2 9 15 Lecture 12 Membrane Potential Figure 6 8 Measure of voltage gradient o Difference in voltage between the inside and the outside of the cell o Vm voltage across membrane Number that equals our membrane potential Cell is more negative on the inside than the outside at rest most cell s Vm 70mV o Due to all charged components o Vm E of all ions o E equilibrium potential Voltage at which elec and conc Gradients for a component are balanced Distribution of a particular ion across a membrane if the ion is allowed to diffuse due to its concentration and electircal gradient Example Vm E Na E K E Cl E Ca Equilibrium potential changes whenever you alter the number of Alter plasma membrane permeability components o Figure 6 15 o Permeability increases ions move change in Vm graded potential When we change our membrane potential Vm we call it a graded The molecule diffuse through remember diffusion has a problem potential with distance This change in our Vm affects voltage gated channels Graded potentials are localized o local change in ion flow Graded Potentials GPs Figure 6 14 Terms changes from rest aka polarized o Rest equates to steady state which is 70mV cells at rest are polarized o Not all graded potentials are depolarizations o Depolarization decrease the negativity of the cell this is a stimulatory stimulus When we depolarize we stimulate the cell Need positive feedback to open the sodium channels and move away from the membrane potential to 0mV Go from 70mV to 0mV o Hyperpolarize increase the negativity of the cell inhibitory stimulus Go from 70mV to an even more negative number In action potentials the cell always hyperpolarizes o Repolarization to rest 2 9 15 Returning to polarization trying to get back to resting polarization Negative feedback drives repolarization o What if graded potential occurs near v gated channels o Figure 6 16 Characteristics o Large stimulus large size so signal The size of the stimulus is going to dictate the size of the response in the cell Stimulus size gets matched with electrical result Large stimulus results in a larger change in the membrane potential o Increase distance decrease size so signal As we move further away from the stimulus the signal diminishes more and more creating an inverse relationship between the distance and size of the signal Travel thru gap junctions Big problem Example our pinky is relatively far from our elbow so are we less likely to receive the stimulus What if graded potentials occur near voltage gated channels o We get an action potential Action Potential In excitable cells graded potentials can cause o Quick large changes in Vm o Figure 6 19 o All or nothing event Like a gun being fired it either happens or doesn t and the bullet comes out at the exact same speed Either we get enough pressure to get the bullet out or we don t We don t have different levels of action potential There is a big depolarization and it comes back down A graded potential triggers an action potential o Components needed What makes it an excitable cell vs a non excitable cell Voltage gated Na channels with inactivator Voltage gated K channels Figure 6 18 Events of an Action Potential Answering the question how do we get this pattern and no other pattern Figure 6 19 1 Membrane at rest 70mV o Voltage gated channels are closed at rest More sodium on outside more potassium on inside o If we somehow stimulate this cell and cause a graded potential 2 9 15 2 Local depolarization reaches threshold potential TP opens up the voltage gated Na channels o Analogy to walking up to a door that opens automatically when you get close enough 3 Na influx more depolarization positive feedback 4 Na channels inactivated K channels open o They are inactivated NOT closed but sodium movement is still open o Ex you open the garage door but something is in there so you can t get your car through o The Na channel is blocked from the cytosol side o The inactivator is a part of the protein that is the ion channel o Potassium channel is slow to open and close Na channel is quick to open and close 5 K outflux cell repolarizes Na channels close 6 Cells hyperpolarize since K is slow to close 7 K channels close negative feedback return to rest Helpful video https www youtube com watch v zLueMYsNMDg Threshold Potentials TP Figure 6 21 o The membrane potential Vm necessary to activate our Na channels o Stimulus the generated by Na influx o Weak depolarizaions subthreshold potentials
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