Electrical Signaling Flashcards
(38 cards)
ion channels act as signaling devices
- channels open and close very quickly on millisecond time scale- every time they open 1000s of ions rush downhill down their electrochemical gradient
- results in change in the voltage, charge difference, membrane potential across the cell membrane very quickly and in fairly large amounts
- this serves a signal that can be sensed by other proteins that would then be activated to trigger downstream events like an AP, muscle contraction, and secretion
signaling events mediated by ion channels
-at the synapse, chemical signals are converted to electrical signals –> neurotransmission
-electrical signals that propagate and give rise to additional electrical signals –> this is how AP moves along the length of a nerve or muscle fiber
-when an electrical signal comes to the end of the nerve, it gives rise to chemical signal leading to excitation contraction or excitation secretion coupling
-ion channels can set up spontaneous electrical signals –> happens in pacemaker of heart
what are three aspects of channel function?
- techniques to measure channel activity –> intracellular microelectrode, voltage clamp, patch clamp
- membrane physiology –> excitable membranes of nerve and muscle cells
- structure and function of ion channels
electrical recordings: intracellular microelectrode
-fine glass capillary pipette that is fired, polished at its end, and drawn out with a tiny opening
-fill capillary tube with Ag and AgCl that’s conducting and you can insert this into a cell
-membrane is very hydrophobic- forms at the edges a tight seal with high resistance so any movement of charges is through the electrode and into the cell
measurement of membrane potential
-muscle cell or nerve fiber sitting in a bath
-you have one of these intracellular electrodes that can be inserted into the cell and to measure charge difference between the inside of the cell and bath you have to have a reference electrode sitting in the bath and the two connected by voltmeter
-at the start voltage is zero then once the electrode enters you see sharp downward deflection at V=-60 mV or -90 mV (inside of the cell is more negative relative to the outside) and we say the cell is polarized
-depolarization- any change that causes inside negative potential to become less
-hyperpolarization- anything that makes inside negative potential even more
-electrode withdrawn and you see voltage that’s measure goes back to zero
-if the depolarization crosses over threshold (currents across membrane), cell fires AP –> negative membrane potential quickly goes toward zero then cross beyond zero (overshoot) then it will rapidly return so membrane is polarized again and it goes further down below resting potential to be hyperpolarized and go back up to resting potential
action potential
-if you were to do this over and over again, you would see that for any particular axon or excitable cell, this AP looks he same always
-once AP is fired or in the process of happening, you couldn’t have a second one come along that would add up and give you something that was twice as large or three times larger –> always this defined thing
-membrane in refractory state and would not fire another AP
-if one depolarized these cells, you would have all or none response- either AP fire or nothing happens
-if you stimulated or depolarized cell at one point of the membrane and you measured way down at the other end, you would see that the AP would propagate and it would be unchanged in its size or shape
voltage clamp
-at every point in the AP, lots of things happening as you might guess from underlying curves
-feedback amplifier- as current goes across membrane and voltage is changing, there’s another electrode that injects charges and removes charges just enough to keep the voltage the same
-compensates for what’s happening through the ion channels in the membrane- either way they could fix the membrane at any voltage that they wanted to and measure the currents
voltage clamp of squid axon
-measured currents after clamping at -9 mV
-positive charges going into the cell is negative current and positive charges going out of the cell is positive current
-diluted seawater so the salt [] fell tenfold and turns out that there was no longer a Na gradient
-when the squid axon was sitting in 10% Na, the Na current was gone –> subtract the two curves to get the difference current
-at the start of the AP, Na ions enter squid axon only briefly b/c then it turns off and they stop
-10x more K inside than outside the cell –> figured there was K ions going out but compared to Na, very slowly activated and stays open
patch clamp technique
-applied microelectrode onto patch of cell and gently applied suction
-if they pulled it away, this patch of membrane would tear off and b.c of the hydrophobic properties of the membrane, it will stick tightly and form a good seal
-in this small patch of membrane, likely one or few ion channels isolated
-put in bath and measured current across one or few ion channels in that patch membrane
-if one pulls enough while keeping it attached, you could break through the cell –> whole cell and when you pull away, membrane reseals but in everted form
patch recording of an ion channel
-if recording and seal are good, you could watch the channel open and close for hours and analyze it
-opening and closing is independent property and described only by probability function –> don’t know at any point when this channel is going to open
-channel is either open or closed with nothing in between- once it opens, it’s free for movement of ions then it closes and current stops
-conductance is finite amount- every ion channel has a certain conductance and can go about as fast as the amount and no more
properties of ion channels: ion selectivity
