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University of Toronto Scarborough
Biological Sciences
Jason Brown

SLIDE 2Neurons have 4 major components dendrites cell body axon presynaptic terminalsFunction of the dendrites is to collect electrical signals from other cells or the environment in generalOther neurons can communicate with the dendrites or sensory cells can communicate with the neuron via dendrites It can also be dendrites with a freeend that interact with chemicals in the environment example a pain neuron in the skin that interacts with chemicals like prostoglandins which produce an inflammation responseThe electrical signals get integrated at the cell body A cell with 4 different dendrites carries 4 different signals and the information it carries gets integrated at the cell bodyIntegration is important because every neuron can either send a signal or not send a signalSo you have information coming in and the neuron either sends a signal as a response or it doesnt theres no such thing as a weak or strong signalThe signal is sent through the cells axon Neurons have a single axon which is the region of the neuron which conducts any electrical signal thats sent down its length to the presynaptic terminalsAt the presynaptic terminals the signals are transmitted to another cell example a neuron a muscle or adrenal glandsThere are 2 basic kinds of potentials that neurons can useoGraded potential restricted to the cell body and dendritesoAction potential the signal occurs at the length of the axon and the presynaptic terminalsSLIDE 3The nature of electrical signal is such that they result from the membrane being polarized theres a charge separation across the plasma membrane of a neuron and that membrane potential can vary considerablyHow is there a membrane potential to begin with and how do you modify the membrane potentialThe extracellular fluid ECF of all animals is rich in sodium and chloride This is because they evolved in seawaterIntracellular fluids ICF are poor in sodium and chloride of most animals Theres lots of potassium and proteins in the ICFSLIDE 4Why isnt the ICF the same composition as the ECF If it was the same composition as the ECF you wouldnt have to expend energy to maintain difference in concentration so why not have a sodium and chloride rich ICFThe answer is summed up by the Double Donnan EffectOn the left you have ICF on the right you have ECF which could be interstitial fluid or hemolymphIf we start with the ICF and ECF both being rich in sodium and chloride we see a problemThe problem is that theres something present in ICF which arent present in the ECF at similar concentrations and those things are proteinsMost proteins under physiological pH have a net negative charge So we have a negatively charged thing in the ICF but its not present at nearly the same concentration if present at all in the ECFThe second important point is that the proteins that are present in the ICF are not membrane permeable and are trapped in the ICFThis leads to donnan equilibrium which says if you have a charged particle trapped on one side of the membrane its going to effect the distribution of all the other charged particles that can permeate the membraneThis means that because the negatively charged proteins are present only in the ICF the chloride is excluded from the inside of the cell due to repelling from the proteins At the same time sodium and potassium are attracted into the cellThe distribution of the ions thus shifts because of the presence of the impermeable negatively charged speciesThe mathamatics of the Double Donnan Effect tells us that because the distribution of these ions occur in such a way that the repulsion of chloride and attraction of sodium and potassium leads to a situation where the total number of solutes in the ICF becomes much greater than the number of solutes in the ECF meaning you now have an osmotic gradient set upRemember animals try to keep the osmotic gradient between ECF and ICF to prevent movement of water but due to the redistribution of ions due to the negatively charged proteins water is drawn into the cell
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