Crayfish Startle Flashcards
MGI
threats from front
jump backwards and retreat
LGI
threats from rear
somersault and end with claws facing threat
LGI command neuron?
collects sensory info and initiates a pattern of movements- good example of CN
advantage of large neurons
large stimulus needed
fast conduction of spikes
LGI excited by disturbance to or near abdomen
each ganglion has L&R LGI
dendrites in ganglion collect sensory info
axons project anteriorly to next ganglion
electrical synapses allow 2 neurons to act as 1
only produces single spikes
within 50ms of stimulus, flexion is complete
one to one correspondance between LGI and spike tail flip
flips elicited by touch or water jets
injecting -ve electrical current will stop it spiking and stops tail flip in response to ab touches
From sense organ to LGI
bristles (140/seg) each attached to 2 sensory neurons
some sensory neurons connect directly to LGI through electrical synapse and prime it to further excitation
others excite small sensory interneurons through chemical synapses which go onto excite LGI after the other neurons
LGI to flexion
LGI excited motor giants which excite all of the flexor muscles (primes for further excitation)
LGI excited segmental giant which excites individual fast flexor motor neurons
all segmental pathway synapses are electrical
swimming triggered by pathway that is slower than LGI’s
LGI plays no direct role in initiating re-extension
electrical stimulation of LGI elicits rapid flecion then re-extension (<1% of these tail flips followed by swimming)
taps on abdomen below LGI threshold often trigger escape swimming
Sensory filtering
sensory neurons respond briefly to sudden touch or water movement but ignore slower ones
larger stimulus= higher number of receptors activated
temporal and spatial summation
individual EPSPs in LGI are short lived so need repetitive ones to imply stimulus
sensory adaptation- sensory receptor cells only respond to beginning of response
sensory habituation- sensory neuron to interneuron chemical synapses weaken
LGI inhibits abdomen extensor motor system
prevents extension interfering with initial fast flexion
prevents premature activation of reflex extension
lasts same as flexion
locations of inhibition; excitatory motor neurons, muscle fibres, muscle receptor organs
LGI inhibits sensory inputs to LGI (&LGI itself)
LGI inhibits sensory input through local interneurons
inhibition delivered at 2 locations
sensory interneurons
sensory neurons- right on presynaptic terminals
this is common in mechano-sensory systems
suppresses synapses (during tail flip)
protects synapse against habituation, so enables startle system to regain responses after flip
LGI inhibits abdomen flexor motor system
arrive when initial flexion ends- as extension starts (and inhibition of extensor system ends)
prevents additional flexor activity during extension
less direct than extensor inhibition
Tail Flip Variability
startle behaviour will over-ride and inhibit any conflicting, competing activity once it has been triggered but sensitivity varies
factors that modulate include social status and degree of hunger
can be sometimes almost impossible to elicit startle (when in fight)
Tail Flip Variability- Krasne & Wine (1975)
harder to elicit response when crayfish is feeding or being restrained
this suppression in restrained animal caused by inhibition of LGI that depends on supraesophageal andsubesophageal ganglia
Tail Flip Variability- Aggression and dominance
aggressiveness influenced by amine neuro-hormones, which also affect aggressiveness in mammals
in crayfish, serotonin increases aggressiveness
also increases responsiveness of LGI
effect is greatest in most dominant crayfish
subordinates likely to retreat using non-giant pathways
dominant animals need to be alert to unexpected threats
