Membrane Potentials PDF Lecture Notes

5.1 Ions Electrochemical Signaling -​ Neurons use a combination of electrical and chemical signals to communicate Ions -​ Ions are elements or molecules which are charged -​ Positive (+) -​ Negative (-) -​ The movement of these ions is how neurons change their v...

5.1 Ions Electrochemical Signaling -​ Neurons use a combination of electrical and chemical signals to communicate Ions -​ Ions are elements or molecules which are charged -​ Positive (+) -​ Negative (-) -​ The movement of these ions is how neurons change their voltage/electrical potential Major Ions for Neuron Signaling -​ Sodium: Na+ -​ Potassium: K+ -​ Chloride: Cl- -​ Calcium: Ca2+ Ion Concentrations -​ There is not an equal concentration of ions inside and outside of the cell -​ This imbalance is called a concentration gradient -​ These concentration gradients are the MAJOR driving force for electrical signaling -​ Easiest way to remember this is: -​ Neurons are like “Bananas in the sea” -​ Containers of K+ -​ In solution of Salt (Na+ & Cl-) -​ Ca2+ is there too 5.2 Ion Channels Membranes -​ Cellular membranes do not allow ions to move in and out of the cell -​ Ions need to depend on specialized proteins called “channels” to pass across the membrane Ion Channels -​ Channels are proteins that are folded into a spiral that leaves a pore in the middle for ions to travel through -​ Electrical signaling in neurons is primarily controlled by the opening and closing of these channels -​ Channels are specific for a given ion -​ Na+ can only pass through Na+ channels -​ While K+ will not be able to pass through a Na+ channel -​ With some seldom exceptions Ion Channel Types -​ There are three major types of ion channels we will talk about -​ Leak -​ Ligand-Gated -​ Voltage-Gated Leak Channels -​ Ion channels that are always open -​ Extremely important for maintaining the membrane potential (voltage of the cell) -​ Keeps the electrical environment constant during rest periods Ligand-Gated Channels -​ Gated channel -​ Closed by default -​ ‘When closed, will not allow ions to flow in or out -​ Channel has ligand binding site on the extracellular (outside) side of the protein -​ Ligand: chemical signal that binds to a binding site of a channel/receptor -​ Ex. neurotransmitters, hormones, axon guidance cues, etc. -​ When a ligand binds to a ligand-gated channel, the protein will shift around, causing the channel to open -​ Ions will flow into/out through the open channel -​ The ligand acts as a key for the channel -​ Each channel has its own specific ligand, different ligands will not activate the cell -​ With exceptions -​ Ligand-gated channels are extremely important for the synaptic signaling in the dendrites Voltage-Gated Channels -​ Like ligand-gated channels, voltage-gated (V-gated) channels are closed in their default position -​ There are opened when activated -​ But do not depend on an external ligand -​ Has a specialized segment of the channel protein on the intracellular (inside) side of the neuron which is a voltage sensor -​ When the cell becomes positive enough, the voltage sensor will detect it, causing the channel to open 5.3 Membrane Potentials Membrane Potentials -​ All neurons are charged -​ Due to the imbalance of positive and negative ions in the cell -​ This electrical charge is called the membrane potential -​ Aka, the cell’s voltage -​ The resting membrane potential (Em) is the default voltage that neurons will always return to overtime -​ Maintained by leak channels -​ A neuron’s Em is typically negative (~ -70 mV) -​ The membrane potential will change as ions flow in and out of the cell -​ Dependent on the opening and closing of channels -​ Ions cannot move across the membrane on their own -​ The ions will always move down their concentration gradient -​ From high concentration to low concentration -​ That movement of ions will change the electrical potential (voltage) because those moving ions are charged (+ or -) Na+ Movement -​ Na+ has a very high extracellular (outside of cell) concentration and a very low intracellular (inside of cell) concentration -​ When Na+ channels open, Na+ will move down its concentration gradient -​ Thus, flowing into the cell Na+ Effect on Membrane Potential -​ Na+ has a positive charge -​ As it flows into the cell, it will cause the cell to become more positive -​ The cell becoming more positive is called: -​ Depolarization -​ Excitatory Postsynaptic Potential (EPSP) Depolarization -​ A cell becoming more positive is called depolarization -​ The membrane potential is getting closer to 0mV -​ “De” = undo -​ “Polar” = far away/opposite EPSP -​ Excitatory Postsynaptic Potential (EPSP) is when a cell becomes more positive -​ Called “postsynaptic potential” because changes in membrane potential is typically recorded from a cell that was stimulated at synapse Na+ Summary & Terminology in Context -​ When Na+ channels open, the cell depolarizes because the positive Na+ ions flow into the cell, thus causing an EPSP EPSP - Time by Voltage Plot -​ Membrane potential is often graphed on a time by voltage plot -​ X-axis = time (msec) -​ Y-axis = voltage (mV) EPSP Plot -​ EPSPs will temporarily make the cell slightly less negative -​ Will return to Em (~ - 70mV) because of constantly open leak channels -​ This allows the cell to maintain a baseline when not stimulated Cl- Movement -​ Cl- has a greater extracellular concentration than its intracellular concentration, so Cl- will flow into the cell -​ Cl- ions are negative, so the influx of Cl- will cause the cell to become more negative Cl- Movement -​ When a cell becomes more negative, it is called: -​ Hyperpolarization -​ “Hyper” = exceed, go past -​ “Polar” = Far away, opposite -​ Inhibitary Postsynaptic Potential (IPSP) IPSP Plot -​ IPSPs will temporarily make the cell slightly more negative -​ Just like EPSP, membrane potential will always return to the Em (~ -70mV) K+ Movement -​ K+ is a positive ion, but the cell becomes more negative when K+ channels open -​ When K+ channels open, the ions move down its concentration gradient -​ High intracellular -​ Low extracellular -​ The flux of positive ions out of the cell causes the cell to hyperpolarize  -​ (+) charges are leaving, so the cell becomes more (-) For Next Lecture -​ Even though Na+ and K+ are both positive ions, they have inverse effects on the membrane -​ Due to concentration gradients -​ Na+ = Depolarize -​ K+ = Hyperpolarize Ca2+ -​ Ca2+ is the funny exception for all these ions -​ Ca2+ flows down its concentration gradient as expected, but it is a weak gradient -​ Very little Ca2+ flows into the cell so it does not depolarize much (when compared to other ions) -​ While it does not change the membrane potential much, Ca2+ is the most influential ions for neurons -​ Ca2+ is the only ion we will cover that activates proteins to do functions -​ Ex: NT release, plasticity, muscle contraction, gene expression, etc. 5.4 ESPS Summation Excitatory Synapses -​ Excitatory axons (aka axons that cause EPSPs) typically form synapses on the dendrites of cells -​ EPSPs cause the dendrites to depolarize (become more positive) EPSP Traveling -​ The positive charges will not stay in the dendrites -​ Positive charges will diffuse through the cell -​ Travel from the dendrite, through the cell body, to the axon hillock Action Potential Threshold -​ If the axon hillock becomes positive enough, the neuron will fire an action potential (AP) -​ The amount of positivity needed to fire an AP is called an AP Threshold EPSP Traveling -​ A single EPSP is a very small change in membrane potential -​ EPSPs will alow slightly decay as it travels to the axon hillock -​ A single EPSP will most likely not reach the axon hillock with enough charge to cause an AP -​ This prevents neurons from being overly sensitive to spontaneous firing -​ Biological systems are prone for random activity -​ Neurons depend on summating multiple EPSPs to fire an AP EPSP Summation -​ There are two forms of EPSP summation -​ Temporal (Time) -​ Spatial (Space, area) Temporal Summation -​ Two (or more) EPSPs on the same dendrite that happen in quick succession -​ The EPSP will summate their depolarization and travel to the axon hillock -​ The following EPSPs will add their positivity to first EPSP  -​ Can only happen when EPSPs are close in time -​ Membrane potential will return to Em if there is too much time between EPSPs Spatial Summation -​ Two (or more) EPSPs that happen at the same time at multiple dendrites -​ The EPSPs from one dendrite will summate on the EPSP from another dendrite EPSP Summation -​ Temporal and spatial summation are not mutually exclusive -​ Both are used in tandem to make the neuron positive enough to cause an AP -​ Gives neurons the means to integrate information from very active axons and/or multiple axons