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NPB162-L15

4 min read

📗 -> 02/27/25: NPB162-L16

Crayfish end
Short fish

🎤 Vocab

❗ Unit and Larger Context

Crayfish Neuroeconomics and Modulations

Fish Outline

Due to limited time, somethings cut

  • Overview
    • Escape C- Response
    • Sensory Anatomy
  • Mauthner Cells’ Properties & Inputs
    • Large Myelinated Club Ending (LMCEs)
    • Passive Hyperpolarizing Potential (PHPs) &
      Ephaptic Synapses
  • Mauthner Cells’ Outputs
  • Mauthner Cell as Command Neurons
    • M-cell and Non-M-cell escape
  • Cognitive Control of Escape Behavior
  • Social Control of Escape Behavior

✒️ -> Scratch Notes

Crayfish

Modulation and Neuroeconomics
  • The probability of GF escape activation is subject to extensive modulation
  • It ranges from practically zero for a restrained adult crayfish, to an almost manic hypersensitivity for free-moving juveniles.
  • Every escape has an opportunity cost (whatever the escapee can’t do instead of escaping) and an energy cost.
Test: cut the connective between the brain and the LG.

Conclusion

  • The reduced responsiveness to threatening stimuli observed in restrained crayfish is due to inhibitory input synapsing on (distal dendrites of) the LG neuron.
  • The escape response then becomes the outcome of balancing/comparing excitatory vs inhibitory inputs.
Social Status Modulates Tail Flip Threshold
  • As many other species, crayfish have a hierarchical system. In this system, dominant individuals get access to more food, shelter and mates.
  • Experiments show that dominant crayfish get more food than a medium-ranked crayfish who themselves get more food than the overall subordinate.
  • When pairing two crayfish (“agonist encounter”), dominance will be determined within seconds to minutes. → This leads or is associated with changes in the nervous system.
    Subordinate animal shows higher likelihood of escape response in agonistic encounters

Fish

Biophysical Properties of M-Cells

Many biophysical properties contribute to its
response properties:

  • High threshold
  • Low input resistance (≈200 kΩ).
    • Changes more readily to input, but needs more combined input
  • Very negative resting potential (≈ –80 mV).
  • Exceptionally short time constant (about 400 μs) which reduces temporal summation.
Mauthner (M) cells
  • Two large dendrites
    • Lateral - Receives inputs from the ear (distal) and lateral line(proximal)
    • Ventral - Receives inputs from the visual and somatosensory systems
  • Large diameter axons
  • Axons cross the midline of the brainstem and projects caudally contralaterally

Large Myelinated Club Ending (LMCEs) - Name given to the synapse between the auditory hair cells and the M-cell dendrite

  • Combination of rectifying electrical synapses and of chemical glutamatergic excitatory synapses
    Commissural Passive Hyperpolarizing Potential (PHP) - Interneurons

  • Feedforward inhibition

  • A sound stimulus activates afferents from the ears and lateral line that directly excite the M-cells, and also the PHP interneurons. The synapse into PHP is an electrical synapse.

  • The PHP neurons themselves inhibit both the contralateral and ipsilateral M-cells through an electrical inhibition (mediated by electric fields, see next slide) + chemical inhibition (glycine).

  • When activated, inhibitory synapse to the same side of thecell

    • Ipsilateral chemical inhibition

  • Has synapses to the cell body of the M cells

    • Prevents movement we don’t want
Two forms of electrical synaptic transmission
  • Gap junction - provide a path of low resistance for ions
  • Field - current transmitted as a result of the electric fields generated by neuronal activity
    Usually the gap junction is taught, but in reality:
  • Diffusion is very slow
  • Neighboring sodium ion is repelled by ions coming in. Which repels the next one and so on.
    • So we get this movement of charge, but not diffusion of ions as much
  • Not the actual ion that’s /moving all the way through, but the charge of the ion moving other ions is how the charge is transmitted
    “Cable theory”
  • Very fast, almost an immediate response
Inhibition by Electric Fields

Axon Cap - A thick, sheath-like capsule composed of glial cells and connective tissue, which surrounds the axon hillock and which has very high electrical resistance
Terminals of the PHP neurons penetrate into the axon cap
It sets the threshold for the startle response that triggers activation of the M-cell

Axodendritic - common chemical transmission. Axon -> Dendrite
Axosomatic - Most effective transmission. Axon -> Cell body.

🧪 -> Refresh the Info

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