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Lecture

Lecture 1 - Neurobiology BIO271.pdf

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Department
Biology
Course
BIO271H1
Professor
Melanie Woodin
Semester
Winter

Description
Lecture 1 - Neurobiology January 7, 2014 5:51 PM BIO271 Page 1 BIO271 Page 2 BIO271 Page 3 BIO271 Page 4 BIO271 Page 5 BIO271 Page 6 BIO271 Page 7 BIO271 Page 8 2 kinds of nervous systems: peripheral and central - Central NS: brain and spinal cord - Peripheral NS: nerves that innervate tissues - Purkinje neuron: shows how complex neurons can be - Back of brain is the cerebellum important for coordinated motor output - Can see a dendritic arbor = very complex - Many cells have a membrane potential which is the difference b/w the inside and the outside of the cell - Excitable cells can rapidly change their membrane potential and a classic example is an action potential (action potential = how the neuron transfers info) depolarizes When - charges comes in --> hyperpolarize BIO271 Page 16 - Need to remember which ions are most concentrated on the inside of the cell vs the outside. - Outside it’s cold and your nose starts dripping…what does it taste like? Salt Nacl. o Na and Cl are most concentrated outside the cell in the extracellular fluid (mucus/blood) o K is high inside of the cell o Ca is not permeable and doesn’t play a role in generating resting mem pot or action potentials! So we leave that aside. - Permeability: if you have a mem a neuron… ions don’t cross easily they need to cross thru a channel so proteins need to carry them …it can’t normally diffuse across Clarification: - Spines are protrusions it’s located on the dendrite…each of the branches are covered with little spines which are the dendritic spines… - Axon hillock is where the action pot is generated and where the axon comes off of the cell body… it’s called the signal integration zone - Spike initiation zone: this is the region where you are most likely to generate an action pot …also called maclonic site? - We can equate the spike initiation zone with the axon hillock and these are where action pot or spikes are generated in the signal integration zone BIO271 Page 17 - We can determine membrane potential experimentally. - Also through this equation. - We need to verify what we had experimentally computationally. - Using the information we know to be true to create a model. This equation is taking into account the 3 major ions that can play a role in maintaining the mem pot… K, Na, Cl - Need to know the conc on the inside and the outside - You know it’s in a solution b/c when you do the experiment you always make up artificial cerebral spinal fluid.. you make up the salt solution that mimics whats in your…there’s a large shelf with all the salts in the lab and you weigh out exactly what the conc of the ions is outside of the cell - How do you know what the conc is on the inside? You may have used that solution and may have perfused it on the inside of the cell. If not use from the textbook. - We also need to know the permeabilty. - There’s a large conc gradient for cl. we already know that b/c cl comes from salt from the outside and there’s almost no permeability to cl when the mem pot is at rest - If you use this equation to calc the resting mem pot the value is going to be close to 0 and you would disregard this component of the equation. This equation takes into account the [] gradients, the permeabilties, for the major ions that are permeable which are Na and K. Equation is not asked to be used in the exam. Allows us to calculate the resting mem pot. - What makes the mem permeable are ion channels and receptors. these are proteins in the mem called transmem proteins which allow ions to cross. - The depolarization and hyperpolarization happens b/c the ions flow thru the mem and it’s the opening of the channels which cause the ions to be permeable - You can have 100-1000s of Na ion channels but they all are closed. And you have no permeabilty for Na. - Ca IS NEVER IMPORTANT IN THE RESTING MEM POT (no permeability to ca) - Ion channels are sometimes called leak channels because they are open all the time… ions just pass cus it’s open all the time - Majority of channels are not open all the time. They open and close = and called gated - These are gated by number of things. Gated by sensory neurons, chemicals etc. when at rets these are clsoed. When the potential changes they open. - Most of these channels are non-selective…they are particular for certain type of ions or a combination of ions - Ions only pass thru the channels if there’s a gradient and that’s called electrochemical gradient - There are 2 things make up the gradient : electrical and chemical and these 2 factors together determines whether the ions will move thru that channel BIO271 Page 18 