Passive membrane properties

Question 1

Neurons produce two types of electrical potentials:

Answer 1

non-propagated local potentials, called electrotonic potentials, and the action potential

Question 2

electrotonic potentials

Answer 2

non-propagated local potentials caused
due to a local change in ionic conductance (e.g. channel opening due to synaptic response). When they spread along the membrane they become exponentially smaller.

Electrotonic refers to the “passive” spread of charge inside a neuron

“Passive” means that voltage-dependent changes in membrane conductance do not contribute.

can sum spatially or temporally

graded response.

.

Question 3

Electrotonic spread and summation of many inputs is
responsible for

Answer 3

depolarizing the voltage of the soma
sufficiently to threshold and trigger the action potential.

Question 4

Electrotonic potentials are unsuitable for longdistance signaling because

Answer 4

conduct faster than action potentials,
but attenuate rapidly

Question 5

Increases in outward or inward current pulses (A1) produce

Answer 5

proportional and symmetrical changes in
membrane potential (Vm) (A2).
Note, that the potential changes more slowly than the current steps

Question 6

An I-V curve is obtained by

Answer 6

y plotting the steady state voltage against the injected current

Question 7

The slope
of the I-V curve defines

Answer 7

the input resistance of the neuron.

Question 8

Voltage is equal to

Answer 8

the product of current and resistance

Question 9

The input resistance of the cell determines

Answer 9

how much the cell will change (both depolarization or
hyperpolarization) in response to a steady current.

Question 10

the neuron with the
higher input resistance will show

Answer 10

a greater change in membrane voltage (both de- or hyperpolarization).

Question 11

In a neuron the input resistance depends on

Answer 11

both the density of the resting (“leak”) ion channels in
the membrane (i.e. the number of open channels per unit area of membrane) and the size of the cell
→ The larger the neuron, the greater will be its membrane surface area and the lower the input
resistance, since there will be more resting channels to conduct ions.

Question 12

outwardly rectifying channel

Answer 12

where positive (outward)
current is larger than the
negative (inward) current for a
given absolute value of voltage.

Question 13

capacitance (unit = farad)

Answer 13

the ability of a body to
hold an electrical charge.

Question 14

capacitor

Answer 14

a device that stores energy in
the electric field created between a pair of conductors on which equal but opposite electric charges have been placed.

A capacitor consists of two conductors
(plates) separated by an insulator
(dielectric)

Question 15

capacitance is directly proportional to t

Answer 15

the surface area of the
conductor plates and inversely proportional to the separation distance between the plates.

Question 16

The action potential is a _______ wave of
Na+ permeability increase

Answer 16

regenerative

Question 17

The length constant (lambda)

Answer 17

a measure of the efficiency of the passive spread of
voltage changes along the neuron (“how far?”)

the distance over which the change in voltage (membrane
potential) decays to about one-third (or specifically 1/e) of its original value

Question 18

The length constant (lambda) depends on the resistences of…

Answer 18

• the membrane (rm),
• the intracellular medium (ri
  ), and
• the outside medium (r(alpha))) (negligible in calculation)

Question 19

For optimal passive spread of current

Answer 19

rm (membrane resistance) has to be high and ri (intracellular medium resistance) and ro (outside medium) low

Question 20

If rm is low, there will

Answer 20

l be substantial ionic (and current) leak

Question 21

The length constant (lambda) is proportional to the…

Answer 21

diameter of the axon/dendrite because diameter determines r

Question 22

The bigger the resistance of the membrane (Rm) and/or the smaller the resistance inside of the axon or
dendrite (Ri),

Answer 22

the less current will escape from across the membrane and the better the signal will propagate.

Question 23

Cells increase Rm by

Answer 23

myelination

Question 24

Cells decrease Ri by

Answer 24

increasing axon diameter.

Question 25

The rate of propagation of an action potential is

Answer 25

inversely related to the product of capacitance and axonal resistance (C * Ri
).

Question 26

A large length constant

Answer 26

helps with the
propagation of the action-potential, as it has to
be regenerated less often

allows for more
spatial summation.

Question 27

the direction of
AP propagation

Answer 27

is determined by The refractory period
(and the distribution of Na+
channels)

Question 28

Myelin from oligodendrocytes acts

Answer 28

as an electrical insulator
to increase the speed of conduction along the axon
→ increases Rm and decreases C

Question 29

Time constant, (tau)

Answer 29

defined as the time when the voltage response (Vt) rises
to 1 – (1/e) (or ~63%) of the steady state membrane response (V∞) (or relaxes to 37%)

Question 30

cm

Answer 30

Charge that is stored in the
capacitance of a cell keeps it
depolarized longer, thus
prolonging the duration of a
signal (e.g. a synaptic input)

Question 31

Spatial summation

Answer 31

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

Question 32

Temporal summation:

Answer 32

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

Question 33

A long time constant (tau)