Why do neurons become unipolar

Nerve cell, unipolar

The term "unipolar nerve cell" is not clearly defined in the specialist literature. The spectrum lexicon of neurosciences, for example, writes [1]that unipolar nerve cells consist only of the soma and a single long primary process, which then takes on the function of an axon. In some unipolar nerve cells, however, this long process is branched, the secondary branches then have a similar function to dendrites and can therefore absorb information.

Other sources refer to such branched unipolar nerve cells as pseudounipolar.

According to Bear, a nerve cell is unipolar when the soma has a single neurite [2]. Baer understands "neurite" to be a generic term for axons and dendrites.

A unipolar neuron according to Bear, redrawn by U. Helmich.

Bear does not address the question of what functions the two branches of the "neurite" have.

The following representation of a unipolar nerve cell can be found in Wikipedia:

A unipolar neuron from Wikipedia, redrawn by U. Helmich.

In this picture all branches look the same; no distinction is made between an axon and the dendrites; there are also no synaptic endbones visible.

With further research, the following representation of a "unipolar neuron" can also be found on the Internet [3]:

Another representation of a unipolar neuron, redrawn by U. Helmich.

Unfortunately, the original source of this drawing was not given in the reference, which I think is very unscientific. Here you can see a neuron in which actually only a single process (neurite) emerges from the soma. However, this extension branches in two places. The first branch then forms the dendrites, the second branch forms the axon collaterals; recognizable by the synaptic end knobs.

In other references it is pointed out that the cones and rods of the vertebrate retina are unipolar nerve cells.

Conclusion: Nobody really knows what a unipolar neuron actually is. Perhaps one should forget these technical terms, some of which come from the early days of neurobiology (late 19th century).