Action and Graded Potentials

Where does the action potential occur in a neuron?

Along axon membrane

When does the absolute refractory period happen?

During depolarization and early repolarization

Why does the absolute refractory period happen? (why can no second A.P. occur)

Voltage-gated Na⁺ channels are inactivated

When does the relative refractory period happen?

During late repolarization and hyperpolarization

Draw and label an action potential.

Depolarization, Repolarization, Hyperpolarization, Rest

Threshold for AP

approximately −55 mV

Depolarization:

voltage rises

Peak of AP

approximately +30 mV

Repolarization:

voltage falls

Hyperpolarization:

voltage dips below −70 mV

What state is the neuron in during the resting membrane potential?

Polarized- the inside is more negative than the outside (about −70 mV).

Why is it called depolarization?

The inside becomes less negative and more positive, so the difference in charge across the membrane decreases.

Why is it called repolarization?

The inside becomes negative again, returning toward the resting membrane potential.

Why is it called hyperpolarization?

The inside becomes even more negative than the resting membrane potential (below −70 mV).

During Action Potential, K⁺ leak channel:

open → K⁺ leaks OUT.

When is the voltage-gated Na⁺ channel open during A.P.

during depolarization

Na⁺ channel activation gate: open or closed during AP?

Open during depolarization.

Na⁺ channel inactivation gate: open or closed?

Open during depolarization, then closes at the peak of the action potential.

When is Na+ permeability high?

during depolarization; Na⁺ flows INTO the cell.

Voltage-gated K⁺ channel: open or closed?

Initially closed; opens during repolarization.

Voltage-gated K⁺ channels during A.P.

Initially closed; opens during repolarization.

When channels for an ion open, permeability for that ion

increases

Depolarization/ Ions

Na⁺ channels open → Na⁺ rushes IN.

Repolarization/ Ions

Na⁺ channels inactivate and K⁺ channels open → K⁺ moves OUT.

Hyperpolarization/ Ions

K⁺ channels close slowly, so extra K⁺ leaves.

which direction is Na⁺ flowing through the voltage-gated Na⁺ channel?

Na⁺ flows INTO the cell.

voltage-gated K⁺ channels close more _____ than voltage-gated Na⁺ channels reset

slowly

What causes hyperpolarization?

K⁺ continues leaving the cell, making the inside temporarily more negative than resting potential

K⁺ permeability _______ during hyperpolarization compared with rest

increases, voltage-gated K⁺ channels remain open, allowing more K⁺ to flow out

RMP during rest =

POLARIZATION- membrane is negative inside (-90 mv)

Conduction

action potential traveling along the axon through the activation of neighboring voltage-gated Na⁺ channels.

How far does the change in voltage due to an action potential spread across the membrane?

A short distance to nearby areas of the membrane; the voltage change decreases as it spreads.

What variable for diffusion limits the spread of voltage?

Distance

How far does the change in voltage need to travel?

Far enough to reach the next voltage-gated Na⁺ channels and bring that area to threshold.

What happens when the voltage reaches the next voltage-gated Na⁺ channel?

The channels open, Na⁺ rushes into the cell, and a new action potential is generated.

How does the action potential conduct a signal along the axon?

depolarizes the next section of membrane to threshold, triggering another action potential. This repeats along the axon.

Analogy of A.P. Propogation

row of dominoes falling: each section of the axon triggers the next.

Where do graded potentials happen?

On dendrites or cell body of a neuron

What is a chemically-gated (ligand-gated) channel?

ion channel that opens when a specific chemical signal binds to it

Mechanically-gated channels

Channels that open or close when the cell membrane is physically stretched/ pressed/ or bent

What stimulus opens a chemically-gated channel?

Chemical, such as a neurotransmitter

Mechanically-gated channels open in response to

physical stimuli like touch or pressure

Mechanically-gated channels change in mV

depends on which ions move through the channel

What happens when pressure deforms a Pacinian corpuscle?

The sensory nerve ending is physically deformed.

What happens if the graded potential reaches threshold?

Voltage-gated Na⁺ channels open and an action potential begins.

What is the sequence from pressure to an action potential?

Pressure → deformation → mechanically gated channels open → depolarization → threshold → action potential.

What voltage change does an excitatory GP cause?

Depolarization (inside becomes more positive)

What ion flow causes an excitatory GP?

Na⁺ enters the cell.

