BILD 2: Multicellular LifeLECTURE #3[Website: http://www.biology.ucsd.edu/classes/bild2.WI11.1]Instructor: Darwin K. [email protected] 48-7 Modeling a mammalian neuronInnerchamberOuterchamber–90 mV140 mM5 mMKCIKCIK+Cl–Potassiumchannel(a) Membrane selectively permeable to K+(b) Membrane selectively permeable to Na++62 mV15 mMNaCICl–150 mMNaCINa+SodiumchannelEK= 62 mV(log5 mM140 mM)= –90 mVENa = 62 mV(log150 mM15 mM= +62 mV)Figure 48-9 Graded potentials and an action potential in aneuron+500–50–100+500–50–100+500–50–100Time (msec)Time (msec)Time (msec)0 1 2 3 4 50 1 2 3 4 501 234 56ThresholdThresholdThresholdRestingpotentialRestingpotentialRestingpotentialHyperpolarizationsDepolarizationsMembrane potential (mV)Membrane potential (mV)Membrane potential (mV)StimuliStimuliStronger depolarizing stimulusActionpotential(a) Graded hyperpolarizations produced by two stimuli that increase membrane permeability to K+. The larger stimulus producesa larger hyperpolarization.(b) Graded depolarizations produced by two stimuli that increase membrane permeability to Na+.The larger stimulus produces alarger depolarization.(c) Action potential triggered by a depolarization that reaches the threshold.KeyNa+K++50ActionpotentialThreshold01451–50Resting potentialMembrane potential(mV)–100TimeExtracellular fluidPlasmamembraneCytosolInactivation loopResting stateSodiumchannelPotassiumchannelDepolarizationRising phase of the action potentialFalling phase of the action potential5Undershoot232134Fig. 48-10: The role of voltage-gated ion channels in the generation of an APAxonPlasmamembraneCytosolActionpotentialNa+ActionpotentialNa+K+K+ActionpotentialK+K+Na+Fig. 48.11:Conduction of an APAn action potential is generated as Na+ flows inward across the membrane at one location.12The depolarization of the action potential spreads to the neighboring region of the membrane, re-initiating the action potential there. To the left of this region, the membrane is repolarizing as K+ flows outward. 3The depolarization-repolarization process isrepeated in the next region of the membrane. In this way, local currents of ions across the plasma membrane cause the action potential to be propagated along the length of the axon.AxonSchwanncellMyelin sheathNodes ofRanvierNode of RanvierSchwanncellNucleus ofSchwann cellLayers of myelinAxon0.1 mFig. 48.12: Schwann Cells and the Myelin SheathCell bodySchwann cellDepolarized region(node of Ranvier)MyelinsheathAxonFig. 48.13: Saltatory ConductionSlight bend:weakstimulusStretchreceptorMembranepotential (mV)AxonDendritesStrong receptorpotentialWeakreceptorpotentialMuscle–50–70Membranepotential (mV)–50–70Action potentialsAction potentialsMembranepotential (mV)Large bend:strongstimulusReceptionTransduction0–700–701 2 3 4 5 6 7Membranepotential (mV)Time (sec)1 2 3 4 5 6 7Time (sec)TransmissionPerceptionBrainBrain perceiveslarge bend.Brain perceivesslight bend.1234123400Fig. 50-2: Sensory Transduction in the Crayfish StretchReceptor Crayfish stretch receptors havedendrites embedded in abdominalmuscles. When the abdomenbends, muscles and dendritesstretch, producing a receptor potentialin the stretch receptor. The receptorpotential triggers action potentials inthe axon of the stretch receptor.A stronger stretch produces a largerreceptor potential and higher frequencyof action potentials.“Hairs” ofhair cellNeuro-trans-mitter atsynapseSensoryneuronMoreneuro-trans-mitter(a) No bending of hairs (b) Bending of hairs in one direction (c) Bending of hairs in other directionLessneuro-trans-mitterAction potentialsMembranepotential (mV)0–700 1 2 3 4 5 6 7Time (sec)SignalSignal–70–50Receptor potentialMembranepotential (mV)0–700 1 2 3 4 5 6 7Time (sec)–70–50Membranepotential (mV)0–700 1 2 3 4 5 6 7Time (sec)–70–50SignalFig. 50-9: Sensory Transduction in the Auditory Hair Cell Vertebrate hair cells havespecialized cilia or microvilli(“hairs”) that bend when sur-rounding fluid moves. Each haircell releases an excitatoryneurotransmitter at a synapsewith a sensory neuron, whichconducts action potentials to theCNS. Bending in one directiondepolarizes the hair cell, causing itto release more neurotransmitterand increasing frequencyof action potentials in the sensoryneuron. Bending in the otherdirection has the opposite effects.Thus, hair cells respond to thedirection of motion as well as to itsstrength and
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