Lecture 9

    Cards (23)

    • Wilfrid Rall's Theory

      Distal synaptic inputs produce EPSPs with smaller amplitudes but longer durations due to dendritic filtering
    • Dendritic Filtering
      Electrical currents flowing through a dendrite decay in amplitude and change in kinetic properties over distance, with distal synapses experiencing more significant amplitude and kinetic filtering compared to proximal synapses
    • Despite Rall's theory, substantial evidence indicates that the dependence of synaptic integration on the location might be minimal
    • Evidence against location dependence
      • Unitary EPSP amplitude is comparable from proximal and distal synaptic inputs
      • Temporal and spatial summation show minimal variation with distance from the soma
    • Passive Properties of Neurons
      Neuron morphology influences the amplitude and shape of local EPSPs, but these passive properties alone are insufficient to fully counteract the dendritic filtering effects
    • Active Properties of Dendrites
      • Voltage-gated ion channels like Na+ and Ca2+ make dendrites electrically excitable, amplifying EPSPs and potentially triggering dendritic action potentials
      • Hyperpolarization-activated and A-type K+ channels modulate the duration and spatial characteristics of EPSPs, reducing the location-dependent effects and supporting uniform synaptic integration
    • Enhanced Synaptic Properties
      • Presynaptic properties like the quantum of synaptic vesicles, the number of vesicles released per site, and the number of release sites per terminal
      • Postsynaptic properties like the density and affinity of receptors, with more receptors potentially placed at distal synapses to compensate for dendritic filtering
    • Hebbian Plasticity (LTP and LTD)

      Ensuring that synapses operate independently of location facilitates consistent firing across the neuron, promoting synaptic plasticity and efficiency
    • Population Synchrony
      Reducing location dependence simplifies the process of achieving synchronous firing across various input patterns, enhancing the neuron's ability to process complex information efficiently
    • Direct Inhibition
      An inhibitory neuron acting directly on a postsynaptic neuron, typically via GABA receptors, leading to the generation of IPSPs and hyperpolarization. The location of the IPSP (axosomatic or near the axon hillock) significantly affects its effectiveness; closer to the axon hillock generally results in greater inhibition of action potential firing
    • Indirect Inhibition
      Inhibition mediated by metabotropic receptors (e.g., GABA_B, mGluR) that influence neurotransmitter release, typically occurring at presynaptic sites where neurotransmitters like GABA and glutamate modulate subsequent neurotransmitter release through feedback mechanisms
    • Mechanisms of Inhibition
      Inhibition of neurotransmitter release is generally facilitated through the interaction of activated Gβγ subunits with Ca^2+ channels and the SNARE complex, reducing synaptic transmission
    • Network Structure
      • Neurons can connect with up to 10,000 presynaptic and postsynaptic neurons, consisting of principal neurons (usually excitatory) and interneurons (usually inhibitory)
      • Interneurons, depending on their types (e.g., basket cells, Martinotti cells, chandelier cells), target different parts of principal neurons to modulate their activity
    • Convergence
      Many presynaptic neurons connecting to a single postsynaptic neuron, allowing for integration of multiple signals
    • Divergence
      The branching of axons from one presynaptic neuron to multiple postsynaptic neurons, facilitating widespread signal distribution
    • Feedforward Excitation
      Sequential relay of information from one neuron to the next, crucial for propagating information throughout the nervous system
    • Feedforward Inhibition
      An excitatory neuron stimulates an inhibitory interneuron, which then inhibits the next neuron in the sequence, limiting further excitation
    • Lateral Inhibition
      Used for edge enhancement, where excitatory signals to one neuron stimulate inhibitory responses in neighboring neurons
    • Feedback/Recurrent Inhibition
      A neuron excites a downstream neuron, which in turn excites an inhibitory neuron that loops back to inhibit the original neuron, regulating activity within the circuit
    • Feedback/Recurrent Excitation
      Supports persistent activity loops within networks, important for functions like memory
    • Specific Neural Circuits and Their Functions
      • Stretch Reflex Circuit
      • Lateral Inhibition in Visual Processing
    • The integration of synaptic excitation and inhibition, facilitated through diverse neuronal network interactions, plays a critical role in processing complex neural information
    • Synaptic inhibition, whether direct or indirect, significantly shapes the neuronal response and is integral to maintaining the balance and proper function of neural circuits
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