Channels normally allows specific ions to move down. Ion channels bins because neuotrnamistters binds to it and cause it to open. Cationic channels: lets Na K ions to pass Generally it is a combination of ions to pass or for a specific ion. Ions travels through channels through gradients. The gradient is made from - Electrical gradient - Chemical gradeint These two factors determines if the ion moves or not. - Mem pot existed at -70 b/c when the mem pot is at rest there are BIO271 Page 19 - Mem pot existed at -70 b/c when the mem pot is at rest there are specific ion channels that are open that allow some ions to pass but if we get a drastic change to the permeability of a particular ion we are going to see that will allow certain ions to flow across and change the mem pot - Mem pot is often gonna go from the equilib value to another value - A lot more Na and Cl outside the cell vs the inside - Relative permeabilty: When the mem is at rest how permeable is the mem to that particular ion? The relative part is important… - K is 30 times more permeable than Na - Na and K both are more permeable than Cl - K is largely responsible for setting the resting mem pot - Why is the inside largely –ve? (-70mV)… it’s b/c there are a lot of negatively charged proteins inside the cell which contributes to the mem potential but they don’t change anything in excitabilty and it doesn’t have to do anything with the depolarization hyperpolarization… this change happens b/c of Na, cl, and K ions - The change happens because of Nacl and K. Relative permeability: When the neurons are rest, how permeable is the membrane for that ion. Relative part is how important these ions are for the rest. Mem pot. K is 30x more permeable than Na. this is usually more permeable for Cl. Na and K hace [] gradients, so they could play a role in the resting mem pot. They are only going to if the mem is permeable for that ion. K is largely responsible for setting the resting mem pot. When the neuron is at rest they have leaky channels for K, and it flows down the [] gradeint. And forms the mem pot at the value of -70mV. What is the inside of the cell at -70mV? That is ebcayse there is lots of negatiively large proteins and they don’t change. They contribute to the mem pot. But - If we look at the inside of the cell vs the outside, we are going to have a BIO271 Page 20 - If we look at the inside of the cell vs the outside, we are going to have a lot more K on the inside and a lot more Nacl on the outside. But you don’t have a permeabilty for them. - Even if you have a huge conc gradient but if you don’t have permeability then you can’t contribute to the resting mem pot - We have to let the ions flow - If you open a K channel.. it wants to go down it’s conc grad… but it’s not just the conc grad.. electrical grad is also important - When positive charge leaves the cell the positive charge builds up on the outside close to the mem. - The mem acts can store charge/ balance charge on either side …positive charges leave the cell and build up on the outside and that’s going to leave excess –ve charge on the inside… so positive charge is still leaving down its conc grad and that +ve charge encounters a build up of +ve charges on the outside so like charges repel eachother… so it repels those positive ions and it goes back…so K establishes an electrical grad which then pushes it back..so the conc grad builds up an electrical grad which repels it back in the opp direc - The outward grad eventually equals the inward grad - So there’s a balance point b/w each individual ion where its conc grad is balanced out by its electrical grad - So remember that we can’t just take the conc grad into account we also need to take the electrical grad into account - Each individual ion has it’s own equilib potential which is the value of the mem pot at which the conc grad is equally balanced by its electrical grad - Equilibrium potential is the value of the mem pot at which that ion will be at equilb - The resting mem pot is made up of k, cl, na and each of those individual ions like to sit at the equilib pot and ions want to be at equalib without the gradient. - So each ion drives its grad’s into the equlib pot - Equlib pot for Na = +60 mV - Equlib pot for K, cl is very negative = below -70 mV = -90 mV - At rest mem is largely permeable to K - If you change the mem pot rapidly you can change the permeability of an ion - As soon as you switch the relative permeability in favour of Na then the mem is gonna rapidly depolarize and go towards the positive value - Individual ions can still come back and forth but there’s no net movement BIO271 Page 21 - So use the Goldman's equation if you wanna calc the resting mem potential - If you want to one ions equilb pot use the Nernst equation. -
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