Lecture 2: Olfaction and Taste Flashcards
The sensation of odors that results from the detection of odorous substances aerosolized in the environment
Olfaction
The sensation evoked by stimulation of taste receptors located in the oropharyngeal cavity
Taste (gustation)
How does the somatosensory system contribute to flavor?
Detecting irritating components in smells like ammonia or the “hot” in spicy foods like peppers, also thermal and texture perceptions through branches of the trigeminal nerve
Orexia
Appetite
Anorexia
LACK of appetite
Anosmia
Lack of smell
Are humans macrosmatic or microsmatic creatures?
Microsmatic - less dependent on smell than many other mammals
Through connections with _____ and _____ structures, the olfactory system plays a role in the pleasures associated with eating and with the many scents that make up our world
Cortical
Limbic
5 taste sensations
Salty, sweet, sour, bitter, umami
Umami is important for
identification of amino acids (tastes like MSG)
Taste, which originates from receptors in the oropharyngeal cavity, is important to determine ______. This information is relayed by neural pathways that underlie various ingestive and digestive functions
the acceptance or rejection of foods
Retronasal smell
Sensation of taste through smell, volatile odorants in food can travel through oropharyngeal cavity to reach the olfactory epithelium
What cells express products of the several hundred olfactory receptor cells?
Bipolar olfactory receptor neurons
A single odor molecule activates how many olfactory receptors?
Several hundred
Where is the olfactory bulb located?
Cribriform plate of the ethmoid bone, inferior to the medial aspects of the frontal bone
Olfactory structures are vulnerable to trauma of what areas?
Facial trauma, particularly of nasal bones, frontal bone, or concha of the nose
The receptors responsible for transduction of odor molecules are found where?
Olfactory mucosa
Where is the olfactory mucosa located?
Roof of the nasal cavity on the inferior surface of the cribriform plate and along the nasal septum and medial wall of the superior turbinate
Composition of olfactory mucosa
Superficial acellular layer of mucus that covers the olfactory epithelium and underlying lamina propria
How is the olfactory epithelium differentiated from respiratory epithelium?
Its faint yellowish color and greater thickness
Olfactory epithelium is pseudo stratified and contains what? (4)
Olfactory receptor neurons, supporting cells (sustentacular cells), basal cells (replacing receptor neurons), and the ducts of small glands (Bowman’s glands)
Bodies of bipolar olfactory neurons are found where?
Basal 2/3 of epithelium
Each bipolar olfactory neuron has a single thin apical dendrite and ______
A basally located unmyelinated axon
10-30 nonmotile cilia on the olfactory receptors arise and protrude into the overlying mucus layer. The apical dendrite of these cells extend to where?
Apical dendrite extends to the surface of the epithelium
Nonmotile cilia contain receptors for what?
Odorant molecules
Unmyelinated axons of an olfactory receptor neuron pass through the lamina proper and group together into bundles called ______ which collectively make ______
Olfactory fila
Olfactory nerve (CN 1)
Olfactory file pass through ______ to terminate in the olfactory bulb
Cribriform plate
Location and attachment of olfactory bulb
Forebrain structure located on the ventral surface of the frontal lobe in the olfactory sulcus and is attached to the rest of the brain by the olfactory tract
Olfactory perception begins when
Volatile odor molecules are inhaled and contact the mucus layer of the olfactory epithelium
Odorants, like hydrophobic odorant musk, cross the mucus by
Interacting with small proteins called odorant-binding proteins
After crossing the mucus, odor molecules bind to
Odorant receptors on the cilia of the olfactory receptor neurons
What are odorant receptors?
Membrane proteins, GPCRs
Humans have as many as ______ types of odorant receptors
1000
Binding of the ligand generates depolarization in the ______ (though the adenylyl cyclase pathway (IP3 pathway) of the olfactory receptor neuron
dendrite
Axons of olfactory receptor neurons communicate with
2nd order neurons of the olfactory tract (mitral cells)
Axons of mitral cells emerge
From the caudal portion of the olfactory bulb to form the lateral olfactory tract
Where do axons of mitral cells terminate?
