Giant Axons

Because an increase in axon diameter increases the conduction velocity of an action potential and because animals often face circumstances in which a rapid response is advantageous for survival, giant axons have evolved in several animal groups. No particular diameter qualifies an axon as giant. Rather, the term is relative: A giant axon is of exceptional diameter in comparison with other axons in the same animal. Some axons are truly giant in cellular dimensions, such as the third-order giant axons in the squid, which may be 1 mm (1000 μm) in diameter. At the other extreme, the giant axons in the fruit fly Drosophila are only about 4 μm in diameter, but they are still an order of magnitude larger than other nearby axons. Box Extension 12.3 describes the structure and function of giant axons in squid and in some other invertebrates.

Giant axons usually mediate behaviors for which speed and short latency of response are paramount, such as escape movements. The role of giant axons in squid locomotion provides an instructive example. A squid moves by jet propulsion. The animal contracts the muscles of its outer mantle to expel a jet of water from the mantle cavity through a moveable siphon. The giant axons serve to ensure a rapid and simultaneous contraction of all the mantle muscles—a necessary condition for fast, effective locomotion.

The squid actually possesses three sets of giant neurons, arranged in series:

Squid giant axon

In the brain are two partially fused first-order giant neurons (see Part 1), either of which can initiate activity of the entire propulsion system. Activation of a first-order giant neuron excites second-order giant neurons (see Part 2), which extend from the brain to paired stellate ganglia at the anterolateral margins of the mantle. Axons of several third-order giant neurons radiate from each stellate ganglion to the mantle muscles (see Part 3), and they are the motor axons that cause these muscles to contract.

The muscles at the posterior end of the elongate mantle are much farther from the stellate ganglion than are the anterior muscles. As an adaptation ensuring simultaneous contraction of these widespread muscles, the radiating axons of third-order giant neurons differ greatly in size. The largest and most rapidly conducting third-order giant axons extend to the most distant, posterior portion of the mantle. It is this arrangement of the giant axons that enables the signal for muscle contraction to arrive simultaneously at all parts of the mantle, as rapidly as possible.

Animals have followed several different evolutionary paths to achieve axons of large diameter. Giant axons can be unicellular (with one soma) or multicellular (with several somata). Unicellular giant axons are found in cockroaches, some annelids (such as Protula), and the medial giant axons of crayfish. Although most giant axons occur in invertebrates, the Mauthner neurons of fish and amphibians are examples of unicellular, myelinated vertebrate giant axons.

Multicellular giant axons may be syncytial or segmented. A syncytium (plural syncytia) results from a breakdown in cell membrane boundaries between cells, forming large multinucleated “cells” of multicellular origin. Squid third-order giant axons develop by syncytial fusion of processes of 300–500 cells that retain discrete somata. In contrast, crayfish lateral giant axons and the giant axons of many annelids are made up of segmentally arranged cells that form low-resistance, end-to-end junctions. These junctions act as electrical synapses that allow direct electrical transmission of action potentials from cell to cell (see Figure 12.1 in the textbook), so the segmented axon functions as if it were one cell.

Clearly giant axons have evolved repeatedly, in diverse forms and developmental origins. This evolutionary convergence indicates a strong adaptive advantage of rapid impulse conduction. Giant axons such as the squid’s are a selective disadvantage only in the neighborhood of Woods Hole, Massachusetts, and a few other locations of marine laboratories, where axonologists exert a negative selective pressure on the creatures that grow big axons, by using them for physiological experiments!