Post on 12-Jan-2016
description
Neural circuitsLecture 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 () Bi-synaptic path ()
Multiplicity – lowers threshold But with safety
factor
Abdominal gangliaMGLG
somata neuropil
neurite
Transverse section
LG to MoG Electrical synapse
LG Motoneuron path Indirect Chemical
Motoneuron filed with procion yellow
LG → SG → MN Segmental giant
Prevents LG MG interaction by rectifying electrical synapse between LG and SG
SG provides chemical excitation of flexor motor neurons
SG acts as amplifier
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 musclecontraction
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
FFMN FI
LG spike
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