Sotnikov's primary sensory neurons

The inside of the body is no more secret than perception of the outside of the body. It is commonly assumed that five senses provide us with information we must have about our bodies and the world, but Sotnikov challenges this position and asks,

"Why are only the sensory cells of the organs of hearing, balance, vision, olfaction, taste, and touch regarded as exteroceptive, while primary extero- and interoceptive neurons within the brain are left out of the discussion?" (O.S. Sotnikov, 2006)

Based on morphological criteria, there is no distinction between the nerves which take messages from ear, eye, nose, skin and taste buds to the brain, and certain groups of nerves which take messages from inside the body as sensory input. Sotnikov argues that the brain, like skin and retina, is innervated by bipolar cells with sensory receptors. Bipolar neurons in the brain have dendrites and other structural features that strongly resemble identified sensory terminals at the periphery. These sensory neurons may be communicating information to us, which our primitive awareness of sight, touch, and smell hasn't described.

Sotnikov identified many interoceptor sensory cells located throughout the brain, based on similar electron microscopic profiles of the terminals of the dendrites of sensory neurons and the sensory terminals of taste and hearing senses. Figure 6 below shows many possible locations for interoceptor sensory cells.

From 06Sotnikov

The list includes Lugaro cells, Cajal-Retzius cells, supraependymal neurons, and supramedullary neurons on the ventrolateral surface of the medulla. Many of these neurons have a bipolar cell shape, and possess specialized dendrites that form bundles. The large external cell membrane surface on the dendrites leads to a high surface/volume coefficient, and thus a high sensitivity for sensing chemical or physical stimuli in the periphery.

Based on the criteria offered by Sotnikov, neurons in the raphe and thalamic reticular nuclei may be primary sensory cells.

References

Sotnikov O. S. (2006). Primary sensory neurons in the central nervous system. Neurosci. Behav. Physiol. 36, 541-548. 10.1007/s11055-006-0053-5

Sotnikov O. S. (2006). Sensory innervation of the brain (primary interoceptor neurons of the brain and their asynaptic dendrites). Neurosci. Behav. Physiol. 36, 453-462. 10.1007/s11055-006-0039-3