Areas on the ventral surface of the frontal lobe, the olfactory cortex
The olfactory system is the only system that bypasses the ______
thalamus
Olfactory system projects directly to the
Cortex
Lateral olfactory tract sends collaterals to the
Subcortical limbic structures
Olfactory cortex also sends projections to ______, which is important for what?
Orbitofrontal cortex and insula either directly or via a relay in the thalamus
Discrimination and identification of odors
Lesions in the orbitofrontal cortex lead to
Loss of discrimination and identification of odors
Insular and orbitofrontal also receive what input
taste
Medial orbitofrontal cortex plays a role in
Integrating olfactory, taste, and other food-related cues that produce the experience of flavor
In addition to neocortical, the olfactory cortex also sends projections directly to
lateral hypothalamus and hippocampus
Lateral hypothalamus projections important for
Feeding behavior
Hippocampus projections important for
Centers concerned with learning and behavior
Conductive olfactory deficits
Can be caused by nasal polyps, septal deviation, and inflammation
Sensorineural olfactory deficits can be triggered by processes that damage
olfactory receptor neurons or parts of the olfactory CNS
Head injuries result in what type of deficit and why?
Sensorineural, the olfactory fila may be torn loose from the olfactory bulb as a consequence of head trauma
Neurodegenerative conditions result in what type of deficit?
Sensorineural
Severe upper respiratory infections result in what type of deficit and why?
Sensorineural, due to permanent damage to olfactory receptor neurons
Seizures originating in the ______ may begin with an illusion of smell or taste, most often an unpleasant one and can include motor phenomena such as chewing movements or smacking of the lips
These are called ______ seizures
Piriform cortex, in the vicinity of the uncus
Uncinate
Loss of smell is often associated with
Polyps
Causes of nasal polyps
Inflamed mucous membranes or allergic reactions and can obstruct the nasal cavities or extend into the nares
Rhinitis or sinusitis
Block the access of odorants to the olfactory epithelium
Head trauma deficits
Damage to the central olfactory pathways or olfactory receptor axons as they pass through the cribriform plate
Fractures along the anterior base of the skull may cause
Drainage of CSF into the paranasal sinuses and through the nose (CSF rhinorrhea)
Cocaine use
perforation of the nasal septum, but normal odor perception
Specific anosmia
Autosomal recessive trait in which healthy individuals with otherwise normal olfactory acuity are unable to perceive the odor of a particular compound or class of compounds
Olfactory hallucination occurs in what conditions
Schizophrenia and Korsakoff psychosis
Chief complaint of patients with chemosensory disturbances
Loss or alteration of taste
Detection of an odorous substance indicates the integrity of the
Peripheral nerve and its pathway
Identification of odor reveals
Intact cortical function
If the patient is aware of a smell but cannot recall the name of the scent
Disorder is likely to reside at higher levels of the sensory system
Olfactory losses may be manifested _____ or _____
Unilaterally or bilaterally
Unilateral deficiencies are typically observed subsequent to ______ or after ______
Nasal cavity disease
Tumor-associated compression of one olfactory bulb or tract
Bilateral anosmia is usually sustained in response to
Head trauma or common cold
Age related declines in olfaction are common or uncommon?
Common
Does age related decline in olfaction occur gradually or suddenly?
Gradually
Olfactory dysfunction is encountered in the early stages of what diseases?
Alzheimer’s, Parkinson’s, Huntington chorea
The sense of taste results from an interaction between ______ and ______ located in sensory organs called taste buds
Gustatory stimuli
Receptor cells
Taste buds are most obvious where? But are found where?
Tongue
Throughout the oral cavity
Fungiform papillae
2-4 dorsally, anterior 2/3 of tongue
Foliate papillae
2-9 clefts, taste buds in clefts, posterior margin of tongue
Circumvallate papillae
8-12, taste buds in cleft
Where are extra-lingual taste buds found
Epithelia, NOT papillae Soft palate (junction of soft and hard palate), pharynx, epiglottis and aryepiglottal folds
Why are there taste buds in the pharynx and epiglottis?
