Introductory Neurobiology: Week 2 Notes
Introductory Neurobiology: Week 2 Notes Biol 3640
Popular in Introductory Neurobiology
Popular in Biology
This 10 page Class Notes was uploaded by lucy allen on Friday January 15, 2016. The Class Notes belongs to Biol 3640 at University of Denver taught by Dr. John C Kinnamon in Fall 2016. Since its upload, it has received 79 views. For similar materials see Introductory Neurobiology in Biology at University of Denver.
Reviews for Introductory Neurobiology: Week 2 Notes
Great notes!!! Thanks so much for doing this...
Report this Material
What is Karma?
Karma is the currency of StudySoup.
You can buy or earn more Karma at anytime and redeem it for class notes, study guides, flashcards, and more!
Date Created: 01/15/16
The Membrane Potential Part I -basic electricity -wires/pipes are analogous to axons -pump and battery supply energy -constrictions in the axon is represented by resistors in pipes -electrical potential (water pressure) -voltage is a measure of the electrical 'pressure' or potential difference that is the energy source for current flow -Vo-3s (v or V or E or e) -10 Volts = 1 millivolt (1mv) -10 Volts = 1 microvolt (1μv) -electrical current -coulombs/sec or Amperes (A) 18 -1 coulomb is equal to the charge carried by 6.24x10 electrons -in electrical circuits or equations, current is usually designated by I or i -current flows from + to - by convention -current shows direction of flow of positive charge -hydraulic analogy: gallons per hour of water flowing through circuit -resistance -factor that determines the relation between voltage and current -resistance (R or r) is measured in ohms (Ω) -the inverse of resistance is conductance (g) g = I/R -hydraulic analogy: diameter of water pipe -in neurons: ion channels -capacitance -the ability of a circuit to store charge C = Q/V -for our purposes, phospholipid bilayer is the insulator and the two conductive plates are the ion-containing solutions o either side of the membrane -capacitors: circuit elements, typically made from two parallel conductive plates separated by an insulator -capacitance introduces a time element -myelination overcomes the reduction in capacitance when the 'plates' are separated by a large distance -symbols in electric circuits: see slide 14 -battery, batteries in series, switch, capacitor, ammeter, voltmeter, resistor, resistors in series, resistors in parallel, ground, amplifier -Ohm's Law V = IR -in hydraulic system: water will choose easiest path for travel, so it does not travel through the 'resistor' area (the constriction of pipe), rather it goes to the capacitor -once charged the capacitor releases the current and forces it to flow through the more difficult path -the resting potential -negative charge with respect to outside of neuron at rest -between -65 and -70 millivolts -established by the unequal distribution of ions on two sides of the membrane -selective permeability of the cell is brought about my ion channels -nerve cell membrane -outside -low potassium -high sodium -inside -high potassium -low sodium -potassium -big player in resting membrane potential -sodium -big player in action potential -sodium-potassium pump -before ATP binds, sodium ions in cytoplasm fit into pump, ATP binds and changes the conformation of the pump, removing the 3 sodium ions and adding two potassium ions -get potassium inside, sodium outside -important for resting membrane potential and action potential -nerve cell capacitance: phospholipid bilayer -comes about as a result of the phospholipid bilayer -extracellular BLANK and the axoplasm are the two plates -dielectric: phospholipid bilayer -myelination separates the plates and makes the capacitance lower -makes the spread of the potential go faster because the capacitors don't have to be charged as long -nerve cell resistance: ion channels -electrical signals in the nervous system are generated by the movement of ions across the nerve cell membrane -these ionic currents flow through pores formed by membrane proteins (ion channels) -ion channels -protein made of subunits -subunits can be the same or different -determines the nomenclature -heteropentamer: five subunits, not the same -homohexamer: five identical subunits -leak channels: always open Channel Selectivity -open and closed channels -always open or always closed -many more of these for potassium and chlorine than for sodium -so at rest, more potassium and chlorine are moving than sodium -how are they moving? -protein repels chlorine so chlorine moves out -potassium is in higher concentration on the inside ,they diffuse out -always open and responsible for permeability when membrane is at rest -specific for one type of ion although not absolute -modes of activation -gated ion channels -open and close because of some stimulus, change the permeability of the cell membrane when they open -voltage-activated (voltage-gated) ion channel -open or close in response to small voltage changes across the cell membrane -voltage-gated sodium channel is responsible for the action potential -at rest: closed -outside of cell is positive with respect to inside -then depolarize the membrane in the vicinity of the ion channel makes the inside of the membrane positive with respect to the outside -causes opening of the ion channel for a very short time -most common are sodium potassium and calcium -common on areas where action potentials develop -axons of unipolar and multipolar neurons -sarcolemma (including T-tubules) of skeletal muscle fibers and cardiac muscle fibers -stretch-activated ion channels -sensory neurons in skin have ion channels that are opened by the mechanical pressure on the skin when touched -sodium comes in and depolarizes the cell, then we feel the