Paraneurons are those cells which have not been designated as neurons but are recognized as closely related to neurons on the basis of their fine structure (possession of neurosecretion-like and synaptic vesicle-like granules/vacuoles), metabolism (production of neurosecretion- and neurotransmitter-like substances) and origin (evidenced or proposed neuroectodermal). Some paraneurons are mainly endocrine in function, while others are sensory. Some should be called interneurons. There are graduations between these types of paraneurons; also a definite boundary between neurons and paraneurons can not be determined.
Different types of paraneurons (together with some related neurons) are shown in Figure 1. Comments and discussions were given on the neuron-like characteristics of the individual paraneurons.