Section 2 Interaction between neuronsSynapse: specialized zone of contact at which one neuron communicate with another. vSynaptic structurevThe classification of synapse vProcess of chemical synapse transmissionvThe synapse transmission featuresvSynapse transmission modulationvSynaptic plasticitySection Outline1. Chemical synapse transmission1.1 Synaptic structure1.1 Synaptic structureVesiclesnOn the basis of connecting area of synapsesvAxo-somaticvAxo-dendriticvAxo-axonicvDendro-dendriticvDendro-axonicvDendro-somatic1.2 Classification of synapseAxodendritic synapsenOn the basis of action on postsynaptic neuronvExcitatory synapse: excitatory neurotransmitterdepolarizationvInhibitory synapse: inhibitory neurotransmitterhyperpolarization 1.2 Classification of synapseExcited or inhibited?Photographs using the electron microscope have shown that synapses can be either asymmetrical (red arrow) or symmetrical (green arrow). In the figure on the left, notice that the red arrow is pointing to a synapse that has one dark band and one lighter band. The green arrow is pointing to a synapse that has two dark bands. Asymmetrical synapses are thought to be excitatory synapses and symmetrical synapse are thought to be inhibitory synapses. The yellow line outlines the dendrite (D).nOn the basis of signal transmission mode between synapsesvChemical synapsevElectrical synapse (gap junction)1.2 Classification of synapse1.3 Process of chemical synapse transmissionvNo structural continuity between pre- and postsynaptic neurons: 20-40 nm.vSynaptic vesicles cluster at active zone.vUnidirection, synaptic delay (1 ms or longer) nChemical synapse1.3 Process of chemical synapse transmission1.3 Process of chemical synapse transmissionnExcitatory postsynaptic potential, EPSPnInhibitory postsynaptic potential, IPSP1.3 Process of chemical synapse transmissionnExcitatory postsynaptic potential, EPSP: Excitatory neurotransmitter released by presynaptic terminal causes depolarization which leads to the excitation and activation of postsynaptic neuron. 1.3 Process of chemical synapse transmissionAction potential arrives at presynaptic neurone (synaptic knob)Action potential causes calcium channels to open (Ca 2+ enters ) Ca 2+ cause synaptic vesicle to move and release excitatory transmitterTransmitter diffuses across synaptic cleftTransmitter binds to receptor on postsynaptic membraneNa+, K+ channels open (Na+K+ )Ionic mechanism of EPSPCauses depolarisation of the postsynaptic membrane (EPSP)Action potential is produced in the initial segment of axonElectrical triggernFor communication between neurones to occur, an electrical impulse must travel down an axon to the synaptic terminal.Ionic mechanism of IPSPAction potential arrives at presynaptic neurone (synaptic knob)Action potential causes calcium channels to open (Ca 2+ enters ) Ca 2+ cause synaptic vesicle to move and release inhibitory transmitterTransmitter diffuses across synaptic cleftTransmitter binds to receptor on postsynaptic membraneCl-, K+ channels openCauses hyperpolarisation of the postsynaptic membrane (IPSP)IPSP moves the membrane potential away from the firing level of the cell decreases excitabilitynOne-way transmissionnSynaptic delaynSummation nChange of excitatory rhythmnAfter dischargenSusceptibility to internal environment and fatigue1.4 Properties of synaptic transmissiontwo neurones action potentials act togetherrapid succesive action potentialsnOne-way transmissionnSynaptic delaynSummation nChange of excitatory rhythmnAfter dischargenSusceptibility to internal environment and fatigue1.4 Properties of synaptic transmissionAfter dischargenOne-way transmissionnSynaptic delaynSummation nChange of excitatory rhythmnAfter dischargenSusceptibility to internal environment and fatigue1.4 Properties of synaptic transmission1.5 Modulation of synaptic transmissionnPresynaptic modulationvCa2+vAutoreceptorvUptake of neurotransmitter nPostsynaptic modulationvAmount of postsynaptic receptorvAffinity of ligand and receptorUp regulationDown regulation Internalization Desensitization1.5 Modulation of synaptic transmissionnPosttetanic potentiationnHabituation and sensitizationnLong-term potentiation (LTP) and long-term depression (LTD)1.6 Synaptic plasticityLong-term potentiationvLTP is an electrophysiological measure of sustained increase in synaptic efficacy when given high-frequency stimulation vCellular and behavioral studies suggest that learning and memory can be modeled by LTPField EPSPsHigh-frequency stimulation200 msec 12 bursts; 4 pulses; 100Hz (TBS 12x)2. Non-synaptic chemical transmissionVaricositynIn PNS: nIn CNS:vEpinephrinergic vDopaminergic vSerotoninergic 3. Electrical synapse transmissionnElectrical synapsevDistance between Pre- and Postsynaptic neurons: 2-3nm.vCytoplasmic continuity: gap junction.vLow resistance, short synaptic delay (0.1ms), bidirectional current flow. nSix connexinsnA channel called a connexonnDiameter 2nmnHighly synchronizedComparison of chemical and electrical synapsesProperties Electrical ChemicalSynaptic cleft 3.5nm 30-50nmCytoplasmic continuity yes noUltrastructure gap junction chemical synapseTransmission ionic current neurotransmitterSynaptic delay no yesDirection bidirection unidirection nLocal circuit neuronnLocal neuronal circuit4. Local neuronal circuitSummary - NeurotransmissionSummarynClassification of synapsenProcess of synaptic transmissionnCharacteristics of synaptic transmissionnModulation of synaptic transmissionnTemporal summation occurs when single synapse receives many EPSPs in a short period of timeSummation of Postsynaptic PotentialsnSpatial summation occurs when single synapse receives many EPSPs from many presynaptic cellsnMore reliable than chemicalnFaster (escape responses)nSynchronizing cellsnIntracellular transfer of molecules (Ca+. ATP and cAMP)nCan be dynamic (Dopamine regulates some in retina)Advantages of electrical synapses1.1 Synaptic structurenSlow postsynaptic potentialvAutonomic ganglia, cardiac and smooth muscle, cortical neuronsvHave a latency of 100-500msvLast several secondsvSlow EPSP: decrease in K+ conductancevSlow IPSP: increase in K+ conductance1.3 Process of chemical synapse transmission