CN I: Olfactory nerve. Olfactory system

In this note, we are going to discuss the anatomy and function of the olfactory system, as well as the olfactory nerve as one of its main components.

Olfactory system

The olfactory system, like any other sensory system, consists of three parts: receptor cells, transmitting structures, and central (or cortical) structures.

We also need to know the term “rhinencephalon” (or the “olfactory brain”), which stands for the whole complex of structures of the central nervous system responsible for olfaction.

The olfactory nerve provides olfaction and, according to the generally accepted classification of functional components, it contains special visceral afferent (SVA) fibers. That means it is a sensory nerve.

Olfactory receptor cells

The peripheral part is located in the mucous membrane of the nasal cavity, called the olfactory epithelium.

This epithelium lines the superior part of the nasal cavity in a semicircular manner, where the superior nasal concha is located on the lateral side, and the nasal septum is located on the medial side.

The olfactory epithelium consists of four types of cells.

The first type is olfactory sensory bipolar neurons. They are the first-order neurons of the olfactory pathway, and each of them has two processes.

The peripheral process, or the dendrite, extends to the surface of the olfactory epithelium, expands, and forms a so-called dendritic knob w ith cilia that capture odor molecules (also called odorants).

The central processes, or axons, are collected into small bundles that enter the cranial cavity via the cribriform plate of the ethmoid bone, which makes up the bony roof of the nasal cavity. Let’s leave this type of cells for now and move on to the next, basal stem cells.

They are located on the basal membrane and differentiate into, and replace damaged receptor cells. This means that, unlike other special sensory neurons, olfactory cells can regenerate throughout life.

The third type of cells is the supporting cells, which are essentially ordinary neuroglia. We can compare them with the maintenance staff.

And the fourth type of cells is secretory cells that form the so-called glands of Bowman, which produce a special liquid or mucus. Molecules of odorous substances are dissolved in this liquid, which increases the efficiency of olfactory perception.

Transmitting structures of the olfactory pathway

So, axons of bipolar neurons pass through small foramina in the cribriform plate of the ethmoid bone and enter the olfactory bulb in the anterior cranial fossa.

Fila olfactoria / olfactory nerve bundles (fila olfactoria / nn. olfactorii)

Cribriform plate of the ethmoid bone (lamina cribrosa ossis ethmoidalis)

These axonal bundles can collectively be called the olfactory nerve, which is the first cranial nerve (CN I). Sometimes they are also referred to as fila olfactoria.

All the other transmitting structures that we are going to discuss, are no longer the nerve, but rather an extension of the brain, or an extension of the central nervous system.

In the olfactory bulb, those axons synapse on second-order neurons, called mitral cells.

The axons of mitral cells form the olfactory tract, which travels posteriorly in the olfactory groove to several areas.

Part of the fibers of the olfactory tract ends up in the olfactory trigone (trigonum olfactorium) located anteriorly to the anterior perforated substance (substantia perforata anterior).

Anterior perforated substance (substantia perforata anterior)

Two other parts branch off from the trigone in the form of the medial olfactory stria (stria olfactoria medialis)

and the lateral olfactory stria (stria olfactoria lateralis).

The medial olfactory stria is formed by two bundles of fibers.

One goes to the area of the septum pellucidum and the subcallosal area,

Neurons in the subcallosal area (area subcallosa)

and the other goes to the contralateral structures through the anterior commissure of the brain and inhibits the activity of the cells of the olfactory bulb. It helps us to determine the location of the odorant more accurately.

Anterior commissure (commissura anterior)

Some collateral branches of the medial olfactory stria form synapses with a small group of neurons called the anterior olfactory nucleus.

The lateral olfactory stria runs to the ipsilateral olfactory cortex.

Olfactory cortex

Broadly speaking, the central part of the olfactory system begins with the olfactory bulb, since it is an extension of the brain. But for educational purposes, it is more reasonable to think of it as a component of the transmitting part.

The lateral olfactory stria projects to the primary olfactory cortex, located deep within the temporal lobe.

It consists of two areas, namely the prepiriform and periamygdaloid areas.

The prepiriform area is located within the uncus of the parahippocampal gyrus, and the periamygdaloid area, as the name suggests, surrounds the amygdala.

The third-order neurons of the olfactory pathway are located in the cortex.

Moreover, some axons of the olfactory tract end up in the so-called olfactory tubercle, which is located on the anterior perforated substance.

In addition, there is the term «piriform cortex», which also includes the secondary olfactory areas, namely the entorhinal cortex, the perirhinal cortex, and the more posterior part of the parahippocampal gyrus.

Parahippocampal gyrus (gyrus parahippocampalis)

From the primary olfactory cortex, information about smell is sent to the various anatomical structures of the brain, especially into the structures of the limbic system. For example, the hippocampal formation, which stores the memory of an odorous substance and associates it with certain emotions.

Or to the brainstem via the medial forebrain bundle (MFB), namely into the reticular formation that triggers autonomic responses involved in appetite, salivation, nausea, and gastric contraction. As a result, all these fibers form a complex network, that interconnects different parts of the cortex and brainstem.

Reticular formation (formatio reticularis)

Unlike other sensory pathways, the olfactory pathway does not project directly to the thalamus.

Instead, it switches indirectly, in the hypothalamus. Information still reaches the thalamus, and from there, it is sent to the orbitofrontal cortex.

This is one of the areas on the inferior surface of the frontal lobe where the conscious processing of olfactory information takes place. At this point, you finally can completely identify the smell with all characteristics.

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