2 Olfaction and Taste

Clinical Terms: *What is the clinical term for impaired taste sensation?* *(Objective #19)*

*Ageusia*: -Loss of taste sense, rare, would require injury to tongue or VII/IX/IX afferent fibers *Hypogeusia*: -Decreased taste sensitivity *Parageusia/dysgeusia*: -Distortions in taste perceptions.

Disorders of the Olfactory System: 17: -Polyps= overgrown nasal mucosal, block access of olfactory senses. *What other pathological conditions or events can cause loss of smell?* *(Objective #9)*

-Loss of smell is usually associated w/ nasal polyps -May result from inflamed mucus membranes or allergic reactions and can obstruct nasal cavities or extend into the nares -Polyps can obstruct flow of the air and compromise olfactory sense

Olfactory Bulb Projections: 11: -Proceed caudally to enter Cerebral Cortex ("Olfactory cortex"), don't need to know subregions. KNOW where olfactory cortex is. In ventral surface of brain, adjacent to Anterior Perforated Substance. Occupy small medial portion of temporal lobe, near uncus. *Anterior Olfactory Nucleus w/in Olfac Tract, communicates w/ contralateral hemisphere* *Where is primary olfactory cortex located?* *(Objective #5)*

-Olfactory cortex is a paleocortex b/c has 3 neuron layers (not 6 like neocortex) -Unique feature is olfactory bulb projects directly to cortex (and passes Thalamus) *Primary Olfactory Cortex is on the ventral surface of the brain, adjacent to Anterior Perforated Substance (medial portion of temporal lobe, near uncus)*

Disorders of the Olfactory System: 15: -conductive= block. -sensorineural= usually from injuries, olfac nerve can be torn, tumors can compress

*Conductive olfactory deficits*: -Deficits caused by nasal polyps septal deviation, inflammation *Sensorineural olfactory deficits*: -Damage to olfactory receptor neurons or parts of the olfactory CNS. (Head injuries- olfactory nerve can be torn. Alzheimer's disease, URI)

Olfactory Structures: Slide 5: -Stress: olfactory bulb and tract on ventral surface. Cribriform plate in ethmoidal bone, olfac nerve fibers pass through openings here to reach olfac bulb

*Olfactory bulb*: -*Lies on cribriform plate of ethmoid bone* *Olfactory structures are vulnerable to facial trauma, particularly that involving the nasal bones, frontal bones, or concha of the nose.

Taste Receptors: 23: -Triggered when molecules come in contact w/ receptors on taste buds. Taste buds= organs that reside in papillae, which are scattered. Most important ones= on tongue, but there are some scattered in oropharyngeal cavities, in mucosa/ epithelium. These ones may have different functions.

-Sense of taste results from interaction btw gustatory stimuli and receptor cells in taste buds -Taste buds are on tongue but also scattered throughout oropharyngeal cavity

29: *Most important slide of this lecture* Anterior 2/3= facial, mostly chorda tymp Posterior 1/3 taste= glossopharyngeal N Small area around epi= vagus N Buds on epiglottis by vagus mostly serve protective function, spoiled food triggers gag reflex, food rejected. facial and glossopharyngeal nerve important b/c may lead to correct dx w/ nerve damage (If Q says decrease anterior 2/3 tongue, will know facial N) -General sensation from Trigiminal N *Name the areas of the tongue innervated by CN VII and CN IX.* *(Objective #14)* *Which nerve carries taste from the epiglottis?* *(Objective #15)*

*Facial N*: -Chorda Tympani innervates *anterior two thirds of tongue* -Greater superficial petrosal nerve innervates the soft pallate. *Glossopharyngeal N*: -Serves *posterior 1/3 of tongue* *Vagus N*: -*Small area around the epiglottis* *General sensation is carried through Trigeminal N, so it also participates in formation of flavor

