GSU NEUR 3000 - NEUR 3000 - Chapter 5 (53 pages)

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NEUR 3000 - Chapter 5



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NEUR 3000 - Chapter 5

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53
School:
Georgia State University
Course:
Neur 3000 - Hon Principles of Neuroscience
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SYNAPTIC TRANSMISSION NEUR 3000 Dr Joseph J Normandin SYNAPTIC 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 synapse TYPES 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 through TYPES OF SYNAPSES TYPES OF SYNAPSES TYPES 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 synchrony TYPES OF SYNAPSES TYPES 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 anchored TYPES 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 membrane TYPES OF SYNAPSES TYPES OF SYNAPSES TYPES 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 vary TYPES OF SYNAPSES TYPES OF SYNAPSES TYPES OF SYNAPSES TYPES 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 endplate TYPES OF SYNAPSES PRINCIPLES 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 cleft PRINCIPLES 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 SYNAPSE PRINCIPLES 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 conditions PRINCIPLES 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 terminal PRINCIPLES OF THE CHEMICAL SYNAPSE PRINCIPLES 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 conformation of docking proteins allowing the vesicle to fuse with the membrane Exocytosis of vesicle contents Fast 0 2 msec Ca2 also mobilizes the reserve pool to move to active zone Secretory granules Not docked found some distance from active zone Require high levels of Ca2 to trigger exocytosis Slow 50 msec Vesicles are recycled by endocytosis PRINCIPLES OF THE CHEMICAL SYNAPSE PRINCIPLES OF THE CHEMICAL SYNAPSE PRINCIPLES OF THE CHEMICAL SYNAPSE Neurotransmitter receptors Two categories Transmitter gated ion channels Ionotropic The receptor is also an ion channel Fast synaptic neurotransmission G protein coupled receptor Metabotropic The receptor activates an intracellular messaging system Slow synaptic neurotransmission PRINCIPLES OF THE CHEMICAL SYNAPSE Neurotransmitter receptors Ionotropic receptors Amino acid and amine neurotransmitters Similar to ion channels we have discussed before Transmembrane proteins of 4 or 5 subunits forming a pore that is gated Binding of neurotransmitter produces a conformational change opening the gate The effect depends on the ion selectivity What are some possibilities PRINCIPLES OF THE CHEMICAL SYNAPSE Neurotransmitter receptors Ionotropic receptors Permeability to Na Produces an excitatory postsynaptic potential EPSP Depolarization of membrane potential Nicotinic Ach receptors are an example Permeable to Cl Produces an inhibitory postsynaptic potential IPSP Hyperpolarization of membrane potential GABA and Glycine receptors are an example PRINCIPLES OF THE CHEMICAL SYNAPSE PRINCIPLES OF THE CHEMICAL SYNAPSE PRINCIPLES OF THE CHEMICAL SYNAPSE Neurotransmitter receptors Metabotropic receptors Amino acid amine and peptide neurotransmitters


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