Cellular Neuroscience (Action Potentials)

Front 1

Action Potential Components

Back 1

- threshold (all-or-none)

- rising phase

- overshoot (> 0mV )

- falling phase

- afterhyperpolarization/undershoot (below RMP)

Front 2

What Action Potentials Result From

Back 2

action potentials are the result of selective (voltage- and time-dependent) increases in the membrane's permeability to Na(+) and K(+)

Front 3

The Refractory Period

Back 3

- brief inactivation of Na(+) channels and activation of K(+) channels, making it harder to produce subsequent action potentials

- limits the number of action potentials that a given nerve can produce per unit tie

- explains why action potentials don't propagate back to the point of origin in the axon

Front 4

Refractory Periods of Different Neurons

Back 4

different types of neurons have different maximum rates of action potentials firing due to different types and densities of ion channels

Front 5

Hodgkin and Huxley

Back 5

(1949-1852)

- use the voltage clamp to describe the ionic conductances that underlie an action potential

- described Na(+) and K(+) conductances mathematically to show that the measured currents represent voltage-dependent changes in membrane permeability

Front 6

Do Membranes have Voltage-Gated Dependent Permeabilities?

Back 6

effect is voltage-dependent

Front 7

Do Ionic Currents Flow Across the Membrane When its Potential is Changed?

Back 7

current flow is observed in response to depolarization, but not hyperpolarization

Front 8

Direction of Current Flow

Back 8

the net movement of positive charge

Front 9

Inward Current

Back 9

- flow of positive charge into the cell or negative charge out of the cell

- Na(+) flowing into the cell along its concentration gradient brings more positive charge into the cell

Front 10

Outward Current

Back 10

- flow of positive charge out of the cell or negative charge into the cell

- K(+) flows out of the cell taking its positive charge with it

Front 11

Voltage-Clamp Experiments

Back 11

the flow of positive charge into the cell (inward current) is typically shown as a downward deflection

Front 12

Current-Clamp Experiments

Back 12

- depicts the change of the membrane potential

- typically shows APs (and synaptic events) as upward deflections (positive potentials are up)

Front 13

Reversal (Equilibrium) Potential

Back 13

- gives clues about the ionic nature of the early current

- relationship Nernst equation

Front 14

Ion-Substitution Experiments

Back 14

confirm the importance of Na(+) ions for the early inward current

Front 15

Experiment for K(+) Current

Back 15

load axon with radioactive K(+) ions and measure efflux in relation to outward current

Front 16

Rectification

Back 16

- the conductance of the membrane to Na(+) and K(+) increases as the membrane is depolarized

Front 17

As Conductance Increases:

Back 17

- so does current (I = gV)

- because V = V-Veq, as the equilibrium potential is approached, the current gets smaller even in the presence of maximal conductance.. thus conductance does not = current

Front 18

With High Conductance:

Back 18

you have low resistance (g = 1/R)

Front 19

Time Course of the K(+) and Na(+) Conductance in Response to Depolarizing Current Steps

Back 19

- gk = slow and non-inactivating

- gNa = fast and inactivating

Front 20

Conclusions Made by Hodgkin and Huxley

Back 20

- Na(+) and K(+) conductance change over time (and "delayed rectifier")

- The Na(+) channels must inactivate because the current goes off even when it is polarized

- The K(+) channels do no inactivate because if the cell is depolarized they are open

- They are voltage-dependent

Front 21

Delayed Rectifier

Back 21

Na(+) and K(+) conductance both require time to turn on, but K(+) is much slower, requiring several ms to reach its peak

Front 22

Phases of the Action Potential

Back 22

1. Rest Potential (Vm)

2. Generator Potential

3. Threshold (-50 mV)

4. Rising Phase "overshoot"

5. Falling Phase

6. After hyperpolarization "undershoot"

7. Return to rest potential

8. Refractory Period

Front 23

Rest Potential (Vm)

Back 23

- 1st phase of an action potential

- due to high resting K(+) conductance

- about -70mV

Front 24

Generator Potential

Back 24

- 2nd phase of an action potential

- depolarizes cell due to opening ligand-gated cation channels

Front 25

Threshold

Back 25

- 3rd phase of an action potential

- membrane potential at which all voltage-gated Na(+) channels begin to open, "all or none"