Taste Bud Distribution: 24: -Mostly fungiform b/c look like mushroom, scattered. Foliate, lateral edge base of tongue. Circumvallate pap, Extra-lin around epiglotis and aryepiglottal folds. Those signal danger and may trigger gag-reflex, protect from inhaling foods *What types of lingual papillae contain taste buds?* *(Objective #11)* *What is the importance of taste buds located in the soft palate, pharynx and epiglottis?* *(Objective #12)*

*Lingual Taste buds*: -*Fungiform papillae*: dorsally, anterior 2/3 or tongue -*Foliate papillae*: In clefts, posterior margin of tongue -*Circumvallate papillae*: In cleft *Extra-Lingual*: -Located in epithelia, not in papillae -Soft palate -Epiglottis and aryepiglottal folds (*May function not for taste but for detection of food and suppression of aspiration into trachea*)

Lateral Olfactory Tract Collaterals and Projections: 13: -Olfactory tract also sends projections to different cortical regions. 1. orbitofrontal cortex and 2. insula. Orbitofrontal cortex recognizes odors. Also receives taste signals, combines these 2 and allows us to experience full flavor of different foods. *Describe the olfactory pathway from receptor to primary olfactory cortex or other targets of the olfactory tract* *(Objective #3)* *What is the functional significance of olfactory information reaching the Orbitofrontal cortex?* *(Objective #6)*

-*Lateral olfactory tract* sends collaterals to *subcortical limbic structures* -*Olfactory cortex* also sends *projections* to *Orbitofrontal Cortex*, either directly or relayed through the Thalamus *Orbitofrontal Cortex*: -*Important for ID of odors* -Insular cortex and orbitofrontal cortex also receive taste input -Medial Orbitofrontal cortex plays role in integrating olfactory, taste, and other food-related cues that produce the experience of flavor.

Olfactory Nerve and Olfactory Bulb: 8: -Axons bundle together and pass through openings of cribriform plate. There are multiple bundles and multiple openings- so composed of several bundles of fibers. Next order sensory neurons located in olfac bulb, where synapse occurs *What is the difference between the olfactory nerve and olfactory tract?* *(Objective #4)*

-*Unmyelinated axons of olfactory receptor neuron* pass through the lamina propria and group together into "Olfactory Fila", which *collectively make up the olfactory nerve*. -Olfactory fila pass through cribriform plate to terminate into olfactory bulb. -*Olfactory bulb is attached to the brain by the olfactory tract*

Olfactory Transduction: 10: -Olfactory receptors directly synapse w/ secondary neurons in bulb. Proceed caudally and enter cerebral cortex. Olf system= only one that bypasses thalamus. Diagram= details not crucial. Neurons in olf bulb are either mitral or tufted, based on shape. Receptors form structures that look like glomeruli. "*Important to know where and how synapse occur"* *Describe the olfactory pathway from receptor to primary olfactory cortex or other targets of the olfactory tract* *(Objective #3)*

-Axons of olfactory receptor neurons then communicate with 2nd order neurons of the olfactory tract ("*Mitral Cells*") -Mitral cell axons emerge from the caudal portion of the olfactory bulb to form lateral olfactory tract -They course caudally to *terminate in areas on ventral surface of the frontal lobe* which are broadly defined as (*olfactory cortex*)

Olfactory Receptors: Slide 7: -Olfactory receptors are neurons, bipolar, peripheral dendrite, ends in 10-30 cilia. Receptors located on cilia. Central processes project up and go past cribriform plate, synapse in receptors w/in bulb. Cillia proturde into mucus *What type of cells are the olfactory receptors?* *(Objective #1)*

-Bodies of *bipolar olfactory receptor neurons* are found in basal 2/3rds of the epithelium -Each has dendrite and unmyelinated axon -Dendrite extends to surface of the epithelium, terminates w/ nonmotile cilia, which protrude into mucus layer. -Olfactory cilia contain receptors for odorant molecules.

