The nose is the protruding structure at the center of the face that serves as the organ of smell and entrance of the respiratory system. The paranasal sinuses (maxillary, frontal, ethmoidal, sphenoid) are air-filled cavities within the bones of the skull that surround the nasal cavity. The nose and the paranasal sinuses provide resonance to the voice and humidify and warm inhaled air. The nasal cavity consists of a respiratory region, which is lined with ciliated pseudostratified columnar epithelium interspersed with goblet cells, and an olfactory region, which is lined with pseudostratified ciliated columnar epithelium interspersed with olfactory receptor neurons. Olfactory information is communicated to the brain via the olfactory nerve (CN I), which arises from the respiratory epithelium. The olfactory receptor neurons in the respiratory epithelium react with odorants via diffusion of odorant-binding proteins. The signal is carried by olfactory receptor neurons, whose axons bundle in fascicles. These fascicles then pass through the cribriform plate of the ethmoid bone and form the olfactory bulb within the forebrain. From there the signal is transmitted on to the target regions in the brain via the olfactory tract. Unlike other sensory tracts, the olfactory tract bypasses the thalamic nuclei and projects directly into the primary olfactory cortex. Diseases affecting the nose and the paranasal sinuses include nasal polyps, epistaxis, choanal atresia, and sinusitis.
- The nose contains the human olfactory system, which is responsible for the sense of smell.
- Paranasal sinuses
- The nose and paranasal sinuses fulfill the following functions:
|Characteristics of paranasal sinuses|
|Frontal sinus|| |
|Ethmoidal sinus|| |
|Sphenoid sinus|| || |
|Maxillary sinus|| |
- Frontal, , , nasal, palatine,
- Roof: composed of the nasal, frontal, ( ), and
- Medial wall (nasal septum): composed of septal cartilage, the vomer, and the perpendicular plate of the ethmoid bone
- Lateral wall: composed of the , , medial pterygoid plate of the , perpendicular plate of the , frontal process of the , nasal surface of the , and the inferior, middle, and superior meatus
|Nasal meatuses and recesses|
|Inferior meatus|| |
|Sphenopalatine recess|| |
- Ophthalmic artery: lateral nasal branches of the anterior and posterior ethmoidal arteries
- Maxillary artery
- Facial artery
- Olfaction: via the (cranial nerve I), which is responsible for the transmission of olfactory signals from the olfactory epithelium to the brain
- Other sensation
- Formed by the lower two-thirds of the nasal cavity
- The anterior part of the vestibule in the nasal cavity is lined with keratinized, stratified, squamous epithelium.
- Posterior to the vestibule, the epithelium becomes pseudostratified, with (respiratory epithelium).
- Contains vibrissae, short and thick hair that filter large particles from inspired air
- The lamina propria is abundantly vascularized.
- Serves to warm, humidify, and clean air that is inhaled
- Found on the roof of the nasal cavity, in the superior nasal meatus, on both sides of the nasal septum
Lined by tall, pseudostratified, ciliated, columnar epithelium
- Olfactory cells
- Sustentacular and microvillar (supporting) cells
- Basal cells
- The lamina propria contains Bowman glands, fibroblasts, blood vessels, and unmyelinated fibers from the olfactory neurons.
- Molecules are inspired and absorbed by the respiratory region of the nose before reaching the olfactory epithelium.
- Smell is the sensation produced when the olfactory epithelium reacts with dissolved odorant substances.
- Mucus secretion is required for odorant molecules to interact with the olfactory epithelium (aids in dissolving odorants).
- The conscious and unconscious perception of smell takes place in different areas of the brain.
- Smell plays an important role in the formation of memory and emotions.
- Smell aids in gustation (sense of taste).
Anatomy of smell
- The olfactory neurons are primarily located in the superior part of the nasal cavity.
- Odorants travel through the superior nasal meatus to reach the olfactory bulb.
- The olfactory system is characterized by direct projections that connect directly to the cortex, bypassing the thalamic nuclei (unlike other sensory systems).
Olfactory receptor cells
- Primary sensory neurons, bipolar neurons that contain chemoreceptors
- Cilia, dendritic knobs, dendrites, and cell bodies of olfactory cells are contained within the olfactory epithelium.
- The unmyelinated axons of olfactory cells penetrate the fenestrated cribriform plate of the ethmoid bone.
- Odorants react with olfactory receptor cells via diffusion or odorant-binding proteins.
- Sustentacular cells: contain microvilli and secretory granules
- Basal cells: stem cells of the olfactory epithelium
- Olfactory glomerulus: neurons with identical olfactory receptors form glomeruli in the olfactory bulb.
- Tufted cells
- Mitral cells
- Periglomerular cell: inhibit mitral cells and tufted cells laterally → sharper contrast between different smells
- Granule cells
|Overview of smell pathways|
|Olfactory tract|| |
|Primary olfactory cortex|| |
|Secondary olfactory cortex|
|Olfactory nuclei|| |