What's so special about special visceral?

Area of Science:

• Neuroscience
• Neuroanatomy
• Functional neuroanatomy

Background:

• Classical brainstem functional columns include special and general somatic and visceral subdivisions.
• The terms 'special visceral motor' and 'special visceral sensory' are applied to branchiomotor nuclei and gustatory/olfactory senses, respectively.
• Existing terminology for brainstem functional columns is questioned due to inconsistencies in neuronal function and sensory system classification.

Purpose of the Study:

• To critically evaluate the established functional column subdivisions of the brainstem.
• To challenge the current classification of 'special visceral motor' and 'special visceral sensory' pathways.
• To propose a revised model for brainstem functional organization.

Main Methods:

• Comparative analysis of branchiomotor neuron function versus general somatic and visceral motoneurons.
• Examination of the homology and characteristics of gustatory and olfactory systems.
• Review of the innervation and function of solitary chemoreceptor cells and other specialized chemoreceptors.

Main Results:

• Branchiomotor neurons exhibit characteristics more aligned with general somatic motoneurons than general visceral motoneurons.
• Gustatory and olfactory systems are not homologous, possess dissimilar receptors, mediate different behaviors, and chemosensory systems are not exclusively 'special' (cranial nerve-limited).
• The designation of taste as 'special visceral sensory' is inconsistent, as other specialized chemoreceptors (e.g., carotid body) are classified as general visceral sensory.

Conclusions:

• The terms 'special visceral motor' and 'special visceral sensory' lack a solid foundation and require re-evaluation.
• A revised functional column model for the brainstem is proposed, comprising six distinct columns: somatic motor, branchial motor, visceral motor, visceral sensory, somatic sensory, and hair cell (dorsolateral placode) sensory.
• This reformulation offers a more accurate and consistent framework for understanding brainstem organization and function.