Active Transporters/ Action Potential Flashcards
Active transporters:
- Maintain ionic concentration gradients
-Translocate ions against their concentration gradients
-Bind and unbind ions
-Slow (several millisecond process)
The Na+/K+ pump
accounts for 20-40% of the…
- 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
Molecular structure of the Na+/K+ pump
- There is no pore
- Pump, not an ion channel
- Pump ions against their concentration gradients
Disruption of the function of the Na+/K+ pump at any point in the cycle can disrupt the efflux of Na+
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
Examples of ion exchangers
- Use electrochemical gradients of co-transported ions as a source of energy
Antiporters
- exchange intracellular and extracellular ions.
- Here intracellular Ca+ and pH are regulated
Co-transporters
- 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
Action potential
- 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.
Phases of the action potential
“all-or-none”
- resting potential
- Rising phase
- Overshoot
- Falling phase
- Undershoot
Resting membrane potential:
- Higher concentration of Na+ ions outside the membrane
- Higher concentration of K+ ions inside the membrane
- Equilibrium/ neuron are at rest
Rising phase
- inward Na+ current
- ENa=+58 mV
What happens when Na+
channels become inactive and K+
channels stay open?
Falling phase= outward K+ current
Action potential firing frequency depends on…
- 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.
Absolute refractory period:
- the period of time (1ms) when it is impossible to initiate another action potential.
- AP’s are inactive
Relative refractory period:
- 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.
The specific characteristics of the voltage-gated Na+ channel determine:
1) The threshold of the action potential
2) The absolute refractory period
The voltage-gated potassium channel
- 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
Voltage-gated K+ channel:
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”
How do voltage-gated Na+ channels and voltage-gated K+ channels differ?
- They are both activated during depolarization of the membrane.
- Na+ channels quickly inactivate.
- K+ channels remain open as long as the membrane is depolarized.
Understanding the action potential in terms of permeability and driving force:
- Pk»PNa
- PNa (up)
- PNa»Pk
- PNa (down)
- Pk»PNa
