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K+ channels open with a delay, Na+ channels open right awayshows the transient inward caused by the Na+ channels immediately opening, and then gradually closing, while the K+ channels open slightly after the Vm reaches threshold. This delayed K+ channel opening causes the delayed outward current. The capacitive current is an artifact that the membrane is depolarizing to 0 mV (charging the membraneProof that the inward current is due to NaMembrane current reverses to outward if extracellular Na+ is removed (following concentration gradient)TEA- blocks K channels TTX- blocks Na channelsWhen add TEA, no delayed outward current, when add TTX no transient inward current.Increases axial resistance, so that the forward path down the axon is more favorable because it has less resistance. Current flows in the path of least resistance.channels only at nodes of Ranvier (which is why the current travels faster and tends to “jump” along axon)There are usually no voltage gated Na+ channels on the dendritesThat NTs are released in vesiclesMEPPs are spontaneous graded depolarization’s below threshold that result from the spontaneous NT release from vesicles into synapseEnd Plate Potential- (EPP) graded potentials whose amplitude is dependent on the [Ca2+] in the extracellular fluid. EPP is the depolarization of skeletal muscle fibers caused by NT binding to the post synaptic membrane in the neuromuscular junction.EPP’s seem to be made up from the random summation of quantal events.The MEPP’s are isolated, spontaneously occurring, quantal eventsThis all supports a model in which the MEPP’s arise from the occasional random release of a single vesicles containing NT molecules.EPP’s arise from the concerted release of several vesicles at once, from a large pool. The total number released per stimulation being dependent on Ca influx during the action potential and so [Ca]oSmall molecule NTs: Axon TerminalSmall molecule NTs are synthesized in the axon terminal from precursors. The synthetic enzyme are carried down from the cell body by slow axonal transportSmall clear core vesicle in the axon terminal typically contain small molecule NTsRespond to high and low frequency APsPeptide NTs: Cell bodySynthesized in the cell body and transported down the axon in vesiclesDense core vesicles in the axon terminal typically contain peptide NTsVesicles release NTs in response to high frequency APsTypical Chemical synapse1. A.p. arrives2. V-dependent Ca2+ channels open3. Ca2+ promotes NT vesicle fusion4. NT released into Synaptic cleft5. NT binds to NT receptor Proteins – opens channels6. Ions enter Postsynaptic cell- Change Postsynaptic VmExcitatory vs. Inhibitory synapse (based on NT and reversal potential)If Erev is greater than threshold than synapse is excitatory, if less than threshold than synapse is inhibitory.So if the postsynaptic channels are sodium selective, calcium selective or cation non-selective, then Erev will be 0mV or greater and the postsynaptic Vm will rise beyond threshold, to trigger an action potential. These are excitatory synapsesIf the postsynaptic channels are potassium selective, or chloride selective, then Erev will be -60 mV or less and the postsynaptic Vm will be forced below threshold inhibiting action potentials. These are inhibitory synapsesTypical excitatory NTs- glutamate, aspartateTypical inhibitory NTs- GABA, GlycinePatch clamps (measuring current through single channel)a) outside out patch clamp, with the outside solution being the extracellular fluid (high Na, low K) and the inside of the electrode being like the intracellular fluid (low Na, high K). Clamp at resting potential or threshold. Add glutamate, expect to see an inward positive current because it’s excitatory.b) Inside out patch clamp, outside of the electrode is the intracellular fluid with no Ca ions, inside of the clamp will be like the extracellular fluid (high Na, low K). Clamp at Vrest or threshold. Expect to see excitatory inward current until add Ca and then no current because channel closes.Other Patch Clamps:Whole cell Method records summed currents from entire similar to a voltage clampInside-out and outside-out methods record single channel activity in excised patchesThe cell-attached patch is generally used to investigate voltage-gated channelsInside-out patches allow the investigation of channels gated by small intracellular molecules or by intracellular calcium ionsOutside-out patches allow the investigation of channels gated by small extracellular molecules like neurotransmittersUse single electrode to record activity in somatosensory cortex neuronStimulate the center receptive field and observe action potential frequencyStimulate the surround (edge) receptive field and observe action potential frequencyIf the receptive field was on center, off surround, then I would expect to see a higher frequency of action potentials when I stimulate the center receptive field and a lower frequency of action potentials when I stimulate the surround. And when they are both stimulated at the same time, I would expect to see no change in action potentials because the surround inhibitory response will cancel out the center excitatory response.Responses of retinal ganglion cells (r.g.c’s) to lightAll RGC’s display some spontaneous a.p. activity – even in darkness.Many RGC’s adapt to prolonged stimuli (The change in a.p frequency decreases with time after light onset)Each retinal ganglion cell has a “receptive field”The area of the retina (or the equivalent section of visual space) within which light elicits a response from the cell.RGC receptive fields are always circular, and cover about 1 degree of visual space. Our entire perceived visual space is made up from the overlapping receptive fields of a million retinal ganglion cells arrayed across the retina.On center, Off center receptive fieldsTwo types of bipolar cellsOn center, off centerLateral InhibitionHorizontal cells communicate with the photoreceptorsHorizontal cells are involved in lateral inhibition through their communication with neighboring photoreceptors. Horizontal cells make a positive synapse with the incoming bipolar cell and an inhibitory synapse with the neighboring synapse, creating the lateral inhibition. When light hits the retina horizontal cells hyperpolarize, inhibiting neighboring photoreceptors.RGCs can be binocular (receive info from both eyes)RGCs are orientation specific ???RGCs are sensitive to bars not


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UMD BSCI 353 - Exam 1

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