MNSR 13

Question 1

Describe currents:

Answer 1

Current is the rate of flow of charge Q.
It has units Amperes [A]:
1 Ampere = 1 Coulomb charge/ second
1 A = 1 C/s
Charge can be electrons (e.g. in wire) or ions (e.g. in neurons)

Question 2

Define voltage:

Answer 2

A potential difference, or voltage, of 1 Volt means that 1 Joule of energy has been expended in moving 1 Coloumb of charge.

Voltage is measured in Volts[V].

In living systems, voltages are on the order of (tens of) millivolts [mV].

Question 3

What is resistance?

Answer 3

Materials which allow easy movement of charge are called good conductors, while materials which do not allow easy flow of charge are insulators.

Question 4

Electrical resistance is measured in…

Answer 4

Ohms [Ω].
Ohms Law:
voltage = current x resistance
or
V = IR

Question 5

When a voltage is applied across a resistor…

Answer 5

A current flows which is proportional to the voltage

Question 6

When electrical charges accumulate on one side of an insulator…

Answer 6

a voltage builds up across the insulator.

capacitance = charge / voltage
C = Q/V

Capacitance depends on the permitivity of the insulator and the distance between the conductor plates.

Question 7

How does the lipid bilayer act as an insulator?

Answer 7

Insulates inside from outside. Ions cannot pass.
If ions accumulate on one side (surplus of charge Q), a voltage builds up: 𝑉=𝑄∕𝐶.
This is called the membrane potential VM .

Question 8

What is the Membrane potential VM?

Answer 8

The membrane potential is the voltage difference between the inside and the outside of the neuron measured at a certain location.

Question 9

In voltage-gated channels, these transitions are favoured/disfavoured by the…

Answer 9

membrane potential.

Question 10

The state of ion channels determines the…

Answer 10

permeability of the cell membrane for different ion types.

Question 11

What drives ions across the membrane?

Answer 11

Drift is a movement driven by difference in electric potential (=voltage)
Caused by Coulomb forces
Affects equally charged ions in the same way

Question 12

What is difussion?

Answer 12

Diffusion is a movement driven by difference in concentration, from high to low
Specific for ion type (Na+, K+, Ca2+, Cl-)
Independent of charge

Question 13

How will ions move between two compartments due to both drift and diffusion?

Answer 13

This is described by the Nernst equation.

Question 14

The Nernst equation is…

Answer 14

𝑉𝑖𝑜𝑛 [𝑚𝑉]=25/𝑧
𝑙𝑛([𝑖𝑜𝑛]_𝑜𝑢𝑡/[𝑖𝑜𝑛]_𝑖𝑛 )

Question 15

Modified Ohm‘s law incl Nernst potential:

Answer 15

𝐼𝑖𝑜𝑛=((𝑉𝑀− 𝑉𝑖𝑜𝑛)/𝑅𝑖𝑜𝑛 )

Question 16

Ions will flow through a resistance Rion according to the…

Answer 16

difference of membrane potential VM (Coulomb force) and their Nernst potential Vion (Diffusion force).

Question 17

The net current of inflowing Na+ ions and outflowing K+ ions charges the membrane capacitance and thus determines how the membrane voltage changes over time.

Describe this in terms of net inflow and outflow.

Answer 17

A net inflow (more Na+ entering than K+ leaving) makes the membrane potential more positive.
A net outflow (more K+ leaving than Na+ entering) makes the membrane potential more negative.

Question 18

When a neuron is resting on the outside of the membrane there is:

Answer 18

• a thin layer of positive charges
• a thin layer of negative charges inside.

Question 19

The AP is a…

Answer 19

change of membrane potential VM over time with a very particular shape (not a specific voltage!)

Question 20

An AP can be ‘triggered‘ by…

Answer 20

the arrival of a signal that raises the membrane potential above a characteristic threshold.

Question 21

The AP has three phases:

Answer 21

depolarization, repolarization, and hyperpolarization. The first two determine the duration of the AP and the latter the ‘refractory period‘.

Question 22

During resting potential, the Vm is at a negative potential, meaning…

Answer 22

• few K+ channels are open, most Na+ channels are closed
• Na+ influx equals K+ efflux
• resting VM

Question 23

During depolarization, the Vm has increased meaning…

Answer 23

• Na+ channels start opening quickly
• strong Na+ influx overcomes modest K+ efflux
• raises VM further, and then quickly further (positive feedback)

Question 24

During repolarization, the Vm is very positive meaning…

Answer 24

• open Na+ channels inactivate; K+ channels open with a delay
• strong K+ efflux overcomes small Na+ influx
• VM starts dropping rapidly

Question 25

During hyperpolarization, the Vm is very negative meaning…

Answer 25

• most K+ start to close, Na+ channels recover from inactivation
• small net influx of ions
• slowly raise VM

Question 26

When the AP returns to rest, the Vm is steady negative meaning…

Answer 26

• few K+ channels are open, most Na+ channels are closed
• Na+ influx equals K+ efflux
• resting VM