Source: Lecture slides, "A Brain-Mind Odyssey" | Textbook: Luo, Principles of Neurobiology
Tags: crayfish escape response, lateral giant interneuron, LGI, motor giant neuron, MoG, command neuron, reflex threshold, all-or-none, electrical synapse, chemical synapse, EPSP, sensory transduction, mechanosensory neuron, neural decision-making, Procambarus clarkii
The crayfish tail flip escape response is a well-understood example of how a simple neural circuit produces a defined behaviour. Mechanosensory neurons on the tail detect water movement, relay that information through interneurons to the Lateral Giant Interneuron (LGI), which integrates the input and makes an all-or-none "decision" to fire. When LGI fires, it triggers motor giant neurons that contract tail segments 1 to 3, flipping the animal away from the threat.
Crayfish tail flip escape response
A reflex behaviour in crayfish (Procambarus clarkii) in which a sudden tap or water movement on the tail triggers rapid tail flexion, pulling the body away from the stimulus. Also called the lateral giant tail flip.
Lateral Giant Interneuron (LGI)
The command/decision neuron in the tail flip circuit. LGI receives summed excitatory input from sensory neurons; when that input crosses spike threshold, LGI fires and the escape behaviour is triggered. LGI has a very large axon (approximately 1 mm diameter).
Motor Giant neuron (MoG)
Large motor neurons that receive input from LGI and drive contraction of tail muscles. Like LGI neurons, they have huge axons and are linked across segments by electrical synapses.
Command neuron
A neuron whose activity is both necessary and sufficient to trigger a specific behaviour. LGI functions as a command neuron for the tail flip: stimulating it causes the behaviour, and preventing it from firing (by hyperpolarisation) blocks the behaviour.
Electrical synapse (gap junction)
A synapse that transmits signals via direct ionic current flow through connexin proteins. Electrical synapses connect LGI neurons across tail segments and connect LGI to MoG neurons. They are fast and can be bidirectional.
Connexins
The proteins that form gap junction channels at electrical synapses.
Chemical excitatory synapse
A synapse that transmits signals by releasing neurotransmitter (e.g. glutamate) across a synaptic cleft, causing an EPSP in the postsynaptic cell. Mechanosensory neurons use chemical synapses to excite interneurons and LGI.
Mechanosensory neuron
A sensory neuron that detects mechanical stimuli. In the crayfish tail flip circuit, these neurons innervate cuticular hairs on the tail fan and respond to water movement. They function somewhat like the cilia in the mammalian inner ear.
All-or-none reflex
A behaviour that either occurs fully or does not occur at all. There is no partial tail flip. The reflex has a sharp threshold: below it, nothing happens; above it, the full escape sequence is executed.
Reflex threshold
The stimulus intensity (and corresponding EPSP amplitude in LGI) at which the tail flip switches from "no response" to "full response." Demonstrated in the Krasne and Lee (1983) data.
Spike threshold
The membrane potential that must be reached in a neuron for it to fire an action potential. Each neuron has a defined spike threshold.
Ganglia
Clusters of neuronal cell bodies. The crayfish tail flip circuit operates within segmental ganglia of the tail, where the cell bodies of LGI, MoG, and interneurons are located.
When something taps the tail of a crayfish or disturbs the water nearby, the animal performs a rapid tail flip: the first three tail segments flex sharply, pulling the body backwards and away from the stimulus.
Key features:
Only tail segments 1 to 3 flex; the remaining segments do not bend
The response is all-or-none (no halfway tail flip)
It is a discrete "decision": the crayfish either escapes or does not
The stimulus threshold follows a sharp cutoff (Krasne and Lee, 1983)
Cuticular hairs on the tail fan are innervated by mechanosensory neurons. These hairs bend in response to water movement, somewhat like cilia in the inner ear.
When the hairs are displaced by rapid water movement, the receptor neurons fire action potentials. The firing rate increases with stronger stimulation. This is the first encoding of the stimulus.