Disorders of the Olfactory System: 16: -Seizures from olfac cortex are preceded by unpleasant smell. Can lead to chewing movements. "Uncinate Seizures"

-Excess activity in olfactory structures can also cause disturbances. -*Piriform cortex* is considered primary olfactory cortex in humans. Seizures originating in the vicinity of the uncus may begin w/ an illusion of smell or taste, usually an unpleasant one. *Uncinate Seizures* -Seizure may include chewing movements or smacking of the lips

27

-In response to taste stimulation, receptor cells secrete ATP -Taste NT excites adjacent type III cells, which form synapses w/ afferent fibers of the sensory neuron -Afferent fibers penetrate the basement membrane and form the sensory nerve fibers

Lateral Olfactory Tract Collaterals and Projections: 14: -Sends projections to hypothalamus and hippocampus. Projections to hypothalamus= trigger sense of hunger. (ie: when smell good food, become hungry) Hippocampus= memory station, different smells are remembered (ie: certain smell reminds you of something) *What is the functional significance of olfactory information reaching the hypothalamus? The hippocampus?* *(Objective #6)*

-Olfactory cortex sends fibers directly to lateral hypothalamus and hippocampus (along with neocortical projections) *Hypothalamus*: -*Feeding behavior* *Hippocampus*: -*Links olfactory input to centers concerned with learning and behavior*

Disorders of the Olfactory System: 20: -Excessive smoking or cocaine use. Olfac hallucination. *What other pathological conditions or events can cause loss of smell?* *(Objective #9)* *What type of pathological event can cause olfactory hallucinations?* *(Objective #10)*

-Olfactory losses are also a result of excessive smoking and prolonged use of cocaine (cause perforation of nasal septum) -However, healthy individuals w/ otherwise normal olfactory acuity are unable to perceive the odor of a particular compound ("anosmia") -Disorders of olfaction are also associated w/ psych disorders. Pt may experience parosmia (dysosmia), a distortion of smell or perception of smell when no odor is present (olfactory hallucination)

Disorders of the Olfactory System: 18: -Head trauma, including anterior fx of cribriform plate, may have leak of cerebrospinal fluid from skull into nose. Suspect fx of base of skull when see this. Pt's w/ allergic rhinitis can also produce clear liquid. *What bone fracture will sever the olfactory nerve fibers?* *(Objective #7)* *What bone fracture would most likely result in anosmia?* *(Objective #8)*

-Rhinitis or sinusitis may block the access of odorants to olfactory epithelium -Head trauma can produce olfactory deficits by damaging 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 cerebrospinal fluid into the paranasal sinuses and through the nose

Taste Receptor Cells: 26: -Receptor cells originate at bottom, have microvili at top w/ receptor, protrude small groove on papillae on tongue, taste pores. When food enter pore, contacts taste receptors

-Taste cells extend from basal lamina to the surface of epi. -Apical ends of these cells are covered w/ microvilli, that extend into taste pore -Pore forms pocket to permit contact btw microvilli and external milleu -New taste cells arise from basal cells

Taste Receptor Cells: 28: -transduction. Increase Ca2+, then expulsion of NT into synapse

-Taste transduction is initiated when soluble chemicals diffuse through the taste pore and interact with receptors located on the apical microvilli of the taste cells. -The interaction between the ligand and the receptor of the taste cell leads *increase in intracellular calcium*, either by the release of calcium from internal stores or by the activation of voltage-gated calcium channels. -Calcium release *results in a release of chemical transmitters* (serotonin) and at the afferent synapse, which in turn leads to an *action potential* in the afferent fiber

Taste Pathways: 30 -Taste receptors located in taste bud communicate w/ first order sensory neurons and project through Facial N through primary order neurons that synapse in Geniculate Ganglion. Lower Petrosal or Glossopharyngeal N and Inferior Ganglion of Vagus N. They all project to rostral Solitary Nucleus, which is the major taste nucleus. Remember, more caudal portions have to do w/ cardiovascular system. -2nd order neuron originate in solitary nucleus and ascend through Tegmental tract, *ipsilaterally*, and reach VPM of thalamus. 3rd order neurons project to taste cortex, in frontal insula and frontal operculum cortex. *Describe the path of the chorda tympani.* *(Objective #16)*

-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 *CNs 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*

