A lobster chews and filters its food with a clock built from about thirty neurons — the stomatogastric ganglion. One small circuit inside it, the pyloric network, produces a rhythm so reliable it runs for the animal’s entire life without ever stopping. The pattern is triphasic: a pacemaker group called AB/PD bursts and, through inhibitory synapses, silences two follower cells, LP and PY; as each follower is released from inhibition it rebounds and fires in turn, giving the fixed sequence PD → LP → PY, cycle after cycle. A network that generates a rhythmic motor pattern with no rhythmic input is called a central pattern generator, and the same kind of circuit drives your own breathing, walking and chewing.
This exhibit runs that circuit as three real relaxation neurons wired in the pyloric pattern with graded inhibitory synapses, and you watch the rhythm fall out of the dynamics on the oscilloscope and in the glowing ganglion. The reason this little circuit became one of the most important systems in all of neuroscience is the knob on the left. Eve Marder and her colleagues spent decades proving that the output of a circuit is not fixed by its wiring — neuromodulators, chemicals that bathe the ganglion, retune the very same neurons and synapses into completely different rhythms. Turn the knob down and the rhythm slows and falls apart; turn it up, or pick a modulator like dopamine or proctolin, and the same three cells beat out a new pattern. The connections never changed.
That is a deep and slightly unsettling idea: the wiring diagram is not the whole story. A brain is a chemical instrument as much as an electrical one, and the same network can be many circuits depending on what is washing over it. From a wasp’s reflex to a mouse’s map to a fly’s compass to this lobster’s clock, the museum’s last room makes one case four ways — that computation lives in the dynamics of neurons, and the dynamics can be tuned.
Specifications
- Circuit
- Pyloric network of the stomatogastric ganglion (~30 neurons)
- Rhythm
- Triphasic PD → LP → PY by pacemaking + post-inhibitory rebound
- Model
- 3 relaxation cells, graded inhibition, real pyloric connectivity
- Lesson
- Neuromodulators retune one circuit into many rhythms (Marder)
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