Active Transporters/ Action Potential

Question 1

Active transporters:

Answer 1

• Maintain ionic concentration gradients
  -Translocate ions against their concentration gradients
  -Bind and unbind ions
  -Slow (several millisecond process)

Question 2

The Na+/K+ pump
accounts for 20-40% of the…

Answer 2

• brain’s energy
  consumption!
• pump is on all the time
  1) Na+ binding
  2) Phophorylation
  3) Conformational change causes Na+ release and K+ binding
• trnaslocated across the membrane, against their concentration gradient
  4) Dephosphorylation-induced conformational changed leads to K+ release

Question 3

Molecular structure of the Na+/K+ pump

Answer 3

• There is no pore
• Pump, not an ion channel
• Pump ions against their concentration gradients

Question 4

Disruption of the function of the Na+/K+ pump at any point in the cycle can disrupt the efflux of Na+

Answer 4

1) Efflux of Na+
2) Na+ efflux reduced by removal of external K+
3) Recovery when K+ is restored
4) Efflux decreased by metabolic inhibitors, such as dinitrophenol, which block ATP synthesis
5) Recovery when ATP is restored

Question 5

Examples of ion exchangers

Answer 5

• Use electrochemical gradients of co-transported ions as a source of energy

Question 6

Antiporters

Answer 6

• exchange intracellular and extracellular ions.
• Here intracellular Ca+ and pH are regulated

Question 7

Co-transporters

Answer 7

• Carry multiple ions in the same direction
• Here, intracellular Cl- concentrations are regulated.
• They use the fact that ions are being exchanged across the concentration gradient

Question 8

Action potential

Answer 8

• a.k.a. nerve impulse, ‘spike’
  -signal that conveys information over distances
  -occurs in the axon of the cell
  -brief and rapid depolarization of the membrane.
  ->For an instant the inside of the membrane
  becomes positively charged.

Question 9

Phases of the action potential
“all-or-none”

Answer 9

• resting potential
• Rising phase
• Overshoot
• Falling phase
• Undershoot

Question 10

Resting membrane potential:

Answer 10

• Higher concentration of Na+ ions outside the membrane
• Higher concentration of K+ ions inside the membrane
• Equilibrium/ neuron are at rest

Question 11

Rising phase

Answer 11

• inward Na+ current
• ENa=+58 mV

Question 12

What happens when Na+
channels become inactive and K+
channels stay open?

Answer 12

Falling phase= outward K+ current

Question 13

Action potential firing frequency depends on…

Answer 13

• the level of depolarization.
• Maximum firing frequency is about 1000Hz (1000 spikes per second).
• Once an action potential is initiated, it is impossible to start another one for about 1ms.

Question 14

Absolute refractory period:

Answer 14

• the period of time (1ms) when it is impossible to initiate another action potential.
• AP’s are inactive

Question 15

Relative refractory period:

Answer 15

• the period of time (several milliseconds) when it is relatively difficult to initiate another action potential. It takes more current during this time to start another action potential.

Question 16

The specific characteristics of the voltage-gated Na+ channel determine:

Answer 16

1) The threshold of the action potential
2) The absolute refractory period

Question 17

The voltage-gated potassium channel

Answer 17

• The repolarizing of the membrane is partly due to inactivation of Na+ channels.
• But there is also a transient increase in K+ conductance that speeds up repolarization.
• K+ channels are responsible for the relative refractory period

Question 18

Voltage-gated K+ channel:

Answer 18

1) Opened by depolarization
2) Opened after a 1 msec delay – so they are “delayed”
3) Their function is to repolarize or “rectify” the membrane…so they are called “delayed rectifiers”

Question 19

How do voltage-gated Na+ channels and voltage-gated K+ channels differ?

Answer 19

• They are both activated during depolarization of the membrane.
• Na+ channels quickly inactivate.
• K+ channels remain open as long as the membrane is depolarized.

Question 20

Understanding the action potential in terms of permeability and driving force:

Answer 20

• Pk»PNa
• PNa (up)
• PNa»Pk
• PNa (down)
• Pk»PNa