Nose - Anatomy & Physiology

Introduction

Olfaction is the sense of smell, which is the ability to perceive and distinguish odours. Most mammals have a good sense of smell, but most birds generally do not. The sense of smell is well-developed in carnivores (predators) and ungulates (prey). Fish also have a fairly well-developed sense of smell.

Olfactory and gustatory receptors can combine to contribute to flavour.

Structure

Nasal Cavities - Copyright David Bainbridge

• The nose consists of the external nares with nasal cartilages, the nasal cavity (including the nasal meatus and conchae), and the paranasal sinuses.
• Borders of the nasal cavity:

- Caudal: cribrifrom plate of the ethmoid bone

- Ventral: continuous with the nasopharynx

- Dorsal: the maxilla and the palatine processes of the incisive bones

• Rostrally, the median septum is a continuation of the ethmoid bone. The median septum is made up of hyaline cartilage, and divides the nasal cavity into left and right halves.
• Nasal conchae are turbinate bones that project into the nasal cavity. Their purpose is to increase the respiratory surface area, and to create turbulence within the passing air. This helps to filtrate and warm or cool the air that passes through. They are cartilagenous or ossified scrolls that are covered with mucous membrane, under which is a layer of anastomosing blood vessels.
• The nasal conchae are more complex in animals with a better sense of smell, as they increase the surface area further.
• There are dorsal and ventral conchae, the dorsal concha extending further into the nasal cavity.
• The conchae divide the nasal cavity into meatuses, which branch out from a common nasal meatus which is adjacent to the nasal septum. There are three nasal meatuses which branch from the common nasal meatus: dorsal, middle and ventral:

- Dorsal nasal meatus: the passage between the roof of the nasal cavity and the dorsal nasal concha

- Middle nasal meatus: between the dorsal and ventral conchae, and communicates with the paranasal sinuses

- Ventral nasal meatus: the main pathway for airflow leading to the pharynx, and is positioned between the ventral nasal concha and the floor of the nasal cavity

- Common nasal meatus: the longitudinal space on either side of the nasal septum

• The Paranasal Sinuses are extensions of the nasal cavity.

Function

Olfactory Epithelium - Copyright David Bainbridge

• Olfactory sensory neurones are present in the olfactory epithelium.
• Those animals with a better sense of smell have more highly innervated olfactory epithelium.
• It is the mucous membrane of the dorsal nasal conchae that is sensitive to smell. Here, odourants dissolve in the mucous membrane, and it is these odourants that are recognised by the olfactory sensory receptors. Sensory cilia, that are present on the surface of the olfactory receptors, project into the film of mucous.
• The mucous also contains antibodies, to prevent infection. This is because the olfactory neurones provide a direct passage, via the olfactory nerve (CN I), to the brain.

Vasculature

• There is a dense network of blood vessels supplying the nasal mucosa.
• The sphenopalatine artery supplies the nasal cavity.
• The lateral and dorsal nasal arteries, which are branches of the facial artery, itself a branch of the external carotid artery, supply the nose.
• The infraorbital artery also supplies the nose.

Innervation

• Sensory innervation is provided by the olfactory nerve (CN I).

Central Olfactory Pathways

Central Olfactory Pathways - Copyright David Bainbridge

• The olfactory receptors are embedded in the mucous membrane within the nasal cavity. They are bipolar neurones, and are covered with cilia (non-motile).
• It is thought that these cilia contain the active sites for the olfactory transduction process.
• The axons from the olfactory receptors join together and become the olfactory nerve (CN I). The axons pass through the perforations in the cribriform plate of the ethmoid bone, and enter the olfactory bulb.
• Once the axons have entered the olfactory bulb, the olfactory nerve synapses on mitral cells. The axons from these cells then project into the olfactory cortex of the cerebral hemispheres, via the olfactory tract.
• The olfactory cortex is the only region within the cerebral hemispheres that receives direct sensory input without any information first passing through the thalamus. This is because the olfactory system evolved before the thalamus.

Species Differences

• Those animals with a better sense of smell have a greater vascular supply to the mucosa of the dorsal nasal conchae.

Links

Nasal Cavity

Diseases of the nasal cavity and sinuses