Olfaction and taste - anatomy, physiology, examination, syndromes
Olfaction (Smell) → special sensory
Anatomy
Olfactory Pathway (CN I)
1.Olfactory receptors in the nasal mucosa (roof of nasal cavity)
2.Olfactory nerve fibers pass through the cribriform plate of the ethmoid bone
3.Olfactory bulb
4.Olfactory tract
5.Primary olfactory cortex (temporal lobe)
Physiology
- Perception and identification of odors
- Contributes significantly to flavor perception
- Closely connected to the limbic system (memory and emotions)
Examination
- Test each nostril separately: Eyes closed. One nostril occluded. Present familiar non-irritating odors (coffee, vanilla, soap). Avoid irritants (ammonia), which stimulate CN V rather than CN I.
Olfactory Syndromes
1.Anosmia: Complete loss of smell.
- Causes:Upper respiratory infections. Head trauma (cribriform plate injury). Tumors. Neurodegenerative diseases (e.g., Parkinson disease)
2.Hyposmia: Decreased sense of smell.
3.Hyperosmia: Increased sensitivity to odors.
4.Parosmia: Distorted perception of odors.
5.Phantosmia: Perception of odors without a stimulus (olfactory hallucinations).
Taste (Gustation)
Anatomy: Taste receptors are located in the taste buds of the tongue, palate, and pharynx.
Cranial Nerves Involved
Pathway
Taste receptors → CN VII/IX/X → Brainstem (nucleus tractus solitarius) → Thalamus → Gustatory cortex (insula)
Physiology
- Primary Taste Modalities: Sweet. Salty. Sour. Bitter. Umami
- Functions: Food recognition. Protective detection of harmful substances. Regulation of appetite
- Examination of Taste
- Test each side of the tongue separately.
- Apply solutions representing: Sweet (sugar). Salty (salt). Sour (lemon). Bitter (quinine/coffee)
- Patient identifies the taste with eyes closed.
Taste Disorders
1.Ageusia: Complete loss of taste.
2.Hypogeusia: Reduced taste sensation.
3.Hypergeusia: Increased taste sensitivity.
4.Dysgeusia: Distorted or unpleasant taste sensation.
Causes of Taste Disorders: Facial nerve lesions (CN VII). Glossopharyngeal nerve lesions (CN IX). Oral infections. Zinc deficiency. Medications. Smoking
Clinical Correlations
Facial Nerve (CN VII) Lesion
- Loss of taste from anterior 2/3 of tongue
- May be associated with facial weakness
Glossopharyngeal Nerve (CN IX) Lesion
- Loss of taste from posterior 1/3 of tongue
- Reduced gag reflex
Neurodegenerative Diseases
- Early anosmia may occur in: Parkinson Disease. Alzheimer Disease