Chemical Senses Flashcards
What are the chemical senses?
Gustation (taste) and olfaction (smell)
- Chemoreceptors respond to chemicals in aqueous solution
- Taste - to substances dissolved in saliva
- Smell - to substances dissolved in fluids of the nasal membranes
What is the function of the taste system?
The taste system evaluates potential food for beneficial (nutritional) and deleterious (poisonous) chemical features
What does flavor depend on?
Flavour of food depends on combination of taste and smell (and texture, temperature, etc)
What does sense of Gustation depend on?
Sense of gustation depends on chemical stimuli, tastants, present in food etc
What is the normal size of taste buds?
70um in height
40um diameter
Note: Each taste bud has 50-150 taste receptor cells
• Adult humans have 3,000-10,000 taste buds, children a few more
• Taste buds degenerate with age
Name the types of taste buds and where they occur?
Occur mostly on the dorsal surface of the tongue in papillae
1. fungiform (anterior)
2. foliate (posterior and lateral)
3. circumvallate papillae (base)
- Each papilla has from one to several hundred taste buds
- Also occur on soft palate, pharynx, & epiglottis
What are the cells of taste buds?
- chemoreceptors
- supporting cells (sustentacular cells)
- basal cells
Describe the structure of a taste bud?
- supporting cells - sustentacular cells
- taste cells - chemoreceptors
- The apex of chemoreceptors have microvilli
- Chemoreceptors synapse with afferent neurons
Name and describe the basic tastes?
- Salty
- Sour
- Sweet
- Bitter
- Umami
Describe the sour taste?
Caused by acids
i.e. by the hydrogen ion concentration
Describe the salty taste?
elicited by ionized salts, mainly by the sodium ion concentration
Describe the sweet taste?
not caused by any single class of chemicals
E.g. sugars, glycols, alcohols, aldehydes, ketones, amides, esters, some amino acids, some small proteins, sulfonic acids, halogenated acids, and inorganic salts of lead and beryllium
Describe the bitter taste?
not caused by any single type of chemical agent
E.g. long-chain organic substances that contain nitrogen, alkaloids (quinine, caffeine, strychnine, and nicotine)
- Some substances that at first taste sweet have a bitter aftertaste
e.g. saccharine (artificial sweetener)
Describe the umami taste?
A Japanese word (meaning “delicious”)
designating a pleasant taste sensation that is qualitatively different from sour, salty, sweet, or bitter
- Umami is the dominant taste of food containing L-glutamate
e.g. meat extracts and aging cheese, soy sauce, fermented foods, tomatoes, grains and beans
What is transduction?
is the process by which an environmental stimulus causes an electrical response in a sensory receptor cell
What is the chemoelectrical transduction of salty taste?
- Na+ flow into taste cells through apical amiloride-sensitive sodium channels [Na+]i
- cell depolarization
- opening of voltage-dependent Ca2+ channels > [Ca2+]i
- neurotransmitter release (serotonin)
Describe the chemoelectrical transduction of sour taste?
- H+ enters cell likely through proton-selective ion channels
- [H+] in receptor cell depolarises chemoreceptor cell (like Na+ in salty taste)
- opening of voltage-dependent C2+ channels
- [Ca2+]i > neurotransmitter release (serotonin)
- [H+] also blocks K+ channels
- increase in [K+]
- chemoreceptor depolarization
Note: Sour sensation can also be elicited by potassium channel blockers
e.g. Tetraethylammonium
Describe the chemoelectrical transduction of bitter taste?
Bitter, sweet, and umami compounds all activate taste receptor cells via G-protein coupled receptors
1. Binding activates phospholipase C IP3 (2nd messenger)
2. Ca2+ release (from ER)
3. neurotransmitter (ATP) release
4. Ca2+ influx not needed because ATP is not released from vesicles
The taste buds are innervates by which 3 cranial nerves?
- chorda tympani/facial (VII)
- glossopharyngeal nerve (IX)
- vagus nerve (X)
Describe the course of all taste fibers?
- 1st order neurons in the gustatory division of solitary nucleus of medulla
- 2nd order neurons synapse in the ventral posterior medial nucleus of thalamus
- 3rd order neurons carry signals to the primary gustatory cortex
What happens to taste fibers after they leave the gustatory nucleus?
From the gustatory nucleus, taste pathways diverge
1. Cerebral cortex
- frontal cortex + somatosensory cortex = conscious perception of taste
2. Brain stem regions involved in swallowing, salivation, gagging, vomiting, and basic physiological functions such as digestion and respiration.
3. Limbic system for food palatability and motivation to eat
- amygdala + hypothalamus = emotional quality of taste
4. Hippocampus for memory of taste
What are the main causes of taste disorders?
Taste disorders are generally the result of
1. certain medical conditions
2. pharmacologic or surgical interventions
3. environmental exposure to toxic chemicals
4. head injury
5. poor oral hygiene
6. advanced age
What is hypogeusia?
Diminished sense of taste
What is ageusia?
The complete loss of taste
What is dysgeusia?
