Moving beyond Type I and Type II neuron types

In 1948, Hodgkin delineated different classes of axonal firing. This has been mathematically translated allowing insight and understanding to emerge. As such, the terminology of ‘Type I’ and ‘Type II’ neurons is commonplace in the Neuroscience literature today. Theoretical insights have helped us realize that, for example, network synchronization depends on whether neurons are Type I or Type II. Mathematical models are precise with analyses (considering Type I/II aspects), but experimentally, the distinction can be less clear. On the other hand, experiments are becoming more sophisticated in terms of distinguishing and manipulating particular cell types but are limited in terms of being able to consider network aspects simultaneously. Although there is much work going on mathematically and experimentally, in my opinion it is becoming common that models are either superficially linked with experiment or not described in enough detail to appreciate the biological context. Overall, we all suffer in terms of impeding our understanding of brain networks and applying our understanding to neurological disease. I suggest that more modelers become familiar with experimental details and that more experimentalists appreciate modeling assumptions. In other words, we need to move beyond our comfort zones.

Type I and Type II Neuron Models Are Selectively Driven by Differential Stimulus Features

Neurons in the nervous system exhibit an outstanding variety of morphological and physiological properties. However, close to threshold, this remarkable richness may be grouped succinctly into two basic types of excitability, often referred to as type I and type II. The dynamical traits of these two neuron types have been extensively characterized. It would be interesting, however, to understand the information-processing consequences of their dynamical properties. To that end, here we determine the differences between the stimulus features inducing firing in type I and type II neurons. We work with both realistic conductance-based models and minimal normal forms. We conclude that type I neurons fire in response to scale-free depolarizing stimuli. Type II neurons, instead, are most efficiently driven by input stimuli containing both depolarizing and hyperpolarizing phases, with significant power in the frequency band corresponding to the intrinsic frequencies of the cell.

Innervation of sheath cells of an insect sensillum by a bipolar type II neuron

A multiterminal bipolar type II neuron is situated centrally in each mandible of an elaterid larva. It is ensheathed by a glial cell to the base of the two terminal scolopophorous sensilla in the terminal mandibular tooth but its terminal branches are naked. These branches extend along the outer surfaces of the inner and outer sheath cells of and the adjacent surfaces of the epidermal cells around both sensilla. The dendrite and its branches contain longitudinal microtubules, peripheral mitochondria, and clear and variously dense vesicles. It has no ciliary region. The dense vesicles are more numerous in newly molted than in intermolt larvae. Unique plates of endoplasmic reticulum and vesiculating bodies occur in the sheath and epidermal cells adjacent to the naked dendritic branches. This neuron may control the secretory activities of the sensillar sheath cells and adjacent epidermal cells through release of appropriate chemical mediators.