pressure on our skin -ligand-activated ion channels -extracellular activation: ligand binds to a receptor on the extracellular face of the membrane -intracellular activation: ligand binds to a receptor on the cytoplasmic face of the membrane Measurement of Single-Channel Currents -the patch clamp technique -got the Nobel Prize in 1991, Neher and Sakmann -measuring current in picoamps (10 -12Amps) -start with a glass micropipette with a small fire-polished (smooth) tip -goal is to attach to the cell membrane without puncture -touch the cell and apply light suction with the pipette (cell- attached patch) -electrode records from a small patch of membrane -removing the electrode will remove a patch of membrane -'inside-out patch' -inside of the membrane is now facing outside -gave the ability to look at second messenger pathways (receptors) and receptors for ion channels which are intracellularly activated -channel conductance -passage of ions though ion channel -if the ion channel is open and there are no ions flowing through it, there is permeability -if there are ions present and they are flowing through the ion channel then there is conductance Significance of Ion Channels -act like resistors in parallel in the nerve cell membrane -exam review Wednesday 1/20/16 5pm-5:45pm in Olin 105 or 205 -exam will be multiple choice, short answer, fill in the blank, true/false and short essay The Origin and Significance of Neuronal Signals The Resting Potential -typically negative in a neuron, about -65mV -due to unequal distribution of ions -resting potential determined via microelectrode penetration of the cell Concentration and Electrical Gradients -opposite charges attract, like charges repel -slide 8 -sodium chloride, cathode (-) and anode (+) -chloride anions migrate towards the anode -sodium cations migrate towards negative cathode -slide 9 -if biological membrane separates the electrodes, the ions are unable to pass through the membrane, impermeable -leak channels make it possible for the ions to pass down their gradient -slide 10 -equal distribution of ions throughout the water without electrode present -at surface of the membrane there is a concentration of positively or negatively charged ions, naturally -attracted to each other -slide 11 -introduction of channels allows movement of ions -eventually equilibrium is reached -slide 12 -introduction of a potassium leak channel allows potassium to flow down its concentration gradient -for each potassium ion through the channel, a positive charge leaves -this eventually sets up an electrical gradient -makes the outside of the membrane positively charged with respect to the inside -eventually for every potassium ion out, another will come in due to the repelling nature of the electrical gradient on the outside (positive repels positive) -like a capacitor -slide 14 -equal amounts of positive and negative ion concentrations (anion/cations), electrode will read zero meaning zero membrane potential -slide 15/16 -introduce a leak potassium channel -potassium flows out of the cell, down its concentration gradient -takes positive charges with it, leaving negative 'holes' behind -after a while, same thing as before occurs + -for every K that leave+ down its concentration gradient, another K will enter going down its electrical gradient -when we reach this equilibrium we record the membrane potential, known as the equilibrium potential -every ion has an equilibrium potential -dependent on membrane potential at point of equilibrium -slide 18 -which has the greater effect, the concentration gradient or the electrical gradient? -electrical gradient! see slides 19 and 20 -slide 19 -water column analogy for the concentration gradient -one molar solution of sucrose will support a column of water 900 feet tall -slide 20 -a 1L box containing 1 mole of electrons would take the weight of 1/6 Earth's weight to hold the lid on the box -slide 21 -at equilibrium, is the actual number of ions that has moved across the membrane significant? -no! see slide 22 -slide 22 -takes 1/10 millionth of the total number of potassium ions available to set off the equilibrium potential The Nernst Equation -how large a potential is necessary to balance the concentration difference across the membrane at equilibrium -for calculating the equilibrium potential for a single ion EK= (RT)/(zF)(ln([K] o[K] in -R: thermodynamic gas constant -T: absolute temperature -Z: valence of the ion -F: Faraday (number of coulombs of charge in 1 mole of a monovalent ion) -simplified form below EK= 61.54mv(log([K] /[Ko ))in -two constants here -use either 58 mv (for room temperature)* -or 61.54 mv (for body temperature)* -add a negative to the answer for the Nernst equation with regards to chloride due to the negative charge of the ion Example: + [K ]o= 5 mM [K ]in 100 mM + + -so if ([K ]o/[K ]in = 1/20 -the log(1/20) = -1.3 -then E = (61.54 mV)(-1.3) = -80mV K -potassium is a BIG player in this! Example: + [Na ] o 150 mM [Na ] in15 mM -so if =150/15 -the log(150/15) = +1 -then E =(61.54mV)(1) = 61.54 mV Na -sodium is not a big player in this -because of the permeability of the leak channel -the permeability is 0.025* -reduced permeability is represented by a constricted ion channel in slide 32 -permeability to Chloride ions is 0.45* -permeability of potassium ions is 1.0* The Goldman Equation -for calculating the membrane potential when there are multiple ions involved -also known as the constant field equation + + - + + V m(58mV)(log((P [K ]K)+ (o [Na ]Na + (P oCl ] )Cl((P in ])+ KP [Ni ] ) Na in + (P Cll ] o)) -see below -still use 61.54mV if body temperature initial conditions -negative V mmans the cell is depolarized -during an action potential the permeability of sodium increases to 3 -the ion concentrations remain the same, change the permeability -membrane potential is now totally depolarized, positive! The Equivalent Circuit -nerve cell membrane -phospholipid bilayer is the capacitor -the ion channel is represented by conductance Time Constant -membrane resistance and membrane capacitance determine the time course of an electrotonic signal (a passive spread of depolarization) -recorded voltage along the axon decreases until it is basically gone, due to leak channels along the axon -current is consistent as ions add to capacitor, but the voltage increases -takes time to charge the capacitors τ=R m m -time following the application of a constant current pulse at which the membrane reaches 63% of its final value (1-(1/e)) -want low capacitance for shorter tao (τ), meaning faster conduction velocity -myelination accomplishes this -don't forget, the closer the plates are, the higher the capacitance, and the lower the conduction velocity -not all neurons have myelination, these small neurons don't need to be big or have a fast conduction velocity so it is ok that they are unmyelinated Length Constant -graded potentials (passive spread of a depolarization) decrease during spatial conduction -steady decrease in measured voltage as current travels along the axon, reaches zero -internal resistance of the axoplasm (cytoplasm of axon), offering resistance to passage of ions -leak channels along the way also contribute to this, leaking current -λ: distance at which the membrane voltage decreases to 37% of its initial value, due to internal resistance and leak channels λ=sqrt(r mr)i -rm=membrane resistance -increase r by having fewer leak channels, will increase λ m -riinternal resistance -decrease riby increasing the diameter of the neuron, will increase λ Neurons Generate and Conduct Electrical Signals Local Potentials/Graded Potentials -graded: of varying intensity; NOT all the same intensity -changes in membrane potential taht cannot spread far from site of stimulation -can result in depolarization or hyperpolarization -depolarization -opening Na channels allows more positive charges to enter thereby making the interior less negative (-70mV --> -60mV) -RMP shifts towards 0mV -hyperpolarization + -opening of K channels allows more positive charges to leave thereby making the interior more negative (-70 --> -80mV) -RMP shifts away from 0mV -repolarization -process of restoring membrane potential back to normal (RMP) -degree of depolarization decreases with distance from stimulation site; called decremental spread -graded potentials occur on dendrites and cell bodies of neurons but also on gland cells, sensory receptors and muscle cell sarcolemma -affect only a tiny area (maybe only 1 mm in diameter) -if so, how do neurons trigger release of neurotransmitter far from dendrites/cell body? -graded potentials passive spread along the axon, decreasing with distance and time due to the effects of capacitance, membrane capacitance, membrane resistance and internal resistance -we don't see action potentials on cell bodies or dendrites, on axons Sources of Ionic Signals -generator potentials -EPSP -excitatory post-synaptic potential -almost always depolarizing -only special circumstances allow for them to be hyperpolarizing -presynaptic cell releases a neurotransmitter onto the post- synaptic cell, opening some ligand-gated ion channels -sodium ions come in and depolarize the cell -takes a spiking neuron towards the threshold, if it reaches it there will be an action potential generated -IPSP -inhibitory post-synaptic potential -takes the neuron away from the threshold -always hyperpolarizing (reducing likelihood of action potential) -receptor potentials -sensory receptors will generate small depolarizations which are termed receptor potentials Characteristics of Sources of These Potentials -graded in amplitude (analog) -spread passively -can be distorted due to the nature of the membrane -membrane resistance and capacitance determine the time course of an electrotonic signal Electrical Wire vs. A Neuron -conductance/resistance are much greater in neurons vs. copper wires -electrons vs. ions -in an axon it is the flow of ions, in a wire it is the flow of electrons -current flow -membrane of a neuron is a poor insulator and leaky to ions -neurons have high internal resistance -difficult for ions to travel through axoplasm -neurons have high capacitance -specifically unmyelinated neurons Neuronal Integration -based on passive membrane properties -a neuron takes in lots of different kinds of information -typical neuron receives synaptic input from 1,000 to 10,000 other neurons -integrates these inputs, makes a decision about whether or not to generate an action potential or whether or not to release neurotransmitter -temporal summation: refers to the integration of inputs coming in one input path but separated in time -if tao is short, temporal summation will not occur -tao must be long -get a response and it takes a long time to go down, before it goes down it repolarizes and get the effect of the second postsynaptic current, to the third, etc. -spatial summation -two presynaptic neurons, one postsynaptic neuron, receiving input simultaneously -passive spread of potential from each neuron -if lambda is large, there will be a large potential at the buteon -two potentials will sum, increasing the probability of action potential propagation or neurotransmitter release *professor said you MUST know these facts for the first exam
Are you sure you want to buy this material for
You're already Subscribed!
Looks like you've already subscribed to StudySoup, you won't need to purchase another subscription to get this material. To access this material simply click 'View Full Document'