Class Notes (839,394)
Canada (511,324)
Psychology (2,794)
PS374 (16)


4 Pages

Course Code
Irene Cheung

This preview shows page 1. Sign up to view the full 4 pages of the document.
NROB60 Chapter 4 Introduction  The frequency and pattern of action potentials constitute the code used by neurons to transfer information from one location to another. Properties of the Action Potential  The Ups and Downs of an Action Potential o See Fig. 4.1. o The first part, called the rising phase, is characterized by a rapid depolarization of the membrane. This change in membrane potential continues until Vmreaches a peak value of about 40 mV o The apart of the action potential where the inside of the neuron is positively charged with respect to the outside is called the overshoot. o The falling phase of the action potential is a rapid repolarization until the membrane is actually more negative than the resting potential o The last part of the falling phased is called the undershoot (after-hyperpolarization). o Finally, there is a gradual restoration of the resting potential o The action potential lasts about 2 milliseconds (msecs).  The Generation of an Action Potential (all-or-none) o Consider you stepping on a thumbtack o The initial chain of events is therefore: 1) The thumbtack enters skin 2) The membrane of the nerve fibers in the skin is stretched 3) Na - permeable channels open. a. Because of the large concentration gradient and the negative charge of the cytosol, Na ions enter the fiber through these channels + b. The entry of Na depolarizes the membrane (i.e. the cytosolic surface of the membrane becomes negative) c. If this depolarization (generator potential) achieves a critical level, the membrane will generate an action potential. The critical level of depolarization that must be crossed in order to trigger an action potential is called threshold. o The depolarization that causes action potentials arises in different ways in different neurons  The Generation of Multiple Action Potentials o The rate of action potential generation depends on the magnitude of the continuous depolarizing current o The firing frequency of action potentials reflects the magnitude of the depolarizing current  This is one way that stimulation intensity is encoded in the nervous system o There is a limit to the rate at which a neuron can generate action potentials  The maximum firing frequency is about 1000 Hz  Once an action potential is initiated, it is impossible to initiate antoehr for about 1 msec.  This period is called the absolute refractory period.  It can be relatively difficult to initiate another action potential for several milliseconds after the end of the absolute refractory period  During the relative refractory period, the amount of current required to depolarize the neuron to action potential threshold is elevated above normal 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 m is 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 =mE . 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 , wKll flow only as long as V ≠ m . TKe driving force on K is defined as the difference between the real membrane potential and the equilibrium potential, and it can be written as M - EK.  The Ins and Outs of an Action Potential + 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 ionic 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 E . K 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 m+mbrane potential would be further aided by a transient increase in g duKing 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  Si
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


Join OneClass

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

Sign up

Join to view


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.