The olfactory nerve, also known as the first cranial nerve, cranial nerve I, or simply CN I, is a

cranial nerve Cranial nerves are the nerves that emerge directly from the brain (including the brainstem), of which there are conventionally considered twelve pairs. Cranial nerves relay information between the brain and parts of the body, primarily to and f ...

that contains sensory nerve fibers relating to the sense of smell. The

afferent nerve fiber Afferent nerve fibers are the axons (nerve fibers) carried by a sensory nerve that relay sensory information from sensory receptors to regions of the brain. Afferent projections ''arrive'' at a particular brain region. Efferent nerve fibers ...

s of the

olfactory receptor neuron An olfactory receptor neuron (ORN), also called an olfactory sensory neuron (OSN), is a sensory neuron within the olfactory system. Structure Humans have between 10 and 20 million olfactory receptor neurons (ORNs). In vertebrates, ORNs are b ...

s transmit nerve impulses about

odor An odor (American English) or odour (Commonwealth English; see spelling differences) is caused by one or more volatilized chemical compounds that are generally found in low concentrations that humans and animals can perceive via their sense ...

s to the

central nervous system The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain and spinal cord. The CNS is so named because the brain integrates the received information and coordinates and influences the activity of all p ...

( olfaction). Derived from the

embryo An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sperm ...

nic nasal placode, the olfactory nerve is somewhat unusual among cranial nerves because it is capable of some regeneration if damaged. The olfactory nerve is sensory in nature and originates on the

olfactory mucosa The olfactory mucosa is located in the upper region of the nasal cavity and is made up of the olfactory epithelium and the underlying lamina propria, connective tissue containing fibroblasts, blood vessels, Bowman's glands and bundles of fine ax ...

in the upper part of the

nasal cavity The nasal cavity is a large, air-filled space above and behind the nose in the middle of the face. The nasal septum divides the cavity into two cavities, also known as fossae. Each cavity is the continuation of one of the two nostrils. The nasal ...

. From the olfactory mucosa, the nerve (actually many small nerve fascicles) travels up through the cribriform plate of the ethmoid bone to reach the surface of the brain. Here the fascicles enter the olfactory bulb and synapse there; from the bulbs (one on each side) the olfactory information is transmitted into the brain via the olfactory tract. The fascicles of the olfactory nerve are not visible on a cadaver brain because they are severed upon removal.

Structure

The specialized

s of the olfactory nerve are located in the olfactory

mucosa A mucous membrane or mucosa is a membrane that lines various cavities in the body of an organism and covers the surface of internal organs. It consists of one or more layers of epithelial cells overlying a layer of loose connective tissue. It is ...

of the upper parts of the

. The olfactory nerves consist of a collection of many sensory nerve fibers that extend from the olfactory epithelium to the

olfactory bulb The olfactory bulb (Latin: ''bulbus olfactorius'') is a neural structure of the vertebrate forebrain involved in olfaction, the sense of smell. It sends olfactory information to be further processed in the amygdala, the orbitofrontal cortex (O ...

, passing through the many openings of the cribriform plate, a

sieve A sieve, fine mesh strainer, or sift, is a device for separation process, separating wanted elements from unwanted material or for controlling the particle size distribution of a sample, using a screen such as a warp and weft, woven mesh or n ...

-like structure of the ethmoid bone. The sense of

smell Smell may refer to; * Odor, airborne molecules perceived as a scent or aroma * Sense of smell, the scent also known scientifically as olfaction * "Smells" (''Bottom''), an episode of ''Bottom'' * The Smell, a music venue in Los Angeles, Californ ...

arises from the stimulation of receptors by small molecules in inspired air of varying spatial, chemical, and electrical properties that reach the nasal epithelium in the nasal cavity during inhalation. These stimulants are transduced into electrical activity in the olfactory neurons, which then transmit these impulses to the olfactory bulb and from there they reach the olfactory areas of the brain via the olfactory tract. The olfactory nerve is the shortest of the twelve cranial nerves and, similar to the optic nerve, does not emanate from the

brainstem The brainstem (or brain stem) is the posterior stalk-like part of the brain that connects the cerebrum with the spinal cord. In the human brain the brainstem is composed of the midbrain, the pons, and the medulla oblongata. The midbrain is ...

