Olfactory Bulb

Structure and/or Key Features

The olfactory bulb is a structure located on the inferior surface of the frontal lobe, on the undersurface of the frontal lobe. Each individual has two olfactory bulbs, one associated with each cerebral hemisphere. The olfactory bulb sits just intracranially to the cribriform plate. It receives input from olfactory nerve bundles located in the nasal epithelium.

The olfactory bulb is the point where olfactory nerve bundles terminate and their central processes coalesce. Their fibers synapse with the mitral cells located in the olfactory bulb.

The olfactory nerve fibers originate from the olfactory epithelium in the superior aspect of the lateral wall of the nasal cavity. These nerves, encircled by dura and arachnoid mater, travel through the cribriform plate of the ethmoid bone via cribriform foramina, to reach the olfactory bulb in the anterior cranial fossa.

The olfactory bulb’s structure has a layered configuration. From external to the internal, the layers include the olfactory nerve layer, glomerular layer, external plexiform layer, mitral cell layer, internal plexiform layer, and the granular cell layer.

The olfactory nerve layer is made up of unmyelinated axons of the olfactory neurons in various stages of development. The glomerular layer is essentially a thin layer of glomeruli, where the olfactory sensory axons entering the olfactory bulb divide and synapse with the dendrites of secondary olfactory neurons (the mitral, tufted, and periglomerular cells). The dendrites of the mitral and tufted cells make up the external plexiform layer, while the cell bodies of the mitral cells make up the thin mitral cell layer. Each of these cell bodies contains a number of granular cell bodies, which project:

—a solitary principal dendrite to a glomerulus;

—a number of secondary dendrites to the external plexiform layer;

—a solitary axon to the olfactory tract.

The internal plexiform layer is composed of axons of mitral and tufted cells, as well as some granular cell bodies. The majority of granular cells are situated in the granule cell layer. The granular cell layer also contains centrifugal nerve fibers. This layer projects into the olfactory tract in the form of multipolar neurons with which the anterior olfactory nucleus is made up of (Standring, 2016). The axons from the anterior olfactory nucleus are continuous with the rest of the olfactory fibers that travel from the bulb to the striae (Moore, Dalley and Agur, 2013; Standring, 2016).

The olfactory tract originates at the posterior margin of the olfactory bulb. It runs parallel to and on either side of the midline on the inferior surface of the brain. The tract runs posteriorly to the anterior perforated substance where it bifurcates and penetrates the brain as the medial and lateral olfactory striae. The medial olfactory stria runs medially, anterior to the anterior perforated substance. It penetrates the cortex and sends axons to the septal nuclei and through the anterior commissure to the contralateral olfactory cortex.

The lateral olfactory stria runs laterally, anterior to the anterior perforated substance. It penetrates the uncus where it sends axons to the primary olfactory cortex.

Afferent neural inputs to the olfactory bulb come from the anterior olfactory nucleus, collaterals of pyramidal neurons, Broca’s area of the forebrain (cholinergic neurons), cerulean nucleus (pontine locus caeruleus), and the mesencephalic raphe nucleus. These afferent inputs are to the granular and glomerular layers of the olfactory bulb. The efferent fibers of the olfactory bulb project to the amygdala, hippocampus, and the orbitofrontal cortex (Standring, 2016).

Function

The olfactory bulb is fundamental in the process of olfactory transduction. It receives neural signals about odors (via odor molecules) detected by olfactory sensory neurons in the nasal cavity. Odor molecules are absorbed and bind to the odor-binding proteins present within the mucus layer of the olfactory epithelium. Odor molecules are then detected by the odorant receptors in the olfactory cilia. This information is subsequently transmitted to the olfactory bulb via the axons of the olfactory sensory neurons, where they are processed. The processing of odor molecules makes it possible to differentiate between various odors (Longstaff, 2005).

Clinical Correlates

—Anosmia

—Hyposmia

References

Dorland, W. (2011) Dorland's Illustrated Medical Dictionary. 32nd edn. Philadelphia, USA: Elsevier Saunders.

Longstaff, A. (2005) Neuroscience. BIOS instant notes 2nd edition: Taylor & Francis.

Moore, K. L., Dalley, A. F. and Agur, A. M. R. (2013) Clinically Oriented Anatomy. Clinically Oriented Anatomy 7th edn.: Wolters Kluwer Health/Lippincott Williams & Wilkins.

Standring, S. (2016) Gray's Anatomy: The Anatomical Basis of Clinical Practice. Gray's Anatomy Series 41st edition: Elsevier Limited.