The basic question is what is the advantage of having a single long axon such as that of sciatic nerve (~1m) compared to a series of neurons?
The primary advantage is surely the conduction speed which will get affected because of chemical synapses. However, gap junction synapses can reduce that delay. The larger volume of the soma of the post synaptic neuron may also reduce the conduction speed, but signal strengthening can also happen. Cell architecture can be adjusted to minimize the loss of speed.
The obvious disadvantage of having a single large cell is that there would be tremendous burden on the nucleus for the cellular maintenance. There would be delays in transfer of non-electrical signals such as biomolecules from axon termini to soma and vice versa. Long axons also means continuous transport along them which in turn demands large number of molecular motors and therefore ATP. Moreover, a small injury can disable the entire conduction channel. It can be imagined that this trait (long axons) can also be selected against; injury leading to inability to move and thereby causing perishment.
EDIT (Based on comments to Anne's answer)
Time is of course a valid advantage for having a single axon as an information conduction channel. However, the energy or the maintenance cost is the factor that I am actually interested in. Maintenance cost would increase with cell volume (in fact surface area) and having serial neurons would demand more energy (cumulative) than a single axon (including costs of maintaining a nucleus). However a long cell would need much higher number of molecular motors to maintain the traffic flow rate. All responses that require transcriptional control would be slow (such as response to injury). Moreover a single soma also imposes a limit on the number of mitochondria. There should be a limit on how long an axon can be. I am especially interested in case of big animals (with long hindlimbs) that also have a good reflex (perhaps camels, even horses).
Has someone tried to analyse the tradeoffs of having a long axon vs series of neurons connected by gap junctions? If not is this problem worth modeling or quite trivial?
Saw this in Quora:
My first thought mostly agreed with the question details, except I'm pretty sure that the dorsal root ganglion (DRG) has a longer axon than the motor information carried in the sciatic nerve (which is the longest nerve in the body, but not axon).
Does this mean that sciatic nerve has serial neurons?