Function

The

s of the

s transmit nerve impulses about

s to the

, where they are perceived as smell ( olfaction). The olfactory nerve is

special visceral afferent A Special visceral afferent fibers (SVA) is a afferent fiber that develop in association with the gastrointestinal tract. They carry the special senses of smell (olfaction) and taste (gustation). The cranial nerves containing SVA fibers are the olfa ...

(SVA).

Clinical significance

Examination

Damage to this nerve leads to impairment or total loss

anosmia Anosmia, also known as smell blindness, is the loss of the ability to detect one or more smells. Anosmia may be temporary or permanent. It differs from hyposmia, which is a decreased sensitivity to some or all smells. Anosmia can be due to a nu ...

of the sense of smell To simply test the function of the olfactory nerve, each

nostril A nostril (or naris , plural ''nares'' ) is either of the two orifices of the nose. They enable the entry and exit of air and other gasses through the nasal cavities. In birds and mammals, they contain branched bones or cartilages called t ...

is tested with a pungent odor. If the odor is smelled, the olfactory nerve is likely functioning. On the other hand, the nerve is only one of several reasons that could explain if the odor is not smelled. There are olfactory testing packets in which strong odors are embedded into cards and the responses of the patient to each odor can be determined.

Lesions

Lesions to the olfactory nerve can occur because of "blunt trauma", such as

coup-contrecoup In head injury, a coup injury occurs under the site of impact with an object, and a contrecoup injury occurs on the side opposite the area that was hit. Coup and contrecoup injuries are associated with cerebral contusions, a type of traumatic bra ...

damage, meningitis, and tumors of the

frontal lobe The frontal lobe is the largest of the four major lobes of the brain in mammals, and is located at the front of each cerebral hemisphere (in front of the parietal lobe and the temporal lobe). It is parted from the parietal lobe by a groove b ...

of the brain. These injuries often lead to a reduced ability to taste and smell. Lesions of the olfactory nerve do not lead to a reduced ability to sense pain from the nasal epithelium. This is because pain from the nasal epithelium is not carried to the central nervous system by the olfactory nerve - it is carried to the central nervous system by the

trigeminal nerve In neuroanatomy, the trigeminal nerve (literal translation, lit. ''triplet'' nerve), also known as the fifth cranial nerve, cranial nerve V, or simply CN V, is a cranial nerve responsible for Sense, sensation in the face and motor functions ...

Aging and smell

A decrease in the ability to smell is a normal consequence of human

aging Ageing ( BE) or aging ( AE) is the process of becoming older. The term refers mainly to humans, many other animals, and fungi, whereas for example, bacteria, perennial plants and some simple animals are potentially biologically immortal. In ...

, and usually is more pronounced in men than in women. It is often unrecognized in patients except that they may note a decreased ability to taste (much of taste is actually based on reception of food odor). Some of this decrease results from repeated damage to the olfactory nerve receptors due likely to repeated upper respiratory infections. Patients with Alzheimer's disease almost always have an abnormal sense of smell when tested.

Pathway to the brain

Some

nanoparticle A nanoparticle or ultrafine particle is usually defined as a particle of matter that is between 1 and 100 nanometres (nm) in diameter. The term is sometimes used for larger particles, up to 500 nm, or fibers and tubes that are less than 10 ...

s entering the nose are transported to the brain via olfactory nerve. This can be useful for

nasal administration Nasal administration, popularly known as snorting, is a route of administration in which drugs are insufflated through the nose. It can be a form of either topical administration or systemic administration, as the drugs thus locally delivered ...

of medications. It can be harmful when the particles are soot or magnetite in

air pollution Air pollution is the contamination of air due to the presence of substances in the atmosphere that are harmful to the health of humans and other living beings, or cause damage to the climate or to materials. There are many different type ...

.Nano air pollutants strike a blow to the brain
Alison Pearce Stevens, Science News for Students, December 17, 2014 In naegleriasis, "brain-eating" amoeba enter through the olfactory mucosa of the nasal tissues and follow the olfactory nerve fibers into the olfactory bulbs and then the brain.

Structure

Function

Clinical significance

Examination

Lesions

Aging and smell

Pathway to the brain

Additional images

See also

Notes

External links