B61 Week 6

• Length constant


  l = √(rm/ri) or (rm/ri)1/2
• rm


  Resistance of unit area, divided by circumference of cylinder
• ri


  Axial resistance per unit length
• As a increases, ri decreases faster than rm and l gets bigger
• Lambda scales with radius
• Typical lambda for pyramidal cell dendrite

  • Rm 0.2 MΩ, resistivity ρ =200Ω, radius 2 um, length 1 cm, lambda = 1 mm
• Giant squid axon


  Has better conduction velocity due to larger radius
• Unmyelinated axon to send signal across 100 mm brain would need 2 cm radius

  
• Long-distance signal transmission requires myelin and non-passive (active) propagation of signals
• Pyramidal cells


  • Have low firing rate, extensive apical and basilar dendritic processes
• Electrical synapses


  Graded passive transmission, fast coupling of neuronal spiking

  
• Structure of gap junctions


  • High conductance non-selective ion channels, opening and closing governed by many factors
• Gap junctions in the wild

  • Ensembles of neurons temporally coupled, marine slug aplysia inking during escape response, synchronization of hippocampal interneurons
• Chemical synapses


  Amplify signals, multi-step process: AP produces Ca+2 influx, vesicles dock and quantal NT release, NT binds to receptor, channels open (ions flow)

  
• Receptors


  Ionotropic (ligand-gated channels), Metabotropic (second messengers)
• Synapse types


  • Glutamate (+): type I, asymmetric, axodendritic on spines. GABA and glycine (-): type II, symmetric, axodendritic, axosomatic, axoaxonic
• Postsynaptic potentials


  Acetylcholine binding to nicotinic receptor produces Na+ influx, Glutamate binding to NMDA receptor produces Ca+2 influx
• Anatomy of synapses in the brain

  • Postsynaptic spines are small (0.5 µm), high density packing (109 synapses, 4.1 km of axon, 500 m of dendrite per 1 mm3), dozens of neurotransmitters and neuropeptides, dozens of receptor subtypes

  
• Chemical signals and knee-jerk reflex

  Flexor and extensor muscles, monosynaptic circuit motif, sensory/motor signaling: integration for behavior, glutamate from 1a sensory neuron released on alpha motor neuron & activates GABA in inhibitory interneuron
• EPSPs and knee-jerk reflex: extensor motor neuron


  Stimulating all 1a sensory fibers creates a larger EPSP that leads to an action potential in extensor motor neuron

  
• IPSPs and knee-jerk reflex: flexor motor neuron

  Population of IPSPs create greater hyperpolarization
• Interaction of postsynaptic potentials


  EPSP from artificially passing current in motor neuron drives a spike, IPSP from interneuron in the motor neuron moves Vm away from the threshold, co-occurrence of EPSP and IPSP fails to reach threshold

  
• Interaction of postsynaptic currents & potentials

  Postsynaptic currents show how Cl- moves across membrane, depending on Vm. Effect of GABA depends on Vm.
• CNS signaling is confined to digital action potentials and their firing patterns
• Passive cable properties dictate EPSP and IPSP spread
• PSP summate together to reach threshold in trigger zone for action potential generation
• Dendrites have an under-appreciated and complex role in neuronal processing
• Interaction of postsynaptic currents & potentials

  Describes how postsynaptic currents and potentials are related
• Voltage clamp experiment


  Experimental technique to measure postsynaptic currents
• Effect of the presynaptic IN depends on

  • Vm in the motor neuron
  • PSP is (-) or (+)

  
• Postsynaptic currents show how Cl-

  • Moves across membrane, depending on Vm
  • Outward (+) current = influx of Cl-
  • Inward (-) current = efflux of Cl-
• Reversal potential for the IPSP


  Identical to ECl
• Effect of GABA


  Depends on Vm
• Outline


  • Primer on synaptic signaling
  • Active dendritic processing
  • Axon-centric view of neuron
  • CNS signaling is confined to digital action potentials and their firing patterns
  • Passive cable properties dictate EPSP and IPSP spread
  • PSP summate together to reach threshold in trigger zone for action potential generation

  
• These are good initial descriptions, but there is a lot of processing happening before an action potential is generated
• Importance of dendrites


  • Dendrites have an under-appreciated and complex role in neuronal processing
• Half truths about dendrites

  • Postsynaptic potentials conduct passively toward the axon hillock
  • EPSPs and IPSPs sum algebraically (i.e. linearly) within dendritic tree
  • The length and time constants of a dendrite are constant
  • Neural computation in circuits is mainly reflected in suprathreshold firing of action potentials

  
• Dendrites

  • Leaky, non-myelinated conductors
  • Low membrane resistance and long distance to soma
• Dendritic regions


  • Apical Dendrites
  • Basal Dendrites
  • Distal
  • Proximal
• Local EPSP amplitude at the synapse

  EPSP amplitude at the soma is lower due to distance-dependent decline