Olfactory Nerve (CN I)

Table of Contents

Introduction

The olfactory nerve is the first cranial nerve (CN I) and is responsible for the sense of smell. It is a sensory nerve that carries information about odours from the nasal cavity to the brain. Olfaction is considered one of the earliest senses in evolutionary history.

This sensory pathway has a unique feature: it reaches the primary olfactory cortex without an initial relay through the thalamus. The olfactory nerve is the first of the 12 pairs of cranial nerves in the human body.

Root

The olfactory nerve begins with specialised sensory cells located in the olfactory lining inside the nasal cavity. Their axons pass through the cribriform plate of the ethmoid bone and reach the olfactory bulb, which lies above the cribriform plate and below the frontal lobe.

The olfactory bulb acts as a relay and processing centre for smell-related impulses before they travel through the olfactory tract to the olfactory cortex and other associated regions of the brain.

Branches

The olfactory nerve fibres pass through the cribriform plate and enter the olfactory bulb. The lateral and medial olfactory striae are divisions of the olfactory tract rather than branches of the olfactory nerve itself. These are:

1. Lateral olfactory stria: Carries olfactory information mainly to the primary olfactory cortex and other areas involved in processing smells.

2. Medial olfactory stria: Carries olfactory information towards medial forebrain regions and connects with areas associated with olfactory processing.

Pathway

The olfactory epithelium is located within the nasal cavity, mainly along its roof, the superior nasal concha, and the upper part of the nasal septum. It contains several types of cells, including olfactory sensory neurons, sustentacular cells, microvillar cells, Bowman’s gland cells, and basal stem cells.

Olfactory sensory neurons are bipolar cells that receive odour-related information through their dendrites at the apical surface and transmit signals through their axons from the basal pole. At the basement membrane, olfactory ensheathing cells surround groups of approximately 15 to 40 axons, forming small nerve fascicles. These fascicles join together to form larger bundles called fila olfactoria.

These bundles pass through the small openings in the cribriform plate of the ethmoid bone and reach the olfactory bulb. Inside the bulb, the axons connect with the dendrites of mitral and tufted cells in structures called glomeruli. The signals then travel through the olfactory tract to the olfactory cortex and other associated brain regions, where odours are processed and recognised.

Function

The olfactory nerve carries sensory information that allows us to detect and recognise different smells. It carries information about odours from the olfactory epithelium in the nasal cavity to the olfactory bulb. From there, the signals travel to other areas of the brain, allowing a person to detect, identify, and recognise different smells.

Clinical Relevance

The olfactory nerve can be injured by traumatic brain injury (TBI), blunt trauma to the head, and certain infections. Damage to the olfactory nerve or its associated pathways may affect a person’s ability to detect and recognise odours.

The anatomical location of the olfactory nerve makes it vulnerable to injury, particularly during trauma or surgery involving the anterior cranial fossa. During surgical procedures, three mechanisms can cause olfactory nerve damage:

1. Detachment of the cribriform plate and fila olfactoria: Retraction of the frontal lobe may stretch or tear the olfactory nerve fibres as they pass through the cribriform plate of the ethmoid bone.

2. Partial or complete section of the olfactory tract: The olfactory tract may be accidentally injured or cut during surgical dissection.

3. Ischaemic injury: Reduced blood supply to the olfactory structures may damage the nerve fibres or surrounding tissues.

Lesions affecting the olfactory nerve or olfactory pathway can lead to the following symptoms:

Anosmia: Inability to detect or perceive any smells.
Hyposmia: Reduced ability to detect odours.
Hyperosmia: Increased sensitivity to smells.
Dysosmia: An altered sense of smell in which odours may seem unusual, unpleasant, or different from normal.
- Familiar odours may smell different or unpleasant, or a person may perceive an unpleasant smell when no corresponding external odour is present.

Assessment

Assessment of the olfactory nerve can be performed at the bedside or in a clinical setting. To test the sense of smell, the patient is asked to close their eyes while one nostril is gently blocked. The patient then smells familiar aromatic substances, such as coffee, vanilla, or cinnamon, and identifies each odour.

