1Nerve SignalsElectrical – within a neuronChemical – between neuronsElectrical – within a neuronAll cell membranes havean electrical potential.Cell membranes havean electrical potential.Why?Differences between ionconcentrations inside and outside thecell.Membrane Potential• Inside cell– Mostly potassium (K+)– Some Sodium (Na+)– Lots of impermeable macromolecules with a negativecharge (-), sulfates (-), & phosphates (-).• Outside Cell– Mostly Na+– Less K+– Lots Chloride (Cl-), but not important to nerve signalsIon concentrations and2Ions cannot dissolve in lipids.Cell membrane is made of lipids.How do ions move across the cellmembrane?Un-gated Ion ChannelsIon PumpsGated Ion ChannelsUn-gated Ion Channels Passive - DiffusionIon PumpsGated Ion ChannelsUn-gated Ion ChannelsIon PumpsGated Ion ChannelsActive - Change shapeUn-gated Ion ChannelsIon PumpsGated Ion ChannelsProteins embedded inthe cell membraneDiffusion of Ionsfollows both achemicalandelectricalgradient.3Un-gated Ion Channels - Diffusion Selectively permeable: K+ > Na+ Diffusion of Ionsleft unregulated wouldlead to anequilibrium concentration ofions that resulted in a lowermembrane potential.How is this condition regulated?Sodium-potassium pumpUses energy to pump ionsagainst their chemical andelectrical gradient.Sodium-potassium Pump – uses ATP Fig. 45.4If we could force more sodium ionsinto the neuron what would happento the membrane potential?1. It would become more negative.2. It would force the membrane toequilibrium.3. The membrane potential would becomemore positive.4. With enough sodium we could make“Neuron jerky”.4Changes in Electric PotentialAcross MembraneorHow to fire a neuronNote: Increasing voltages can be either –or + . Voltage increases as it moves in either (+or -) direction from 0.Diffusion of Ionsleft unregulated wouldlead to anequilibrium concentration ofions that resulted in a lowermembrane potential.How is this condition regulated?Sodium-potassium pumpUses energy to pump ionsagainst their chemical andelectrical gradient.Sodium-potassium Pump – uses ATP Pattern and ProcessAction potential & Movement ofions across cell membraneAll cells have a membrane potential.What so special about neurons?They can modulate it!Voltage-gated ion channels5Potentialin/out balancedPotential inside(+)}Potentialinside(-)}Potentialchanges over timeHyperpolarizationPotential insidebecomes morenegative• Hyperpolarization– Gated K+ channelsopen → K+ diffuses outof the cell → themembrane potentialbecomes more negative.6DepolarizationPotential insidebecomes lessnegative.Depolarization– Gated Na+ channelsopen → Na+ diffusesinto the cell → themembrane potentialbecomes less negative.The Action Potential:All or NothingDepolarization– If graded potentialssum to ≈-55mV athreshold potential isachieved.• This triggers anaction potential.–Axons only.What causes the change inmembrane potential associatedwith an action potential?A change in concentration ofions inside the cell.What controls the changes inmembrane potential associated withan action potential?Voltage-gated Ion ChannelsVoltage-gated Ion Channels• Na+– A Fast opening gate– A slow closing gate• K+– A slow opening gate7Voltage-gated Ion Channels& DiffusionOnce channels open, ion followchemical and electrical gradients.Stages of Action Potential1) Resting State2) Threshold3) Depolarization Phase of Action Potential4) Repolarization Phase of Action Potential5) Undershoot6) Back to Resting StateAction Potential – Voltage-gated Ion ChannelsSerin is a nerve gas that disables theslow acting voltage-gated K+ channels.How would this toxin effect neuronfunction?1. It would cause the Na+ levels to increase inside the cell and prevent theneuron from depolarizing.2. K+ levels would not decrease inside the cell and neuron would be remaindepolarized.3. The K+ would decrease inside the cell and the neuron would repolarize.4. The Na+ levels would remain the same inside the cell and the cell