Neuroanatomical and neurophysiological foundations of the human olfactory system

The aim of this review is to provide an overview of the knowledge concerning the neuroanatomical and neurophysiological foundations of the human olfactory system. The authors describe the neuroanatomical route via which chemosensations received by receptors located in the nasal cavity are transmitted to brain structures along with a description of the processing sites. Air passage through nasal structures and subsequent transmission of the chemosensory activation from the olfactory epithelium to the olfactory bulb are described. The olfactory bulb is a structure creating olfactory maps which are transmitted to other brain centers for further processing purposes. According to some neurodegenerative disease conceptions, it is one of the first locations in the brain where the process of accumulation of amyloid plaques and Lewy bodies starts in the course of Alzheimer’s disease and Parkinson’s disease. The olfactory bulb is connected with the diffused cortical and sub-cortical structures constituting the primary olfactory cortex. The piriform cortex is the main component of the primary olfactory cortex. The anterior olfactory nucleus plays an important role, as it connects the olfactory bulb with the primary olfactory cortex and the homologous centers of the brain hemispheres with each other. Nine out of 22 centers of the secondary olfactory cortex are directly connected with the primary olfactory cortex without involvement of the thalamus, which is engaged in olfactory information processing, but not to such a degree as in the case of other sensory modalities. Attention is paid to the structure of the trigeminus and the role in transmission of somatosensory information. Another section of this paper depicts the tertiary olfactory cortex structure and role, with special focus on the temporal lobes, insula, and orbitofrontal cortex. A discussion on peripheral and central olfactory centers’ lateralization closes the article.