Neuronal Signals

What are neuronal signals?

Brief changes in the electrical potential difference across the plasma membrane of neurons.

What is electrical potential difference?

Voltage measured in volts, which is a force.

What does resistance in a circuit mean?

The property of the circuit that opposes the movement of charge.

What is current?

The rate of movement of charge.

What are action potentials?

All or none signals that propagate long distances regeneratively.

What does the all or none principle of an action potential mean?

It means that increasing the strength of the stimulus will not make for a larger action potential; the size is fixed.

What are postsynaptic potentials?

Graded, local signals that vary with the strength of the stimulus.

What is the resting membrane potential difference?

-90 millivolts, meaning the inside of the cell is 90 millivolts more negative than the outside.

How can action potentials be activated?

By a stimulus that passes a threshold or critical point.

What follows after the threshold is crossed during an action potential?

The cell rapidly depolarizes, becomes less negative, passes 0 V, and may rise to positive 40 mV.

What is hyperpolarization?

The phase when the membrane potential drops below the resting membrane potential.

How do action potentials propagate?

They regenerate themselves as they travel throughout the cell.

What influences the ionic basis of the membrane potential (Vm)?

The concentrations of potassium and sodium inside and outside the cell.

What is the sodium-potassium pump's function?

It sends 3 Na+ out and 2 K+ in, maintaining ion concentration gradients.

How does the potassium channel affect the membrane potential?

It allows potassium to flow out of the cell, contributing to the negative charge inside.

What happens when a cell membrane is only permeable to potassium?

The membrane potential would be -90 mV, which is the resting potential.

What occurs when sodium channels are open?

Sodium flows rapidly into the cell, possibly making the inside positive, around +50 mV.

What is the result of simultaneous open K and Na channels?

The membrane potential would be around halfway between the two equilibrium potentials for sodium and potassium.

What is the resting potential of a nerve cell?

-70 mV.

What is the effect of opening the activation gate of the Na⁺ channel?

Sodium rushes in rapidly, depolarizing the membrane.

What happens at the peak of the action potential?

The Na⁺ channel activation gate is open but the inactivation gate closes, stopping Na⁺ inflow.

What marks the beginning of repolarization?

K⁺ channel activation gates open, allowing K⁺ to flow out.

What is hyperpolarization in action potentials?

K⁺ continues to flow out, overshooting the resting potential.

What restores ion gradients after an action potential?

The sodium-potassium pump (Na⁺/K⁺-ATPase) using ATP.

What is the initial concentration of sodium and potassium inside the cell?

High K⁺ inside and high Na⁺ outside.

What is the primary cause of the negative charge inside a cell?

The diffusion of potassium ions out, leaving behind chloride ions.

What role do chloride ions play in membrane potential?

They contribute to making the inside of the cell more negative when potassium ions diffuse out.

What happens during depolarization?

The membrane potential becomes less negative as Na⁺ floods into the cell.

What triggers the opening of Na⁺ channels?

A change in voltage across the membrane.

What is the effect of a strong stimulus during action potentials?

It leads to a stronger depolarization and can generate an action potential.

What limits the amplitude of action potentials?

The all or none principle means they have a fixed size.

What is the significance of the sodium-potassium pump?

It actively maintains concentration gradients for sodium and potassium ions.

What type of signals are primarily generated in the brain?

Graded signals that are depolarizations.

How do changes in membrane potential affect channel openings?

Changes in potential can cause gates of channels to open or close rapidly.

What determines the direction of potassium flow when channels are open?

The concentration gradient always drives potassium out of the cell.

What are the typical ion concentrations inside and outside the cell for potassium?

High concentration inside (150 mM) and low concentration outside (5 mM).

What occurs during the repolarization step of an action potential?

K⁺ channels remain open, allowing K⁺ to exit, while Na⁺ channels are inactivated.

What occurs to the membrane potential during hyperpolarization?

It drops below the resting potential of -70 mV.

What is the sequence of events during an action potential from resting to peak?

Resting state, depolarization, peak action potential, repolarization, hyperpolarization, return to resting state.

How does the depolarization of the cell affect the action potential propagation?

It regenerates along the myelinated segments quickly.

What does the term 'graded' refer to in postsynaptic potentials?

The response varies with the amplitude of the stimulus.

What is a critical feature of action potentials that differentiate them from graded potentials?

Action potentials are all-or-none, while graded potentials vary in amplitude.

Why is the resting potential important for nerve function?

It sets the stage for action potentials and the transmission of nerve impulses.

What is the effect of neurotransmitter release at synapses?

It is crucial for communication between neurons, facilitated by calcium influx.

What determines whether a channel is open or closed?

Hydrogen bonds, hydrophobic interactions, and the voltage across the membrane.

What is the primary function of calcium selective channels?

To enable the influx of calcium ions, important for neurotransmitter release.

What is the ratio of sodium to potassium ions moved by the sodium-potassium pump?

3 sodium ions out for every 2 potassium ions in.

What is the role of glial cells in restoring ion balance?

They take up excess potassium ions from the extracellular space.

What can happen if the action potential doesn't reach the threshold?

No action potential will be generated.

What is the relationship between concentration gradients and membrane potential?

Concentration gradients drive ions in and out of the cell, affecting the potential.

What factors influence the membrane potential in a neuron?

Ion concentrations, channel states, and gating mechanisms.

