L5: membrane properties and CAPs Flashcards
any change in membrane potential, + or -, graded (proportional to stim), declines as it propagates, has no threshold
local potential
amplitude independent of stim, regenerated as it propagates, has threshold
action potential
3 differences between local and action potential
-graded vs not graded -dissipates vs regenerates -no threshold vs threshold
generator potential =
local potential
electrotonic potential =
local potential
what is the difference between synaptic potential and receptor potential
-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)
what changes membrane potential
what changes membrane potential
what composes the capacitor plates vs the dielectric in a cell membrane?
plates = polar heads dielectric = hydrophobic tails
capacitance =
Q/V (charges separated and stored at a voltage)
E Eo A/dx (dielectric x constant x area/thickness)
how is capacitance related to membrane surface area?
C ~ A
how is capacitance related to membrane thickness?
C ~ 1/dx
how is membrane resistance related to conductance?
R ~ 1/g
the capacitance of cell membranes is near the same for all cell types, at __ μF/cm^2
1 µF/cm^2
1 Farad =
1 Coulomb / 1 Volt
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
R and C in parallel with electrodes on opposite sides
\_\_\_\_|\_\_\_\_
| |
R C
|\_\_\_\_\_\_\_\_|
|
R ~ all ion channels C ~ lipid bilayer
how does membrane current flow when exhibiting passive electrical behavior (dV is not opening or closing additional ion channels)
-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
what happens to membrane resistance as membrane area increases?
decreases (more ion channels available to carry current)
what are the units of membrane resistance?
Ω / cm^2
Ω / cm^2 are units for…
membrane resistance
T/F when a membrane is stimulated with an electrical current, ∆Vm is instantaneous
false - change in Vm is hyperbolic
how does voltage increase with time during charging of a capacitor
Vt = Vmax [1 - e^(-t/RC)]
how does voltage decrease with time during discharge of a capacitor
Vt = Vmax e^(-t/RC)
time constant τ for Vm while capacitor charges or discharges
τ = RC
= time to 63% Vmax on charging
= time to 37% Vmax on discharge
1 - 1/e =
.63
what determines the time course with which a local potential rises and decays?
R and C of the cell membrane
how does time constant relate to axonal conduction velocity?
v ~ 1/τ (depolarization can reach threshold faster if time constant is lower)
eq for decrement of local potential over distance
Vx = Vo e^(-x/λ) x = distance λ = length or space constant λ = sq.rt (Rm / Ri + Ro)
membrane length constant
λ = sq.rt (Rm / Ri + Ro)
= how far current will spread along axon (same as space constant)
membrane space constant
λ = sq.rt (Rm / Ri + Ro)
= how far current will spread along axon (same as length constant)
how does length or space constant relate to how far current will flow?
larger λ = further flow
how does distance of current flow relate to membrane resistance?
more Rm, further flow
current will flow through ICF or ECF instead of leaking across membrane
how does distance of current flow relate to ICF or ECF resistance?
more Ri and Ro, shorter current flow
current will leak across Rm instead
what is the resistance of the ECF when it is not a restricted volume?
very small compared to Ri… so Ro ~ 0
when ECF is not a restricted volume, λ (length constant) can be expressed as…
λ = sq.rt (Rm / Ri)
T/F Ro is negligible in an unrestricted extracellular volume
true
how is Ri related to the cross sectional area of the axon?
Ri ~ 1 / πr^2
how is Rm related to the circumference of the axon?
Rm ~ 1 / 2πr
how is λ related to axonal radius?
λ ~ sq.rt (r)
if Rm decreases with axonal radius, and Ri also decreases with axonal radius, then what happens to λ as radius increases?
λ increases λ ~ sq.rt. r Rm ~ 1 / 2πr Ri ~ 1 / πr^2 λ ~ Rm / Ri
T/F local potentials will travel further in large axons
true
λ ~ sqrt r
how does λ relate to conduction velocity?
larger λ, faster v
what happens to τ for a local potential as distance from the injection point increases?
τ increases as current leaks out over distance
the area of the cell body connecting the axon
axon hillock
what happens at the axon hillock
this is where local potentials initiated at dendrites travel to combine and initiate action potentials
2 factors that can help a dendritic local potential reach the axon hillock with greater amplitude and faster time course
- nearer to axon hillock (τ remains lower)
- thicker dendrite (λ remains larger)
the extracellular recording resulting from the combined action potentials from multiple axons in the nerve
compount action potential (CAP)
how does an extracellular action potential measurement look different from an intracellular action potential measurement?
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
describe a compound action potential
- 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
how does the magnitude of a CAP compare to that of an AP?
CAP is much smaller
2-3 mV vs 100-150 mV
why is the magnitude of a CAP 2-3 mV while that of an AP is 100-150 mV ?
because CAP is an extracellular recording and extracellular resistance and current density are low, producing only a small voltage drop
what happens to a CAP signal if you crush the nerve between the two sensory electrodes?
monophasic CAP (upward only) ∆V does not reach second electrode
what happens to a CAP if you apply a local anesthetic between the two sensory electrodes?
monophasic CAP (upward only) ∆V does not reach second electrode
T/F a CAP is an all-or-none response
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
a stimulus that activates all possible neurons and reaches max CAP is called…
suprathreshold stimulus
how is neuron diameter related to threshold stimulus?
larger diameter, lower threshold
because larger diameter = lower longitudinal R and more current will flow for a given stimulus voltage
what happens to a CAP signal as distance from stimulus to recording electrodes is increased?
distribution widens and if distance is increased enough the CAP will separate into multiple peaks
(faster fibers reach electrodes earlier than slow fibers)
what kind of fibers dominate a CAP record?
large fibers
lower τ so get there faster
larger λ so greater amplitude when they get there
T/F small, slow conducting fibers dominate a CAP record because they are more numerous than large, fast conducting fibers
false - despite being less numerous, large fibers dominate a CAP because they get there fast and with larger amplitude (lower τ, higher λ)
