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When it comes to flexibility, even the most limber among us can't begin to compare with the octopus. Joints like the shoulder, elbow and wrist limit maneuverability in humans and other creatures with articulated appendages. The octopus, in contrast, lacks joints. As a result, its arms possess virtually limitless freedom of movement. Such flexible arms require especially complex control, however. New research suggests that to meet those demands, the octopus allocates some of that motor control to neural circuitry embedded in the arms themselves, instead of relying on the brain alone. The findings appear today in the journal Science.
When an octopus reaches for something, it does so in a highly stereotypical and invariant way, unfurling its arm such that a bend propagates along the arm toward the tip (see images at right). Binyamin Hochner of the Hebrew University in Jerusalem and colleagues tested octopuses whose arm-brain connection had been severed and found that they could elicit the characteristic arm movement by stimulating the arms mechanically or electrically.
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Based on these observations, the researchers conclude that the octopus arm contains an independent peripheral nervous system. If so, it may well be that the brain governs the general direction of planned arm movement and the peripheral system handles the details. It is this division of neural labor and the use of highly stereotypical movements, they assert, that allow the octopus to maintain control over its exceptionally flexible appendages.