The complete pathway:
Sensory neurons → Interneurons (A, C) → LGI neurons → Motor Giant neurons (MoG) → Muscles
All of this happens within the segmental ganglia of the crayfish tail.
Sensory neurons to interneurons:
Mechanosensory neurons on the tail fan connect to interneurons (labelled A and C in the circuit diagram) via chemical excitatory synapses
These interneurons provide a graded response that scales with stimulus intensity
Interneurons to LGI:
Interneurons excite the LGI neuron via chemical excitatory synapses
The EPSP in LGI grows larger as the stimulus gets stronger (more sensory neurons firing, stronger input)
LGI across segments:
LGI neurons in different tail segments (1, 2, 3) are connected to each other by electrical synapses (gap junctions using connexins)
This means that when one LGI fires, the signal propagates rapidly and reliably across all three segments
LGI to MoG:
LGI excites Motor Giant neurons via electrical synapses
This is an extremely fast connection, appropriate for an escape reflex
MoG to muscles:
Motor Giant neurons release glutamate at the neuromuscular junction (analogous to ACh release at the NMJ in mammals)
Muscles in segments 1 to 3 contract, producing the tail flip
The LGI neuron integrates all sensory input and converts it into a binary decision.
The evidence that LGI is both necessary and sufficient:
Sufficient: Electrical stimulation of the LGI neuron alone is enough to trigger contraction of tail segments 1 to 3 and produce the full escape behaviour.
Necessary: In experiments by Olson and Krasne (1981), an in vitro isolated nerve cord was used. When a second electrode was used to hyperpolarise LGI (injecting current to hold it below spike threshold), sensory stimulation still drove EPSPs in LGI but LGI could not spike. Without the LGI spike, no action potentials appeared in the motor nerve, and no tail flip occurred. The EPSP drive was present but the spike was blocked, so the behaviour was blocked.
This confirms that spiking in LGI is the required signal to initiate the behaviour.
The decision mechanism is straightforward:
Sensory input generates a graded network response (the more sensory neurons fire, the larger the summed EPSP in LGI)
LGI integrates all of this input
If the total EPSP reaches LGI's spike threshold, LGI fires an action potential (all-or-none)
Once LGI fires, the motor programme is triggered automatically and the full tail flip occurs
If the EPSP stays below threshold, nothing happens
The graph from Krasne and Lee (1983) shows this clearly: at low stimulus intensities, EPSPs are small and no tail flip occurs (purple dots). At a critical intensity, EPSPs cross threshold and every subsequent stimulus produces a tail flip (red dots). The transition is sharp.
The circuit uses both major synapse types:
Chemical synapses (slower, unidirectional, modulatable):
Mechanosensory neurons → interneurons
Interneurons → LGI
Electrical synapses (fast, can be bidirectional, via connexins):
LGI → LGI across segments
LGI → MoG motor neurons
The speed of electrical synapses is particularly important here because escape must be fast.
Sensory neurons produce a graded response that scales with stimulus strength
The LGI command neuron integrates the input and generates an all-or-none spike if threshold is reached
Motor neurons create the specific muscle activation pattern for the reflex (contraction of segments 1 to 3 only)
This is a clean example of how sensory encoding, decision-making, and motor output are implemented by a neural circuit. In more complex animals, decision circuits involve far more neurons and are considerably more elaborate, but the basic principles (summation, threshold, all-or-none output) remain the same.
Crayfish lateral giant tail flip sequence (showing progressive tail flexion of segments 1 to 3)
Circuit diagram: mechanosensory neurons → interneurons (A, C) → LGI → MoG → muscles, with chemical and electrical synapses labelled
Krasne and Lee (1983) threshold graph: EPSP amplitude in LGI vs stimulus intensity, with tail-flip-occurred (red) and no-tail-flip (purple) data points
Olson and Krasne (1981) hyperpolarisation experiment: comparison of LGI recordings with and without hyperpolarisation, showing that blocking LGI spike eliminates the motor response
Summary flow diagram: graded sensory response → LGI integration and all-or-none spike → specific motor pattern → muscle contraction
⚠️ LGI is both necessary and sufficient for the tail flip. Know the experimental evidence for each (electrical stimulation for sufficiency; hyperpolarisation block for necessity).
