BIOL630 TERM PROJECT
NERVOUS SYSTEM
When it comes to the brain and the nervous system, octopuses, cuttlefish, and squid have the most evolved, largest, and most complex of the cephalopods. The nervous system for the octopus is commonly divided into three parts: 1) the central brain, 2) two optic lobes, and 3) the peripheral nervous system located in the arms (Hochner, 2012). The central brain and optic lobes are considered the central nervous system. A diagram of this nervous system layout can be seen in figure 1.
Figure 1. Diagram of the octopus nervous system layout and breakdown of neuron distribution. (Courtesy of Hochner, 2012)
Breaking this system up piece by piece, the optic lobes are connected to the octopus’ retinas through the photoreceptors. It connects to the retinas of the eyes via the optic nerve and contains three “cortical layers” that are similar in arrangement to the vertebrate retinas (Hochner et al, 2006).
The central brain of the octopus is made up of 40 lobes, thought to have developed from separate ganglia with different functions (Broadwell, 2019). Interestingly enough, it has been found that the octopus’s brain is unmyelinated (Adamo, 2019). Myelin insulates the nerves in the human brain and spinal cord and allows the action potential to move quickly along the axon (Hill, 2018). However, when unmyelinated like in the octopus, the speed of the action potential being sent slows greatly – about 100 times more slowly when the axon is as big as it is in an octopus (Zalc et al, 2008). This is speculated to be the reason evolutionary-wise how more neurons are located in other parts of the body than in the brain. In order to get messages relayed quickly, spreading out the neurons in areas other than the brain is beneficial.
More impressively, out of the 500 million total neurons in an octopus, two-thirds of the neurons in an octopus are distributed amongst the eight arms. Each sucker on the arms contains at least 10,000 neurons on their own (Godfrey-Smith, 2017). The nerves in each of the eight arms are capable of both slow and fast innervation, limited to the area where the dorsal nerve comes in contact with the muscle (Gutfreund et al, 2006). Even when amputated from the octopus’s body, researchers have found “extraordinary mobility”, (Gutfreund et al, 2006).