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Janelle Leboutillier

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NROB60 - LEC06 key SYMBOLS RMP - resting membrane potential MP - membrane potential AP - action potentials VG - voltage-gated ANNOUNCEMENTS - lecture test is NOT cumulative - final exam >- 4,5,6, all of 7 and appendix - keep e-mail questions v.brief if close to examinations - developmental section is excluded from C7 [?] What page range is this? - some figures will be excluded (see slides) [1] LECTURE OVERVIEW OUTLINE OF LECTURE 1. BRIEF REVIEW 2. THE AP - In Theory - In Reality - do NOT need to know Box 4.2 (The Patch-Clamp Method) [2] BRIEF REVIEW FIRST 1. What is RP (resting membrane potential) the consequence of? a) difference in [ion]'s inside vs. outside of neuron b) membrane's semipermeable nature 2. Membrane mainly permeable to K+ ions - RP is in range of -60 to -70mV when (eq) is reached b/ween: a) diffusion P forcing K+ out of cell, and b) electrostatic P forcing K+ into cell => that is why its called "potassium membrane" at rest - inside of memb is primarily [K+], as a result of it being mainly perm. to K+ - v.close to (eq) pot. of K+ at rest 3. Na/K pump (sodium potassium pump) used to main ion balance b/c there is gradual leakage of ions across membrane - what maintains pot. is using sodium-potassium pumps - some K+ channels leaky (not a total seal), so need to constantly maintaining this balance [3] PROPERTIES OF ACTION POTENTIAL Now we are looking at: what happens when RP is altered, to the extent that an action potential may fire? THE UPS AND DOWNS OF AN ACTION POTENTIAL - oscilloscope is used to visualize the AP - 4 main phases of AP - RISING - OVERSHOOT - MP goes above 0 (becomes +ve) - FALLING - MP dropping back to RP - UNDERSHOOT - MP falling below RMP Questions we are going to be answering - going to look at what happens at each particular phase of AP - what is going on wrt ion channels? - as potential is changing, what is changing on membrane? ANIMATION OF AP BY VOLTAGE-GATED Na CHANNEL - To measure elec current, one technique is to insert 2 electrodes into neuron - one for recording MP - other to inject current, or divert current away => can either depolarize (make cell more +ve) or hyperpolarize (make cell more -ve) - elec. current gen'ed has to be large enough to gen. AP - hyperpolarization: drives MP in -ve dir. - makes MP more and more -ve - voltage response dir. prop to magnitude of current - ie. the extent of alteration in voltage is prop to amt of current affecting it How is AP gen'ed? - if stimulus is sufficient to push it (MP) further than firing threshold, then AP is gen'ed - depolarization of memb past threshold sets changes in memb conductances, which ultimately leads to gen'ion of AP NOTE: [!] AP is an all-or-none event - amplitude doesn't change with size of stimulus - once AP gen'ed, it doesn't vary when related to size of stimulus by its amplitude and duration => will prod. similar amplitude and duration, regardless of size (aka str.) of stimulus, or mag. of current incoming RESTING PHASE (aka "steady state") - only "leaky" K channels open - are constantly in open state RISING PHASE - incr'ing +vity of memb. pot by opening of more and more Na+ channels - Na+ are entering into neuron, going down their []grad - this inward Na+ current is depolarizing the membranal voltage (ie. making it more +ve) - Na+ conductance >> K+ conductance OVERSHOOT PHASE - when MP goes beyond 0mV - when its +ve MP, two processes are occurring: 1. Na-VG channels start closing => sodium conductance starts to decline 2. K-VG channels start opening - these K+ channels are different from the leaky K+ channels, b/c unlike leaky, they are normally closed at RP - open in response to depolarization (more particularly, at overshoot) FALLING PHASE - MP rapidly returning to RMP - activation of K-VG channels is at maximum - # of Na-VG channels opened is dramatically reduced UNDERSHOOT PHASE - AP repolarizes BEYOND RMP of voltage (ie. it went lower than it shouldve) - hyperpolarization undershot (went below RMP) - some K-VG channels still open, s.t. total K+ conductance of neuron is greater than when it was in resting state RECOVERY PHASE -= memb returns to original steady state resting pot, this occurs as K-VG channels close - memb. pot is determined by other channels normally open at resting pot (leaky)
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