action potential

What is an action potential?

A rapid, temporary reversal of membrane potential that propagates along a neuron's membrane to transmit information.

What is another term for an action potential?

A spike or nerve impulse.

What is the resting membrane potential before an action potential?

Approximately -70 mV.

What provides the conditions necessary for action potentials to occur?

The electrical and chemical gradients established at the resting membrane potential.

What is a graded potential?

A local change in membrane potential whose size varies according to stimulus strength.

Can graded potentials be depolarising or hyperpolarising?

Yes.

What is depolarisation?

A reduction in the negativity of the membrane potential.

What is hyperpolarisation?

A change that makes the membrane potential more negative than resting potential.

How does the size of a graded potential relate to stimulus strength?

Stronger stimuli produce larger graded potentials.

What is meant by a neuron being polarised?

The inside of the cell is negative relative to the outside.

What happens when a small region of membrane becomes depolarised?

Local currents spread depolarisation to adjacent membrane regions.

What causes local current flow?

Positive ions moving towards nearby negative regions.

What allows a wave of depolarisation to spread along a membrane?

Local current flow between adjacent membrane regions.

What is threshold potential?

The membrane potential at which an action potential is triggered.

What happens if threshold is not reached?

Only a graded response occurs.

What happens when threshold is reached?

An action potential is generated.

How does increasing stimulus strength above threshold affect action potential size?

It does not increase action potential size.

How does increasing stimulus strength above threshold affect firing?

It increases the number or frequency of action potentials.

What does all-or-none mean?

Once threshold is reached, the action potential occurs with a fixed amplitude.

What are the main phases of an action potential?

Resting state, depolarisation, overshoot, repolarisation, and undershoot (afterhyperpolarisation).

What is the rising phase of an action potential?

Depolarisation.

What is the falling phase of an action potential?

Repolarisation.

What is the overshoot phase?

The part of the action potential where membrane potential becomes positive.

What is afterhyperpolarisation?

A temporary period when membrane potential becomes more negative than resting potential.

What is the approximate peak membrane potential during an action potential?

Approximately +30 to +40 mV.

What initiates an action potential?

A depolarising current reaching threshold.

Where do action potentials normally travel?

Down the axon away from the cell body.

What change in permeability initiates depolarisation?

A rapid increase in Na+ permeability.

Which ion is primarily responsible for depolarisation?

Na+.

Which ion is primarily responsible for repolarisation?

K+.

What type of ion channels generate action potentials?

Voltage-gated ion channels.

Are the voltage-gated K+ channels involved in action potentials the same as resting K+ leak channels?

No.

What are the two gates found on voltage-gated sodium channels?

Activation gate and inactivation gate.

What is the state of voltage-gated Na+ and K+ channels at rest?

Both are closed.

What happens to Na+ channel activation gates during depolarisation?

They open rapidly.

What is the consequence of opening Na+ activation gates?

Na+ rushes into the neuron.

Why does Na+ enter the neuron during depolarisation?

It moves down its electrochemical gradient.

What happens to membrane potential as Na+ enters?

It becomes increasingly positive.

What happens to Na+ channels during repolarisation?

Their inactivation gates close.

What happens to K+ channels during repolarisation?

They open.

Why does K+ leave the neuron during repolarisation?

It moves down its concentration gradient.

What causes membrane potential to become negative again during repolarisation?

K+ efflux.

Why does afterhyperpolarisation occur?

K+ channels remain open longer than necessary.

What happens to Na+ channels during afterhyperpolarisation?

They remain closed.

What restores the resting state after an action potential?

Voltage-gated K+ channels close and membrane potential returns to resting levels.

What is meant by permeability (P)?

How easily a specific ion crosses the membrane.

How does Na+ permeability change during an action potential?

It rises sharply then rapidly falls.

How does K+ permeability change during an action potential?

It rises later and remains elevated longer.

Which permeability rises first during an action potential?

Na+ permeability.

Which permeability remains elevated during afterhyperpolarisation?

K+ permeability.

Why is the action potential self-propagating?

Local currents depolarise adjacent membrane regions to threshold.

How do action potentials communicate information between neurons?

They trigger neurotransmitter release at synapses.

What structure releases neurotransmitters when an action potential arrives?

The presynaptic axon terminal.

What is the synaptic cleft?

The gap between communicating neurons.

What is the role of neurotransmitters?

To transmit signals between neurons.

What is the absolute refractory period?

A period when no second action potential can be generated.

Why can no action potential occur during the absolute refractory period?

Voltage-gated Na+ channels are inactivated.

What is the relative refractory period?

A period when a stronger-than-normal stimulus is required to generate another action potential.

Why is a stronger stimulus needed during the relative refractory period?

The membrane is hyperpolarised and K+ channels remain open.

What important function does the refractory period serve?

It ensures unidirectional propagation of action potentials.

Why can't an action potential travel backwards under normal conditions?

The membrane behind it is refractory.

What does unidirectional propagation mean?

The action potential travels in only one direction along the axon.

How are larger stimuli encoded if action potentials are all-or-none?

By increasing action potential frequency.

What determines action potential amplitude?

It is fixed once threshold is reached.

What determines action potential frequency?

Stimulus strength.

What is myelin?

A lipid-rich insulating sheath surrounding axons.

What is the effect of myelination on conduction velocity?

It greatly increases conduction speed.

What is saltatory conduction?

Action potentials appearing to jump between nodes of Ranvier.

What are nodes of Ranvier?

Gaps in the myelin sheath where voltage-gated channels are concentrated.

Why are nodes of Ranvier important?

Action potentials are regenerated at these locations.

Why does myelin increase conduction speed?

It prevents current leakage across the membrane.

Where are voltage-gated Na+ and K+ channels concentrated in myelinated axons?

At nodes of Ranvier.

What happens to local current flow under myelin?

It travels rapidly with minimal leakage.

What would happen if myelin were lost?

Action potential conduction would become slower and less efficient.

What are the main ionic gradients underlying action potentials?

High Na+ outside, high K+ inside.

Which ion has a higher extracellular concentration?

Na+.

Which ion has a higher intracellular concentration?

K+.

What generates the ionic gradients required for action potentials?

The Na+/K+ pump.