LE 48-13_5Resting potentialThresholdMembrane potential(mV)ActionpotentialTime–100–50+500PotassiumchannelExtracellular fluidPlasma membraneNa+Resting stateInactivationgateActivationgatesSodiumchannelK+CytosolNa+DepolarizationK+Na+Na+Rising phase of the action potentialK+Na+Na+Falling phase of the action potentialK+Na+Na+UndershootK+Na+• After the depolarization of an action potential,repolarization occurs due to the– A. closing of sodium activation and inactivationgates.– B. opening of sodium activation gates.– C. refractory period in which the membrane ishyperpolarized.– D. delay in the action of the sodium-potassiumpump.– E. opening of voltage-gated potassium channels andthe closing of sodium channels.LE 48-5DendritesCell bodyNucleusAxon hillockAxonSignaldirectionPresynaptic cellMyelin sheathSynapticterminalsSynapsePostsynaptic cellConcept 48.4: Neuronscommunicate with other cells atsynapses• In an electrical synapse, current flows directly fromone cell to another via a gap junction• The vast majority of synapses are chemicalsynapses• In a chemical synapse, a presynaptic neuronreleases chemical neurotransmitters stored in thesynaptic terminalLE 48-16PostsynapticneuronSynapticterminalsof pre-synapticneurons5 µm• When an action potential reaches aterminal, the final result is release ofneurotransmitters into the synaptic cleftAnimation: SynapseAnimation: SynapseLE 48-17Postsynaptic cellPresynaptic cellSynaptic vesiclescontainingneurotransmitterPresynaptic membraneVoltage-gatedCa2+ channelCa2+Postsynaptic membranePostsynaptic membraneNeuro-transmitterLigand-gatedion channelNa+K+Ligand-gatedion channelsSynaptic cleftDirect Synaptic Transmission• Direct synaptic transmission involves bindingof neurotransmitters to ligand-gated ionchannels• Neurotransmitter binding causes ion channelsto open, generating a postsynaptic potential• Postsynaptic potentials fall into twocategories:– Excitatory postsynaptic potentials (EPSPs)– Inhibitory postsynaptic potentials (IPSPs)• After release, the neurotransmitterdiffuses out of the synaptic cleft• It may be taken up by surrounding cellsand degraded by enzymesSummation of PostsynapticPotentials• Unlike action potentials, postsynapticpotentials are graded and do notregenerate• Most neurons have many synapses ontheir dendrites and cell body• A single EPSP is usually too small totrigger an action potential in apostsynaptic neuronLE 48-18Postsynaptic neuronTerminal branchof presynaptic neuronE1E1AxonhillockE1E2E1IActionpotentialE1E1 + E2Spatial summationof EPSP and IPSPSpatial summationIE1 + IActionpotentialE1Temporal summationE1Threshold of axon ofpostsynaptic neuronE1Subthreshold, nosummationE1RestingpotentialMembrane potential (mV)–7006. Neurotransmitters categorized as inhibitorywould not be expected to– A. bind to receptors.– B. open K+ channels.– C. open Na+ channels.– D. open Cl- channels.– E. hyperpolarize the membrane.• If two EPSPs are produced in rapidsuccession, an effect called temporalsummation occurs• In spatial summation, EPSPs produced nearlysimultaneously by different synapses on thesame postsynaptic neuron add together• Through summation, an IPSP cancounter the effect of an EPSPIndirect SynapticTransmission• In indirect synaptic transmission, aneurotransmitter binds to a receptor that isnot part of an ion channel• This binding activates a signal transductionpathway involving a second messenger inthe postsynaptic cell• Effects of indirect synaptic transmissionhave a slower onset but last longerNeurotransmitters• The same neurotransmitter can producedifferent effects in different types of
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