The sense of smell is mediated via the olfactory nerve, which is one of the 12 pairs of cranial nerves. The olfactory nerve is purely sensory and has no motor component. It is the first cranial nerve and is also known as cranial nerve I. The olfactory nerve enters the brain and synapses (terminates) within the glomerular layer of the olfactory bulb. This is where an odor is first detected, filtered, and identified. The meaning or perception of the odor is determined in other regions of the brain in association with olfactory cortices. Collectively, structures involved in olfaction are part of the rhinencephalon, which translates from Latin as “nose brain.”
General Functional Component
The olfactory nerve has one component, special sensory (special afferent). Its function is the sensation of olfaction, or the sense of smell. The inability to smell is called anosmia, which can be a congenital (present at birth) defect or associated with degenerative neurological diseases such as Parkinson’s and Alzheimer’s diseases.
Neurons that make up the olfactory nerve are found in the olfactory epithelium within the nasal passageway, specifically in the roof of the nasal cavity, the superior nasal conchae, and the nasal septum. These are special sensory neurons as they include the sensory receptor (a specialized protein) on the anterior end and the posterior end (axon) makes up the nerve. These neurons are called olfactory sensory neurons, which are activated when a specific chemical component of an odor binds to the receptor. When an odor enters the nostril, olfactory gland secretions dissolve the scent. These secretions also keep the olfactory epithelium moist.
Roughly 20 axons bundle together making several small branches of the olfactory nerve. These branches pass through the numerous foramina (holes) of the cribiform plate of the ethmoid bone. The axons then terminate on the secondary sensory neurons (mitral and tufted cells) of the olfactory bulb. Here the odor is filtered and identified before its signal is relayed to other brain regions, such as the limbic system that includes the amygdala and entorhinal cortex. The olfactory system is unique as it is the only sensory system that bypasses the thalamus, which is a deep structure in the brain that modulates neuronal signals (or alters the perception of the sense). Instead, the olfactory message directly enters the limbic system, which is associated with memories. This is why a specific smell, like a perfume, can instantly bring back memories of a certain place (for example, Grandma’s house) or feelings.
The olfactory system is also associated with the autonomic nervous system, particularly for visceral responses. The smell of food cooking will induce salivation if it is a pleasant odor, while unpleasant odors induce nausea. This is due to the complex medial forebrain bundle, the stria medullaris thalami, and stria terminals pathways.
Most animals have a very strong sense of smell, as it is important for finding food and potential mates as well as to determine nearby predators. Thus, nonhuman mammals and reptiles have two systems: the main olfactory system to detect volatile odors and an accessory olfactory system to detect fluid-phase stimuli. Fluid-phase stimuli are commonly known as pheromones, and it is important to note that some pheromones can be detected via the main olfactory system. The accessory olfactory system has a special organ to sense the fluid-phase stimuli. This structure is a bone that separates the left and right nasal cavities (between the nose and the mouth) and is called the vomeronasal organ. Mammals curl back their upper lip (flehmen) to stimulate the vomeronasal organ, while snakes stick out their tongue while touching the vomeronasal organ to detect prey.
Injury to cranial nerve I may occur during a fall or injury to the head especially if the cribiform plate breaks. This is because the nerve passes through the foramina of the cribiform plate and is easily severed. Tumors in the base of the anterior skull can also damage the olfactory nerve. The major symptom of a damaged nerve is the inability to smell in one (unilateral) or both (bilateral) nostrils. Anosmia may improve over time as the nerve regenerates. Persons with anosmia generally have a loss of appetite because they can no longer smell and their food lacks flavor.
Jennifer L. Hellier
See also: Anosmia; Cranial Nerves; Olfactory Bulb; Olfactory Sensory Neurons; Olfactory System; Sensory Receptors
Liang, Barbara. (2012). The 12 cranial nerves. Retrieved from http://www.wisc-online.com/objects/ViewObject.aspx?ID=AP11504
Wilson-Pauwels, Linda, E. J. Akesson, & Patricia A. Stewart. (1988). Cranial nerves. Anatomy and clinical comments. Philadelphia, PA: B. C. Decker.