lecture 5 physio.docx
0 likes16 views
There are two main types of synapses: electrical and chemical. Electrical synapses allow direct electrical transmission between neurons through gap junctions, making them much faster than chemical synapses but also less common. Chemical synapses rely on neurotransmitters and are the main type of synaptic transmission. Neurotransmitters can be small molecules like acetylcholine for fast synaptic transmission, or larger molecules like peptides for slower synaptic transmission. A single neuron can use both electrical and chemical synapses.
lecture 5 physio.docx
- 1. TOPIC 31 Synapsesand TheirTypes: Electrical Synapses Synapses The junctionsbetweenaxonterminalsof one neuronandthe dendritesof anotherneuronare knownas synapses.Atthese junctions,transmissionof signalstakes place fromone neuronto another. Types ofSynapses There are twotypesof synapses: • • Electrical synapses • • Chemical synapses Electrical Synapses At electrical synapses,the plasmamembranesof the pre- and postsynapticcellsare inclose apposition. Theyare electricallycoupledbyparticularproteinscalledgap junctionspresentwithinthe membranes.Electrical current can flowdirectlyfromone cell intothe otherthroughthese gap junctions. Electrical synapsestransferinformationbetweencellsmuch rapidlyascomparedto the chemical synapseswhichtransmit signalsatmuch slowerpace.However,electrical synapsesare relativelyrare andmostsignalingbetweenneuronstakes place at chemical synapses. Signal Transmissionthrough Electrical Synapses The transferof informationacrosselectrical synapsesoccurspurelybyelectrical meansanddoesnot involve anychemical transmitter.Itinvolvesthe passive spreadof local circuitcurrentthatdepolarizes and excitesthe regionahead.Thiswayof transmissionisverymuchlike signal transmissionalonga single axon. Limitedefficacyof electrical synapses The electrical signal inthe presynapticcell producesasimilar,butsomewhatattenuated,signal inthe postsynapticcell.Due tothisattenuationasingle presynapticactionpotentialmightbe unable to provide enoughlocal circuit currentacrossan electrical synapse toelicitanactionpotential inthe postsynapticcell.Thisreducesthe efficacyof electrical synapsesandisconsideredasthe basicreason whyelectrical synapsesare lesscommonthanchemical synapses. Advantage of electrical synapses Electrical synapsesconductsignalsmuchmore rapidlythandochemical synapses.Thisgivesthem definiteadvantageswhere rapidsignal transmissionisimportant. The rapidityof electrical synapsesmakesthemparticularlyeffective inthe synchronizationof electrical activitywithinagroupof cells. It isalso effective forrapidlytransmittinginformationacrossaseriesof cell-cell junctions.Forexample, inthe myocardiumof the vertebrate heart,inwhichsignalsare passedbetweenmusclecells. Occurrence ofelectrical synapses Electrical transmissionhasbeendiscoveredbetweencellsinthe vertebratecentral nervoussystemand inthe vertebrate retina,betweensmoothmuscle fibers,betweencardiacmuscle fibers,between receptorcells,andbetweensome axonsof PNS.
- 2. TOPIC 32 Synapsesand TheirTypes: Chemical Synapses The chemical synapses The chemical synapsesare involvedinchemical synaptictransmissioninvolvingneurotransmitters.This mode of synaptictransmissionismore commonascomparedto the electrical synaptictransmission. Types ofchemical synapses • • Fastchemical synapses • • Slowchemical synapses Fast Chemical Synapses Fast chemical synapsesare foundinthe central nervoussystemandatthe neuromuscularjunctions. Althoughthistransmissioniscalled"fast"itisinfactconsiderablyslowerthantransmissionacross electrical synapses. Neurotransmittersat Fast Chemical Synapses • • The neurotransmittersinvolvedinfastchemical synapsesare typicallysmallmolecules(e.g. Acetylcholine). • • Neurotransmittersare storedinsynapticvesiclesthatare small,clearvesiclesinthe axon terminals. Release ofNeurotransmitters • • Ata fastchemical synapse,actionpotentialsinthe presynapticneuroncause the release of neurotransmittermoleculesatthe axonterminals. • • Neurotransmittersare releasedbyexocytosisinto the synapticcleftthroughspecializedsites on the membrane. • • The synapticcleftisa narrow,fluid-filledspace,about20 nm wide thatseparatesthe membranesof pre- andpostsynapticneurons. Mode of Action of Neurotransmitters • • Neurotransmittermoleculesbindtospecificproteinreceptorswhichare ligand-gatedion channelsinthe postsynapticmembrane.
