2 2 Passive Potentials All cells have a resting membrane potential but neurons are able to manipulate this electrical charge The membrane of a neuron has special properties that make it excitable or able to rapidly change its potential Although this property is unusual it is not unique to neurons muscle cells eggs upon fertilization and some plant cells are also excitable Neurons use changes in membrane potential as a signal in electrical form to communicate with each other and other tissues over long distances in a short amount of time This electrical signal passively spreads through some parts of the neuron the dendrites and soma and is actively propagated through other parts the axon At rest the membrane is polarized to approximately 70 mV When Vm changes we refer to those changes relative to this original resting membrane potential Increases in Vm are depolarization decreases in Vm are hyperpolarization and returns to the resting potential are repolarization The neuron can control the voltage of its membrane by controlling its permeability to particular ions Recall that Na is more concentrated outside the cell and its Keq is positive so at 70 mV its electrochemical gradient points into the cell If the neuron increases the permeability of its membrane to Na it will tend to flow into the cell until Vm Keq or permeability is decreased This inward flow of cations makes the inside of the cell more positive depolarizing the membrane Note that more positive is synonymous with less negative On the other hand because K is more concentrated inside the cell and its Keq is approximately 90 mV increasing permeability to K at 70 mV will allow K to flow out of the cell until Vm Keq or permeability is decreased This outward flow of cations makes the inside of the cell less positive or more negative This hyperpolarizes the membrane Opening and closing channels has predictable effects on membrane potential because flow of that particular ion will drive Vm closer to its Keq Note that in the examples above we were dealing with movement of only one ion When the membrane is permeable to multiple ions simultaneously Vm will never go all the way to the Keq of any single ion but it will get closer Mathematically recall that in the GHK equation this is due to increased relative permeability The chemical gradient for that particular ion now contributes to Vm more heavily The neuron is able to control the permeability of its membrane and thus depolarization hyperpolarization etc by opening and closing channels in its membrane More long term control such as altering the resting membrane potential is possible by changing the number of channels present in the membrane by altering expression of the genes that code for the channels There are several different types of ion channels Some are always open referred to as leak channels but others are gated they open and close in response to specific stimuli This gating mechanism allows the neuron to changes its membrane potential in response to changes in its cellular environment By converting external stimuli into an electrical signal the neuron is said to transduce these signals A more intense stimulus will open more gated channels allowing more ions to flow across the membrane which will cause a greater change in membrane potential These graded potentials whose size correspond directly to the stimulus intensity occur along the membrane of the dendrites and soma Let s look at what happens when a ligand gated Na channel opens in the membrane of the soma The chemical to which this channel is specific binds the protein channel This is transduced into an electrical signal the channel opens and Na follows its electrochemical gradient at 70 mV flowing into the cell This makes the inside of the cell more positive depolarizing the membrane The flow of Na or Na current spreads through the cytosol of the cell repelling other positive charges as it goes causing this depolarization to travel along the soma membrane However as you can see in the graph and as is indicated by the color gradient in the cell the change in electrical charge is most intense close to the site of stimulation As you get farther from the channel the current is effectively diluted by the volume of the cell body This is why the dendrites and soma of the cell are said to have passive properties of excitability The membrane in these regions can rapidly change voltage but this change passively spreads throughout the cell The electrical signal is limited by distance It will also decay over time as the channel closes and the membrane returns to its resting state via leak channels and Na K pumps Gated channels are found all along the membrane of the dendrites and soma This creates a large surface area over which the neuron can receive stimuli and transduce them into electrical signals As multiple channels are opened all over the membrane conducting ions into or out of the cell their small effects on Vm add up in a process called spatial summation The cumulative effect of the gated channels in the dendrites and soma is seen at the axon hillock where the axon is attached to the soma In the neuron below the membrane has two different types of ligand gated channels one that conducts Na and one that conducts K If they are both exposed to their respective ligands they will both open The resulting inward Na current will depolarize a small area of the membrane The outward K current will hyperpolarize a small area of the membrane Both of these local changes in charge will spread passively through the cytosol summing at the membrane around the axon hillock In this example the two currents are equal in magnitude but in opposite directions so the net effect of opening both channels is zero Electrical signals can sum over space they can also sum over time a process called temporal summation Normally a gated channel opens only briefly either because the stimulus stops or in the case of a ligand gated channel the ligand falls off After each of these changes in Vm the leak channels and Na K pump restore the resting membrane potential However if the neuron is stimulated multiple times in rapid succession the membrane does not have sufficient time to return to its resting potential The graded potentials will spread through the soma and sum at the axon hillock just as in spatial summation The passive properties of the dendrite and soma membrane are insufficient to send electrical signals quickly over long distances The axon
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