A simple way to study the nervous system is by looking at small worms, C. elegans. They have a very specific number of neurons (302) and a consistent pattern of how they connect and where they are located. They have a “brain” (term used loosely) which is a collection of nerves in their head- a ganglia is just a term for a bunch of nerves grouped together. We have ganglia too- for instance the doral root ganglia, which is a bunch of sensory neurons along the spinal cord.
Anyway, why do we care about C. elegans? Well, the simplicity of their nervous system (302 total neurons as compared to our ~100 billions neurons) means that it is much easier to look at and identify specific neurons. They are an ideal model for neuronal development and neurodegeneration/neurogenesis. You can see what types of genes, for example, might disrupt the normal development of their nervous system. You can also sever a single neuron and see what it needs to grow back to where it was. This is a much smaller scale injury as compared to human spinal cord injury, but it may help develop treatments that could be used to treat human developmental disorders or spinal cord injuries.