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Lecture 3

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Michael Inzlicht

LECTURE 3: Synaptic integration & neuromodulator Principles of synaptic integration  A muscle fiber receives 1 excitatory input from 1 motor neuron  Central neuron receive inputs from 100s of neurons  can be both excitatory and inhibitory  A neuron decides to fire an action potential or not  depending on the membrane potential at the spike trigger zone is above or below threshold  Action potential in the central nervous system is triggered by 1000s of EPSP and IPSP generated at different parts of complex dendritic tree  must travel down the axon hillock  Synaptic integration  EPSPs and IPSPs add up to determine the membrane potential at the spike initiation zone  Dendrites are the mere conduits delivering synaptic potentials to the site of integration Temporal summation  If a synaptic input occurs before a previous potential has decayed then the two will summate causing a high frequency excitatory input producing a high EPSP Spatial summation  If two synapses are located close together , immediate inputs will summate, however if they are distant then the potential may decay before they meet, preventing spatial summation  if the synapse inputs are further away then it doesn’t work  2 different inputs, 2 different sites Synaptic Integration  A synaptic input located relative to the spike initiation zone  depends on the branching architecture of the dendritic tree; can distal inputs 1mm away from the axon influence action potential generation?  Synaptic input is located relative to active dendritic conductance  Backpropagation of action potential  Synaptic input is located relative to other EPSP/IPSP, which is to do with synaptic placement  Frequency of synaptic input  Initial size of the EPSP/IPSP Dendrites  Apical  more distant from soma  Basal  very close to soma  Dendrite also have little branches called ‘spying’  These spying increase synaptic input and surface area of the dendrite Distal and basal dendrites  EPSP and IPSP decay with distance from their point of origin at synapses  Post-synaptic potentials from the distal synapses may decay more and therefore, contributing less to the hyperpolarization or depolarization Cable theory  If the post-synaptic potential is rely solely on the passive electrical properties of dendrites then we will see a decay of the PSP along the dendrites  meaning without the voltage gated ion channels  Further you move from the soma  decay of EPSP amplitude  PSP rate dependent : a) Length constant  long length constants aid spatial summation because EPSPs decay less over distance The length constant dependent of Rm and Ri Length constant achieved when the Rm high and Ri low  Rm : membrane resistance  depend on how leaky the membrane is  too leaky , Rm decreases  Ri : internal axial resistance  thin dendrite means bigger the Ri; any bumps or any obstacle  more conflict  Cm : membrane capacitance  storage of charges  operates like a battery depend on the diameter of dendrite b) Time constant  a time constant that describes how fast PSPs change, or decay in time  if a EPSP decay slowly, greater temporal window for summation with the next EPSP Time constant depend on Rm and Cm Closer the charged plates, higher the capacitance Greater the T, longer it will take to reach maximal voltage change; decay of voltage is slower c) Dendritic morphology - If the EPSP spread from synapse toward a closed end or a branch that is much finer  there’s less decay than predicted - ESPS spreads toward an expansion  spatial decay is more extreme - Spatial decay is direction-dependent or asymmetric EPSP will decay more over the same distance as it spreads toward the cell body from a dendrite than as it spreads towards the tip of the branch or a finer dendritic branch  At more distal sites, the PSP response to synaptic input has a higher amplitude than the PSP response to synaptic input at more proximal sites to the cell body  CA1 pyramidal neuron  further away the synaptic location was from the soma, the bigger the size of the EPSP  ‘dendritic democracy’ the distal synapse is deciding the outcome of an axonal output would count just as much as that of the proximal synapse  Distal dendrites tend to be more narrower than proximal, creating a larger local input resistance Ri and lower local membrane capacitance Cm that together act to increase the amplitude of local voltage response  Neocortical layer 5 pyramidal neurons  comparison with real EPSP showed an absence of dendritic democracy proving the probability of AP generation is determined directly by synapse location with the apical dendritic tree (close to the soma) Distal site doesn’t make a big difference; it is entirely dependent on passive  CA1 hippocampal pyramidal neurons real data showed site-independence of somatic EPSP, an equality of dendritic EPSP impact on AP output  dendritic democracy  Neocortical 5 layer shower an absence of dendritic democracy while CA1 hippocampal pyramidal shows presence of dendritic democracy  There is an increase density of post-synaptic receptorsAMPA receptors at more distal sites of dendrites [apical dendrites] Active conductance in dendrites can amplify EPSPs  Dendrites possess voltage-sensitive conductance that can generate spikes, that are mediated by voltage-gated Na+ and Ca 2+ channels also NMDAR channels  Dendritic excitability depends on the distribution of these channels along with the dendrites in addition to dendritic morphology  In hippocampal pyramidal cells, Ca 2+ mediated found at more distal dendritic sites, and Na + mediated spikes occur at more proximal
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