Anatomy and function
Olfactory Nerve
In this guide
The olfactory nerve is the first cranial nerve, commonly called CN I. It is a special sensory nerve: its fibres carry information about odours from the nose into the brain. Smell also contributes to flavour, helps us notice environmental substances, and can become linked with memories and emotions.
Origin and course of the olfactory nerve
The pathway begins in the olfactory epithelium, a specialised area near the roof of the nasal cavity. Olfactory receptor neurons extend tiny cilia into the mucus covering this tissue. Odour molecules dissolve in the mucus and interact with receptors on the cilia.
The receptor neurons send slender axons through small openings in the cribriform plate, a part of the ethmoid bone. These fibres reach the olfactory bulb, where they connect with second-order neurons in organised structures called glomeruli. Signals then travel along the olfactory tract towards olfactory regions of the brain.
Read more about the Peripheral Nervous System
Classification and functional components
CN I is a pure sensory nerve. It carries special visceral afferent information related to smell and has no motor component. Unlike most sensory systems, the initial olfactory pathway reaches cortical areas without first making its main relay in the thalamus.
Smell begins when receptor neurons detect airborne molecules. The experience of an odour depends on how the brain interprets the resulting pattern of activity.
Olfactory receptors
Olfactory receptor neurons are bipolar nerve cells. Their receptor-bearing cilia project into the nasal cavity, where they encounter volatile molecules carried in the air. The receptors are G-protein-coupled receptors. When activated, they begin a chemical sequence that changes ion flow across the cell membrane and produces an electrical signal.
Different receptor neurons respond more strongly to different molecular features. The brain combines these patterns rather than treating every odour as a single simple signal. This helps explain why people can distinguish many complex smells and why the same odour may be experienced differently in different settings.
Olfactory pathways
After signals reach the olfactory bulb, the olfactory tract distributes them to several connected areas. These include the primary olfactory cortex and parts of the limbic system, which are involved in emotion, motivation, and memory. Further connections support conscious identification and comparison of odours.
From detection to recognition
A smell can trigger a rapid response before we have named it. With attention and experience, the brain can compare the pattern with stored information and recognise an odour as familiar, pleasant, unusual, or potentially concerning.
Why the sense of smell matters
Smell contributes to the flavour of food and can help us notice smoke, spoiled food, or irritating substances. It also interacts with appetite and emotional memory. Smell is not an isolated sense: information from the nose is interpreted alongside taste, touch, vision, and context.
Changes in smell
Changes in smell can have many possible causes, including nasal inflammation or blockage, infection, head injury, exposure to irritants, medication effects, and neurological conditions. The terms below describe patterns rather than identifying a cause.
Anosmia and hyposmia
Anosmia means a complete loss of smell, while hyposmia means a reduced ability to smell. Some cases are temporary; others may persist. A new or unexplained change should be discussed with a qualified healthcare professional.
Parosmia and dysosmia
Parosmia describes a distorted experience in which familiar odours smell different or unpleasant. Dysosmia is a broader term for an altered sense of smell.
Phantosmia
Phantosmia describes sensing an odour that is not present in the surroundings. Recurrent or troubling phantom smells warrant medical assessment, particularly when they occur with other new symptoms.
Specific odour loss
Some people have difficulty detecting particular odours while retaining other smell abilities. Differences in receptor biology and individual experience can contribute to this pattern.
Summary
The olfactory nerve is cranial nerve I and carries special sensory information from receptor neurons in the upper nasal cavity. Its fibres pass through the cribriform plate to the olfactory bulb, then continue through the olfactory tract to brain regions involved in smell, memory, emotion, and recognition.
Smell depends on receptor activity, neural pathways, and the brain’s interpretation of changing patterns. Loss or distortion of smell can have several causes, so persistent or sudden changes should be assessed by an appropriately qualified professional.