Detection of food and suppression of aspiration into trachea
Type 1 taste bud cells
mainly perform supportive functions
Type 2 taste bud cells
Receptor cells, posses G-protein coupled receptors for bitter, sweet, and umami compounds
Type 3 taste bud cells
Type III: secrete neurotransmitters
Type 4 taste bud cells
Basal cells, are progenitor cells
Taste cells extend from
basal lamina to surface of epithelium
Apical ends of taste cells are covered with
Microvilli of various lengths that extend into a taste pore
The taste pore forms a pocket to
Permit contact between the microvilli of the taste cell and the external milieu
Taste pore is filled with
Protein rich substance though which substances must pass to reach the taste cell microvilli
Life span of taste cell
10-14 days
New taste cells arise from
Basal cells
In response to taste stimulation, receptor cells secrete
ATP
ATP does what in taste?
Excited adjacent type 3 cells, which form synapses with the afferent fibers of the sensory neuron
Afferent taste fibers penetrate the basement membrane and form
Sensory nerve fibers
Taste transduction is initiated when
soluble chemicals diffuse through taste pore and interact with receptors located on the apical microvilli of the taste cells
Interaction between ligand and receptor of the taste cells causes
Increase in intracellular calcium, either by release of calcium from internal stores or activation of voltage-gated calcium channels
Calcium release in the taste transduction pathway results in
A release of chemical transmitters (serotonin) at the afferent synapse, which leads to AP in afferent fiber
Facial nerve (CN 7)
Chorda tympani = anterior 2/3 of tongue
Greater superficial petrosal nerve innervates soft palate
Glossopharyngeal nerve (CN IX)
Posterior 1/3 of tongue
Vagus nerve (CN X)
Small area around the epiglottis
General sensation detecting (hot, etc.) is carried through what nerve?
Trigeminal (CN V)
TASTE PATHWAY
The cell bodies of facial nerve fibers serving taste are located in the geniculate ganglion, and their central processes enter the brainstem at the pontomedullary junction in the intermediate nerve, which is actually a part of the facial nerve.
These primary afferent taste fibers enter the solitary tract and synapse on cells of the rostral solitary nucleus.
Taste fibers in cranial nerves IX and X have their cell bodies of origin in the inferior ganglia (petrosal and nodose, respectively) of these cranial nerves.
The central processes of these fibers, enter the medulla, travel in the solitary tract, and terminate on neurons in the adjacent solitary nucleus.
Is taste ipsilateral or contralateral?
Ipsilateral
Principal visceral afferent nucleus of the brainstem is
Solitary nucleus
Divisions of solitary nucleus
Rostral (gustatory) and caudal (visceral or cardiorespiratory)
What terminates in rostral SN
Taste fibers traveling in CN VII, IX, and X
Axons arising from 2nd order taste neurons in the gustatory nucleus ascend in the ipsilateral central tegmental tract and terminate in the
VPM (ventral posteromedial nucleus) of the thalamus
Axons from the VPM travel through the ipsilateral posterior limb of the internal capsule to terminate in
The inner portion of the anterior insular cortex and on the lateral frontal operculum convexity of the post central gyrus
What pathway is responsible for discriminative aspects of taste and is exclusively ipsilateral?
Solitary nucleus -> VPM -> cortex
Sensation we perceive when eating and drinking is the combination of what 3 different kinds of input?
Direct chemical stimulation of taste buds, Stimulation of olfactory receptors by vapors from food, and stimulation of chemical-sensitive and somatosensory free nerve endings of the trigeminal and other nerves in the mucous membranes of the oral and nasal cavities
Ageusia
Loss of taste sense (rare, would require injury to tongue or CN VII, IX, or X afferent fibers)
Hypoguesia
Decreased taste sensitivity
Paraguesia/dysguesia
Distortions in taste perceptions
Clinical scenarios for decreased taste perception/sensitivity
Radiation/chemotherapy, head/brain trauma, surgical injury, or medications
