neural circuits

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