Physiology Of The Nose And Sinuses

Physiology of the Nasal Airway

Nasal Cycles and Respiratory Airflow

• nasal airflow is regulated through the volume of the venous sinusoids (capacitance vessels) in the nasal erectile tissue (located primarily in the inferior turbinate and to a lesser extent in the anterior septum)

• the hypothalamus continuously stimulates a sympathetic tone (via the superior cervical sympathetic ganglia) to maintain a level of nasal vasoconstriction

• inspired air is warmed to body temperature and is humidified to almost 100% humidity

• Sneeze Reflex: induced by allergens, ammonia, viral infections, exercise, and other irritants which stimulates trigeminal afferents, complex efferent input results in a slow inspiratory phase, glottic and velopharyngeal closure (increases subglottic pressure), followed by a sudden glottic opening (sneeze)

• Regulation Response Types

1. Asymmetrical Congestive Response (The Nasal Cycle): normal physiological congestion/decongestion cycle alternating between nasal sides every 2—7 hours

2. Symmetrical Congestive Response: temporary bilateral congestion induced by exercise, changes in body position, hyperventilation, cold air, sulfur, histamine, and other irritants; lasts 15-30 minutes

Microvasculature

• regulates nasal volume, humidity, and heat exchange

• Resistance Vessels: arterioles and precapillary sphincters, regulate blood flow to the nasal mucosa

• Subepithelial Capillaries: fenestrated vessels allow for transport of solutes and fluids

• Venous Sinusoids: capacitance vessel, determines blood volume and nasal congestion

• Arteriovenous Anastomoses (AVA): regulate nasal blood flow by allowing blood to flow directly from the resistance vessels to the venous sinusoids

Regulation of Nasal Microvasculature

• Sympathetic Innervation: provides vasoconstrictor tone to arteries and capacitance veins, mediated through Norepinephrine (primary neurotransmitter), Neuropeptide Y (a weak vasoconstrictor, enhances effects of norepinephrine), and Avian Pancreatic Polypeptide (APP)

• Parasympathetic Innervation: controls secretions and dilates resistance vessels, mediated through Acetylcholine (primary neurotransmitter), Vasoactive Intestinal Peptide (VIP), and Peptide Histamine Isoleucine (PHI)

Nasal Valves

• External Nasal Valve (Nasal Vestibule): anterior nostril (nasal alar cartilage, columella, and nasal sill), potential cause of obstruction during inspiration

• Internal Nasal Valve (Limen Nasi): bordered by septum, anterior edge of the inferior turbinate, and caudal edge of upper lateral cartilage; narrowest segment (50% of total nasal resistance), potential site of obstruction secondary to Bernoulli's principle (narrowed segment accelerates nasal airflow resulting in a decrease in intraluminal pressure)

Mucociliary System

• Function: humidification, cleaning of inspired air, eliminating debris and excess secretion from paranasal sinus and nasal airway

• Mucociliary Flow: mass motion of mucous layer in the paranasal sinus of the mucous blanket at 1 cm/min (eg, migration in the maxillary sinus begins at the floor of maxillary sinus — natural ostium — nasal cavity — nasopharynx)

• Components

1. Ciliated, Pseudostratified Columnar Epithelium: anterior border begins at limen nasi

2. Double Layered Mucous Blanket: deep, less viscous, serous periciliary fluid (sol phase) and superficial, more viscous, mucous fluid (gel phase)

3. Mucous Producing Glands: goblet cells (columnar cells, basal nucleus, secretory granules at lumen end), deep and superficial seromucinous glands (serous or mucous acini with cuboidal lined duct complexes), and intraepithelial glands (20—50 mucous cells around a single duct)

• Major Composition of Nasal Mucus: 95% water, 3% glycoproteins (mucin), 2% salts, immunoglobulins (IgA), lysozymes (bacteriolytic), and lactoferrin (bacteriostatic)

Olfactory Physiology

• olfaction requires turbulent airflow from the anterior nares or the choanae

• pungent odors (vinegar, ammonia) may be perceived through trigeminal nerve fibers (via substance P pain fibers)

• Olfactory Epithelial Cell Types

1. Ciliated Olfactory Receptor Cells: club shaped bipolar neurons with axons that synapse to the olfactory bulb

2. Microvillar Cells: neuronal cells of unknown function

3. Supporting Cells: sustenacular cells

4. Basal Cells: allow capability of olfactory fiber regeneration (unlike other sensory cells)

• Olfactory Mechanism: odorant enters olfactory cleft ^ odorant dissolves in mucus ^ odorant binding proteins (OBP) concentrate the solubilized odorant ^ binds to olfactory receptor at the sensory cilia ^ receptor binding results ^ stimulates a specific G-protein (cAMP dependent) cascade for depolarization ^ synaptic connections form a complex network of secondary neurons (suggesting peripheral processing) before entering the brain (dentate and semilunate gyri)

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