- about -50 mV

Front 26

Rising Phase

Back 26

- 4th phase of an action potential

- approaches Na(+) equilibrium potential

- "overshoots" 0 mV

Front 27

Falling Phase

Back 27

- 5th phase of an action potential

- K(+) channels open, Na(+) channels close

- membrane potential returns to near K(+) equilibrium potential

Front 28

After Hyperpolarization

Back 28

- 6th phase of an action potential

- membrane potential becomes negative to rest potential

- "undershoots" the resting potential

Front 29

Return to Rest Potential

Back 29

- 7th phase of an action potential

Front 30

Refractory Period

Back 30

- 8th phase of an action potential

- time during which a 2nd action potential cannot be evoked (axon resistant to further excitation)

- gNa(+) inactivated

- gk(+) activated over rest (-> undershoot)

- Absolute Refractory Period and Relative Refractory Period

Front 31

Generator Potentials Which Cause Small Depolarizations

Back 31

are not enough to open voltage-gated Na(+) channels

Front 32

If the Generator Potential is Large Enough

Back 32

- cell will reach threshold membrane potential and open voltage-gated Na(+) channels

- opening Na(+) channels produces more depolarization which opens more Na(+) channels

- eventually leads to all of the voltage-gated Na(+) channels opening and the membrane potential moves toward the Na(+) equilibrium potential (rising phase)

Front 33

Absolute Refractory Period

Back 33

- time during which it is impossible to get another action potential (a 2nd AP cannot be initiated under any circumstances)

- corresponds to the falling phase when Na(+) channels are inactivated

Front 34

Relative Refractory Period

Back 34

- time during which only a strong stimulus can evoke another action potential (a 2nd AP is inhibited but not impossible)

- corresponds to afterhyperolarization: Na(+) channels have recovered from inactivation but the membrane potential is below resting potential due to open K(+) channels

Front 35

Firing Frequency of Action Potentials Depends On

Back 35

- refractory periods

- several hundred Hz is generally the limit

Front 36

Properties of (Protypical) Na(+) and K(+) Channels

Back 36

- show ion selectivity

- both are voltage-gated

- have voltage-sensor

- Na(+) channel has mechanism for inactivation

Front 37

Voltage-Sensor

Back 37

- used by voltage-gated ion channels

- depolarization increases open probability, while hyperpolarization closes them

Front 38

Properties of Single Na(+) Channels

Back 38

- currents carried by Na(+) inward -> potentials to be more negative than ENa and reverse their polarity above ENa

- time course of opening, closing and inactivation matches macroscopic current -> stochastic events to be averaged many times

- opening/closing of channels is voltage-dependent (E)

Front 39

Macroscopic Currents

Back 39

- sum of all (microscopic) currents in the cell

- shows how multiple channels work to create an action potential

- electrode records current

- can view smaller currents (ie. channels) with micropipette technique

Front 40

Micropipette Technique

Back 40

allows view of smaller currents (ie. channels)

Front 41

Amplitude of Current is Dependent On:

Back 41

Na(+) concentration

Front 42

Tetradoxin

Back 42

blocks both microscopic and macroscopic Na(+) currents

Front 43

Cycle of Na(+) Channel States

Back 43

- fast response- two gates (activation and inactivation -> biphysical properties of proteins)

- rapid opening (activation) followed by slower closing (inactivation)

- refractory period

resting (closed) -> activated (open) -> inactivated (closed) -> resting (closed)

Front 44

Voltage Sensor of the Na(+) Channel

Back 44

- S4 (positively charged amino acids)

Front 45

Changes in Na(+) Channel

Back 45

depolarization cause a conformational change in the channel

Front 46

Na(+) Activation Gate

Back 46

opened by voltage

Front 47

Na(+) Inactivation Gate

Back 47

causes refactory periods

Front 48

Properties of Na(+) Channels

Back 48

- have activation/inactivation gate (biophysical property of proteins)

- 24 membrane spanning proteins

- 4 different domains form pores

Front 49

Voltage-Gated Ion Channels

Back 49

- Na(+) channels have more kinetics than K(+) (faster at opening than K(+) channels)

Front 50

Impulse Propagation is Unidirectional

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- kept unidirectional by trailing Na(+) channel inactivation (doesn't mean can't go other way)

- the only Na(+) channels that are available to open are the ones in front of the nerve impulse

