Electrophysiology 2. The basis of bioelectricity

Question 1

Define Ionic equilibrium potentials

Answer 1

The potential at which there is no net movement of that ion across the membrane, and this exists when the electrical gradient balances the concentration gradient

Question 2

Explain transmembrane transporters (pumps) and channels

Answer 2

• Pumps (transporters) maintain concentration gradients over the long term
  
  • not directly involved in neuronal signalling
• channels allow selective movement of ions across the membrane (’downhill’ - no energy consumption)
• direction depends on both concentration and electrical gradients
• ion flow constitutes an electrical current

Question 3

What is Q, Charge?

Answer 3

• Coulombs
• Anions and cations

Question 4

What is I, current?

Answer 4

• current is movement of charge
• amps
• I = dQ/dt

Question 5

What is V, voltage

Answer 5

• voltage/potential is separation of charge
• volts

Question 6

What is R, resistance?

Answer 6

• conductance
• ohms
• g is 1/R

Question 7

What is C, capacitance?

Answer 7

• capacity to store and separate charge
• farads
• C = Q/V

Question 8

Vm controlled by?

Answer 8

Vm controlled by individual ionic equilibrium ⇒ independent ionic batteries ⇒ depend on ionic concentration gradient ⇒ maintained pumps

Question 9

Describe Ion distributions across the membrane

Answer 9

• Membrane potential results from separation of positive and negative charges across the cell membrane
• Excess of positive charges outside the membrane and negative charges inside the membrane of a nerve cell at rest represents a small fraction of the total number of ions inside and outside cell

Question 10

State the Nernst equation

Answer 10

Ex = RT/zF In ((X)out/(X)in)
58log

Question 11

What is the electrochemical driving force

Answer 11

-It is equal to difference between equilibrium potential and actual membrane potential at any time
- 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 different, the greater the driving force!

Question 12

What is the main point?

Answer 12

• an ionic current tries to change Vm so that it is closer to the equilibrium (reversal) potential for that ion
• if there is an increase in conductance to a given ion, the change in Vm will be towards the equilibrium potential for that ion

Question 13

State Ohm’s law

Answer 13

I = V/R = gV
I ion = g ion (Vm -Eion)

Question 14

What are Sign conventions

Answer 14

• current is defined as movement of positive charge
• outward current is positive charge leaving the cells
• inward current is positive charge entering the cell

Question 15

What is Ek with normal physiological concentration

Answer 15

• At equilibrium net out and inflows are equal
• driving force due to concentration gradient is outward
• with Vm at -92mV inflow and outflow would be equal and K in equilibrium
• therefore at equilibrium force due to voltage gradient must be inward

Question 16

What is ENa with normal physiological concentration

Answer 16

• At equilibrium net out and inflows are equal
• driving force due to concentration gradient is inward
• with Vm at +60mV inflow and outflow would be equal and Na in equilibrium
• therefore at equilibrium force due to voltage gradient must be outward

Question 17

What happens if Vm is different from Eion?

Answer 17

• force due to conc is outward
• forced due to voltage gradient
• net flow is the difference: electrochemical driving force

Question 18

Explain the concept of reversal potential of anionic current and its significance in determining the nature (excitation or inhibition) of synaptic potentials

Answer 18

IPSP OR EPSP?
-likely to increase or decrease probability of reaching threshold
-receptor potential amplitude is limited by the reversal potential of the ionic current that generates it (0mV)