Olfactory Transduction: 9: -Some signal from outside detected by receptors, leads to generation of electrochemical signal. Mucus traps/ attracts particles, hydrophobic molecules are bound by proteins and transported through mucus to bind to receptor, once achieved, Electrochemical signal triggered *Describe the olfactory pathway from receptor to primary olfactory cortex or other targets of the olfactory tract* *(Objective #3)*

-Volatile odor molecules are inhaled and contact mucus layer of olfac epi. -Odorants (hydrophobic ones) cross mucus by interacting with "*Odorant-binding proteins*" -Odor molecules then bind to *odorant receptors* (G protein receptors) on the cilia of olfactory receptor neurons -Binding of ligand generates depolarization in the dendrite of the olfactory receptor neuron

Taste Receptor Organ: 25: -Type I= supportive type II receptor, villi contain receptors, but do not trigger signal, communicate w/ TIII which secrete NT, type IV replace receptor cells *What are the four types of cells in a taste bud?* *(Objective #13)*

4 types of cells w/in taste bud: 1. Type I: -*Supportive* functions 2. Type II: *Receptor* cells w/ G protein-coupled receptors for bitter, sweet, and umani compounds 3. Type III: -*Secretory* (of NTs) 4. Type IV: -*Basal/ progenitor* cells

Slide 2: Emphasize: Somatosensory includes thermal, texture, etc

Anorexia: -Lack of appetite Anosmia: -Lack of smell

Olfactory Receptors: Slide 6: -Olfac receptors are in olfactory mucosa, in the roof of nasal cavities. Border (superior) ethmoidial, (laterally) superior turbinate, (medially) nasal septum. This mucosa is thicker b/c covered by mucus. Basal cells proliferate and differentiate, and can replace olfactory receptor cells. Small glands (Bowman's glands) secrete mucus *Where is the olfactory epithelium localized?* *(Objective #2)*

Olfactory Receptors: -Receptors responsible for transduction of odor molecules are found in the olfactory mucosa -Covered in mucus, more yellow and thicker than other respiratory epithelium *Olfactory epithelium = located in roof of nasal cavities* Contains: 1. Olfactory receptor neurons 2. Supporting cells (Sustentacular cells) 3. Basal cells (Replacing receptor neurons) 4. Ducts of small glands (Bowman's glands)

Slide 4: -Have different receptors, and can also get different combo of these receptors- which leads to diversity of smell

Retronasal smell: -How smell contributes to the sensation of taste -Bipolar olfactory receptor neurons in olfactory mucosa express the products of several hundred olfactory receptor genes -Single odor molecule usually activates several olfac receptors, enabling a virtually infinite number of different odors to be identified through combinatorial processing.

Slide 3:

Taste: -Originates from receptors in the oropharyngeal cavity -Important to determine the acceptance or rejection of foods -Info is relayed by neural pathways that underlie various ingestive and digestive functions.

31: -look at pathway/ synapses. *Ipsilateral pathway* *Which nucleus is the "taste nucleus" in the brainstem? Where in the brainstem is it located?* *(Objective #17)* *Does the taste pathway cross the midline?* *(Objective #18)*

The *solitary nucleus* is the *principal visceral afferent nucleus of the brainstem*. On the basis of functional characteristics, it is divided into a rostral (gustatory) nucleus and a caudal (visceral or cardiorespiratory) nucleus. Taste fibers traveling in cranial nerves *VII, IX, and X* *terminate* primarily in the *rostral portions of the solitary nucleus*. Axons arising from *second-order taste neurons* in the gustatory nucleus *ascend in the ipsilateral central tegmental tract* and *terminate* in the ventral posteromedial nucleus of the thalamus (*VPM*). Axons from these neurons in the VPM travel through the *ipsilateral posterior limb* of the internal capsule to terminate in the *inner portion* of the and *anterior insular cortex* and on the *lateral frontal operculum convexity of the postcentral gyrus*. *This pathway (solitary nucleus → VPM → cortex) is responsible for the discriminative aspects of taste and is exclusively ipsilateral!!*