Olfactory Nerve

Figure 1-1. Overview of olfactory nerve.


The olfactory nerve has only a special sensory component. 

Special sensory (special afferent)- Functions in the special sense of smell or olfaction. 

The olfactory system consists of the olfactory epithelium, bulbs and tracts along with olfactory areas of the brain collectively known as the rhinencephalon.

Olfactory Epithelium

Figure 1-2. Olfactory Epithelium

Figure 1-2. Olfactory epithelium.

The olfactory epithelium is located in the roof, superior conchae, and septum of the nasal cavity.

The epithelium is kept moist by the secretions of olfactory glands. 

Olfactory receptor cells are bipolar nerve cells with a peripherally directed dendrite which terminates in a knob from which project numerous cilia. 

The olfactory chemoreceptors are located on these cilia.

Sensory Transduction & Peripheral Course of the Olfactory Nerve

Figure 1-3. Peripheral course of the olfactory nerve.

Inhaled aromatic molecules dissolve in the moisture lining the olfactory epithelium and stimulate its chemoreceptors.

Olfactory receptor cells initiate action potentials in response to these chemical stimuli. Intracellular studies show the presence of a slow rising receptor (generator) potential followed by a spike discharge from the receptor cell. 

The peripheral processes of the receptor cells assemble into small bundles and pass through the cribiform plate of the ethmoid bone to synapse on secondary sensory neurons in the olfactory bulb.

Central Course of the Olfactory Tract

The olfactory bulb, which contains the cell bodies of the secondary sensory neurons, is an enlargement of the rostral end of the olfactory tract. The olfactory tract consists of the axons of these secondary sensory neurons. 

Just rostral to the anterior perforated substance the olfactory tract divides into the lateral, intermediate, and medial striae at the olfactory trigone. The axons of the olfactory tract project to the lateral (primary), intermediate, and medial (secondary) olfactory areas of the cortex via these striae.

There are two principal types of secondary sensory neurons:

  1. Mitral cells project primarily to the lateral (primary) olfactory area of the rhinencephalon. These neurons also give off collateral fibers to the anterior olfactory nucleus. Tufted cells - project to the anterior olfactory nucleus and to the lateral, intermediate and medial olfactory areas of the rhinencephalon.
  2. Postsynaptic fibers from the anterior olfactory nucleus project along with the mitral and tufted cell axons to the rhinencephalon or via the anterior commisure to the contralateral anterior olfactory nucleus.

Areas of the Rhinencephalon

There are three olfactory areas of the cortex:

  1. Lateral (primary) olfactory area: consists of the cortex of the uncus and the anterior part of the hippocampal gyrus (entorhinal area). Most of the axons of the olfactory tract project here.
  2. Intermediate olfactory area: located beneath the olfactory trigone. A limited number of fibers project to this area and it is thought to be insignificant in man.
  3. Medial olfactory area: located in the septal region of the medial surface of the frontal lobe. Via its connections with the limbic system, this area is thought to mediate the emotional response to odors.

Major Projections of the Olfactory Areas

Major projections of the olfactory areas.

Figure 1-6. Major projections of the olfactory areas.

All three olfactory areas interface with various autonomic centers via a complex communications network.

The principal pathways include:

  • Medial forebrain bundle: provides information from all three olfactory areas to the hypothalamus.
  • Stria medullaris: carries olfactory information from the rhinencephalon to the habenular nucleus.
  • Stria terminalis: carries information from the amygdala to the anterior hypothalamus and preoptic area of the cortex.

From the hypothalamus and habenular nucleus information is sent to the reticular formation and various cranial nerve nuclei involved in visceral responses. Examples include the dorsal motor nucleus of the vagus which mediates nausea, gastric motility and secretion, and intestinal peristalsis and the superior and inferior salivatory nuclei which mediate salivation.