PPP - Resting Membrane Potential and Action Potential

Question 1

Membrane needs what three things for diffusion of ions?

Answer 1

Membrane requires:

1. Pumps
2. Capacitance
3. Conc gradient

If no channel, then no conc gradient.
To separate charge on membrane, you need to do work which is usually done by hydrolysing ATP to ADP.

Question 2

Equation to work out conc gradient of +ve ions:

Answer 2

Gradient = [C]out/[C]in

For negative ions, it is the other way round.

Question 3

Resting potential?

Definition for equilibrium potential of ion:

Answer 3

Resting potential = -70mV depending on Na/K conc.

Equilibrium potential of ion is the membrane voltage that a cell needs to be at to prevent movement of ion down conc gradient.

If +ve inside = Na can’t enter
Ena = +50mV
if -ve inside = K can’t leave
Ek= -90mV

Question 4

Membrane potential: Vm

Answer 4

Vm closer to Ek than Ena as membrane more permeable to K than Na. More open channels of K.

At constant Vm, net flow of ions is zero as leak of K out and leak of Na in is equal.

If a cell becomes more permeable to that ion then Vm would move towards that eqm of that ion.

Question 5

Driving force equation:

Answer 5

Driving force = Vm - Eeq

It is the force that cause an ion to move e.g. K transported out due to driving force. 
K= -70 - (-90) = +20mV
Na= -70 - (+50) = -120mV
Positive driving force = in to out
Negative driving force = out to in

At eqm both dorces act simultaneously. K more permeable to membranes so charges balance out with Na which is harder to get in.

Question 6

Definition of Permeability and Conductance:

Answer 6

Permeability - is the ease of an ion to get across membrane. Depends on no. of channels.

Conductance - amount of ions/material that get across (quantitative).

High conductance means more ions pass through membrane. Low conductance means that membrane can have same permeability/low permeability but fewer ions pass through.

Question 7

Goldman Hodgkin Katz Equation:

Answer 7

Nernst eqn deals with one ion at a time and doesn’t take relative permeability into account.

Katz eqn consider relative permeability of monovalent ions.
Vm=58log[Peq[C]out/Peq[C]in]

Permeability of membrane to K is 50 times grater than to Na. So Pk=50 and Pna=1

Question 8

Properties of AP:

Answer 8

1. Caused by depolarisation
2. Has to reach threshold
3. All or nothing
4. Propagates without decreasing
5. At peak: Vm close to Ena
6. Membrane inexcitable during refractory period

AP caused by Na/K flow. Channels are ion-selective and open and close depending on voltage across channel.

Question 9

Structure of channels:

Answer 9

When there is A.P, it causes helix structures in channel to move –> open and close channels.

During AP, membrane more permeable to sodium first and then potassium. Vm driven to Ena and then Ek.

Question 10

Vm to Ena:

Vm to Ek:

Answer 10

Vm to Ena:
Depolarisation cause Na channels to open fast. Na influx. further depolarisation. This is positive feedback.
If prolonged, it can cause confirmational change and close Na channel.

Vm to Ek:
Depolarisation leads to opening K channels slowly. K efflux from inside to outside. Leads to repolarisation and hyperpolarisation (more +ve taken out than in).

Question 11

Conductance (mmho/cm2) against time graphs:

Answer 11

Vm close to gna. 
  _
 /  \  _
/    \    \
Occurs between 0-4ms. AP generated rapidly. 

Charge - Q
Voltage - V
Capacitance - F
Q = V x F

Each mole of a monovalent ion has 10^5 coulombs of charge.
Faction of mole required: Charge/(10^5)

Question 12

Propagation of AP:

Answer 12

Membrane +ve inside when AP generated. Na open so electric current movement from +ve to -ve. Electric current due to +ve and -ve charge.

Membrane -ve at rest. Charges can get spaced out showing depolarisation.

AP generated at roughly +50mV.

Question 13

Local circuits:

Answer 13

Movement of electric current represents local circuits.
Cause axon to depolarise up ahead when AP generated.
Na channels are open. +ve charge flow out.

Question 14

Ways to increase speed of conduction:

Answer 14

1. Insulation
2. Saltatory conduction where current found at nodes so AP jump from node to node.

If not myelinated, current lost easily. Demyelination means you lose current easily and not held long in neuron.

Question 15

Unidirectional AP: Hitting Elbow

Answer 15

AP doesn’t usually go backwards but can. Hit elbow and can feel tingle in fingers. Sends AP down to fingers and brain - Antidromic stimulation.