3 2 Action Potentials and Cable Properties Passive graded potentials are initiated at the dendrites and soma when ligand and mechanically gated ion channels are opened These small potentials travel via electrotonic conduction through the cytosol down the neuron membrane and can sum at the axon hillock Due to the resistance of the cytosol to current flow and loss of signal through leak channels these passive potentials decay over space and time Although they spread incredibly rapidly they are typically able to travel only 2 3 mm before they die out For many neurons this is insufficient to send electrical signals down the entire length of the axon to the axon terminal where the neuron contacts its target For example your sciatic nerve which runs from the base of your spine to your foot has an axon that is about a meter long So how do electrical signals travel through the entire axon Recall that in the axon we have leak channels but no ligand or mechanically gated channels 1 Increase the diameter of the axon increases 2 Insulate the axon increases 3 Actively propagate the signal down the membrane 1 When the diameter of the axon increases both Rm and Ri decrease For a given amount of current it will be harder to push the charges through a smaller space As you increase the space for the current to flow through there is less resistance As axon diameter increases so do the surface area of the membrane and the volume of the axon so they impede the flow of charge less However these values decrease at different rates The surface area of the membrane changes in the form of 2 r while the volume of the axon changes as r2 thus changes in diameter 2r will change surface area slower than volume This corresponds to changing Rm slower than Ri So as these both decrease the denominator drops away faster resulting in a larger ratio Thus as axon diameter increases so does the length constant Although Rm decreases with greater axon diameter often does not As the surface area of the membrane increases so does its capacitance Cm These two changes often balance out thus an increase in axon diameter often has no effect on 2 If the axon is insulated this will also increase Rm If ion channels are blocked by a layer of insulation the membrane will be more resistant to current flowing across it This will keep more current running down the membrane increasing Some neurons have myelinated axons which have specialized nerve cells called Schwann cells wrapped around them These are not neurons they are cells of the nervous system with highly fatty outgrowths myelin that act as insulation By insulating the membrane they prevent leakage of ion currents and increase the distance a signal can travel before it decays Although Rm increases with myelination again often does not Recall that a capacitor like the phospholipid bilayer accumulates charge and can store it and as the plates of a capacitor get larger it can store more charge The capacitance of a capacitor is also affected by the insulating layer that separates the plates If the layer is thicker the two sides of the capacitor cannot interact as well thus its capacitance is decreased This is similar to the effect of myelinating the axon membrane By making the insulating layer within the capacitor thicker the capacitor is less efficient at holding charge Thus myelination decreases Cm However because it also increases Rm as explained above these two changes often balance out thus myelination often has no effect on Sections 3 2 3 3 from UT Health Science Center s online Neuroscience textbook http neuroscience uth tmc edu s1 chapter03 html http neuroscience uth tmc edu s1 chapter03 html are a helpful review of the time constant length constant and effects of myelination on signal conduction 3 Finally a neuron can increase its ability to send electrical signals by actively propagating them down its axon Recall from lesson 2 2 Passive Potentials that graded potentials can sum up over time and space at the axon hillock If these sum to depolarize the membrane at the axon hillock to 55 mV an action potential will be initiated This is an explosive brief depolarization in the membrane that travels down the axon It occurs only if the membrane at the axon hillock is depolarized from 70 mV to 55 mV which is the threshold potential Action potentials are due to a different kind of gated channel that is present only in the membrane of the axon voltage gated channels Instead of opening or closing in response to ligands binding or mechanical disturbance these channels open in response to particular changes in the electrical gradient Vm Let s stick an electrode into a point on the axon and measure what happens over time during an action potential We ll start with the membrane at rest 70 mV Graded potentials initiated at the dendrites and soma sum to 55 mV at the axon hillock This voltage causes voltage gated Na channels in the axon to open just locally close to the axon hillock Recall that Na is more concentrated outside the cell so its chemical gradient is pointed inward At a negative voltage the electrical gradient for Na is also pointed inward When these channels open Na will passively flow down its electrochemical gradient into the cell causing the bit of membrane just past the axon hillock to depolarize further Voltage gated Na channels like all membrane channels are proteins The amino acids that comprise the gate for this channel are positively charged Under resting conditions these amino acids are attracted to the negative charge of the membrane However upon depolarization to threshold this ionic interaction becomes too weak and the gate opens conducting an inward Na current All voltage gated Na channels in the local membrane close to the axon hillock will open in response to depolarizing to threshold resulting in an all or none response The action potential is not graded Due to the nature of voltage gated channels it occurs either fully or not at all As Na continues to flow inward and the membrane continues to depolarize Vm eventually becomes positive At this point the direction of the electrical gradient on Na reverses but is not sufficient to counteract the strong chemical gradient However it grows stronger getting closer to balancing out the inward chemical gradient as Vm approaches the Keq for Na An action potential will continue until the membrane has depolarized to 30 mV About 1 2 msec after they open voltage gated Na channels will start to inactivate Voltage gated Na
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