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Physiology 1 (WPH) > L5: membrane properties and CAPs > Flashcards

L5: membrane properties and CAPs Flashcards

1
Q

any change in membrane potential, + or -, graded (proportional to stim), declines as it propagates, has no threshold

A

local potential

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2
Q

amplitude independent of stim, regenerated as it propagates, has threshold

A

action potential

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3
Q

3 differences between local and action potential

A

-graded vs not graded -dissipates vs regenerates -no threshold vs threshold

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4
Q

generator potential =

A

local potential

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5
Q

electrotonic potential =

A

local potential

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6
Q

what is the difference between synaptic potential and receptor potential

A

-synaptic potential occurs at a neuron - neuron synapse (may involve receptors) -receptor potential can be stimulated at a location other than the synapse (e.g. stretch receptors in muscle, optic receptors in eye, etc)

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7
Q

what changes membrane potential

A

what changes membrane potential

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8
Q

what composes the capacitor plates vs the dielectric in a cell membrane?

A 
plates = polar heads
dielectric = hydrophobic tails
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9
Q

capacitance =

A

Q/V (charges separated and stored at a voltage)

E Eo A/dx (dielectric x constant x area/thickness)

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10
Q

how is capacitance related to membrane surface area?

A

C ~ A

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11
Q

how is capacitance related to membrane thickness?

A

C ~ 1/dx

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12
Q

how is membrane resistance related to conductance?

A

R ~ 1/g

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13
Q

the capacitance of cell membranes is near the same for all cell types, at __ μF/cm^2

A

1 µF/cm^2

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14
Q

1 Farad =

A

1 Coulomb / 1 Volt

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15
Q

describe an equivalent electrical circuit that approximates the passive electrical behavior of a cell, i.e. the responses of cells when ∆V is not large enough to open ion channels

A 
R and C in parallel with electrodes on opposite sides 
\_\_\_\_|\_\_\_\_
|             |
R           C
|\_\_\_\_\_\_\_\_|
       |

R ~ all ion channels C ~ lipid bilayer

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16
Q

how does membrane current flow when exhibiting passive electrical behavior (dV is not opening or closing additional ion channels)

A

-initially all Im flows to charge the lipid bilayer (C)
-as C becomes charged, more Im begins to flow across ion channels (R)
-when stimulation ends, C is discharged across Ir
Ic + Ir = Im
at t = 0, Im = Ic
at t = large, Im = Ir

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17
Q

what happens to membrane resistance as membrane area increases?

A

decreases (more ion channels available to carry current)

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18
Q

what are the units of membrane resistance?

A

Ω / cm^2

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19
Q

Ω / cm^2 are units for…

A

membrane resistance

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20
Q

T/F when a membrane is stimulated with an electrical current, ∆Vm is instantaneous

A

false - change in Vm is hyperbolic

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21
Q

how does voltage increase with time during charging of a capacitor

A

Vt = Vmax [1 - e^(-t/RC)]

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22
Q

how does voltage decrease with time during discharge of a capacitor

A

Vt = Vmax e^(-t/RC)

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23
Q

time constant τ for Vm while capacitor charges or discharges

A

τ = RC
= time to 63% Vmax on charging
= time to 37% Vmax on discharge

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24
Q

1 - 1/e =

A

.63

25
Q

what determines the time course with which a local potential rises and decays?

A

R and C of the cell membrane

26
Q

how does time constant relate to axonal conduction velocity?

A

v ~ 1/τ (depolarization can reach threshold faster if time constant is lower)

27
Q

eq for decrement of local potential over distance

A 
Vx = Vo e^(-x/λ)
x = distance
λ = length or space constant
λ = sq.rt (Rm / Ri + Ro)
28
Q

membrane length constant

A

λ = sq.rt (Rm / Ri + Ro)

= how far current will spread along axon (same as space constant)

29
Q

membrane space constant

A

λ = sq.rt (Rm / Ri + Ro)

= how far current will spread along axon (same as length constant)

30
Q

how does length or space constant relate to how far current will flow?

A

larger λ = further flow

31
Q

how does distance of current flow relate to membrane resistance?