Avoid using pungent substances like alcohol or ammonia, as they can activate the trigeminal nerve and may not accurately assess the sense of smell. The same procedure is repeated with the other nostril to compare the sense of smell on both sides.

Odour identification tests help assess the patient’s ability to recognise different smells. Additional tests may evaluate how well the patient detects odours at progressively lower concentrations.

If necessary, the physician may recommend imaging tests, such as magnetic resonance imaging (MRI) or computed tomography (CT), to identify structural abnormalities, injuries, or other conditions affecting the olfactory nerve and its associated pathways.

Treatment

In most situations, treating the underlying condition may improve the ability to smell. For example, medications can help manage sinusitis and reduce inflammation that interferes with the sense of smell. If certain medications contribute to smell loss, adjusting or changing them under medical supervision may help improve symptoms.

If something is obstructing the nose, such as a nasal polyp or a deviated nasal septum, surgery may be necessary in selected cases. The appropriate treatment depends on the underlying cause and the severity of the olfactory dysfunction.

Prevention

Anosmia is not always preventable because it can result from various factors, including head injuries, infections, and neurological conditions. However, preventing respiratory infections, practising good hygiene, and protecting the head from injury may help reduce the risk of certain causes of smell loss.

Prognosis

Anosmia is often associated with an underlying medical condition, and the ability to smell may improve when that condition is treated. Recovery depends on the cause, severity, and duration of the smell loss. Congenital anosmia is a rare condition in which a person is born without a sense of smell. Treatment options are limited, particularly when the olfactory structures have not developed normally.

Living With Olfactory Nerve Dysfunction

How do I take care of myself?

If you have anosmia, you may be unable to detect potentially dangerous odours, such as smoke, gas leaks, or spoiled food. However, you can take several precautions to protect yourself:

Install smoke alarms: Fit smoke detectors in suitable locations around your home and workplace, and test them regularly to ensure they are working properly.
Check food expiry dates: Do not rely only on the appearance of food to decide whether it is safe to eat. Check expiry dates and follow proper food storage guidelines.
Use additional safety measures: Ensure that gas appliances are properly maintained and consider installing suitable gas detectors.
Maintain food hygiene: Keep food at safe storage temperatures and throw away anything that appears spoiled or has not been stored under proper conditions.

These simple precautions can help reduce everyday risks and make it easier to live safely when the sense of smell is reduced or absent.

FAQs

How do you treat anosmia?

How is anosmia treated? Treating the underlying issue typically improves the sense of smell. For example, medicines can help clear sinusitis. If some drugs impair your sense of smell, changing medications may alleviate your anosmia symptoms.

How to learn to smell again?

The training consists of smelling four scents (rose, eucalyptus, lemon, and clove) twice a day and following these steps.
1. Smell one perfume for 15 seconds, attempting to recall what it used to smell like.
2. Pause for 10 seconds.
3. Smell the next aroma for 15 seconds.
4. Pause for 10 seconds.

Can I live without a sense of smell?

Living with a diminished or lost sense of smell may have a significant impact on your health, happiness, and overall quality of life, and it can happen to anybody.

Can an ENT help with loss of smell?

Endoscopic sinus and nasal surgery produces remarkable results by treating scent loss at the source and eliminating the polyps that are causing the problem.

References:

Contributors to Wikimedia projects. (2026). Olfactory nerve – Wikipedia. Wikimedia Foundation, Inc. https://en.wikipedia.org/wiki/Olfactory_nerve
Cleveland Clinic medical professional. (2025). Olfactory Nerve: Overview, Function & Anatomy. Cleveland Clinic. https://my.clevelandclinic.org/health/body/23081-olfactory-nerve
(n.d.). Olfactory_Nerve. physio-pedia.com. https://www.physio-pedia.com/Olfactory_Nerve
(2025). The Olfactory Nerve (CN I) – Pathway – Anosmia – TeachMeAnatomy. TeachMeAnatomy. https://teachmeanatomy.info/head/cranial-nerves/olfactory-cni/
Nitesh Dhameliya. (2021). 1ST CRANIAL NERVE- OLFACTORY. SAMARPAN PHYSIOTHERAPY CLINIC AHMEDABAD. https://samarpanphysioclinic.com/1st-cranial-nerve-olfactory/