1. Resting Membrane Potential

Resting state where all voltage-gated channels are closed. The membrane potential is typically around -70mV.

2. Depolarizing Stimulus

Stimulus causes initial depolarization. Sodium channels begin to open, and the membrane potential moves towards threshold (typically around -55mV).

3. Rapid Depolarization (Action Potential Upstroke)

If the threshold is reached, many voltage-gated sodium channels open, causing a rapid influx of Na^+. The membrane potential becomes positive.

4. Repolarization Begins

Sodium channels inactivate and voltage-gated potassium channels open. K^+ begins to exit the cell.

5. Repolarization Phase

Potassium efflux continues; more K^+ exits than Na^+ entered, restoring the negative resting membrane potential.

6. Hyperpolarization

Potassium channels remain open longer than necessary, causing a transient hyperpolarization. The sodium-potassium pump (Na^+/K^+ ATPase) works to restore the original ion concentrations.

Sodium Channel - Resting State

Closed but capable of opening.

Sodium Channel - Open State

Open, allowing Na^+ to flow into the cell.

Sodium Channel - Inactivated State

Channel is closed and cannot be opened immediately. This state is crucial for unidirectional propagation of the action potential.

Potassium Channel - Resting State

Closed at resting membrane potential.

Potassium Channel - Open State

Open during repolarization, allowing K^+ to flow out of the cell.

Absolute Refractory Period

The period during which another action potential cannot be triggered, regardless of the strength of the stimulus. Corresponds to the period when sodium channels are inactivated.

Relative Refractory Period

A period when a stronger-than-normal stimulus is required to elicit an action potential. Some sodium channels have recovered from inactivation, but potassium channels are still open.

Threshold Potential

The minimum depolarization required to open a sufficient number of sodium channels to trigger the positive feedback loop, leading to an action potential.

Summation

The combined effect of multiple sub-threshold signals to reach threshold. Can be spatial or temporal.

Action Potential Propagation

The action potential travels down the axon. The influx of Na^+ depolarizes adjacent areas of the membrane, causing the action potential to propagate.

Saltatory Conduction

In myelinated axons, the action potential jumps between Nodes of Ranvier, which are gaps in the myelin sheath with a high concentration of voltage-gated sodium channels. This increases the speed of conduction.

Axon Diameter Effect

Larger diameter axons have lower internal resistance, which allows faster propagation of the action potential.

Myelination Effect

Myelination increases membrane resistance and decreases membrane capacitance, resulting in faster action potential propagation.

Sodium-Potassium Pump ($$

Enzyme that helps maintain resting membrane potential by transporting 3 Na^+ ions out of the cell for every 2 K^+ ions into the cell, using ATP.

1. Resting Membrane Potential - Channel States

All voltage-gated Na^+ and K^+ channels are closed. Leak channels maintain the resting potential.

2. Depolarizing Stimulus - Channel States

Some voltage-gated Na^+ channels open in response to the stimulus. K^+ channels remain closed.

3. Rapid Depolarization (Action Potential Upstroke) - Channel States

Many voltage-gated Na^+ channels open, causing a large influx of Na^+. K^+ channels are still mostly closed.

4. Repolarization Begins - Channel States

Na^+ channels start to inactivate. Voltage-gated K^+ channels begin to open, allowing K^+ to exit.

5. Repolarization Phase - Channel States

Na^+ channels are inactivated. Voltage-gated K^+ channels are fully open, causing rapid efflux of K^+, which brings the membrane potential back down.

6. Hyperpolarization - Channel States

Voltage-gated K^+ channels remain open longer than necessary, causing hyperpolarization. Na^+ channels are closed, and the Na^+/K^+ pump helps restore the resting ion concentrations.

Sodium Channel - Resting State - Gate Positions

Activation gate is closed, inactivation gate is open.

Sodium Channel - Open State - Gate Positions

Both the activation and inactivation gates are open, allowing Na^+ to flow into the cell.

Sodium Channel - Inactivated State - Gate Positions

Activation gate is open, but the inactivation gate is closed, blocking Na^+ flow.

Potassium Channel - Resting State - Gate Positions

Gate is closed at resting membrane potential.

Potassium Channel - Open State - Gate Positions

Gate is open during repolarization, allowing K^+ to flow out of the cell.

Absolute Refractory Period - Channel States

Most Na^+ channels are in the inactivated state, and cannot be opened regardless of stimulus strength.

Relative Refractory Period - Channel States

Some Na^+ channels have recovered from inactivation (activation gate closed, inactivation gate open), but some K^+ channels are still open.

Threshold Potential - Channel States

Sufficient number of Na^+ channels open to trigger the positive feedback loop. There is a rapid influx of Na^+, initiating the action potential.

Summation - Channel States

Multiple sub-threshold signals can cumulatively open enough Na^+ channels to reach the threshold.

Action Potential Propagation - Channel States

Influx of Na^+ further opens Na^+ channels in adjacent areas.

Saltatory Conduction - Channel States

Action potential jumps between Nodes of Ranvier, where there is a high concentration of voltage-gated Na^+ channels.

Axon Diameter Effect - Channel States

Larger diameter reduces resistance, allowing faster and more efficient opening of Na^+ channels.

Myelination Effect - Channel States

Myelination insulates the axon, minimizing leakage of charge and maximizing the effect of $$Na^+