How do nerves work? We think, we touch, we move according to the interactions of our brains and a very complex network of nerves, ganglia and synapses. Fifty years ago almost nothing was known about how nerves functioned. How the touch of a feather on the fingertip was transmitted to the brain. Mammalian nerves proved very difficult to study. They are very small and delicate and trying to measure electrochemical properties, particularly with the equipment available, was very difficult.

Mammalian neurons convey messages to the brain by passing an eletrochemical signal from one small cell to another, like a relay race, until the message reaches the brain. These thin and myriad connections have limits as to how fast and how much of a signal can be transported to and from the brain. The survival of squids like Loligo depend in a large part on their ability to move very quickly when danger arises. Lightning fast reflexes, rapid acceleration, and quick direction changes are all features of Loligo's defense strategy. The mantle of the squid can be expanded and contracted, drawing water into the cavity and rapidly expelling it through the siphon. This jet propulsion provides the squid with great speed but requires a coordinated and almost simultaneous activation of the muscle fibers throughout the entire mantle to provide the required thrust.

The means to accomplish this comes from a nervous system featuring some of the largest nerve cells found in nature. Instead of small nerve cells activating mantle contraction, Loligo features some "giant axons" axons that can be 10 cm in length. The cells may be over 100 times the diameter of a mammalan axon. This system provides the squid with the means to get it's body moving away from danger. It has been demonstrated that conduction velocity is directly proportional to the axon diameter. Thus, the axons that must activate muscle at the farthest reaches of the mantle are the largest in diameter, while the shorter distances nearer the stellate ganglion have proportionately smaller diameters. This allows contraction impulses to reach the muscle fibers throughout the mantle at the same time. The largest of these axons, the medial posterior mantle nerve, has been the focus of much of the science of neurobiology. Vertebrates solved this problem by evoloving a myelin sheath around each axon to speed conduction, so that "giant axons" are not common in vertebrate animals.

Researchers interested in the basic questions regarding nerves have found this giant axon to be invaluable as an experimental tool. It's large size allows it to be examined and studied in ways impossible with mammalian nerves. Early research in neurobiology was given a huge boost by the contributions of the giant axon to shed light on the electrical properties of nerves. Researchers today are using the giant axon to understand the constituent proteins and properties of the axoplasm, the inside of the axon.

Some MBL publications that explore these questions can be found here.