Ion channels

  • Created by: lridgeway
  • Created on: 11-11-20 10:31

Patch clamping

Attach a very sensitive electrode to a patch of membrane, and measure the current across the membrane keeping the voltage clamped. This process can be used to measure channels as you only get a current if a channel is open. Observations need to be seen using a microscope. 

In order to this this first you need to place a pipette tip on the membrane surface and this forms a tight seal over the end of the pipette.

If you pull from here you get inside-out patch clamping, where the internal membrane surface is exposed and you can change things on teh internal surface to see how it is affected. 

If you push on the pipette here the whole cell is sealed onto the pipette not just a patch of membrane, allowing you to view the channels in the whole cell. 

If you push on the pipette and then pull you get outside-out patch clamping, where the membrane inverts on the pull so the external surface is exposed and you can view things like ligand gated channels. 

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Results of patch clamping

You get a current reading with levels at fixed positions.  The lowest is when there is no current and therefore no channels open. The next level will be when there is one channel open and then the next is when there is two etc. These fixed position levels show that all channels have the same current when open. 

Opening an dclosing of channels is essentially random. The avergae opening and closing rates are specific to the channel and dtermine how long the channel is open for. 

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Voltage gated Na+ channels

Voltage gated so the opening and closing of the channel is determined by voltage flowing across the membrane. Normal resting potential of the cell is -60mV (closed channel) and the channel begins to open at -40mV and the maxium ion flow (channel fully open) when the potential is 0mV or positive. 

The sodium channel also has a 'plug' which closes the channel after 1ms and the detaches after the potential has returned to normal. 

Process of opening and closing 

  • Initial depolarization, movement of the voltage sensor up opening the channel. 
  • Plug insertion and return of the voltage sensor to resting potential inactiviating the channel. 
  • Repolarization of the membrane, displacement of 'plug' and closure of gate. 
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Voltage gated K+ channels

  • Selective for potassium 
  • Similar to Na+ channel, closed when membrane potential is negative and opens when potentiall becomes less negative
  • Slower than Na+ channels so can be called a delayed K+ channel
  • Also has a plug
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Other channels

Na+/K+ pump 

ATP-dependent pump which pumps 3 Na out of and 2 K into the cell to maintain the intra and extraceullar concentrations of both. 

Resting K+ channels (aka leak channels) 

Open slightly even when cell is at 'rest' allowing K+ to constantly leak out. This makes the membrane potential negative because there is a K+ gradient across the membrane and ions are moving to try to equalise those concentrations. Due to their charge they leave a negative charge inside the membrane and a positive one is created outside the membrane. 

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