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Biological Sciences
Kenneth Lam

Lec7 Vm=-92, whie lEk=-58 Review the goldman equation o Combine all the ions together Concentration and permeability o In frog, Pk is extremely larger than Pna Why?cuase the leak channel of K+ is open in resting axonmain reason of the Vm ( leak channel only refers to the K+) Pcl can be ignored m, since almost impermeable o Compare with Nernst equation N only gives you Ek , not thinking about permeability ,only concentration Its linear While goldman, takes into account multiple ions Not linear More close to observed values Take into account f permeability CAP o Mini stimulasu intensity The intensity necessary to trigger at least one or a few axon to fire AP Max stim: max # of axons fire max CAP TM: The Hodgkin cycle o Positive feedback loo Current applied from outside ( electrode stimulus on the neuron), to trigger just a few Na+ channel to open, but not enough to trigger a all-or non-AP Then more Na+ channel( voltage-gated Na+ cchannel, activated by depolarization open due to depolarization ) Lead to more depolarization o So its the voltage gated ion (Na+) channel, that lead to AP happening Compare the Na+ and K+ channel o Na+ rapidly activated by depolarization, but K+ is slower o Na+ become inactivated even if Vm is depolarized(leading to refractory period) , but K+ become inactivated slowly, and not completely if even Vm remains depolarized, (leading to the hyperpolarization) Equilibrium potential of the membtrnae potential o Def: when no net movement of any specific ion o Look t Na+ : Start with Vm resting at -70 concentration gradient of Na+ always want Na+ to flow in ---so always inward of Na+ while the electrical gradient changes direction of ion movement (Na+) as Vm changes: Ena is +58, and Vm at resting is -70 so when you are starting from resting state, you want to move more + ions(Na+) inward to drag the Vm up to +58 once your Vm pass +58(wrong! Should be 0, since a voltage of 0 means the outside and inside charge are the same, and hence no electrical gradient/difference) , from their onward, your electrical gradient flip the direction of ions to outward , but still smaller than the force of concentration gradient once you reach Ena, the two gradient becomes equal o look at K+: start with Vm at near Ena+(+58), but for K+ wants to drag it down to its Ek=-90 so concentration gradient is always out, since K+ inside is larger than outside so electrical gradient FIRST is out( K+ positive ions outward), and then no at Vm=0, and then flip to in after becomes negative o Na+/K+ pump Its effect is 3Na+ outward,with 2K+ inward, so a net negative charge inside the cell, hencemaintain a negative Vm Not directly cause the repolarization Though it is activated by AP phenomenon(Na+ inward and K+ outward due to AP), but needs many AP before the pumping activity becomes noticeable o What do you see in refractory period?exctitability of the AP/Na+ voltage gated channels are reduced This is talking about CAP, as you look at a population of Na?K+ channels Threshold of AP is higher Because not all the Na+ voltage gated channels are active during refractory period, some of them are deactivated(rapid closing of voltage gated Na+ channel) recall the comparision between Na+ and K+ channels Ie , not enough synchronization of the opening Na+ channels o This can also be resulted from a slowly stimulation Ie, the population ( each individual Na+ V gated channel) varies wrt to open and inactivation time Hodgkin cycle may not at all, or is harder to get started higher threshold of AP o Length constant(lambda) Distance Over which Vm decays by 63% So the larger, the faster the conduction velocity Squid giant to increase rm(numerator) Myelination to decrease rl(total longtitudinal resistance) High memebrane resistance + reduce membrane capacitance Lead to a very high threshold for AP whre myelin is present So only NOR can generate APsaltatory conduction o ELECTRICAL SYNAPSE SIGNAL IN CELL B looks like in cell A graded potential If its depolarization above threshold, then magnitude of AP is smaller in B, but still above threshold Rectifying ES Due to one of the two connexon is only functional to respond to depolarization to open so one way only o Chemical synapse short answer-to compare Fast: ligand gated channels cleft is small NT is small vesicles releaed at specialized active zone NT recycling de: NT deactivated and removed from synapse o where the NT is endocytosed in the axon terminus ( ie the presynase) eg: Ach bind to bicotinic synapseslead to post-synapstic depolarization , GABA o desensitization of the nicotinic channel normal: Ach bind->channel open, note open to both Na= and K+, so no difference between Na+ and K+unlike the ones seen in single cell (pre0synapse) but desensitized: if Ach binds, then channel will close, and stay close hence needs the Acetylchoniesterase to break down Ach in the cleft and recycle into the axonal terminal slow NT is larger Not released at specialized active site Indirect post-synapse action o The G protein channel Direcly or indirectly(2 messenger) cuase ion channel conductance ( the ionic conductance of ions , Ina+ ad Ik+, by changing the permeability g ion) Channel open=increase in permeabiilyty Can cause longer term effect of the ion channel conductance to ion moeveent Can cause amplification of the effect NT recycling o Where is the endocytosis of the NT In the soma need intracellular trafficking o EMF for an ion: The driving force for the movement of an ion Vm-Eion o Ionic conductance for an ion I=g*EMF G=1/R, R is the inveser of resistance G is the permeability of the membrane to this ion when theion wanna move across the membrane Because at resting state, membrane is not permeable to Na+, but leaky to K+, so Ionic conductance for K+ is larger than Ina+ restting state. Hence at resting state, a tendency for K+ to flow out Thats why the Vm resting is closer to Ek, but as depolarzaition happens a bit, it will reverse, and Vm will get closer to Ena o This reversal is caused by Ach acting on thenicotinic post channels o Opposite to the GABA fast chemical synapse o Reversal potential for nicotinic channel When the I for a pari of ions ( IK=-Ina+) is Equal Opposite Beucase gk and gNa+ is the same ( nicotinic channel is the same permeable to both ions) Erev for nicotinic channel=1/2(EK+ + Ena+)=-16V o RICT the four functional neural zones Reception(dendrites and soma)(, integration ( at axon hillock)(signal converted to AP) , conduction , transmission (at the terminus) Why integration point is not at the soma, but the AH ( axon hillock)? Cause the threshold to generate AP is lower at AH o Due to a denser population of voltaged gated Na+ channels at AH than soma
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