Neurophysiology: Resting Potential and Action Potential

These question-and-answer flashcards review key facts about resting membrane potential, ion distribution, action potentials, propagation, synaptic transmission, EPSPs/IPSPs, summation, and related pharmacology from the lecture notes.

What is the typical range of the resting membrane potential in neurons?

Approximately –50 to –80 millivolts (mV).

Why is the interior of a resting neuron more negative than the exterior?

Because of unequal ion distribution (more K+ and negatively-charged proteins inside; more Na+ and Cl– outside) and the action of the Na+/K+ pump.

Which ions are most concentrated outside a neuron at rest?

Sodium (Na+) and chloride (Cl–).

Which ion is most concentrated inside a neuron at rest?

Potassium (K+).

What two forces drive ions across the neuronal membrane?

Diffusion (concentration gradient) and electrostatic pressure (electrical gradient).

What is selective permeability, and which ion passes most freely at rest?

The membrane allows certain ions to pass more easily; at rest, K+ passes through leak channels most freely.

What does the sodium–potassium pump do?

It uses ATP to pump 3 Na+ out and 2 K+ into the cell, helping maintain the resting potential.

How does pumping 3 Na+ out and 2 K+ in affect membrane charge?

It makes the interior more negative because more positive charges leave than enter.

Define equilibrium potential.

The membrane potential at which the net flow of a particular ion across the membrane is zero because diffusion and electrostatic forces are balanced.

What is depolarization?

A decrease in membrane potential in which the inside of the neuron becomes less negative (closer to zero).

What is hyperpolarization?

An increase in membrane potential in which the inside of the neuron becomes more negative than at rest.

At approximately what membrane potential does an action potential threshold occur?

Around –40 mV.

Where do action potentials originate?

At the axon hillock.

Which ion first rushes into the neuron during an action potential?

Sodium (Na+).

Which channels open first when threshold is reached?

Voltage-gated Na+ channels.

To what value does the membrane potential usually rise at the action-potential peak?

About +40 mV (Na+ equilibrium potential).

Which ion exits the neuron to repolarize the membrane after the peak?

Potassium (K+).

Which channels open to allow K+ to leave during repolarization?

Voltage-gated K+ channels.

What is the absolute refractory period?

A brief time after an action potential when another AP cannot be initiated because Na+ channels are inactivated.

What is the relative refractory period?

A period following the absolute refractory period when a stronger-than-normal stimulus is required to trigger another AP because the membrane is hyperpolarized.

Why do action potentials travel in only one direction along an axon?

Because the membrane behind the advancing AP is in a refractory state and cannot fire again immediately.

What is saltatory conduction?

The jumping of action potentials from one Node of Ranvier to the next along a myelinated axon.

Where are voltage-gated Na+ channels concentrated on a myelinated axon?

At the Nodes of Ranvier.

How does myelin affect conduction velocity?

It increases velocity by insulating the axon and allowing saltatory conduction.

How does axon diameter influence conduction velocity?

Larger diameter reduces internal resistance, speeding up action-potential propagation.

State the all-or-none law of action potentials.

An AP either occurs fully or not at all, and its amplitude is independent of stimulus strength.

How is stimulus intensity encoded in neurons?

By the frequency (number per second) of action potentials, not by their size.

What are graded potentials?

Local changes in membrane potential whose amplitude is proportional to stimulus strength and that decay with distance.

What is an excitatory postsynaptic potential (EPSP)?

A depolarizing graded potential in the postsynaptic neuron, often due to Na+ influx, that increases the likelihood of firing an AP.

What is an inhibitory postsynaptic potential (IPSP)?

A hyperpolarizing graded potential in the postsynaptic neuron, often due to Cl– influx, that decreases the likelihood of firing an AP.

What ion influx typically produces an EPSP?

Sodium (Na+) (or sometimes Ca2+).

What ion influx typically produces an IPSP?

Chloride (Cl–).

Define spatial summation.

The addition of postsynaptic potentials that occur at different locations on the neuron’s membrane at the same time.

Define temporal summation.

The addition of postsynaptic potentials that arrive at the same location in rapid succession.

