The octopus, an invertebrate known for its remarkable problem-solving abilities, possesses a highly decentralized nervous system. The majority of its neurons, approximately two-thirds of the total, are located outside of the head. This distribution gives rise to the popular concept of “nine brains” and allows the octopus to manage eight highly flexible limbs without overwhelming its main computational center.
The Central Brain: Location and Function
The centralized nervous tissue is located in the head, nestled between the two large eyes. This main brain is protected by cartilage, which functions similarly to a skull. An unusual anatomical detail is that the animal’s esophagus passes directly through the center of the brain mass, a constraint inherited from its mollusk ancestry.
The central brain is responsible for all higher-order cognitive functions, including learning, memory formation, and sophisticated decision-making. It also controls the instantaneous and dramatic camouflage displays for which the octopus is famous. Although it holds less than half of the total neurons, this mass acts as the main director for the animal’s overall behavior and strategy.
The Distributed Intelligence: Ganglia in the Arms
The uniqueness of the octopus nervous system lies in the massive neural networks embedded within its eight arms. Each arm contains the axial nerve cord, which runs the entire length of the limb and holds large clusters of nerve cells called brachial ganglia. The total number of neurons housed in these eight arms is estimated to be around 350 million, far exceeding the count in the central brain.
The brachial ganglia function as local processing centers, effectively acting as “mini-brains” for their respective arms. Each axial nerve cord is a complex, segmented network capable of processing sensory input and generating motor output. This distributed system offloads the computational burden of controlling eight infinitely flexible limbs.
Independent Action: What Arm Autonomy Looks Like
The decentralized structure grants the arms a profound degree of autonomy, allowing them to make reflexive decisions based on local sensory input. This independence is demonstrated by the suckers, which are active sensory organs lined with chemoreceptors. This allows the arm to “taste” and touch its environment simultaneously, deciding whether to grasp an object without consulting the central brain.
For instance, an arm can locate food and move it toward the mouth based on local processing within its ganglia. Laboratory observations show that a severed arm can continue to move, respond to stimuli, and grasp objects for up to an hour after separation. Furthermore, the local ganglia recognize a chemical cue that prevents the suckers from attaching to the octopus’s own skin.
Sensory Integration and Coordinated Movement
Despite their independent capabilities, the eight arms must still work together for coordinated actions like locomotion or hunting. The central brain functions as the high-level director, initiating a general command, such as “move forward” or “reach for that object.” This instruction is transmitted to the arms via the cerebrobrachial tracts.
Once the command is received, the individual brachial ganglia take over the complex, fine-grained details of the movement. This involves determining the precise bend of the limb, the sequence of sucker activation, and the force applied, all based on the sensory feedback the arm is receiving locally.