An alteration or distortion in the perception of taste
What is phantogeusia?
phantom taste, often a bitter or metallic taste when no gustatory stimulus is present
- Schizophrenia
- Seizure aura for focal epilepsy
What is the purpose of olfaction?
Olfaction serves as an early warning system for the detection of fire, dangerous fumes, and polluted environments, and it largely determines the flavour and palatability of foods and beverages
- 5-6 million receptors in humans
What is olfactory epithelium?
• Located in olfactory (nasal) mucosa
• Surface area increased by conchae/turbinate (shell-like bone shelf)
• 2.4 square cm in each nostril
• The olfactory bulb is the part of the brain that receives direct input from primary olfactory neurons (in olfactory epithelium)
What are olfactory chemoreceptors?
Olfactory chemoreceptors are bipolar neurons
1. The apical surface of olfactory chemoreceptors has cilia
2. the base is a neuron
- Each cell has 4-25 cilia, 200 um in length, immobile
Note: Olfactory receptors are interspaced between sustentacular cells
What is the role of sustentacular cells?
- physical support
- nourishment
- electrical insulation
- help detoxify chemicals
Describe the effect of SARS-CoV-2 on olfactory chemoreceptors?
- SARS-coV-2 infects sustentacular cells and not chemoreceptors or olfactory bulb neurons
- To infect a cell SARS-CoV-2 must bind to the angiotensin-converting enzyme 2 (ACE2) receptor in the cell membrane.
- ACE2 is expressed by sustentacular cells in the human olfactory epithelium but not by olfactory sensory neurons
What is the lifespan of olfactory chemoreceptors?
60 days
Describe potential infections of the CNS through the olfactory epithelium?
Harmful particles can potentially invade sensory neurons in the olfactory epithelium and can be transported into the CNS via the olfactory nerve
- Neurogenesis replaces damaged neurons
Name 6 odour qualities?
- floral e.g. roses
- ethereal e.g. pears
- musky e.g. perfume, aftershave
- camphor e.g. eucalyptus
- putrid e.g. rotten eggs
- pungent e.g. vinegar
Note: Humans recognize 10,000 separate odours (20% pleasant, 80% bad)
Name odorless gases?
- Nitrogen
- hydrogen
- ethane
- carbon monoxide
- radon
- helium
- cooking gas (LPG, ethanethiol added for odour)
Molecules are introduced to the olfactory system through what actions?
- Sniffing
- Air
- Diffusion from the mouth
Describe the olfactory transduction mechanism?
- Odorants Bind to membrane odorant receptor proteins
- Stimulate G-protein receptor protein (Golf)
- Activate adenylyl cyclase
- Form cAMP in the presence of ATP
- Bind cAMP to a cyclic nucleotide-gated cation channel
- Open cation channels and allow influx of Na+ and Ca2+
- Open Ca2+ - activated CI- channels
- Cause current flow and membrane depolarization (receptor potential)
What is the olfactory bulb?
The bulb is an outgrowth of brain tissue from the base of the brain
Describe the cells of the olfactory bulb?
- mitral cells
- tufted cells
- Mitral cells and tufted cells are the output neurons of the olfactory bulb - interneurons (granule and periglomerular cells)
- Granules and periglomerular cells are inhibitory interneurons
What is the olfactory bulb made of?
Nerve fires leading backwards from the bulb form the cranial nerve I / olfactory nerve
- Glomeruli contain axonal ends of olfactory receptors and apical dendrites of 2nd order afferent fibres
What makes olfaction special from other major sensations?
Olfaction is the only major sensation relayed to the cerebral cortex without first passing through the thalamus
- Axons of mitral and tufted cells pass posteriorly through the intermediate
olfactory stria and lateral olfactory stria to the olfactory cortex (temporal lobe)
Describe central pathways of olfaction?
- project to entorhinal cortex
- responsible for olfactory memories - project to the frontal lobe (medial and intermediate olfactory areas)
- Medial olfactory area responsible for visceral and emotional reactions to odors - Connections with limbic system (amygdala)
Describe what each area connected to the olfactory system stimulates?
- Frontal cortex - conscious perception of smell
- Hypothalamus + amygdala - motivational and emotional aspects of smell
- Hippocampus - odor memory
What is anosmia?
Absence of smell
What is hyposmia?
Diminished sensitivity of smell
What is dysosmia?
Distortion of normal smell
What is phantosmia ?
A type of dysosmia refers to perception of odour in absence of an odour stimulus
What is parosmia?
Parosmia refers to distortion of odour perception when an odour is present
- when the smell of something familiar is distorted, or when something that normally smells pleasant now smells foul e.g. long COVID
What are the causes of disorders of olfaction?
- Aging
- respiratory infections
- smoking
- growths in the nasal cavities
- head injury
- hormonal disturbances
- dental problems
- exposure to certain chemicals
e.g. insecticides and solvents - numerous medications
e.g. antibiotics and antihistamines - radiation for treatment of head and neck cancers
- CNS disorders
