The Hitchhiker’s guide to the rhinencephalon

Pathology of the rhinencephalon has been a subject of interest in the fields of neurodegenerative diseases, trauma, epilepsy and other neurological conditions. Most of what is known about the human rhinencephalon comes from comparative anatomy studies in other mammals and histological studies in primates. Functional imaging studies can provide new and important insight into the function of the rhinencephalon in humans but have limited spatial resolution, limiting its contribution to the study of the anatomy of the human rhinencephalon. In this study we aim to provide a brief and objective review of the anatomy of this important and often overlooked area of the nervous system.

olfactory cortex; anatomy; neuroanatomy

Authorship

Laura Silveira-Moriyama

Universidade Nove de Julho, Programa de Pós-graduação em Medicina,Sao PauloSP,Brazil;Universidade Nove de JulhoBrazilSao Paulo, SP, BrazilUniversidade Nove de Julho, Programa de Pós-graduação em Medicina,Sao PauloSP,Brazil;

UCL Institute of Neurology,Reta Lila Weston Institute,London,UK;UCL Institute of NeurologyUKLondon, UKUCL Institute of Neurology,Reta Lila Weston Institute,London,UK;

Universidade Estadual de Campinas,Faculdade de Ciências Médicas,CampinasSP,Brazil;Universidade Estadual de CampinasBrazilCampinas, SP, BrazilUniversidade Estadual de Campinas,Faculdade de Ciências Médicas,CampinasSP,Brazil;

Universidade de São Paulo,Faculdade de Medicina,Departamento de Neurologia,Sao PauloSP,Brazil;Universidade de São PauloBrazilSao Paulo, SP, BrazilUniversidade de São Paulo,Faculdade de Medicina,Departamento de Neurologia,Sao PauloSP,Brazil;

Philip Glass

Universidade Estadual do Sudoeste da Bahia,Faculdade de Medicina,Vitoria da ConquistaBA,Brazil;Universidade Estadual do Sudoeste da BahiaBrazilVitoria da Conquista, BA, BrazilUniversidade Estadual do Sudoeste da Bahia,Faculdade de Medicina,Vitoria da ConquistaBA,Brazil;

Suraj Rajan

Premier Health Miami Valley Hospital,The Clinical Neuroscience Institute,DaytonOH,USA.Premier Health Miami Valley HospitalUSADayton, OH, USAPremier Health Miami Valley Hospital,The Clinical Neuroscience Institute,DaytonOH,USA.

Rafael Carvalho

Fabiano Reis

Carlos A. A. Penatti

Valeria Muio

Correspondence: Laura Silveira-Moriyama; Postgraduate Research Program in Medicine, Universidade Nove de Julho (Uninove); Rua Vergueiro, 235/249; 01504-001 São Paulo; SP, Brasil; E-mail: laurasm@uninove.br

Conflict of interest: There is no conflict of interest to declare.

SCIMAGO INSTITUTIONS RANKINGS

UCL Institute of Neurology,Reta Lila Weston Institute,London,UK;UCL Institute of NeurologyUKLondon, UKUCL Institute of Neurology,Reta Lila Weston Institute,London,UK;

Figures | Tables

Figures (6)
Tables (1)

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Figure 1
Search results on PubMed (www.ncbi.nlm.nih.gov/pubmed) using search terms (“visual”, “motor”, “auditory” and “olfactory”). (A) shows the number of search results for the term “olfactory” alone, using the number of citations up to two thousand, and demonstrating the exponential increase of search results using the term “olfactory”. (B) shows the comparative number of citations for the terms “olfactory”, “motor”, “auditory” and “visual”, clearly demonstrating that despite the increase, the number of search results is still very small when compared to the terms “visual” and “motor”, and almost half of the volume for term “auditory”.

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Figure 2
Schematic view of the levels of processing in the rhinencephalon. From bottom to top, the levels are showed in sequential order from more peripheral to more central.

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Figure 3
Simplified structure of the olfactory mucosa. Olfactory receptor neurons (ORN) have their cilia in the olfactory mucus and contact with odorant substances, which yield action potentials. This information is transmitted through their axons across the cribriform plate and synapse into the olfactory bulb.

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Figure 4
Illustrative photomicrographs showing: histology of glomerular layer (A) and periglomerular dopaminergic cells (B), and appearance of large neurons in islands of the anterior olfactory nucleus inside the olfactory bulb (C), compared to the smaller cells in the granule cell layer (D). Scale bar: 100 µm in A, 50 µm in B, and 30 µm in C and D. (A), (C) and (D) used haematoxylin and eosin stain, and (B) used immunohistochemistry for tyrosine hydroxylase counter stained by haematoxylin.

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Figure 5
Illustrative images showing location of some areas of the primary olfactory cortex. Insert (A) shows a schematic drawing with the main landmarks which are easily visible both in histology slides and neuroimaging, while insert (B) shows the corresponding location of the olfactory tubercle, piriform cortex and anterior portion of the entorhinal cortex (blue highlight). Insert (C) shows a coronal section of a half brain stained for luxol fast blue and counterstained with cresyl violet, showing a slightly more posterior location in which the periamygdaloid cortex can be visualized as well.

