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BIOL 3020 (2)
Chapter 4

Chapter 4 Introduction

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BIOL 3020
Scott Brandon

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
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