-pore of the channel is charged –> K channel likely has acidic residues like Aspartate and glutamates that would attract the positive ions and help it through but repels Cl- ions
-size of the pore also determines the ions that get through
-charge density and consider hydration shell –> diameters are different so the Li has the highest charge density with all that charge []ed across tiny ions whereas rubidium has the weakest, lowest charge density
-water is a dipole so in the presence of a charge, water will arrange itself around the ion like a coat and this is the hydration shell
-if ion has to be stripped of its water molecules before it can go through pore of a channel then it’ll take energy to do that
-for the K ion going through the K channel, it’s perfectly matched and the hydration shell can be replaced by the walls of the channel and coordination of the negative charges like the Aspartate and the carbonyl groups of the backbone
-ion selectivity is ultimately determined by charge density, which affects hydration shell
conductance
-ease with which an ion can go through channel is the inverse of resistance
-ion channel- conducting device in a circuit
-as you increase the driving force (voltage) across a membrane, ions are pulled by the gradient
conductance example without [] gradient
-membrane with equal number of K ions on both sides –> no [] gradient then one side becomes positive so the ions will go from the more positive to the negative side
-more positive you make it, the more positive current you have and at zero, thee’s nothing so you can reverse the voltage and you see reverse current
-as you add more channels, it’s like adding them in parallel and this is based on Ohm’s law
ohm’s law
g (1/R) = I/E where E is the voltage
conductance example with [] gradient
-pump- battery in this circuit
-at zero voltage, with more K on one side than the other, you have [] gradient and immediately ion channel opens and there’s current
-if there’s a K gradient directed outwards, you see a K current @ intersection
-as the K ions move, the membrane potential starts to develop and eventually it hits zero current but if inside becomes even more negative, the K moves against its [] gradient and you get negative current
conductance is voltage dependent
-kinds of ion channels we need to look at to understand membrane potential their conductance is itself dependent on the membrane potential
-they open and shut in response to voltage
-rectifiers- prefers to pass current in one direction
-you see break in conductance and the slope isn’t constant
-if slope is close to zero, conductance is zero
channels have high conductance
-Ach receptor channel- channel opened when Ach binded to it –> receptor and channel
-ion channel moves ions very fast
gating
-channels opening and closing
-can be one of two ways
-activation gate- something activates channel and the gate opens
-inactivation gate- channel could be activated but channel still closes
-for channel to be conducting ions, both activation and deactivation gates have to be open
-after channel opens, it can spontaneously close b/c inactivation gate is shut and it has to undergo series of conformational changes that are slow before coming back to closed resting state
what are the two types of gating?
- voltage- as membrane potential changes, the channel could be primed to open or shut- bilayer is very thin and field across it is so high b/c i’s so thin and this is large enough to be felt by the protein
- ligand- anything can bind to ion channel- neurotransmitter, small G protein, ATP, Ca, and IP3 –> receptor channels and when ligand binds, they open
what are the three ion channels APs need?
- leak K channels
- Na channels
- delayed rectifier K channels
leak K channels
-property is that it’s open at rest with low conductance and when cell is depolarized, it shuts
-opens when it’s repolarized with high conductance upon hyperpolarization
Na channels
-unlike K channels, closed @ rest and when inside starts to become positive, it opens
-once it’s open it’s committed to closing and will spontaneously inactivate within one to 2 milliseconds of opening
delayed rectifier K channels
-like the Na channel, closed @ rest
-opens upon depolarization but takes some time (tenfold slower to open than Na channels)
time course of AP
-starting at rest, the inside of the cell is more negative since the leak K channels are open so small amount of K always going out then inside becomes a little less negative compared to outside –> if K was the only conductance, this would keep the cell at Ek
-injection of positive charges somewhere else starts to cause membrane potential to get depolarized
-as the membrane depolarizes, leak channel closes and Na channels open –> if Na channels open, positive ions come into the cell and the inside will go towards zero even faster that will depolarize the cell even more and open more Na channels
-threshold- Na ions coming in balance the leak K channels going out
-if at this point more Na ions come in compared to small conductance of K going out, then the Na ions overwhelm the leak and membrane rapidly depolarizes
-causes burst of opening of all the Na channels and peak will correspond to the sharpest portion of the rise
-delayed K opens, K starts going out the clel
-when it gets repolarized, the delayed K shuts and stimulus to activate it (depolarization) is no longer there and membrane potential drifts up
-at rest, leak K channel is open and as the membrane gets depolarized with requirement of an activating or stimulus in neuron, the leak channels start to close and Na channels start to open and beyond threshold, all the Na channels open and the memrbane potential shoots up and becomes positive then the Na channels inactivate and delayed K channels open and membrane potential swings back down and becomes repolarized
