Stereotyped neuropil branching of an identified stomatogastric motor neuron

Anatomical studies of the crab stomatogastric ganglion (STG) have suggested only minimal organization within the neuropil of this structure. Here, we present evidence that, for at least one intrinsic neuron type, the ventricular dilator (VD) neuron, a highly organized and stereotyped branching structure exists within the stomatogastric neuropil. Specifically, we show the morphology of the VD neuron consists of a single primary neurite that projects from the soma into the neuropil and bifurcates into a pair of subprimary neurites, which in turn exit the neuropilar region, one entering the left and the other the right medial ventricular nerve. Nearly all secondary neurite branching of the VD neuron is from the subprimary neurites. There are approximately 22 secondary branches/neuron (range 14-28), with no significant difference between the number of secondary branches off the right vs. the left subprimary neurite, although the ratio of secondary branches between subprimaries varies (range 0.4-1.6). The fine neurites that branch from the secondary processes segregate hemispherically within the neuropil, based on the subprimary neurite of origin. Within this hemispherical organization, another level of fine neurite segregation is present, namely, the fine neurites derived from each secondary branch are restricted to discrete regions of the hemisphere with only minimal overlap with those derived from other secondary branches. Monte Carlo simulations show that this segregation differs significantly from a random distribution. The organization of branching seen in the VD neuron may play a critical role in the electrotonic and local computational organization of this neuron and sets the stage for physiological experimentation addressing these issues.