EXSC%224:%Week%of%1/16/17%%Textbook%Notes%Ch.%11.5%&%11.6%11.5$Graded$potentials$are$brief,$short-distanced$signals$within$a$neuron$• Graded$potential:$short-lived,$localized$changes$in$membrane$potentials$o Can$be$a$depolarization$or$hyperpolarization$o “graded”$because$their$magnitude$varies$directly$with$stimulus$strength$o triggered$by$a$stimulus$that$opens$gated$ion$channels$• classified$by$where$they$occur$and$function$o receptor/generator$potential:$when$a$sensory$neuron$is$excited$by$a$form$of$energy$(heat,$light,$other)$o postsynaptic$potential:$when$the$stimulus$is$a$neurotransmitter$released$by$another$neuron$11.6$Action$potentials$are$brief,$long-distance$signals$within$a$neuron$• action$potential:$brief$reversal$of$membrane$potentials$o total$change$in$voltage$of$about$100mV$(from$-70$to$+30$mV)$o also$called$a$nerve$impulse$o voltage$gated$channels$along$the$axon$open$and$close$in$response$to$changes$in$membrane$potential$o axon$hillock:$where$the$transition$from$local$graded$potentials$to$an$action$potential$occurs$• stages$of$an$action$potential$o resting$state:$all$gated$Na+$and$K+$channels$are$closed$o Depolarization:$Na+$channels$open,$K+$channels$stay$closed$o Repolarization:$Na+$close,$K+$channels$open$o Hyperpolarization:$some$K+$channels$remain$open$and$Na+$channels$reset$$Lecture%1/17%Notes%Graded$potentials$• Short-lived$• Localized,$only$changes$the$membrane$potential$near$the$channel$• Depolarizes$or$hyperpolarizes$• Spreads$as$local$currents$change$the$membrane$potential$of$adjacent$regions$• Occurs$when$gated$ion$channels$open$• Change$in$membrane$potential$is$greatest$at$the$spot$nearest$to$the$ligand$channel$o Voltage$declines$with$distance$from$the$stimulus$• Have$to$have$a$lot$of$graded$potentials$to$excite$the$neuron$Action$potential$• Found$in$axon$hillock,$axon,$and$axon$terminal$• Only$present$if$graded$potential$is$large$enough$(there$is$an$adequate$stimulus)$• Dependent$on$presence$of$graded$potentials$$$$$$$$$$$$$$$$$$$$• 1=resting$state:$before$the$action$potential,$the$membrane$potential$is$at$its$resting$value$of$-70$mV$• 2=depolarization:$the$neuron$becomes$more$positive$as$Na+$enters$the$cell,$threshold$is$reached$at$-55$mV$(red$dashed$line)$and$the$membrane$potential$peaks$at$+30$mV$• 3=repolarization:$the$neuron’s$membrane$potential$decreases$as$K+$exits$the$cell$• 4=hyperpolarization:$the$membrane$potential$decreases$past$the$resting$membrane$potential,$it$overshoots$its$mark$• afterwards,$the$sodium$potassium$pump$helps$restore$it$to$its$resting$state$Sodium-Potassium$Pump$• repolarization$o active$transport$o pump$ions$up$concentration$gradient$o restores$resting$electrical$conditions$of$the$neuron$o doesn’t$restore$resting$ionic$conditions$o restored$by$the$thousands$of$sodium$potassium$pump$o located$everywhere$in$the$neuron$at$threshold:$• membrane$depolarized$by$15-20$mV$• Na+$permeability$increases$• Na$influx$exceeds$K+$efflux$• The$positive$feedback$cycle$begins$• All$or$none:$AP$sent$only$if$threshold$is$reached$• Action$potentials$are$all$the$same$magnitude$• Large$stimulus=more$action$potentials$• Frequency$of$action$potentials$increases$with$an$increased$magnitude$of$the$stimulus$• Can$have$900$action$potentials$per$second$Conduction$velocity$• Conduction$velocity$can$vary$widely$• Effect$of$axon$diameter$o Larger$diameter$fibers$have$less$resistance$to$local$current$flow$and$have$faster$impulse$conduction$• Effect$of$myelination$o Continuous$conduction$• Action$potential$propagated$down$the$axon$by$voltage$gated$ion$channels$o Action$potential$at$every$channel$o So$more$than$one$action$potential$is$sent$down$the$axon$• No$channels$are$present$where$myelin$is$o Voltage$gated$channels$located$at$the$nodes$of$Ranvier$$Textbook%Notes%Ch.%11.7-11.9%11.7$Synapses$transmit$signals$between$neurons$• Chemical$synapses$o Most$common$o Has$two$parts:$axon$terminal$(presynaptic$neuron)$and$receptor$region$(postsynaptic$neuron)$o Neurotransmitter$is$used$to$convey$message$between$the$two$neurons$o Synaptic$cleft:$fluid-filled$space$between$the$neurons,$about$30-50$nm$o Steps$§ Action$potential$arrives$at$axon$terminal$§ Voltage$gated$Ca2+$channels$open$and$Ca2+$enters$the$axon$terminal$§ Ca2+$entry$causes$synaptic$vesicles$to$release$neurotransmitter$by$exocytosis$§ Neurotransmitter$diffuses$across$the$synaptic$cleft$and$binds$to$specific$receptors$on$the$postsynaptic$membrane$§ Binding$of$neurotransmitter$opens$ion$channels,$creating$graded$potentials$§ Neurotransmitter$effects$are$terminated$by$• Reuptake$into$the$presynaptic$terminal$• Degradation:$enzymes$break$it$down$• Washed$away$from$the$synapse$• Electrical$synapses$o Less$common$o Gap$junctions$o Channel$proteins$connect$the$cytoplasm$of$adjacent$neurons$and$allow$ions$and$small$molecules$to$flow$directly$from$one$neuron$to$the$next$$$11.8$Postsynaptic$potentials$excite$or$inhibit$the$receiving$neuron$• EPSP:$excitatory$postsynaptic$potential$o Neurotransmitter$binds$and$depolarizes$the$postsynaptic$membrane$o Action$potential$sent$• IPSP:$inhibitory$postsynaptic$potential$o Neurotransmitter$binds$and$hyperpolarizes$the$postsynaptic$membrane$o Action$potential$not$sent$$11.9$The$effect$of$a$neurotransmitter$depends$on$its$receptor$• Neurotransmitters$o “language$of$the$nervous$system”$o how$neurons$communicate$with$each$other$o more$than$50$neurotransmitters$have$been$identified$o classified$chemically$and$functionally$o see$table$11.3$in$textbook$for$detailed$list$of$neurotransmitters$o excitatory$vs$inhibitory$§ excitatory:$cause$depolarization$§ inhibitory:$cause$hyperpolarization$o direct$vs$indirect$§ direct:$binds$and$open$ion$channels$§ indirect:$act$through$second$messenger$molecules$(typically$G-proteins)$$Lecture%1/19%Notes$Location$of$different$channel$types$on$the$neuron$• Dendrites:$ligand,$leakage,$and$mechanical$channels$o Sodium-potassium$pump$constantly$trying$to$correct$imbalances$o No$myelin$here$so$graded$potentials$don’t$travel$far$• Axon:$voltage$gated$ion$channels,$leakage$channels$o Voltage$gated$channels$are$close$together.$if$one$fires,$they$all$do$like$dominoes$• Axon$with$myelin:$voltage$gated$ion$channels$at$the$nodes$of$Ranvier$Neural$transmission$of$unmyelinated$vs$myelinated$axon$•