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Transcript
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? Why escape behaviour? Simple behaviour Short duration startle response simple nervous system Abdominal ganglia with about 400 neurons 2 Escape behaviours 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 Abdominal tap Ventral nerve cord Contains lateral giant LG Stimulated by tap LG Causes motoneurons Then muscles to be active Neural response Neural circuit – anatomy Neural circuit - schematic Sense organs Tactile hairs activated by water movement Sense organs Excite Sensory interneuron Direct path (a) Bi-synaptic path (b) Multiplicity – lowers threshold But with safety factor Abdominal ganglia Transverse section LG MG neurite somata neuropil LG to MoG Electrical synapse LG Motoneuron path Indirect Chemical Motoneuron filed with procion yellow LG → SG → MN Segmental giant LG MG interaction by rectifying electrical synapse between LG and SG SG provides chemical excitation of flexor motor neurons SG acts as amplifier Prevents FF Motoneuron 9 Fast Flexor motor neurons Individually identifiable All excited by LG via SG Rectifying synapse MG and LG separated LG & motoneurons Summary so far Excitatory pathway sense cell to muscle contraction Preventing second escape Turn off hair cell afferents CDI neurons produce delay and postsynaptic inhibition of the SI 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 Inhibition of Posture MRO normally excites extensor motoneuron and flexor inhibitor MRO turned off twice Accessory cell Fast extensor End of escape Inhibition of the flexion system LG spike FFMN FI 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 Other systems Locust & Drosophila jump Cockroach running Fish C-start Drosophila Rapid activation of GF Photoactivation of GF Flies cannot see http://www.sciencedirect.com/science/ MiamiMultiMediaURL/B6WSN-4FWM4P4-J/ B6WSN-4FWM4P4-J-4/7051/ d542b7199c07d3f274131cb29e173241/Movie_S2..mov Cockroach Arthropod – escapes from toads, etc Responds to air movement Cockroach Air movement hairs give directionality Escape correct way! Giant fibres Teleost fish Mauthner cell Large hindbrain, descending cell Responds acoustically Feed forward pathway Receptor – interneuron or Receptor – Mauthner ? C-start startle response But note Mauthner cell only used in some fast starts, other homologous cells exist in other neuromeres 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
