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-Inhibitory Interneuron Postsynaptic Neuron
+Excitatory Interneuron Postsynaptic Neuron
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Interneurons
An Introduction toExcitatory and Inhibitory Interneurons
andIntegration
By Noel Ways
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Excitatory interneuron that produces an Excitatory postsynaptic potentials (EPSP)
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Excitatory Interneuron Postsynaptic Neuron
Inhibitory interneuron that produces an Inhibitory postsynaptic potentials (IPSP)
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Inhibitory Interneuron Postsynaptic Neuron
If the interneuron is inhibitory, it's effect will be to bring the postsynaptic neuron further away from threshold due to the influx of negatively charged calcium ions (Cl-) into the neuron; and the efflux of positively charged potassium ions (K+) out of that neuron.
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INHIBITORY AND EXCITATORY INTERNEURONS
Inhibitory and excitatory interneurons play a crucial role in determining whether a postsyn-aptic neuron reaches the threshold. If the interneuron is excitatory, it will bring the postsyn-aptic neuron closer to threshold due to the in�ux of positively charged sodium ions (Na+).
+K-Cl -Cl
-Cl -Cl
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Na+
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Na+Na+
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PO4-3
PO4-3 Protein
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-Cl -Cl -Cl-Cl -Cl-Cl-Cl
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PO4-3
PO4-3 Protein
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Excitatory Interneuron
Inhibitory Interneuron
Chem. SensitiveChloride Gate
Chem. SensitiveSodium Gate
Voltage Sensative Gates
Voltage Sensative Gates
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-40 Threshold is ReachedSodium ActivationGate opens
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Temporal SummationA single excitatory stimulation may not allow enough positively charged sodium (Na+)into the postsynaptic neuron enabling it to reach the threshold. However, repeatedstimulation through time (temporal) may result in threshold due to the sum ofall the sodium that accumulates.
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Likewise, repeated inhibitory stimulation can be summed up decreasing the likelihoodof an action potential being generated as the membrane potential gets further and furtheraway from threshold.
Remember, these effects are on the postsynaptic neuron, NOT the interneuron.
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Spatial SummationThe summation can also be spatial (over the space or the area of the postsynaptic neuron).In the convergent circuit illustrated below, if all the presynaptic neurons simultaneouslyexcite the postsynaptic neuron, this may result in it quickly reaching the threshold,followed by an action potential.
Spatial Summation may also occur with inhibitory neurons, and of course, the generation of an action potential is most unlikely (not shown).
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-AxonHillock
IntegrationA postsynaptic neuron is frequently a point of integration for both inhibitory and excitatory stimulation, where both spatial and temporal summation may occur concurrently. The determining factor, however, is the state of the axon hillock, where the electrical potential of the hillock will reflect the sum of effects "upstream". One can view this as a sort of "tug of war" between inhibitory and excitatory stimulation. regardless, no action potential will be generated until the threshold is reached. For the postsynaptic neuron, this is an all or nothing principle.
InhibitoryNeuron
InhibitoryNeuron
( - )IPSP
IPSP( - )
ExcitatoryCircuit
InhibitoryCircuit
Diaphragm andExternal Intercostals
Application: Respiratory Rhythmicity Center(A hypothetical / Simplistic Model)
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