6 Synaptic transmission

6 Synaptic transmission - SYNAPTIC TRANSMISSION page 1...

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SYNAPTIC TRANSMISSION A C Brown page 1 A7a INTRODUCTION A. Definitions 1. Synapse: junction between two excitable cells through which one cell influences the excitability of the other cell 2. Presynaptic : refers to the cell which influences the activity of the other cell 3. Postsynaptic: refers to the cell whose activity is influenced B. Types 1. Chemically mediated synapses: presynaptic neuron releases a chemical or neurotransmitter that influences the excitability of the postsynaptic cell 2. Electrical synapses: electrical current flow due to presynaptic action potentials passes directly to the postsynaptic neuron through gap junctions (cytoplasmic continuity), and so influences the postsynaptic neuron excitability Note: chemically mediated synapses are by far the most common and important type in the mammalian nervous system, and will be the focus of the following material Note: a synapse is often schematically represented as
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SYNAPTIC TRANSMISSION A C Brown page 2 A7a INTRODUCTION (continued) C. Morphology 1. Site of synapse a. axosomatic : presynaptic axon to postsynaptic soma b. axodendritic: presynaptic axon to postsynaptic dendrite c. axoaxonal: presynaptic axon to postsynaptic axon 2. Presynaptic ending : enlargement, sometimes termed the synaptic bouton ; contains neurotransmitter-containing vesicles 3. Synaptic cleft : narrow space between presynaptic and postsynaptic cells 4. Axon hillock or Initial Segment: start of the axon where it leaves the soma
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SYNAPTIC TRANSMISSION A C Brown page 3 A7a INTRODUCTION (continued) D. Anatomical Organization 1. Divergence: each presynaptic axon generally has a number of branches that synapse with a number of postsynaptic cells 2. Convergence: each postsynaptic neuron generally has synapses from a number of presynaptic axons Note: Usually both are exhibited in nuclei and ganglions; that is, most presynaptic cells innervate a number of postsynaptic cells and most postsynaptic cells are innervated by a number of presynaptic cells. E. General Properties 1. One-way conduction: a synapse transmits information from presynaptic to postsynaptic cell only , thus establishing a direction for nervous system information transfer 2. Synaptic delay: small delay (about 0.5 ms) between the time an AP arrives at the presynaptic terminal and the time the post synaptic cell's activity is altered 3. Spatial summation: summation of the effects of several presynaptic cells on the postsynaptic cell 4. Temporal summation: summation of the effects of several
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