Electrophysiology Lecture 6 Propagation Of Action Potentials

Front 1

3 ways to depolarize a cell

Back 1

1. increase potassium in extracellular area

2. add drugs to open cationic channels

3. inject positive current into the cell

Front 2

2 ways to hyperpolarize a cell

Back 2

1. add drugs to open potassium channels (for K+ efflux)

2. inject negative current into the cell

Front 3

what happens when you inject positive current into a cell?

Back 3

it depolarized until the current injection stops (minus the capacitance charge and discharge at beginning and end)

Front 4

what is equivalent to the capacitor and resistor of a cell?

Back 4

capacitor = cell membrane

resistor = leakage, (channels)

Front 5

time constant (tau)

Back 5

time it takes to rise 67% (1-1/e) or fall to 33% (1/e) of its final value

tau = Rm*Cm

time constant = membrane resistance * membrane capacitance

Front 6

what changes occur when the amount of injected current is increased?

Back 6

- voltage is affected

- Rm = same

- Cm = same

Front 7

what is the normal membrane capacitance?

Back 7

Cm = 1 uF/cm^2 usually; Cm changes based on the cell type

Front 8

how can you calculate membrane capacitance?

Back 8

tau=Rm*Cm

Rm = slope of line on IV plot

tau needs to be given

then solve for Cm

Front 9

what happens to Vm when you increase distance from injection point? (passive axon)

Back 9

- Vm decreases as you increase distance

- plotting the decay of Vm by distance = passive

Front 10

distance constant (lambda)

Back 10

- lambda = distance over which a voltage step decays to 1/e (33%) of its original value

- lambda = sq rt (rm/ri)

- if rm >ri, lambda increases and current will flow further down the inside of the axon

Front 11

what happens in a voltage vs space plot in a passive axon?

Back 11

voltage peaks at the injection point and then decays bi-directionally

Front 12

in a passive axon, what happens when the diameter increases?

Back 12

- as diameter increases, internal resistance increases, so Vm decreases

- as diameter increases, length constant increases

Front 13

what is the cable theory for a fiber?

Back 13

- describes how decay occurs as distance increases

- replication of space and time

- cable theory gives the time and length constants

Front 14

how large is the outside resistance of a cell?

Back 14

negligible

Front 15

what happens when you increase capacitance in a passive axon?

Back 15

the time constant increases (it takes longer to reach steady state)

- overall, decreasing capacitance is more efficient for the whole system

Front 16

what defines an active axon?

Back 16

active axon = voltage-gated ion channels and action potentials

- can be unmyelinated or myelinated

Front 17

how does an AP propagate in an active axon?

Back 17

- the AP is an impulse and is transient; the voltage changes are regenerative and propagate along the fiber with regeneration from ion channels as necessary to keep it above threshold

Front 18

what does a higher initial brief stimulation do in an active axon?

Back 18

it depolarizes the cell faster (b/c it charges the capacitor faster), resulting in a faster generation of an action potential

Front 19

what happens when a long current pulse is injected into an active axon?

Back 19

- there are no changes because the channels are inactivated

- new AP cannot be generated until channels are repolarized

- long current pulse keeps AP from returning completely to resting membrane potential

Front 20

what are the factors involved with an active unmyelinated axon (with Na and K channels)?

Back 20

- axial resistance (internal resistance)

- outside resistance (negligible)

- temperature

- an AP is reflected across the entire axon and the Vm is all the same

Front 21

active unmyelinated axon:

what occurs for action potential versus space?

Back 21

- membrane potential is the same across the distance of the axon

- large diameter = short distance before AP peak (very fast)

- small diameter = longer distance before AP peak (slower)

Front 22

active unmyelinated axon:

how does temperature affect the propagation?

Back 22

- shape of AP changes

- temp increase = voltage (AP) duration decreases

- temp increase = velocity of AP increases

Front 23

active unmyelinated axon:

what happens in a sodium-dependent AP?

Back 23

- all-or-none propagation at the same amplitude

- peak of the AP depends on sodium ratio (inside: outside); it adjusts to the sodium equilibrium potential

Front 24

active unmyelinated axon:

why is the propagation active?

Back 24

- active b/c the action potential is reinitiated at each point along the length of the axon

Front 25

active unmyelinated axon:

why is propagation unidirectional?

Back 25

unidirectional because the channels behind it are in the inactive state

Front 26

active unmyelinated axon:

what does the refractory period do?

Back 26

refractory period (b/w 2 APs) limits the "max frequency of firing"

Front 27

myelinated axon - what is it made of?

what are the resistance and capacitance properties?

Back 27

- tight spiral of glial cell membranes (oligodendrocytes)

- high resistance (hard to leak current)

- low capacitance (many receptors in series)

- tau (time constant) doesn't change

Front 28

what are nodes of ranvier and what are their key properties?

Back 28

- high density of sodium channels (compared to unmyelinated axons)

- resting membrane conductance is higher in the node

- peak sodium current has much higher amplitude

- rapid depolarization

Front 29

what happens to the time constant with myelin?

Back 29

Rm increases

Cm decreases

tau = same

tau = Rm*Cm

Front 30

what happens to the length constant with myelin?

Back 30

Rm increases

Ri depends on diameter of axon

if diameter remains the same, then length constant increases

lambda = sq rt (rm/ri)

Front 31

what happens with the propagation of an AP in a myelinated axon?

Back 31

- still all-or-none

- still has same amplitude

- reinitiated at nodes of ranvier

- unidirectional b/c of inactivation behind it

- refractory period still exists, so there is still a limit to max frequency of firing

- impulse spreads out, over multiple nodes at once

Front 32

myelinated axon: membrane voltage vs space (based on temperature)

Back 32

-lower temperature at the peak = constant voltage across distance

- higher temperature at the peak - much shorter voltage spread

Front 33

what happens when more myelin is added?

Back 33

propagation is faster (velocity increases)

myelin allows velocity to increase at a much smaller diameter