Physiology of taste and smell Flashcards
What type of receptors allow for taste?
Chemoreceptors
What is the function of taste?
Stimulation of taste receptors can induce a pleasurable or objectionable sensation, acting as a checkpoint for quality control (Off foods and poisons taste bad)
It also influences the flow of digestive juices and is influenced also by smell
What makes up a taste bud?
Tightly packed receptor cells and support cells
Receptor cells end in microvili, branching through the pore in the tongue
What is the lifespan of a taste bud?
They have a lifespan of around 10 days are replaced by basal cells from within the taste buds
What are some locations of tastebuds?
Tongue
Mucosa of the palate
Epiglottis
Pharynx
Nasal cavity
Where on the tongue are tastebuds found?
In the papillae of the tongue
What are the 4 types of papillae on the tongue?
- Filiform - Most numerous, do not contain taste buds
- Fungiform - Contain taste buds
- Vallate - Contain taste buds
- Folliate - Contain taste buds
Describe the process of taste
- Binding of tastant chemicals to receptor cells
- Initiates action potentials in afferent nerve fibres
- Cranial nerve transmit APs via the brainstem and thalamus to the cortical gustatory areas
What cranial nerves are responsible for transmitting taste?
Chorda tympani branch of CN VII - Anterior 2/3rds tongue
CN IX - Posterior 1/3rd of tongue
CN X - Epiglottis and pharynx
What are the 5 primary tastes?
- Bitter
- Sour
- Salty
- Sweet
- Umami (Meaty or savoury taste)
What causes bitter taste?
Stimulated by diverse group of tastants including alkaloids, poisonous substances and toxic plant derivatives
What causes a sour taste?
Stimulated by acids containing free H+
What causes salty taste?
Stimulated by chemical salts, especially NaCl
What causes sweet taste?
Stimulated by configurations of glucose
What causes umami taste?
Stimulated by amino acids
What is ageusia?
Loss of taste
What is hypogeusia?
Reduced taste
What is dysgeusia?
Distortion of taste
What are some causes of ageusia?
- Nerve damage
- Local inflamation (E.g. glossitis, radiation, tobacco)
- Some endocrine disorders
What are some causes of hypogeusia?
- Chemotherapy
- Medications
What are some causes of dysgeusia?
- Glossitis
- Gum infection
- Tooth decay
- Reflux
- URTI
- Medications
- Neoplasms
- Zinc deficiency
What are the functions of smell?
Stimulation of smell receptors can induce a pleasurable or objectionable sensation, acting as a checkpoint for quality control (Off foods and poisons taste bad)
It also influences the flow of digestive juices and influences taste
Smell is also useful in communication (E.g. mother to infant)
What are the 4 neural systems in the left and right sides of the nose?
- Main olfactory system - CN I
- Trigeminal somatosensory system - CN V
- Accessory olfactory system - E.g. Vomernasal
- Nervus terminalis - CN 0
What does CN I sense in smell?
CN I mediates the common odours such as roses, chocolate and vanilla
What does CN V sense in smell?
CN V has a chemical and non-chemical stimulus such as irritation, burning, cooling and tickling
It also has the protective effect of halting inhalation if you sniff something harmful
Where does the olfactory neuroepithelium sit?
Within a small region of the nasal mucosa in the olfactory cleft, just below the cribriform plate of the ethmoid bone, covering small parts of the superior and middle nasal concha and septum
What percentage of inhaled air reached the olfactory neuroepithelium?
10-15%
How are swallowed foods smelt?
Particles can move up the pharynx into the nasal cavity
How do odourants reach the olfactory neuroepithelium during normal breathing?
By diffusion as the olfactory mucosa is above the path of normal air intake
How does sniffing enhance smelling?
By drawing air currents upwards within the nasal cavity
What are some characteristics of a substance that allow it to be smelled?
- Sufficiently volatile (Some of its molecules can enter the nose with inspired air)
- Sufficiently water soluble (Can dissolve in the mucus coating of the olfactory mucosa
What type of cell surface is the olfactory neuroepithelium?
Pseudostratified columnar epitheium
What are some structures found within the olfactory neuroepithelium?
- Bi-polar sensory neurones extending odourant receptor-containing cilia into the mucus
- Supporting cells to insulate and protect
- Duct cells of bowmans glands, which secrete mucus
- Basal cells (Stem cells)
Describe the path of an odourant to a chemical signal
- Odourant enters the nose
- Passes to the olfactory cleft
- Moves from our phase to the aqueous phase through mucous
- Odourants then either diffuse or are transported by specialised proteins to the receptor
- This causes an AP in the olfactory nerves
- This passes through the cribriform plates in fascicles of around 20-30 neurones, which release dendrites containing receptor cilia
What is anosmia?
Inability to smell
What is hyposmia?
Reduced ability to smell
What is dysosmia?
Altered sense of smell
What is phantosmia?
Smell percieved in the absense of a stimulus (Olfactory halucination)
What are some conductive causes of olfactory abnormality?
- Nasal polyps
- Rhinitis
- Nasal mass
What are some sensorineural causes of olfactory abnormality?
- Viral
- Head trauma
- Neurological (E.g. Parkinsons, Alzheimers)
- Brain tumours
- Medications
What test can be used to assess olfaction?
UPSIT test - Scratch and sniff cards
What is a supportive management option in anosmia lasting >2 weeks?
Smell retraining
How is life-expectancy affected by anosmia and ageusia?
It is decreased
What are some possible affects of anosmia?
Loss of libido
Impact on occupation
What are some occupations that may be affected by anosmia?
Fire fighters (Can’t smell smoke)
Chefs (Can’t smell or taste food)
Sommeliers (Can’t smell wine)