Front 51

What Can Increase Neurons Spike

Back 51

- for ultrafast spiking, voltage-gated Na(+) channels must be rapidly activating and inactivating

- voltage-gated K(+) channels must match kinetics to minimize relative refractory period

- (Na(+) and K(+) channels allow for re-depolarization by rapidly closing after potential fires)

Front 52

Neurons as a Cable

Back 52

- a very bad cable

- membranes are leaky (some channels are always open, current leaks out)

Front 53

Passive Membrane Properties

Back 53

the length constant (λ)

Front 54

The Length Constant (λ)

Back 54

the length constant is a measure of the efficiency of the passive spread of voltage changes along the axon (the distance over which the change in voltage or membrane potential decays to 1/3 or 1/e of its original value)

Vx = V0 e ^ (-x/λ)

λ = sqrt( rm / (ro + ri) )

Front 55

Voltage vs. Distance

Back 55

decrement in voltage with distance

Front 56

Length Constant (λ) Depends On

Back 56

the resistances of:

- the membrane (rm)

- the intracellular medium (ri)

- the outside medium (ro) (negligible in calculation)

Front 57

For the Optimal Passive Spread of Current:

Back 57

- rm must be high

- ri and ro must be low

Front 58

ri is smaller when:

Back 58

axon diameter is smaller

Front 59

rm is greater when:

Back 59

the axon is more myelinated

(if low rm then there will be substantial ionic and current leak)

Front 60

A Large Length Constant Makes it:

Back 60

- easier for distant synapses to influence the activity of the neuron

- increases spatial summation with a neuron

Front 61

ri deceases when:

Back 61

decreases in proportion to the square of the increased diameter (area is proportional to diameter squared and resistance decreases with an increase in area)

Front 62

Spatial Summation

Back 62

- synaptic potentials generated in different regions of the neuron are added together

- two inputs far away from each other can sum together to reach a threshold

Front 63

"Saltatory" Conduction

Back 63

- how impulses travel

- myelin sheath prevents leakage of charge out of the axons

- Nodes of Ranvier are breaks in the myelin where ions can flow across the membrane (propagation is extremely fast between nodes)

- high density of voltage-gated ion channels at the nodes (none under myelin)

Front 64

Myelinated Axons Take Advantage Of:

Back 64

passive current to conduct signals over a greater distance

Front 65

Number of Voltage-Gated Sodium Channels in Cell Areas:

Back 65

- cell body: ~1 VGNaC per square micron

- axon hillock and initial segment of the axon: ~100-200 VGNaC per square micron

- nodes of Ranvier: ~1000-2000 VGNaC per square micron

Front 66

The Time Constant (τ)

Back 66

- as the time when the voltage response (Vt) rises to 1-(1/e) (or 63%) of the steady state membrane response (V∞) (or Vmax: maximal voltage response)

- temporal summation

- given in thirds (rise of 2/3 or decrement of 2/3)

τ = (rm)(cm) where cm=capacitance, rm=resistance

Front 67

A Long Time Constant (τ):

Back 67

means the rise and fall of the action potential takes more time (eg. because voltage gated channels with a long open time contribute to the action potential)

Front 68

Capacitance (cm)

Back 68

- a membrane property

- important for axon

- for practical purposes the only membrane property that changes capacitance is the presence or absence of myelin

Front 69

Myelination vs. Capacitance (cm)

Back 69

- myelination decreases capacitance because it increases the effective thickness of the membrane

- therefore you have a smaller time constant with myelination

Front 70

Resistance (rm)

Back 70

- the inverse of conductance

- important for dendrites

- the number of open channels at rest contributes to the measure of rm

Front 71

Open Channels vs. Resistance (rm)

Back 71

- more K(+) leak channels open at rest contributes to a large conductance and therefore a small resistance

- this would decrease the time constant

Front 72

Temporal Summation

Back 72

- synaptic potentials generated in the same membrane patch as a result of successive stimulation are added together

Front 73

Time Constant (τ) vs. Temporal Summation

Back 73

a long time constant allows for more temporal summation

Front 74

Effects of Passive Membrane Properties (λ and τ)

Back 74

affect signal propagation and synaptic integration

Front 75

Role of Summation

Back 75

- allows for getting above the threshold

- without summation signals could not be sufficient to induce an action potential