- 3. • • Thisbindingresultsinthe openingof the channelsthatallowsanioniccurrenttoflow intothe postsynapticcell. • • The postsynapticcurrentcausesa change inthe membrane potential of the postsynapticcell. • • If the change inmembrane potentialexceedsthe threshold,anAPisinitiated. Slow Chemical Synapses Transmissionthroughslowchemicalsynapsestakesplace by adifferentpostsynapticmechanism. The onsetof postsynapticresponseisslower(hundredsof milliseconds),butitcan lastmuch longer (fromsecondstohours). Neurotransmittersat Slow Chemical Synapses: The neurotransmittersinvolvedinslow synaptictransmissionare: • • typicallylarge molecules. • • synthesizedfromone ormore aminoacids. • • of two types: Biogenicamines:are synthesizedfromasingle aminoacid Neuropeptides:Consistof several aminoacidresidues. Packaging and Release ofNeurotransmitters: • • Vesiclesinthe slowsystemare larger. • • Theyare usuallysynthesizedin the cell body,afterwhich they are transportedtothe nerve terminals. • • Vesiclesmayreleasethe transmittermoleculesatmany sitesinthe presynapticterminal thatlackmorphological specialization.Thesesitesare locatedawayfromthe sitesof release of fastneurotransmitters. Mode of Action of slowNeurotransmitters Slowresponse transmittersdon’tactthroughligand- gatedchannels. Theyact throughG protein-linkedreceptors. Whena neurotransmitterbindstoitsreceptor,the neurotransmitter-receptorcomplexactivatesaG protein. The G protein, in turn, activates a signal transduction pathway through a second messenger e.g. cAMP. The secondmessengersmodifythe functionsof channelsandotherintracellularprocesses. A NeuronMay Produce Both Typesof Transmission Physiological andanatomical evidence indicatesthatsinglepresynapticneuronmayparticipateinboth kindsof neurotransmissionandasingle neurotransmittermayaffectpostsynapticneuronsbothby meansof ligand-gatedchannelsandbymeansof G-protein-coupledreceptors.
- 4. TOPIC-35 Neurotransmitters:Diversityand Classification Diversity By the mid-1960s, onlythree neurotransmittershadbeenidentified.These wereacetylcholine, norepinephrineandγ-aminobutyricacid(GABA).Today,more than100 neurotransmittershave been identifiedthatvaryinsize andmolecularweightandbelongtovariouschemical categories. ClassificationBasedon Chemical Structure Basedon theirchemical structure,neurotransmitterscanbe sortedintotwogroups: 1. Small,low molecularweightNeurotransmitters. Theyinclude: • • Acetylcholine • • Aminoacids Include GABA,Glycine,Glutamate,Aspartate • • Biogenicamines Include Norepinephrine,Epinephrine,Dopamine,SerotoninandHistamine. • • Gases e.g.Nitricoxide (NO) andCarbonmonoxide(CO) 2. Large, highmolecular weightNeurotransmitters Theyare derivedfromaminoacidsandinclude: • Neuropeptides Theyare largermoleculesthatare constructedof aminoacids.More than40 neuropeptidetransmitters have beenidentifiedinthe mammalian central nervoussystem.Theseinclude manyhypothalamicand pituitarypeptidehormones,Substance-P,endorphins,enkephalinsandmanyotheraminoacid derivatives. ClassificationBasedon Mode ofAction: 1. Fast, Direct Neurotransmitters These transmittersactdirectlyonionchannel proteinsandchange the conductance of postsynaptic membrane forvariousions. These include acetylcholine,aminoacids(glutamate,aspartate,glycineandγ-Aminobutyricacid). 2. Slow,Indirect Neurotransmitters These transmittersworkthroughanindirectbiochemical pathwaywithinthe postsynapticcell that involvesGprotiens.Theychange the state of a secondmessengerthatresultsinchangesinconductance of ionchannel proteins. Theyinclude biogenicamines andneuropeptides. Many of these neurotransmittersmayalsoact as neuromodulators.Neuromodulatorscanaffect neighboringneuronsandmodifytheirbehavioratonce.