Ionic Basis of Excitation

Question 1

What is the name for the voltage difference across cell membrane?

How is it used?

Answer 1

membrane potential

used as a bettery and for signaling purposes

Question 2

What are the typical intracellular and extracellular concentrations of:

Na+

K+

Ca2+

Cl-

Answer 2

• Na+
  
  • intracellular: 15 mM
  • extracellular: 140 mM
• K+
  
  • intracellular: 140 mM
  • extracellular: 4mM
• Ca2+
  
  • intracellular: 0.0001 mM
  • extracellular: 2.5 mM
• Cl-
  
  • intracellular: 6 mM
  • extracellular: 100 mM

Question 3

What is the relationship between energy and concentration gradients?

Answer 3

It takes energy to move something against its concentration gradient, but it releases energy to move something down its concentration gradient

Question 4

What two forces are generated by the uneven distribution of ions acros the membrane?

Answer 4

chemical and electrical

Question 5

What are the 4 factors influencing teh ion distribution across the cell membrane?

Answer 5

1. Membrane permeability (ion channel)
2. Concentration gradient (chemical force)
3. Voltage gradient (Vm; electrical force)
4. Active transport of the ion (Na-K-Pump)
   
   1. restores membrane potential from (Na/K movement due to leak or action potentials)

Question 6

Nenst potential

Answer 6

Equilibrium potential (ion specific)

membrane potential at which there is no net movement of ions

electrical and chemicla forces are equal but opposite in direction

chemical > electrical

Question 7

Based on the Nerst euqation, what is the deiving force and current flow when the following ion channels are opened?

K+

Na+

Ca2+

Cl-

Answer 7

• K+: -25 mV : outward current; negative
• Na +: +130 mV: inward current; positive
• Ca2+: +205 mV: inward current; positive
• Cl-: -5 mV: outward current; negative

Question 8

What is the definition of ionic current?

Answer 8

direction of cation flow?

• Inward (+) current:
  
  • inward movement of cations
  • outward movement of anions
• Outward (-) current:
  
  • outward movement of cations
  • inward movement of anions

Question 9

If conductance in 0, what is the driving force and current?

Answer 9

there is no driving force, there is no current flow

Regardless of driving force, if there are not open channels, then ther will be no current

Question 10

Moving the membrane potential more positive?

Moving the membrane potential more negative?

Answer 10

positive: depolarization
negative: hyperpolarizaion

Question 11

Influence of Eion on Vm

for K+, Na+ and Cl-

Answer 11

Question 12

Vm is closest to the equilibrium potential of the ion with the highest what?

What ion is this?

Answer 12

Permeability

K+

Question 13

What types of stimulation can induce channel opening?

Answer 13

• mechanical (stretch)
• electrical (voltage-gated)
• transmitter
• G-proteins
• pH
• drugs

Question 14

What channels are most important for altering Vm?

Answer 14

• Potassium channels
  
  • to establish resting Vm–> some always open for K+; some to restore Vm following excitation (voltage-gated)
• Sodium channels
  
  • elicit action potentials (voltage-gated)f
    
    • Lidocaine: topical anesthetic that blocks Na channels
• Na-K pump

Question 15

How does the Na-K ATPase work?

Answer 15

• ATP-dependent electorgenic displacement K+ and Na+
  
  • 3 Na+ out for eery 2 K+ in
• blocked by Ouabain (cell depolarize)
• If activity decreases
  
  • [Na+] will increase
  • [K+] will decrease
  • Vm will depolarize

Question 16

Consider a neuron in the central nervous system. What would you typically expect in regards to the following properties?

A high number of negatively charged anions that do not cross teh cell membrane are found

• A: on both sides of cell membrane
• B: on inside of the cell
• C: on the outside of the cell
• D: nowhere, anoins usually carry a positive charge

Answer 16

B: on inside of the cell

Question 17

Consider a neruon in the central nervous system. What would you typically expect in regards to the following properties?

Vm with approdimately -70mV

• A: leak potassium currents
• B: sodium permeability
• C: the equilibrium potential of chloride
• D: the ion with the lowest permeability
• E: Na-K pump

Answer 17

A: leak potassium currents

b/c at resting state, mostly determined by leak potassium current

Na-K pump, important to restore ion concentrations

Question 18

• What type of change is required to alter Ex?

Answer 18

• Only a ‘global’ change in [X]in and or [X]out alters Ex
  
  • ‘local’ events (EPSP/IPSP/AP) have not effect on Ex

Question 19

• What is the only variable with regardsto ion X can change Vm?

Answer 19

• Only a change in membrane permeability for ion X can influence Vm
  
  • closed ion channels cannot determine Vm (however, closing of ion channel can change Vm

Question 20

What is hyper/hypokalemia?

What pathologies are associated?

Answer 20

increase/decrease of extracellular K+ concentration

• Pathologies
  
  • changes in Ek with cocomitant changes in membrane excitability
  • for increa K+: dyspnea, arrhythmias, palpations, chest pain, paraesthesia, diarrhea
  • acidosis, alkalosis coupeld to exracellular K+

Question 21

What are examples of channelopathies?

What channelopathy causes “the fainting goat”?

Answer 21

epilepsy, long QT syndrome, myotonia congenita (fainting goat)

• fainting goat (in humans = Thomsen types and Becker type)
  
  • mutation CIC-1, major skeletal muscle Cl- channel
    
    • exagerate response to stimula dur to failure to restore normal resting Vm
      
      • CIC-1 helps to re-establish resting Vm
      • delayed relaxation of the muscles
      • hereditary genetic disorder