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Olfactory Nerve (CN 1) & System
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Olfactory Nerve (CN 1) & System

CN 1 is responsible for smell.
Numerous formal tests exist that will quantitatively evaluate the patient’s sense of smell. These tests are widely used in research. At the bedside, a more simplistic approach is applied. Simply compress one of the patient’s nostrils, ask the patient to close his/her eyes, present the odiferous substance, and asks the patient to sniff and report the odor. Then repeat the test on the opposite nostril. In regards to the odor itself, pay attention to the following principles:
1. The substance being tested should be aromatic. Common substances include: coffee grounds, soap, and lemon. For practical reasons, “scratch and sniff” cards are often used instead of an actual substance. 2. The odor should be nonirritating to avoid triggering the trigeminal nerve pathway. 3. The odor should be easily recognizable to avoid making the test an evaluation of the patient’s verbal ability.
The olfactory system
Landmarks: midline nasal cavity, temporal lobe, anterior corpus callosum, frontal lobe, cribriform plate and olfactory epithelium.
  • The cribriform plate separates the cranial vault from the nasal cavity.
  • Fracture to the cribriform plate (or more commonly to the ethmoid air cells posterolateral to the cribriform plate) is a common cause of rhinorrhea — cerebrospinal fluid leak from the nasal cavity.
olfactory bulb & tract
  • Lie underneath the frontal lobe.
    • The olfactory bulb is often distinguished as the main olfactory bulb because the majority of vertebrates also have an accessory olfactory system. However, the role and existence of the accessory olfactory system (aka vomeronasal system) in humans is disputed.
The Olfactory nerve, Bulb, & Tract: Essentials
  • Bipolar primary olfactory neuron: dendrites project to the olfactory epithelial surface and the centrally-mediated axon (the olfactory nerve) extends through the cribriform plate to innervate the olfactory bulb.
  • Within the olfactory bulb, lie bipolar secondary olfactory cells, which connect with the olfactory nerve in the inferior olfactory bulb and also send axons down the olfactory tract: at the posterior end of the olfactory tract, lies the olfactory trigone.
  • CN 1, the olfactory nerve, refers to the primary olfactory neuron/nerve, only.
    The olfactory bulb and tract are extensions of the cerebrum, itself.
The olfactory system bypasses the thalamus as it projects to the cerebral cortex, which is unique. Auditory, visual, somatosensory, and gustatory sensory pathways all relay within the thalamus prior to synapsing in the cerebral cortex.* The olfactory tract divides into a medial olfactory stria, which innervates the medial olfactory area in the subcallosal (aka septal) region, and a lateral olfactory stria, which innervates the primary olfactory cortex in the basal frontal and anteromedial temporal lobes. Olfactory impulses also extend across the anterior commissure to the opposite side of the cerebrum.*
The Olfactory nerve, Bulb, & Tract: Connections
  • Cilia from the apical dendrite interact with the mucus layer of the olfactory epithelial surface.
Key constituents of the olfactory epithelium include:
  • The sustentacular cells, which are olfactory supporting cells.
  • The basal cells, which renew the primary olfactory neurons and sustentacular cells.
  • The Bowman's glands, which secrete a serous, watery odor dissolvent.
  • CN 1 comprises an unmyelinated nerve bundle to interact with bipolar secondary olfactory neurons.
Two principal forms of secondary olfactory neuron exist: tufted cells and mitral cells. Less notable interneurons (eg, periglomerular and granule cells) also exist within the olfactory bulb.*
  • The glomerular layer refers to the layer of spherical glomeruli which encompass the interaction between dendrites from the secondary olfactory neurons and primary olfactory axons within the inferior olfactory bulb.
  • The secondary olfactory neurons project axons that travel either directly down the olfactory tract to synapse in the olfactory cortex or first to the anterior olfactory nucleus, which projects its axons down the olfactory tract to the olfactory cortex.

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