Lecture 3 - Electrophysiology of the cell membrane

Question 1

Chemical gradient

Answer 1

Concentration gradient

the energy provided by the difference in concentration across the plasma membrane

Question 2

Electrical gradient

Answer 2

Potential difference

the energy associated with moving charged molecules across the membrane - when a membrane potential exists

Question 3

Resting membrane potential

Answer 3

Charge difference between the two sides of the membrane, seen as a membrane potential of about -70mV

Question 4

Outside the cell

Answer 4

high Na+, low K+ and high Cl-

Question 5

Inside the cell

Answer 5

Low Na+, high K+ and low Cl-

Question 6

Potential difference across the cell membrane is principally generated by..

Answer 6

a sodium-potassium ATPase

which pumps 3 sodium out for every 2 potassium in (2 potassium in increases intracellular potassium and decreases extracellular potassium, 3 sodium out increases extracellular sodium and decreases intracellular sodium). The primary regulator is the Na+K+ ATPase but there are also other ways things can move across such as passive transport (Na+ channel, K+ channel etc)

Question 7

Electrochemical gradients

Answer 7

Set up by the ionic distribution across the membrane

Only works if specific ion channels open and (a few) ions flow

Question 8

Maintenance of resting membrane potential

Answer 8

K+ and Na+ are moved against their concentration gradient - energy from ATP

Question 9

Nernst equation

Answer 9

Eion=60/Z log (Co/Ci)

Eion = equilibrium potential 
60 = constant (that accounts for temp, R, F) 
Z= valence (charge) 
Log = base 10 log 
Co= concentration outside 
Ci=concentration inside

Question 10

Equilibrium potential

Answer 10

The equilibrium potential is the energy (expressed in mV) of a concentration gradient of an ion OR The electrical potential (in mV) that exactly balances the concentration of an ion

It is the charge pushing the ion in one direction and balancing the concentration gradient pushing it in the other direction and then charge you need to do that is the equilibrium potential

Question 11

Equilibrium potential of sodium

Answer 11

+65mV

This is the potential difference across the membrane for sodium that would exactly balance the concentration gradient trying to push sodium inside of the cell

Question 12

Equilibrium potential of potassium

Answer 12

-89 mV

Question 13

Resting membrane potential

Answer 13

Is largely an OUTWARD K+-mediated equilibrium potential modified by a much smaller INWARD equilibrium potential for Na+ ions

Flow direction for potassium is our and charge gradient for potassium is in

Flow direction and charge gradient are in the same direction for sodium

Vm = -70mV (negative inside)

Question 14

What would happen if sodium channels were open in terms of the equilibrium potential for sodium?

Answer 14

the equilibrium potential for sodium was about +65mV in our generic neuron so if the sodium channels were open there would be a tendency for the cell to be polarised towards +65mV so you get an overshoot beyond 0mV

Question 15

Afterhyperpolarisation reaches…

Answer 15

the equilibrium potential for potassium

Question 16

Depolarisation

Answer 16

Becoming more positive

Depolarisation refers to a sudden change in membrane potential – usually from a (relatively) negative to positive internal charge. In response to this chemical stimulus, sodium channels open within the membrane.

Question 17

Repolarisation

Answer 17

Becoming more negative but still above RMP

Repolarisation refers to the restoration of a membrane potential following depolarisation (i.e. restoring a negative internal charge). Following an influx of sodium, potassium channels open within the membrane. The efflux of potassium causes the membrane potential to return to a more negative internal differential.

Caused as a result of the stimulus being removed and excess sodium ions being transported out of the cytosol

Question 18

Hyperpolarisation

Answer 18

More negative than RMP

Hyperpolarization is when the membrane potential becomes more negative. It is the opposite of depolarisation.

At this level the sodium channels begin to close and voltage gated potassium channels begin to open. After hyperpolarization the potassium channels close and the natural permeability of the neuron to sodium and potassium allows the return to resting membrane potential.

Question 19

Overshoot

Answer 19

More positive than zero

Question 20

Local potentials

Answer 20

Local potentials are graded, decremental, reversible, and can either excite or inhibit the membrane.

