BB Lecture 14: Intro to Electrophysiology

Question 1

What defines biological currents?

Answer 1

the movements of positive charges

Question 2

What produces a potential during an idealized experiment with a voltmeter?

Answer 2

pushing the electrode from the extracellular space into a cell

Question 3

What is Veq?

Answer 3

equilibrium potential

the value of Vm where the concentration gradient and the electrostatic force are balanced

Question 4

What are the typical Veq values for the common biological ions?

Answer 4

K+: -91 mV
Na+: +67 mV
Ca2+: +125 mV
Cl-: -78mV

Question 5

What happens when a membrane is depolarized?

Answer 5

Vm is made less negative by the entry of cations or the exit of anions

Question 6

What happens when a membrane is polarized/hyperpolarized?

Answer 6

Vm becomes more negative upon the exit of cations or the entry of anions

Question 7

How can you calculate Veq?

Answer 7

Nerst Equation

Veq(s) = (58/z) ln([S]o/[S]i)

were:
z is the valence of the solute
[S]o is concentration outside
[S]i is concentration inside

Question 8

What determines the size of the current carried by a given ion species?

Answer 8

if you assume the concentration gradient across the membrane remains constant

1. the electrochemical gradient for this ion (driving force)
2. the ease with which this ion can pass through the membrane

Question 9

How do you calculate the driving force?

Answer 9

Driving force = |Vm-Veq|

Question 10

What impacts the ease at which this ion can pass through the membrane?

Answer 10

how many channels for the particular ion are available to carry current

Question 11

What is conductance?

Answer 11

availability of a material to carry the current

Question 12

What is Ohm’s law?

Answer 12

describes the simple relationship between current, voltage, and conductance
I=gV

where:
I= membrane current for a given ion species
g= conductance of the membrane for that ion
V= driving force for that ion

Question 13

How do you calculate the current for an ion?

Answer 13

1. determine whether the current will be inward (depolarized) or outward (hyperpolarizing), based on the direction in which the ion must move to bring Vm closer to its own Veq
2. calculate the size of the current using Ohm’s law

Question 14

What is the current-clamp method?

Answer 14

the technically simplest experimental approach for studying the effects of currents on Vm
uses a microelectrode to impale a cell, inject a salt solution (depolarize or hyperpolarize the membrane), and measure Vm

Question 15

What is capacitance?

Answer 15

the ability to redistribute electrical charges within the bilayer
the reason why real membranes don’t exhibit instantaneous behavior