Class Notes (839,150)
Canada (511,218)
Psychology (3,977)
PSYC 4750 (80)
Lecture

Action Potential.docx

4 Pages
49 Views

Department
Psychology
Course Code
PSYC 4750
Professor
Linda Hunter

This preview shows page 1. Sign up to view the full 4 pages of the document.
Description
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
More Less
Unlock Document

Only page 1 are available for preview. Some parts have been intentionally blurred.

Unlock Document
You're Reading a Preview

Unlock to view full version

Unlock Document

Log In


OR

Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


OR

By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.


Submit