Slide 1Synaptic transmissionTypes of synapsesTypes of synapsesTypes of synapsesTypes of synapsesTypes of synapsesTypes of synapsesTypes of synapsesTypes of synapsesTypes of synapsesTypes of synapsesTypes of synapsesTypes of synapsesTypes of synapsesTypes of synapsesTypes of synapsesPrinciples of the chemical synapsePrinciples of the chemical synapsePrinciples of the chemical synapsePrinciples of the chemical synapsePrinciples of the chemical synapsePrinciples of the chemical synapsePrinciples of the chemical synapsePrinciples of the chemical synapsePrinciples of the chemical synapsePrinciples of the chemical synapsePrinciples of the chemical synapsePrinciples of the chemical synapsePrinciples of the chemical synapsePrinciples of the chemical synapsePrinciples of the chemical synapsePrinciples of the chemical synapsePrinciples of the chemical synapsePrinciples of the chemical synapsePrinciples of the chemical synapsePrinciples of the chemical synapsePrinciples of the chemical synapsePrinciples of Synaptic integrationPrinciples of Synaptic integrationPrinciples of Synaptic integrationPrinciples of Synaptic integrationPrinciples of Synaptic integrationPrinciples of Synaptic integrationPrinciples of Synaptic integrationPrinciples of Synaptic integrationPrinciples of Synaptic integrationPrinciples of Synaptic integrationPrinciples of Synaptic integrationPrinciples of Synaptic integrationPrinciples of Synaptic integrationPrinciples of Synaptic integrationSlide 53SYNAPTIC TRANSMISSIONNEUR 3000Dr. Joseph J. NormandinSYNAPTIC TRANSMISSION•The process by which information is transferred between neurons•At some synapses electrical signals pass through physical connections between neurons•Electrical synapses•At other synapses electrical signals are converted into a chemical signal•Chemical synapse•In both systems we have:•A presynaptic neuron before the synapse•A postsynaptic neuron after the synapseTYPES OF SYNAPSES•Electrical synapses•The pre- and postsynaptic cells are physically connected by gap junctions•The cells are only 3 nm apart•Gap junctions are formed by the protein connexin•Six connexin, forming a ring, comprise a connexon•Two connexons, one from each cells, form the gap junction channel•The channel is large enough for ions and small molecules to flow throughTYPES OF SYNAPSESTYPES OF SYNAPSESTYPES OF SYNAPSES•Electrical synapses•Information flow is bi-direction as electric current can flow both ways•Very fast conduction•A presynaptic action potential will produce a postsynaptic potential in the postsynaptic neuron•In the mammalian brain, this potential is small at about 1 mV•Summation of postsynaptic potentials from many cells can result in an action potential•Synaptic integration•An action potential in the postsynaptic neuron will result in a PSP in the (previously) presynaptic neuron•Present in systems requiring synchronyTYPES OF SYNAPSESTYPES OF SYNAPSES•Chemical synapses•Pre- and postsynaptic cells are not continuous with one another•The space in between is referred to as the synaptic cleft•20-50 nm wide•A fibrous matrix between the two cells keeps the synapse anchoredTYPES OF SYNAPSES•Chemical synapses•The presynaptic element is usually an axon•Contains synaptic vesicles with neurotransmitters•Large, neuropeptide-containing vesicles are called dense-core vesicles•A pyramid shaped active zone on the intracellular side of terminals is where vesicles bind•The postsynaptic element is usually a dendritic spine•The postsynaptic density, containing receptor proteins, lines the membraneTYPES OF SYNAPSESTYPES OF SYNAPSESTYPES OF SYNAPSES•Chemical synapses in the CNS•Several different pre- to postsynaptic connections•Axodendritic•Axosomatic•Axoaxonic•Dendrodendritic•Two types of pre- to postsynaptic arrangements•Gray’s Type I synapse: tends to be excitatory, large postsynaptic density (asymmetrical)•Gray’s Type II synapse: tends to be inhibitory, small postsynaptic density (symmetrical)•The size and shape of pre- and postsynaptic processes can also varyTYPES OF SYNAPSESTYPES OF SYNAPSESTYPES OF SYNAPSESTYPES OF SYNAPSES•Chemical synapses outside the CNS•Neurons make connections with glands, smooth muscle, cardiac muscle and skeletal muscle•Neuromuscular junction•The synapse between motor neurons from the spinal cord and skeletal muscle•The muscle fibers are the postsynaptic cells and are excitable like neurons•The postsynaptic membrane is called the motor end-plateTYPES OF SYNAPSESPRINCIPLES OF THE CHEMICAL SYNAPSE•What is required of a chemical synapse?Neurotransmitter synthesisLoad neurotransmitter into synaptic vesiclesVesicles fuse to presynaptic terminalNeurotransmitter spills into synaptic cleftBinds to postsynaptic receptorsBiochemical/Electrical response elicited in postsynaptic cellRemoval of neurotransmitter from synaptic cleftPRINCIPLES OF THE CHEMICAL SYNAPSE•Neurotransmitters•Three categories•Amino acids•Small organic molecules•Released from synaptic vesicles•Amines•Small organic molecules•Released from synaptic vesicles•Peptides•Larger amino acid chains•Released from dense-core vesicles (secretory granules)PRINCIPLES OF THE CHEMICAL SYNAPSEPRINCIPLES OF THE CHEMICAL SYNAPSE•Neurotransmitters•“Fast” synaptic transmission•In the CNS, most often mediated by:•Glu, GABA, Gly•At the neuromuscular junction:•ACh•“Slow” synaptic transmission•Mediated by all three categories of neurotransmitters•Neuropeptides are often present in the same terminals as amino acid or amine neurotransmitters•Release of neuropeptides / neurotransmitters under different conditionsPRINCIPLES OF THE CHEMICAL SYNAPSE•Neurotransmitter synthesis and storage•Amino acid and amine neurotransmitters•Glutamate, glycine, GABA, and amines•Precursor molecules and enzymes for synthesis sent to the axon terminal in vesicles•Some precursor molecules may be taken up by transporters on axon terminal•Synthesis completed in terminal and packed into synaptic vesicles via transporters•Neuropeptides•Synthesized in ER, packed into secretory granules in Golgi apparatus, sent to the terminalPRINCIPLES OF THE CHEMICAL SYNAPSEPRINCIPLES OF THE CHEMICAL SYNAPSE•Neurotransmitter release•Synaptic vesicles•Docked at the active zone or in a reserve pool•An action potential activates voltage-gated Ca2+ channels•Ca2+ alter the