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Neural circuits

Neural circuits. Lecture 3. Cellular neuroscience. Nerve cells with ion channels and synapses How do neurons interact? How is activity patterned? How is appropriate activity selected? How is sensory input used? How is motor output coordinated and generated?. Why Crayfish?.

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Neural circuits

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  1. Neural circuits Lecture 3

  2. Cellular neuroscience • Nerve cells with ion channels and synapses • How do neurons interact? • How is activity patterned? • How is appropriate activity selected? • How is sensory input used? • How is motor output coordinated and generated?

  3. Why Crayfish? • Why escape behaviour? • Simple behaviour • Short duration startle response • simple nervous system • Abdominal ganglia with about 400 neurons

  4. 2 Escape behaviours

  5. 2 Escape behaviours • Anterior tap • Goes back • All segments bend • Tail tap • Goes up • Segments 1-3 bend • Differences in physiology match differences in adaptive behaviour

  6. Abdominal tap • Ventral nerve cord • Contains lateral giant • LG Stimulated by tap • LG • Causes motoneurons • Then muscles to be active

  7. Neural response

  8. Neural circuit – anatomy

  9. Neural circuit - schematic

  10. Tactile hairs activated by water movement Sense organs

  11. Excite Sensory interneuron Direct path (a) Bi-synaptic path (b) Multiplicity – lowers threshold But with safety factor Sense organs

  12. Abdominal ganglia MG Transverse section LG neurite neuropil somata

  13. LG to MoG • Electrical synapse

  14. LG  Motoneuron path • Indirect • Chemical • Motoneuron filed with procion yellow

  15. LG →SG →MN • Segmentalgiant • Prevents LG  MG interaction by rectifying electrical synapse between LG and SG • SG provides chemical excitation of flexor motor neurons • SG acts as amplifier

  16. FF Motoneuron • 9 Fast Flexor motor neurons • Individually identifiable • All excited by LG via SG • Rectifying synapse • MG and LG separated

  17. LG & motoneurons

  18. Summary so far • Excitatory pathwaysense cell to musclecontraction

  19. Preventing second escape • Turn off hair cell afferents • CDI neurons produce delay and postsynaptic inhibition of the SI

  20. Preventing second escape • Turn off hair cell afferents • CDI neurons produce delay and postsynaptic inhibition of the SI • CDI neurons produce delay and also presynaptic inhibition of the receptors

  21. Inhibition of Posture • MRO normally excites extensor motoneuron and flexor inhibitor • MRO turned off twice • Accessory cell • Fast extensor

  22. End of escape • Inhibition of the flexion system LG spike FI FFMN

  23. Major features of net • Need sensory coincidence to fire LG • Ensures safety if single cell accidentally fires • Lowers behavioural threshold below single neuron threshold (law of averages) • Fast • Multiple, parallel pathways • Combination of electrical feed-forward and chemical excitation • Chemical allows amplification of signal • Chemical allows modulation of pathway

  24. Other systems • Locust & Drosophila jump • Cockroach running • Fish C-start

  25. Drosophila

  26. Rapid activation of GF

  27. Photoactivation of GF • Flies cannot see http://www.sciencedirect.com/science/MiamiMultiMediaURL/B6WSN-4FWM4P4-J/B6WSN-4FWM4P4-J-4/7051/d542b7199c07d3f274131cb29e173241/Movie_S2..mov

  28. Cockroach • Arthropod – escapes from toads, etc • Responds to air movement

  29. Air movement hairs give directionality Escape correct way! Giant fibres Cockroach

  30. Teleost fish • Mauthner cell • Large hindbrain, descending cell • Responds acoustically

  31. Feed forward pathway • Receptor – interneuron or • Receptor – Mauthner ?

  32. C-start startle response • But note Mauthner cell only used in some fast starts, • other homologous cells exist in other neuromeres

  33. Conclusions • Apparently simple behaviour has complex neural circuit • Giant fibers for fast response • Feed-forward pathways • Safety features so only escape when needed • Chemical systems • Amplification • Modulation • Inhibition

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