium channels open, more potassium exits than enters.

How does potassium efflux affect the cell?

The loss of positively charged potassium ions causes the cell to return toward its resting state.

What happens during hyperpolarization in an action potential?

Hyperpolarization makes the cell more negative than its typical resting membrane potential.

Why does the cell hyperpolarize during action potential?

Potassium channels stay open a little longer during hyperpolarization, allowing more positive ions to exit the neuron.

What happens when the potassium channels close after hyperpolarization?

When the potassium channels close, the sodium-potassium pump works to reestablish the resting membrane potential.

What are refractory periods?

Refractory periods are brief periods during which nerve or muscle cells are temporarily unable to respond to additional electrical or chemical stimuli.

Why are refractory periods important?

Refractory periods are crucial for regulating the timing and coordination of nerve impulses and muscle contractions.

What do refractory periods allow neurons to do?

Refractory periods give the neuron time to replenish neurotransmitter packets at the axon terminal so it can keep passing the message along.

What is the absolute refractory period?

The absolute refractory period is a brief time after an action potential during which a neuron cannot generate another action potential.

Can a neuron generate another action potential during the absolute refractory period?

No, the neuron cannot generate another action potential, regardless of the stimulus strength

What primarily causes the absolute refractory period?

The inactivation of voltage-gated sodium channels primarily causes the absolute refractory period.

What is the relative refractory period?

The relative refractory period follows the absolute refractory period and is characterized by a heightened threshold for generating a new action potential.

What happens during the relative refractory period?

Some voltage-gated potassium channels are still open during the relative refractory period, making it harder to trigger a new action potential.

What is required to trigger an action potential during the relative refractory period?

A stronger stimulus is required to trigger an action potential during the relative refractory period.

How is an action potential sustained along an axon?

Depolarization at one segment of the axon triggers the opening of voltage-gated Na⁺ channels in the next segment, creating a wave of depolarization that travels forward.

What prevents backward conduction of an action potential?

The segment behind enters a refractory period, preventing backward conduction.

What is the function of myelin in myelinated axons?

Myelin is a lipid-rich insulating sheath that increases membrane resistance (Rm) and decreases capacitance (Cm).

Where do action potentials occur in myelinated axons?

Action potentials occur only at the Nodes of Ranvier, which are gaps in the myelin.

How does myelination affect action potential conduction?

Myelination allows faster and more energy-efficient conduction through saltatory conduction.

How does action potential propagation work in unmyelinated axons?

In unmyelinated axons, action potentials propagate by continuous conduction, where every segment of the membrane depolarizes and repolarizes.

What are the downsides of unmyelinated axons?

Continuous conduction in unmyelinated axons is slower, more energy-consuming, and less efficient, making them more susceptible to signal attenuation or failure over long distances.

What is the space constant (λ)?

The space constant (λ) is the distance over which a change in membrane potential (e.g., depolarizing current) decays to ~37% of its original value along the axon. It shows how far a change in membrane potential spreads before decaying significantly.

What is the time constant (τ)?

The time constant (τ) is the time it takes for the membrane potential to change by ~63% of its final value in response to a stimulus. It indicates how fast the membrane responds to a stimulus.

What are the two major factors that govern how quickly an action potential moves down an axon?

The two major factors are the axon’s diameter and how heavily myelinated it is.

What is the general path of depolarization through the heart?

Depolarization moves through the myocardium in the following sequence: Sinoatrial (SA) node → Atria → Atrioventricular (AV) node → Bundle of His → Purkinje fibers → Ventricles.

What is saltatory conduction?

Saltatory conduction is the process by which action potentials (APs) propagate along myelinated axons by "jumping" from one Node of Ranvier to the next.

What is the electrotonic spread in saltatory conduction?

Electrotonic spread is when the local depolarization from one node spreads passively and rapidly through the insulated internode to the next node.

What is the physiological advantage of saltatory conduction?

The physiological advantage of saltatory conduction is much faster conduction velocity than in unmyelinated axons.

How is saltatory conduction energy-efficient?

Saltatory conduction is energy-efficient because fewer ions move, leading to less Na⁺/K⁺ pump activity needed to restore resting gradients.

What factors affect the speed of saltatory conduction?

The speed of saltatory conduction scales with axon diameter and the degree of myelination.