BISC 307L 1st Edition Lecture 4 Current Lecture Electrical Signals Neurons Things to remember o Figure 8 1 General organization of nervous system o Fig 8 2 8 3 8 4 Parts and general types of neurons o Fig 8 5 8 6 Types and functions of glial cells Nature of neuronal signals o Neurons and muscle cells have electrical properties that are used to communicate information generate signals o ALL Living cells generate an electrical potential difference Vm across their plasma membranes membrane potential o Unlike most cells neurons also generate signals consisting of brief changes in Vm membrane potential o Signals make everything possible Two types Responses to a stimulus Lowering of the membrane potential Positive up inside of cell is positive Negative down inside of cell is negative 1 Graded local signals Graded Amplitude varies with the strength of the stimulus Local signal generated in one place on the membrane the amplitude decreases as it moves away from that one place All or non regenerative signals action potential All or none once you give a threshold stimulus then it will be really positive really fast and this is called the action potential All or none because if you give at least a threshold stimulus the action potential will always be the same size Regenerative As it spreads away the amplitude does not decay with distance keeps regenerating itself Vm when one ion is permeable o Where do membrane potentials come from 2 conditions 1 Unequal distribution of an ion across the membrane 2 Plasma membrane must be permeable to the ions rates of movement continue until they are equal and opposite equilibrium o Nernst equation for K Equilibrium potential high concentration inside and low outside diffuses out down its concentration gradient and in down its electrical gradient If neuron membrane will be negative inside if only permeable to K E K 61log K o K i mV o Nernst Equation for Na Equilibrium potential low concentration inside cell and high outside diffuses in down concentration gradient and out down its electrical gradient If neuron membrane will be positive inside if only permeable to Na E Na 61log Na o Na i mV Vm when several ions are permeable o Goldman Hodgkin Katz Eq uation PK K P Na out PConsider out Na Cl What if more than one ion is permeable 3 Na Cl and Cl K inK close to 90mV Na close to 50 or 60mV and Cl is negative Depends on the permeability of each ion P the more permeability the more effect it has 2 Depends on the relative concentrations of each ion Vm 61log P K K out P Na Na out P Cl Cl in P K K in P Na Na in P Cl Cl out mV If the permeability of a specific ion is very low then the term essentially drops out because it wont have much effect on the membrane potential Cell controls membrane potential by varying the permeability of these three ions Something to remember o If the permeability for a particular ion increases the membrane potential will change tending to move toward the equilibrium potential for that ion IMPORTANT Ion leakage and Na K pumps o For a typical cell both K channels and Na channels are open so for the typical cell Neither ion is in equilibrium o If Vm 70 mV neither ion is in equilibrium so Na leaks in K leaks out o Na K pumps counteract ion leakage maintaining gradients and Vm o What is the force making the ion move When not at equilibrium there is a net force the difference between the potential of the membrane and the ions equilibrium potential THE DRIVING FORCE For Na 70 60 130mV inward net For K 70 90 20 mV outward net Vm E ion Driving Force for cations negative is inward and positive is outward To fix this the sodium potassium pump maintains the ionic distribution of ions across the membrane by pumping Na out and pumps K in Requires ATP Neurons have a lot of Na K pumps in their membranes so they need a lot of ATP o Babies need this they are very active and continuously growing so they need a lot of calories to keep up with the ATP need in their brains Phylogeny of V gated ion channels o Voltage gated ion channels o 1 Simplest ion channel 2 transmembrane segments Found in prokaryotes and potassium channel called the inwardly rectifying potassium channel Kir o 2 Addition of 4 more transmembrane segments 6 membrane motif structure most channels today K channels Many different kinds o 3 Voltage gated sodium and calcium channels one single protein that contains 4 subunits Early Ca2 channels diverged into two types by amount of depolarization required to open channel 1 High voltage activated HVA Separated into two types o 1 L type sensitive to set of drugs that block the Ca2 channels dihydropyridine important in heart stop raise in blood pressure o 2 Non L type not sensitive to this drug dihydropyridines 2 Low voltage activated LVA o 4 Sodium Channels most recent many types Voltage gated Na channel Na v o Big alpha subunit with 4 transmembrane subunits o Pore is down middle of each subunit
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