Where does the neuron integrate EPSPs and IPSPs to decide whether to fire?

At the axon hillock.

List the basic steps of chemical synaptic transmission.

1) AP reaches terminal, 2) voltage-gated Ca2+ channels open, 3) Ca2+ triggers vesicle fusion, 4) neurotransmitter released, 5) NT binds postsynaptic receptors, 6) EPSP/IPSP generated, 7) NT removed by degradation or reuptake.

What ion enters the presynaptic terminal to trigger neurotransmitter release?

Calcium (Ca2+).

What are ligand-gated ion channels?

Receptors that open to allow ion flow when a specific ligand (e.g., neurotransmitter) binds.

Name two ways neurotransmitter action is terminated.

Enzymatic degradation (e.g., AChE) and reuptake by presynaptic transporters.

How do local anesthetics like lidocaine work?

They block voltage-gated Na+ channels, preventing action potentials and thus pain signals.

How do many general anesthetics reduce neuronal activity?

They open K+ channels wider, keeping neurons hyperpolarized and less excitable.

Give two key differences between action potentials and synaptic potentials.

APs are all-or-none, occur on axons, and have refractory periods; synaptic potentials are graded, occur on dendrites/soma, and can summate.

What stops voltage-gated Na+ channels from reopening immediately after an AP?

Their inactivation gates remain closed during the absolute refractory period.

During rest, which way would K+ move along its concentration gradient?

Out of the cell.

During rest, which way would K+ move along the electrical gradient?

Into the cell (toward negative interior).

During rest, which way would Na+ move along its concentration gradient?

Into the cell.

During rest, which way would Na+ move along the electrical gradient?

Into the cell (toward negative interior).

What is meant by ‘propagation’ of an action potential?

The active regeneration of the AP as it travels along the axon, opening new voltage-gated channels in adjacent segments.

Why can a stronger-than-normal stimulus trigger an AP during the relative refractory period?

Because some Na+ channels have reset, but the membrane is hyperpolarized, requiring more depolarization to reach threshold.

What is the effect of blocking Na+ channels on neuronal excitability?

Prevents initiation and propagation of action potentials, silencing the neuron.

What enzyme breaks down acetylcholine in the synaptic cleft?

Acetylcholinesterase (AChE).

What is a ligand in neurophysiology?

A molecule that binds to a receptor to activate or block it; neurotransmitters are endogenous ligands.

How do EPSPs and IPSPs differ in size with increased neurotransmitter release?

Their amplitudes increase (they are graded).

Does an EPSP always trigger an action potential?

No; it must depolarize the axon hillock to threshold, often requiring summation with other EPSPs.

Where is the Na+/K+ pump especially important on neurons?

Throughout the membrane, including nodes of Ranvier and dendrites, to restore ionic gradients.

What happens to Cl– during an IPSP?

Cl– enters the postsynaptic neuron, making the interior more negative.

Why can’t action potentials summate like EPSPs?

Because each AP is an all-or-none event followed by refractory periods, preventing overlap.

What property of the membrane determines the amplitude of an action potential?

It is fixed by ionic equilibrium potentials; thus amplitude does not vary with stimulus strength.

What effect does opening additional K+ channels have on membrane potential?

It hyperpolarizes the cell, making firing less likely.

How does the presence of negatively charged proteins inside the cell contribute to resting potential?

They cannot cross the membrane, leaving excess negative charge inside, promoting K+ retention and overall negativity.

Which two refractory periods together ensure unidirectional propagation?

Absolute refractory period and relative refractory period.

What determines whether a synaptic potential is excitatory or inhibitory?

The type of ion channel opened and the ions’ equilibrium potentials relative to threshold.

What effect do transporter proteins have in synaptic clefts?

They remove neurotransmitter molecules from the cleft via reuptake, ending synaptic signaling.

Which type of synaptic summation relies on multiple synapses at different locations?

Spatial summation.

Which type of synaptic summation relies on rapid successive inputs from the same synapse?

Temporal summation.

What initiates vesicle fusion with the presynaptic membrane?

Binding of Ca2+ to vesicular and membrane proteins (e.g., synaptotagmin and SNAREs).