A

more Rm, further flow

current will flow through ICF or ECF instead of leaking across membrane

32
Q

how does distance of current flow relate to ICF or ECF resistance?

A

more Ri and Ro, shorter current flow

current will leak across Rm instead

33
Q

what is the resistance of the ECF when it is not a restricted volume?

A

very small compared to Ri… so Ro ~ 0

34
Q

when ECF is not a restricted volume, λ (length constant) can be expressed as…

A

λ = sq.rt (Rm / Ri)

35
Q

T/F Ro is negligible in an unrestricted extracellular volume

A

true

36
Q

how is Ri related to the cross sectional area of the axon?

A

Ri ~ 1 / πr^2

37
Q

how is Rm related to the circumference of the axon?

A

Rm ~ 1 / 2πr

38
Q

how is λ related to axonal radius?

A

λ ~ sq.rt (r)

39
Q

if Rm decreases with axonal radius, and Ri also decreases with axonal radius, then what happens to λ as radius increases?

A 
λ increases
λ ~ sq.rt. r
Rm ~ 1 / 2πr
Ri ~ 1 / πr^2
λ ~ Rm / Ri
40
Q

T/F local potentials will travel further in large axons

A

true

λ ~ sqrt r

41
Q

how does λ relate to conduction velocity?

A

larger λ, faster v

42
Q

what happens to τ for a local potential as distance from the injection point increases?

A

τ increases as current leaks out over distance

43
Q

the area of the cell body connecting the axon

A

axon hillock

44
Q

what happens at the axon hillock

A

this is where local potentials initiated at dendrites travel to combine and initiate action potentials

45
Q

2 factors that can help a dendritic local potential reach the axon hillock with greater amplitude and faster time course

A
  • nearer to axon hillock (τ remains lower)

- thicker dendrite (λ remains larger)

46
Q

the extracellular recording resulting from the combined action potentials from multiple axons in the nerve

A

compount action potential (CAP)

47
Q

how does an extracellular action potential measurement look different from an intracellular action potential measurement?

A

CAP vs AP
CAP is sigmoidal, biphasic waveform resulting from multiple combined APs from multiple axons in nerve
AP is spiked signal from single axon within nerve

48
Q

describe a compound action potential

A
  • records summed response of multiple neurons within nerve with two surface electrodes
  • ∆V reaches first reference electrode, and V2-V1 is positive
  • ∆V reaches second electrode, and V2-V1 is negative
  • CAP signal is biphasic waveform
49
Q

how does the magnitude of a CAP compare to that of an AP?

A

CAP is much smaller

2-3 mV vs 100-150 mV

50
Q

why is the magnitude of a CAP 2-3 mV while that of an AP is 100-150 mV ?

A

because CAP is an extracellular recording and extracellular resistance and current density are low, producing only a small voltage drop

51
Q

what happens to a CAP signal if you crush the nerve between the two sensory electrodes?

A 
monophasic CAP (upward only)
∆V does not reach second electrode
52
Q

what happens to a CAP if you apply a local anesthetic between the two sensory electrodes?

A 
monophasic CAP (upward only)
∆V does not reach second electrode
53
Q

T/F a CAP is an all-or-none response

A

false - an AP is an all-or-none response. a CAP is a summation of various APs and will therefore increase in amplitude as more APs are activated

54
Q

a stimulus that activates all possible neurons and reaches max CAP is called…

A

suprathreshold stimulus

55
Q

how is neuron diameter related to threshold stimulus?

A

larger diameter, lower threshold

because larger diameter = lower longitudinal R and more current will flow for a given stimulus voltage

56
Q

what happens to a CAP signal as distance from stimulus to recording electrodes is increased?

A

distribution widens and if distance is increased enough the CAP will separate into multiple peaks
(faster fibers reach electrodes earlier than slow fibers)

57
Q

what kind of fibers dominate a CAP record?

A

large fibers
lower τ so get there faster
larger λ so greater amplitude when they get there

58
Q

T/F small, slow conducting fibers dominate a CAP record because they are more numerous than large, fast conducting fibers

A

false - despite being less numerous, large fibers dominate a CAP because they get there fast and with larger amplitude (lower τ, higher λ)

Physiology 1 (WPH) (18 decks)

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