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Figure 6
Coronal images extracted from T1-weighted magnetic resonance imaging showing location of some areas of the primary olfactory cortex. Inserts show the coronal slices with main landmarks labelled in white on the left half, and the rhinencephalon areas labelled in black and marked with arrows on the right. For a reference for coronal slices see the Paxinos & Mai atlas5, coronal slices numbered from the anterior commissure (AC). Insert (A) approximately 1cm anterior from AC, insert (B) slightly anterior to AC, and insert (C) slightly posterior from AC.

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Table
Different acronyms for the subdivisions of primary olfactory cortex found in the literature. The table displays the most commonly used name on the first column, and other common acronyms in the second column, followed by the number of citations found using the acronym as a search term on the Google Scholar (www.googlescholar.com) database.

Figure 1 Search results on PubMed (www.ncbi.nlm.nih.gov/pubmed) using search terms (“visual”, “motor”, “auditory” and “olfactory”). (A) shows the number of search results for the term “olfactory” alone, using the number of citations up to two thousand, and demonstrating the exponential increase of search results using the term “olfactory”. (B) shows the comparative number of citations for the terms “olfactory”, “motor”, “auditory” and “visual”, clearly demonstrating that despite the increase, the number of search results is still very small when compared to the terms “visual” and “motor”, and almost half of the volume for term “auditory”.

Figure 2 Schematic view of the levels of processing in the rhinencephalon. From bottom to top, the levels are showed in sequential order from more peripheral to more central.

Figure 3 Simplified structure of the olfactory mucosa. Olfactory receptor neurons (ORN) have their cilia in the olfactory mucus and contact with odorant substances, which yield action potentials. This information is transmitted through their axons across the cribriform plate and synapse into the olfactory bulb.

Figure 4 Illustrative photomicrographs showing: histology of glomerular layer (A) and periglomerular dopaminergic cells (B), and appearance of large neurons in islands of the anterior olfactory nucleus inside the olfactory bulb (C), compared to the smaller cells in the granule cell layer (D). Scale bar: 100 µm in A, 50 µm in B, and 30 µm in C and D. (A), (C) and (D) used haematoxylin and eosin stain, and (B) used immunohistochemistry for tyrosine hydroxylase counter stained by haematoxylin.

Figure 5 Illustrative images showing location of some areas of the primary olfactory cortex. Insert (A) shows a schematic drawing with the main landmarks which are easily visible both in histology slides and neuroimaging, while insert (B) shows the corresponding location of the olfactory tubercle, piriform cortex and anterior portion of the entorhinal cortex (blue highlight). Insert (C) shows a coronal section of a half brain stained for luxol fast blue and counterstained with cresyl violet, showing a slightly more posterior location in which the periamygdaloid cortex can be visualized as well.

Figure 6 Coronal images extracted from T1-weighted magnetic resonance imaging showing location of some areas of the primary olfactory cortex. Inserts show the coronal slices with main landmarks labelled in white on the left half, and the rhinencephalon areas labelled in black and marked with arrows on the right. For a reference for coronal slices see the Paxinos & Mai atlas5, coronal slices numbered from the anterior commissure (AC). Insert (A) approximately 1cm anterior from AC, insert (B) slightly anterior to AC, and insert (C) slightly posterior from AC.

AC: Anterior commissure; Acc : accumbens nucleus; Aco : anterior cortical nucleus of the amygdala; CC: corpus callosum; Cd: Caudate head; GP: Globus pallidus; IC: internal capsule; PAC: periamygdaloid cortex; PiF; PiT: frontal and temporal parts of piriform cortex; Pu: Putamen; Tu: olfactory tubercle.

Table Different acronyms for the subdivisions of primary olfactory cortex found in the literature. The table displays the most commonly used name on the first column, and other common acronyms in the second column, followed by the number of citations found using the acronym as a search term on the Google Scholar (www.googlescholar.com) database.

Most commonly used acronym	Other acronyms found in the literature	Cites
Piriform cortex (25000 cites)	Pyriform cortex	6,09
	Prepyriform cortex	2,31
	Piriform area	544,00
	Pyriform area	391,00
	Prepiriform area	134,00
	Lateral olfactory gyrus	97,00
	Prepyriform area	95,00
Anterior olfactory nucleus (6,990 cites)	Retrobulbar region	613,00
Anterior olfactory nucleus (6,990 cites)	Anterior olfactory cortex	142,00
Olfactory tubercle (19,500 cites)	Anterior perforated substance	1,43

AC: Anterior commissure; aEnt: anterior entorhinal cortex; CC: corpus callosum; Cd: Caudate head; Cla: claustrum; GP: Globus pallidus; IC: internal capsule; Och: optic chiasm; PAC: periamygdaloid cortex; PiF; PiT: frontal and temporal parts of piriform cortex; Pu: Putamen; Tu: Olf tubercle.

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The Hitchhiker’s guide to the rhinencephalon

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[1] Correspondence: Laura Silveira-Moriyama; Postgraduate Research Program in Medicine, Universidade Nove de Julho (Uninove); Rua Vergueiro, 235/249; 01504-001 São Paulo; SP, Brasil; E-mail: laurasm@uninove.br