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Chapter_8_NT - Chapter 8 Synaptic Transmission Neural...

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Chapter 8 Synaptic Transmission & Neural Integration Synapses Junction where nerve terminal meets a neuron, muscle cell, or gland Presynaptic cell (sends signal), synaptic cleft and postsynaptic cell (receives signal) 2 types Electrical – electric charge freely flows through gap junctions from cell to cell Chemical – neurotransmitter acts as signal from presynaptic to postsynaptic cell Why is the recpetor on the outside of the cell? Homophobic, amino acids Electrical Synapses 2 neurons linked together by gap junctions Function in photoreceptors, astroglia & most early cells in development Rapid communication Rapid eye movement Bidirectional communication Primarily excitation Cannot amplify a signal Electrical Synapse Impulses can be regenerated without interruption in adjacent cells. Gap junctions: Adjacent cells electrically coupled through a channel. Examples: Smooth and cardiac muscle, the brain Chemical Synapse Location of Chemical Synapses figure 8.1 Has a single axon that stimulates the dendrite of another cell Advantages of Chemical Synapses NT can trigger amplifying cascade NT can generate inhibition via Cl - channels NT can transmit longer using G-proteins & 2 nd messengers to prolong effects NT have plasticity ; stimulated by learning & memory Anatomy of a Synapse **FIG 8.2a** Communication Across a Synapse 1) Action Potential 2) Voltage-gated Ca channels open 3) Calcium triggers exocytosis 4) NT diffuses and binds to receptor 5) Response in cell Synaptic Delay 1-5 msec between arrival of AP and change in post-synaptic Vm Caused by changes in [Ca] Not related to diffusion of Fast Responses at Ionotropic Receptorsfig 8.3a Slow Responses at Metabotropic Receptors: Direct G Protein Coupling fig 8.3b
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Slow Responses at Metabotropic Receptors: Second Messenger Coupling fig 8.3c Adelate cylcase: most common second messenger Binding of neurotransmitter changes membrane potential of postsynaptic cell Excitatory postsynaptic potential (EPSP) Brings membrane closer to threshold potential Inhibitory postsynaptic potential (IPSP) Takes membrane farther from threshold potential (hyperpolization) or stabilization Synaptic signal ends when neurotransmitter broken down by enzymes or taken back into presynaptic cell for reuse Fast EPSPs fig 8.4 Slow EPSPs EPSPs Are Graded Potentials Higher frequency of action potentials y More neurotransmitter released y More neurotransmitter binds to receptors to open (or close) channels y Greater increase (or decrease) ion permeability y Greater (or lesser) ion flux y Greater depolarization Inhibitory Synapses Neurotransmitter binds to receptor Channels for either K or Cl open If K channels open K moves out à If Cl channels open, either Cl moves in à Cl stabilizes membrane potential Fast Inhibitory Synapse Involving Potassium Channels fig 8.5
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