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Unformatted text preview: Chapter 5 Synaptic Transmission Types of Synapses o Electrical synapses Occur at specialized sites called gap junctions 3mm gaps spanned by clusters of special proteins called connexins o 6 connexins combine to form a channel called a connexon and two connexons combine to form a gap junction channel The channel allows ions to pass directly from the cytoplasm of one cell to the other Pore is relatively large (1-2nm diameter), big enough for all major cellular ions and many small organic molecules to pass through Most gap junctions allow ionic current to pass equally well in both direction Bidirectional Cells connected by gap junctions are said to be electrically coupled Transmission is fast and if synapse is large, failsafe Action potential in pre-synaptic neuron almost instantaneously makes an action potential in the postsynaptic neuron When ionic current flows across gap junction channel to postsynaptic neuron, the current causes a postsynaptic potential (PSP) in 2 nd neuron When 2 nd neuron generates action potential, a PSP is induced in the 1 st neuron due to bidirectional movement o PSP generated by single electrical synapse is usually small (~1mv) and may not be large enough to trigger action potential in postsynaptic cell o Several PSPs occurring simultaneously may strongly excite a neuron o Chemical synapses Separated by synaptic cleft 20-50nm wide, 10times width of gap junctions Filled with matrix of fibrous extra-cellular protein o Matrix makes pre and postsynaptic membranes adhere to each other Presynaptic side of the synapse (presynaptic element) is usually an axon terminal o Typically contains dozens of small membrane-enclosed spheres, each about 50nm in diameter called synaptic vesicles Store neurotransmitters o Larger vesicles about 100nm in diameter called secretory granules contain soluble protein that appears dark in the electron microscope so sometimes called dense-core vesicles Dense accumulations of protein next to and within membranes of synaptic cleft are called membrane differentiations Active zones are sites of neurotransmitter release Synaptic vesicles are clustered in the cytoplasm next to the active zone Postsynaptic density is a protein thickly accumulated in and just under the postsynaptic membrane that contains neurotransmitter receptors, which convert intercellular chemical into intracellular signal CNS Synapses If postsynaptic membrane is on dendrite, synapse is axodendritic If postsynaptic membrane is on cell body, synapse is aid to be axosomatic If postsynaptic membrane on another axon, the synapses are axoaxonic In certain specialized neurons, dendrites form synapses with each others and are called dendrodendritic Two general categories o Synapses which membrane differentiation on the postsynaptic side is thicker than that on the presynaptic side are called asymmetrical synapses, or Grays type I synapses...
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This note was uploaded on 04/17/2008 for the course NEURO 101 taught by Professor Mosh. during the Fall '08 term at NYU.
- Fall '08