Electrical and Chemical SynapsesSynapse(突觸) is a specialized junction between a neuron and a target cellPresynaptic neuron is the signaling cellPostsynaptic cell (neuron, gland or muscle) is the targetElectrical synapses signal via gap junctionsChemical synapses signal via neurotransmitters released into the synaptic cleftSlower than electrical synapses (synaptic delay)Operate in one direction onlyAllow for various kinds of signaling events-----Neuron-to-Neuron SynapsesChemical synaptic transmission (Fast synapses)Neurotransmitter is stored in synaptic vesicles(突觸小泡) in synaptic knob(球型突出物) of presynaptic neuronAction potential reaches axon terminal of presynaptic neuronOpens voltage-gated Ca++ channels in the synaptic knob, allowing Ca++ to enterNeurotransmitter is released by exocytosisChemical synaptic transmissionNeurotransmitter diffuses across the synaptic cleft(裂口) and combines with receptors on the subsynaptic membraneBinding triggers opening of chemically-gated ion channels in subsynaptic membrane-----Neuron-to-Neuron SynapsesExcitatory(刺激) synapsesBinding of neurotransmitter opens nonspecific cation channels (Na+ and K+)Electrochemical gradients favor inward movement of Na+ leading to depolarizationExcitatory postsynaptic potential (EPSP)-----Inhibitory(抑制) synapsesBinding of neurotransmitter opens specific K+ or Cl- channelsOutward movement of K+ or inward movement of Cl- leads to hyperpolarizationInhibitory postsynaptic potential (IPSP)-----Neurotransmitters are quickly removed from the synaptic cleftDiffuse away from the synaptic cleft orInactivated by a specific enzyme orTransported back into the axon terminal by reuptake carriersNeurotransmitters in “slow” synapses activate intracellular second messengersResponses take longer and last longer than “fast” synapses