⚠️ The tail flip is all-or-none. There is a sharp reflex threshold. This is a direct consequence of LGI's spike threshold acting as the decision point.
⚠️ Know which synapses are chemical (sensory → interneuron → LGI) and which are electrical (LGI to LGI across segments; LGI to MoG). Be able to explain why electrical synapses are advantageous for escape reflexes (speed).
⚠️ Only segments 1 to 3 flex. The motor programme is specific.
⚠️ Connexins form gap junction channels at electrical synapses. This is a term worth remembering.
⚠️ The crayfish uses glutamate at its neuromuscular junction, whereas mammals use acetylcholine (ACh). Do not mix these up in an exam context.
⚠️ This circuit illustrates general principles that apply to more complex brains: EPSP summation, spike threshold as a decision mechanism, and the existence of command/decision neurons.
Q: What is the crayfish tail flip escape response, and what triggers it?
A: It is a rapid reflex in which the crayfish flexes tail segments 1 to 3 to pull its body away from a threat. It is triggered by a sudden tap on the tail or water movement detected by mechanosensory neurons on the tail fan.
Q: Trace the neural circuit for the crayfish tail flip from sensory input to muscle contraction.
A: Mechanosensory neurons on the tail → interneurons (A, C) via chemical synapses → LGI command neuron via chemical synapses → LGI spreads across segments via electrical synapses → Motor Giant (MoG) neurons via electrical synapses → tail muscles via glutamate at the neuromuscular junction → contraction of segments 1 to 3.
Q: What evidence shows that the LGI neuron is sufficient to trigger the tail flip?
A: Direct electrical stimulation of the LGI neuron causes contraction of tail segments 1 to 3 and produces the full escape behaviour, even without sensory input.
Q: What evidence shows that LGI spiking is necessary for the tail flip?
A: In the Olson and Krasne (1981) experiment, hyperpolarising LGI with current injection prevented it from spiking even though sensory stimulation still produced EPSPs. Without the LGI spike, no action potentials appeared in the motor nerve and no tail flip occurred.
Q: Why is the tail flip described as an "all-or-none" reflex?
A: Because there is no partial response. Below the reflex threshold, no tail flip occurs. Above it, the full escape sequence is executed every time. This reflects the all-or-none nature of the action potential in the LGI command neuron.
Q: What are the two types of synapses used in the tail flip circuit, and where does each occur?
A: Chemical excitatory synapses are used from sensory neurons to interneurons and from interneurons to LGI. Electrical synapses (gap junctions, formed by connexins) connect LGI neurons across segments and connect LGI to MoG motor neurons.
Q: Why are electrical synapses particularly well-suited for this escape circuit?
A: Electrical synapses transmit signals faster than chemical synapses because current flows directly through gap junction channels without the delay of vesicle release and receptor binding. Speed is critical for escape from predators.
Q: How does the LGI neuron implement a "decision"?
A: LGI sums excitatory postsynaptic potentials from the sensory network. If the total depolarisation reaches spike threshold, LGI fires and the tail flip is triggered. If it does not reach threshold, nothing happens. The spike threshold acts as the decision boundary.
crayfish tail flip, lateral giant interneuron, LGI, motor giant neuron, MoG, command neuron, escape reflex, escape response, Procambarus clarkii, mechanosensory neuron, cuticular hair, all-or-none reflex, reflex threshold, spike threshold, EPSP summation, electrical synapse, chemical synapse, gap junction, connexin, ganglia, sensory transduction, decision neuron, Krasne and Lee 1983, Olson and Krasne 1981, hyperpolarisation experiment, neuromuscular junction, glutamate, neural decision-making, circuit computation, NEU 100B, UC Berkeley