Octopus arms have segmented nervous systems

Octopus arms are a marvel of nature. Their 8 appendages can move with a near infinite degree of freedom and are packed with hundreds of suckers, which can change shape independently, and taste and smell the things they touch.

So, how do octopuses control all that complexity?

Inside each octopus arm is a massive nervous system, and now researchers have shown that it’s segmented.

“If you’re going to have a nervous system that’s controlling such dynamic movement, that’s a good way to set it up,” says Clifton Ragsdale, professor of neurobiology at the University of Chicago in the US and senior author of a new study published in the journal Nature Communications.

“We think it’s a feature that specifically evolved in soft-bodied cephalopods with suckers to carry out these worm-like movements.”

There are more neurons distributed across an octopus’ arms than in its brain. They are concentrated in a large axial nerve cord (ANC) that travels down the length of the arm.

When Ragsdale and collaborators studied lengthwise strips of the arms of California two-spot octopuses (Octopus bimaculoides) under the microscope, they made an unexpected discovery.

The neuronal cell bodies of the ANC are packed into columns that form discrete segments, like in a corrugated pipe or worm.

“Thinking about this from a modelling perspective, the best way to set up a control system for this very long, flexible arm would be to divide it into segments,” says Cassady Olson, a graduate student in computational neuroscience who led the study,

“There has to be some sort of communication between the segments, which you can imagine would help smooth out the movements.”

The ANC segments are separated by gaps called septa, where nerves and blood vessels exit to nearby muscles. Nerves from multiple segments connect to different regions of muscles, suggesting the segments work together to control movement.

Nerves for the suckers also exit the ANC through these septa and attach to the outer edge of each sucker.

The team also studied the longfin inshore squid (Doryteuthis pealeii) to see if this structure exists in other soft-bodied cephalopods. The species has 8 arms with muscles and suckers like an octopus. They also have 2 tentacles with long, sucker-free stalks that terminate in clubs (which do have suckers), which they use to grab prey while hunting.

They found that, while the ANC isn’t segmented within the tentacle stalk, it is segmented in the sucker-covered club at its end. This suggests that a segmented ANC is specifically built for controlling any type of dexterous, sucker-laden appendage in cephalopods. 

The squid ANC has fewer segments per sucker compared to octopuses, which the researchers think is likely because they do not use suckers in the same way as octopuses.

Whereas octopuses prowl the ocean floor and use their sensitive arms as tools for exploration, squid rely more on their vision to hunt in the open water.

“Different cephalopods have come up with a segmental structure, the details of which vary according to the demands of their environments and the pressures of hundreds of millions of years of evolution,” says Ragsdale.