Electrophysiology 2. The basis of bioelectricity

Question 1

Compare the function of ion transporters and ion channels:

Answer 1

Transporters:

• Actively move ions against concentration gradient
• Create ion concentration gradients

Ion channels:

• Allow ions to diffuse down concentration gradient
• Cause selective permeability to certain ions

Question 2

Define the electrical terms = Q, I, V, R and C along with their units:

Answer 2

Q = charge, Units = coulombs

I = current = movement of charge, Units = amps

V = voltage = separation of charge, Units = volts

R = resistance, Units = ohms

C = capacitance = capacity to store and separate charge, Units = farads

Question 3

Describe the use of the Nerst equation:

Answer 3

Used to find cell potential at any moment during a reaction

Or at conditions other than standard state

Question 4

Define Vm:

Answer 4

Electrical potential difference across the cell membrane

That exactly balances the concentration gradient for an ion

Question 5

Describe the link between ionic equilibrium potential and [ion]:

Answer 5

Ionic equilibrium potential is proportional to the log of concentration ratio for that ion

Question 6

What is Ek with normal physiological concentration:

Answer 6

At eq = net out and inflows are equal

Driving force due to known concentration gradient is outward

Thus, at eq, force due to voltage gradient must be inward (due to Vin being more negative)

Question 7

What is Ena with normal physiological concentration:

Answer 7

At eq = net out and inflows are equal

Driving force due to known concentration gradient is inward

Thus, at eq, force due to voltage gradient must be outward (due to Vout being more negative)

Question 8

What happens if Vm is different from Eion (describe net flow):

Answer 8

Force due to voltage gradient will be towards whichever (Vin/Vout) is more negative, in the case of K+ will be inward

Force due to known concentration gradient will be outward (for K+)

Net flow will be the difference = known as electrochemical driving force

Question 9

Describe the link between absolute difference and driving force:

Answer 9

A potential that drives ion flow across the membrane

Potential: voltage

Ion flow: current

Driving force is difference between Vm and Eion

The greater the absolute of this difference, the greater the driving force

Question 10

Describe the link between conductance to a given ion and Vm:

Answer 10

An ionic current ‘tries’ to change Vm so that it is closer to the equilibrium (reversal) potential for that ion

This means that if there is an increase in conductance to a given ion

Then the change in Vm will be towards the equilibrium potential for that ion

Question 11

Describe how to calculate ion current:

Answer 11

More formally, from Ohm’s law

I = V/R = gV (since conductance (g) is the reciprocal of resistance (g = 1/R) )

So ionic current is membrane conductance for that ion times the driving force on that ion

I_ion = g_ion (Vm – E_ion)

Question 12

Describe a synaptic potential:

Answer 12

Potential difference across postsynaptic membrane that results from the action of neurotransmitters at a neuronal synapse

“incoming” signal that a neuron receives

Two forms of synaptic potential: excitatory and inhibitory

Question 13

Describe the function of EPSPs and IPSPs with relation to each other:

Answer 13

EPSPs depolarize membrane and move potential closer to threshold for AP to be generated

IPSPs hyperpolarize membrane + move potential farther away from threshold

Decreasing the likelihood of an action potential occurring.

In order to depolarize a neuron enough to cause an action potential

There must be enough EPSPs to both depolarize the postsynaptic membrane from its resting membrane potential to its threshold

And counterbalance the concurrent IPSPs that hyperpolarize the membrane