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Synaptic Transmission

Classical Mediated by Neurotransmitter Gated Ion Channel aka ionotropic receptors. Neuromodulatory Mediated by Metabotropic Receptors. Synaptic Transmission. Both cause a post-synaptic potential, ie a change in the Membrane potential of the post-synaptic plasma membrane

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Synaptic Transmission

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  1. Classical Mediated by Neurotransmitter Gated Ion Channel aka ionotropic receptors Neuromodulatory Mediated by Metabotropic Receptors Synaptic Transmission Both cause a post-synaptic potential, ie a change in the Membrane potential of the post-synaptic plasma membrane The psp can be depolarizing or hyperpolarizing

  2. EPSP: excitatory post-synaptic potential IPSP: inhibitory post-synaptic potential Temporal Summation Spatial Summation Synaptic Potentials and Their Integration

  3. Classical Neurotransmission • Effects due to direct gating of ion channel • Direct postsynaptic effects last for tens of milliseconds • No secondary effects • Postsynaptic electrical effects are fast and strong

  4. Neuromuscular Junction • is always excitatory • is one for one • 1 AP in presynaptic MN= 1 AP in post-synaptic muscle NMJ caused by release of 200 synaptic vesicles • In the rest of the NS, it is not 1 for 1, the psp is so small that an AP is not always triggered at the hillock • AP can cause release of 1 synaptic vesicle

  5. Excitatory Transmission • Synaptic transmission that causes depolarization of the postsynaptic neuron • Increases the probability that the post synaptic neuron will fire an action potential • Increases amount of neurotransmitter released from post synaptic neuron by presynaptic facilitation

  6. Excitatory Post-synaptic Potential= EPSP • Depolarization of the post-synaptic membrane caused by the neurotransmitter brings the membrane potential close to the threshold for firing an action potential • Can increase sodium or calcium permeability or can be caused by decreasing potassium permeability

  7. Inhibitory Transmission • Synaptic transmission that causes transient hyperpolarization of the postsynaptic neuron • Decreases the probability that the post synaptic neuron will fire an action potential • This is called an inhibitory post-synaptic potential ipsp

  8. I.P.S.P. • Caused by increase in potassium permeability similar to the undershoot of the action potential • Increase in chloride permeability • If ECl=Vr then no change in Vr will be observed, however an epsp would be smaller if the Cl permeability is still high

  9. Neuronal Integration • Summing of all ipsp and epsp to determine if threshold has been met for AP generation • Based on temporal summation • Time constant • Based on spatial summation • Space constant

  10. Temporal Summation • Rapid firing from a single presynaptic input leads to repeated post-synaptic potentials in a short period of time • Causes repeated depolarization of membrane without time to go back to resting state • Allows a weak presynaptic input to generate an action potential in post synaptic neuron

  11. Time Constant • The amount of time that a psp will last at a given membrane location= tau • tau=membrane resistance x membrane capacitance • Time it takes for constant applied voltage to build up to 63% of its final value

  12. Temporal Summation • Neurons with membranes that have long time constants show more temporal summation for conduction of psp • Typical values are 10 msec • Membrane resistance is reflected by number of open channels and channel density

  13. Membrane Capacitance

  14. Spatial Summation • The simultaneous firing of multiple individual presynaptic neurons to one post-synaptic neuron. • The post-synaptic effects sum and can bring the post synaptic membrane closer or further away from threshold.

  15. Length constant • Distance that a psp can spread along the membrane= lambda • Lambda= resistance of membrane/resistance of cytoplasm • Distance along a neurite at which a constant applied voltage will decay to 37% of its original value. Common value is 100-300 um to mm. • The greater the membrane resistance, ie no channels the longer the psp travels

  16. Synaptic Integration • Look at Geometry of Inputs and the liklihood that any synapse will lead to an action potential in the axon of the post-synaptic neuron

  17. PreSynaptic Inhibition and Facilitation • Requires 3 synapses • The middle synapse can be active or inactive

  18. Types of CNS Synapses • Axodendritic • Axosomatic • Axoaxonic • Dendrodendritic

  19. Functional/Structural SynapseClassification • Gray’s Type I • Post-synaptic membrane is thicker than pre-synaptic • Asymmetrical • Excitatory • Gray’s Type II • Symmetrical synapse, pre & post-synaptic densities are similar thickness • Inhibitory

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