As I am planning to implement mechanics for the Axon organelle in the Multicellular Stage, and considering it’s listed for the upcoming 1.7 release, I took the time to read through the surface-level view of these topics for inspiration (and to avoid unnecessary inaccuracies). And I figured I might as well compile Thrive-relevant information here.
Most of this is literally just from reading the wikipedia page on neurons.
- Neurons consist of a main cell body that supports the metabolic activity of the cell, and neurites, which are the recognisable spindly projections of the cell membrane (and cytoplasm) sticking out from the cell, that actually carry the signals.
- Axons carry signals away from the cell body. If the cell has a distinct Axon, there is almost always only one. They can branch a lot afterwards though. Axons can also be extremely long, this basically just depends on which target they’re trying to reach.
Myelinated axons with cells as sheaths around them are cool and recognisable (like our current Axon icon), but they’re a vertebrate invention, and even there not all axons have them. Invertebrates that need faster transmissions make do by just making the axons thicker, which speeds things up because physics/chemistry. - Dendrites pick up signals and carry them towards the cell body. There may be one or many of them.
- In some cases, there aren’t specific identifiable axons or dendrites, and they just get called neurites.
- Axons carry signals away from the cell body. If the cell has a distinct Axon, there is almost always only one. They can branch a lot afterwards though. Axons can also be extremely long, this basically just depends on which target they’re trying to reach.
- While there is one most stereoptypical form, neurons can take quite a few different shapes in terms of usage of the above structures:
- Multipolar: One axon, lots of dendrites.
- Unipolar: One Neurite sticks out. It seems like most typically this then divides into dendrite and axon branches. But there seem to also be cases where it’s all a bit of an undifferentiated mess.
- Bipolar: One dendrite, one axon.
- Pseudounipolar: IMO Bipolar with extra steps. Short neurite/axon coming off the cell body, that than forms two branches, one to pick up the signal, and one to deliver it in the other direction. But, both are more axon-like. Likely developed from bipolar.
- Which morphology is used for which purposes though, is a very mixed bag across different between branches of life. For example, the central nervous system in vertebrates is mostly made up of multipolar neurons (though with the other types mixed in sometimes), while for invertebrates it’s unipolar neurons.
- The first neurons (with neurites) might have evolved from sensory cells to affect more than just the cells directly around them (to control cilia in this example), or even by splitting skin+muscle+sensory cells into separate muscle cells and sensory cells, leaving a connection in between.
In this case, the first neurites would indeed have been axons. But other sources are less certain that the above is true. - Neurons with axons also have something called an axon hillock in the cell body, containing the infrastructure needed to support the axon itself.