Small local ion fluxes (channel openings) across the membrane means that the membrane potential changes over time. These changes are called local potentials - because they are localised to a subsection of the cell membrane

Question 21

Measuring membrane potential

Answer 21

Can be measured by sticking an electrode inside the cell and then having a reference electrode outside the cell and then having a voltmeter showing the potential difference between the cells

Current injected by experimenter and what this causes is different changes in the membrane of the neuron

Positive current injected - change in membrane potential is positive (become more positive) and the same occurs for negative current injected (chang in membrane potential is negative, membrane potential becomes more negative)

This causes local potential which are passive changes in membrane potential caused by the flow of current across the membrane - in this instance by current injected by the experimenter but in real life it is caused by the opening of channels when synapses are active

Question 22

Action potential vs local potential

Answer 22

Local potentials are graded, decremental, reversible, and can either excite or inhibit the membrane. In contrast, action potentials are all-or-none, nondecremental, irreversible and always excitatory.

Question 23

Local potential features

Answer 23

Graded in size
Decrease amplitude over distance
Summate in time (temporal summation) and area (spatial summation)
Can depolarise or hyper polarise
Influence the generation of an action potential

Question 24

Passive potentials features

Answer 24

Since local potentials are not actively propagated they can also be called passive potentials

Length constant
As you move away in distance the voltage decays with a constant (length constant) and it is essentially the distance over which the voltage decays to 37% of its original size

Axial resistance
Resistance along its axis or the internal resistance
Bigger the pipe, the more easily it will flow therefore said to have lower axial resistance
Smaller diameter means that length constant is shorter so the voltage will decay much quicker away from the point of origin which is to do with the intrinsic physical properties of the axial or internal resistance

Passive conduction will not transport a signal from one end of an axon to the other unless the axon is very short (i.e., on the order of its length constant) because passively conducted signals decrease in size rapidly with distance from their origin.

Question 25

Length constant

Answer 25

As you move away in distance the voltage decays with a constant (length constant) and it is essentially the distance over which the voltage decays to 37% of its original size

Question 26

Axial resistance

Answer 26

Resistance along its axis or the internal resistance
Bigger the pipe, the more easily it will flow therefore said to have lower axial resistance
Smaller diameter means that length constant is shorter so the voltage will decay much quicker away from the point of origin which is to do with the intrinsic physical properties of the axial or internal resistance

Question 27

Action potential features

Answer 27

Threshold - often approx 10mV depolarised from RMP (estimate, different for different neurons)
Overshoots beyond zero towards ENa (equilibrium potential for sodium)
All-or-nothing
Regenerative - propagated without decreasing amplitude

Question 28

Afterhyperpolarisaiton define

Answer 28

Afterhyperpolarization, is the hyperpolarizing phase of a neuron’s action potential where the cell’s membrane potential falls below the normal resting potential. Hence, hyperpolarization persists until the membrane K+ permeability returns to its usual value.

Question 29

Equilibrium potential calcium ***

Answer 29

equilibrium potential is positive which means that it is going into the cell (its concentration gradient is that way)

Question 30

Equilibrium potential chlorin ***

Answer 30

equilibrium potential is negative which means that it is going out of the cell (its concentration gradient is that way)

Question 31

If the RMP is less than the equilibrium potential of Na+ ….

Answer 31

then moving Na+ out of the cell will not occur via diffusion must be an active process to move it against electrochemical gradient therefore use the Na+-K+ ATPase

Question 32

Membrane permeabilities to ions

Answer 32

Membrane is essentially impermeable to sodium but is somewhat permeable (leaky) to potassium at RMP when the Na+-K+ ATPase is active (3 sodium out to 2 potassium in)

Question 33

If the RMP us greater than the equilibrium potential of Na+

Answer 33

If the Na+ equilibrium potential was +65mV and the membrane potential was more positive than this e.g. +100 mV then ions would flow down the electrochemical gradient without active transport

Question 34

If the RMP is -60mV it means

Answer 34

that the inside of the cell is -60mV if the outside of the cell is 0mV