How does an excitatory GP affect the chance of an action potential?

Increases the chance of reaching threshold.

What voltage change does an inhibitory GP cause?

Hyperpolarization (inside becomes more negative).

Excitatory Graded Potential

A change in voltage that makes the inside of the cell more positive, increasing the chance of an action potential.

Inhibitory graded potential

A change in voltage that makes the cell less likely to reach threshold, decreasing the chance of an action potential.

What voltage change does an inhibitory GP cause?

Hyperpolarization (inside becomes more negative).

What are two ion flows that can cause an inhibitory GP?

• Cl⁻ enters the cell.

• K⁺ leaves the cell.

How does an inhibitory GP affect the chance of an action potential?

Decreases the chance of reaching threshold.

Stimulus magnitude/ number of channels

Stronger stimulus = more gated channels open.

Number of channels → graded potential amplitude

More channels open = larger graded potential.

Membrane permeability → graded potential amplitude

Greater permeability to an ion = a larger effect of that ion on membrane voltage.

Stimulus magnitude → graded potential amplitude

Stronger stimulus = larger graded potential.

unlike an action potential, a graded potential does not have

all or nothing - Its amplitude depends on the stimulus.

Where are action potentials generated?

Trigger zone (axon hillock) and along the axon

Where are neurotransmitters released?

Axon terminals

When an action potential reaches the axon terminal, what channels open?

Voltage-gated Ca²⁺ (calcium) channels.

When voltage-gated Ca²⁺ channels open at the axon terminal, which ion moves and in what direction?

Ca²⁺ enters the axon terminal.

Where are neurotransmitters stored before release from the presynaptic neuron?

In synaptic vesicles in the axon terminal.

What triggers synaptic vesicles to release neurotransmitters into the synaptic cleft?

Ca²⁺ entering the axon terminal.

After neurotransmitters cross the synaptic cleft, how do they affect the postsynaptic cell?

They bind to receptors and change the postsynaptic cell's membrane potential.

What type of receptors or channels do neurotransmitters bind to on the postsynaptic cell in this process?

Chemically gated (ligand-gated) ion channels.

What two types of graded potentials can neurotransmitters produce in the postsynaptic cell?

EPSPs (excitatory) and IPSPs (inhibitory).

How does the amount of neurotransmitter released generally affect the amplitude of the postsynaptic graded potential?

More neurotransmitter = more receptors, producing a larger graded potential.

Sequence of Action Potential at the synapse

Action potential reaches axon terminal → Ca²⁺ channels open → Ca²⁺ enters → neurotransmitters are released → neurotransmitters bind postsynaptic receptors → graded potentials form.

Does action potential amplitude change based on stimulus strength?

No. Action potentials are all-or-none, so their amplitude stays the same.

What is the absolute refractory period?

period when another action potential cannot be generated, no matter how strong the stimulus.

What characteristic of voltage-gated Na⁺ channels causes the absolute refractory period?

Na⁺ channels become inactivated and cannot reopen immediately.

relative refractory period

period when another action potential is possible, but requires a stronger-than-normal stimulus

What characteristic of voltage-gated Na⁺ channels contributes to the relative refractory period?

Some Na⁺ channels have reset and are ready to open again.

What characteristic of voltage-gated K⁺ channels contributes to the relative refractory period?

K⁺ channels remain open briefly, allowing K⁺ to leave the cell.

What characteristic of membrane voltage makes this a refractory period?

The membrane is hyperpolarized, making threshold harder to reach.

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

a stronger-than-normal stimulus to bring the membrane to threshold.

During hyperpolarization, will a normal threshold stimulus trigger an action potential?

No, a stronger-than-normal stimulus is needed.

How does stimulus strength affect action potential frequency?

stronger stimulus = higher frequency of action potentials, not larger action potentials.

How does action potential frequency affect neurotransmitter release?

Higher action potential frequency = more neurotransmitter to be released over time.

Stronger stimulus →

more frequent action potentials → generally more neurotransmitter released.

Describe the proportional relationship between stimulus strength, graded potentials, action potentials, and neurotransmitter release.

Stronger stimulus → larger graded potentials → higher action potential frequency → generally more neurotransmitter released.

How do voltage-gated Na⁺ and K⁺ channels contribute to action potential frequency?

Na⁺ channels cause depolarization

K⁺ channels cause repolarization.