Neuron Classification

Neurons could be classified into four different types: unipolar, bipolar, multipolar, and pseudounipolar neurons. These four neurons could transmit signals through many specific structures, including dendrites, axons, and synapses.

• Types of Neurotransmitters

The communication of neurons with others is through junctions, also known as synapses. At a synapse, one neuron sends a message to a target neuron. Many synapses communicate using chemical messengers. In some electrical synapses, ions flow between cells directly. Presynaptic nerve endings generally contain many rather spherical so-called synaptic vesicles containing a special substance: the synaptic transmitter. There are many different kinds of neurotransmitters, and new ones are still being discovered. As shown in table 1, some of the most well-known conventional neurotransmitters are listed. More recently, several classes of neurotransmitters have been identified that do not follow all of the usual rules. These are considered “unconventional” or “nontraditional” neurotransmitters. There are two unconventional transmitters classes: the endocannabinoids and the gasotransmitters (soluble gases such as nitric oxide, NO, and carbon monoxide, CO). These molecules are stored in synaptic vesicles, cross the cell membrane, act directly on molecules inside the cell, and carry messages to the presynaptic neuron.

Table.1 Conventional neurotransmitter and the receptor types.

High-Throughput Classification Methods

Classification of neurons facilitates genetic access for functional studies and analyses of development, evolution, and disease. Neurons have various molecular, morphological, and functional properties. Dendritic and axonal shapes and branching patterns have been the most informative of the morphological properties of neurons; however, features such as soma size and spine density are also used. The physiological properties of neurons include the resting potential, biophysical properties, and firing rate. Of the many molecular properties that can be considered, the most useful are protein composition (generally assayed immunohistochemically) and mRNA composition (measured by in situ hybridization and, increasingly, RNA sequencing).

Fig.1 Criteria by which neurons can be classified. (Zeng, 2017)

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Reference

1. Zeng, H., Sanes, J. R. Neuronal cell-type classification: challenges, opportunities and the path forward. Nature Reviews Neuroscience. 2017, 18(9), 530-546.