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Membrane Currents and Conductances.docx

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PHGY 313
Sebastien Breau

The Action Potential, In Theory  Depolarization of the cell during the action potential is caused by the influx of sodium ions across the membrane, and repolarization is caused by the efflux of potassium ions.  Membrane Currents and Conductances o See Fig. 4.4. o The membrane of this cell has three types of protein molecules: sodium-potassium pumps, potassium channels and sodium channels. o Begin by assuming that both the potassium channels and the sodium channels are closed and that the membrane potential, V ,mis equal to 0 mV.  Opening the potassium channels only causes the potassium ions to flow out of the cell, down their concentration gradient, until the inside becomes negatively charged and V m E . k  This movement raises three points: 1) The net movement of potassium ions across the membrane is an electrical current 2) The number of open potassium channels is proportional to an electrical conductance 3) Membrane potassium current, I , will flow only as long as V ≠ E . The driving + K m K force on K is defined as the difference between the real membrane potential and the equilibrium potential, and it can be written asMV -KE .  The Ins and Outs of an Action Potentia+ o What’s happening with the Na ions concentrated outside the cell?  The membrane potential is so negative with respect to the sodium equilibrium potential and + there is a driving force on Na  However there can be no net movement of sodium ions as long as the membrane is impermeable to Na + o When the channels are open, however,:  The ion+c permeability of the membrane, g Na is high and there is a large driving force pushing on Na .  Assuming that the membrane permeability is now far greater to sodium than it is to potassium, + this influx of Na depolarizes the neuron until Vmapproaches E , Na mV. o How could we account for the falling phase of the action potential?  Simply assume that sodium channels quickly close and the potassium channels remain open, so the dominant membrane ion permeability switches back from Na to K . The potassium ions would flow out of the cell until the membrane potential again equals EK. The Action Potential, In Reality  When the membrane is depolarized to threshold, there is a transient increase in g .Nahe increase in g Na + allows the entry of Na ions, which depolarizes the neuron.  Restoring the negative membrane potential would be further aided by a transient increase in g dKring the falling phase, allowing K ions to leave the depolarized neuron faster.  The Voltage-Gated Sodium Channel o Sodium Channel Structure  The voltage-gated sodium channel is created from a single long polypeptide  It has four distinct domains (I-IV) each consisting of six transmembrane alpha helices (S1 – S6).  The pore is closed at the negative resting membrane potential.  When the membrane is depolarized to threshold, the molecule twists into a configuration that allows the pass of Na+ through the pore. + +  The sodium channel is 12 times more permeable to Na than to K .  The sodium channel is gated by a change in voltage across the membrane o Functional Properties of the Sodium Channel  Changing the membrane potential from -65 to -40 mV causes these channels to pop open  See Fig. 4.9.  These voltage-gated sodium channels have a characteristic pattern of behaviour: 1) They open with little delay 2) They stay open for about 1 msec and then close (inactivate) 3) They cannot be opened again by depolarization until the membrane potential returns to a negative value near threshold  The fact that single channels do not open until a critical level of membrane depolarization is reached explains the action potential threshold.  The rapid opening of the channels in response to depolarization explains why the rising phase of the action potential occurs so quickly  The short time the channels stay open before inactivating partly explains why the action potential is so brief.  Inactivation of the channels can account for the absolute refractory period  Single amino acid mutations in the extracellular regions of one sodium channel have been shown to cause a common inherited disorder in human infants known as generalized epilepsy with febrile seizures.  Generalized epilepsy with febrile seizures is a channelopathy, a human genetic disease caused by alterations in the structure and function of ion channels. o The Effects of Toxins on the Sodium Chann+l  Tetrodotoxin (TTX) clogs the Na - permeable pore by binding tightly to a specific site on the outside of the channel.  TTX blocks all sodium-dependent action potentials  Voltage-Gated Potassium Channels o There are many different types of voltage-gated potassium channels. o Most of them open when the membrane is depolarized and function to diminish any further depolarization by giving K ions a path to leave the cell across the membrane o The channel proteins consist of four separate polypeptide subunits that come together to form a pore between them  These proteins are sensitive to changes in the electrical field across the membrane o When the membrane is depolarized, the subunits are believed to twist into shape that allows K ions to pass through the pore.  Putting the Pieces Together o The key properties of the action potential can be explained using these terms:  Threshold: membrane potential at which enough vo
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