Olfactory Neuro Notes: Special Senses & Chemical Senses PDF

Chapter 17 The Special Senses 2  Special Senses All sensory organs contain receptors for increasing sensitivity to the environment. In Chapter 16, we considered the relatively simple receptors and pathways for the general senses of touch, pressure, vibration, temperat...

Chapter 17 The Special Senses 2  Special Senses All sensory organs contain receptors for increasing sensitivity to the environment. In Chapter 16, we considered the relatively simple receptors and pathways for the general senses of touch, pressure, vibration, temperature, pain, and proprioception. The special senses include smell, taste, vision, hearing and equilibrium. Special sensory afferent pathways in many ways resemble general sensory pathways. However, a major point of differentiation is the arrangement of special sense receptors in complex sensory organs, specifically the nose, tongue, eyes, 3 4  Chemical Senses Olfaction (smell) and gustation (taste) Considered chemical senses because of the interaction of molecules (chemicals) with receptor cells Because the sensations of smell and taste have a strong connection to the limbic system, certain odors and/or tastes can evoke strong emotional responses or memories. 5  Olfactory 1 square inch of Epithelium membrane holding 10-100 million receptors Within a total area of 5 cm2 (a little less than 1 in.2) Covers superior nasal cavity and cribriform plate 3 types of cells 6 Within the nasal cavity, the turbinates or nasal conchae serve to direct the inspired air toward the olfactory epithelium in the upper posterior region. 7  Olfactory Olfactory receptors are the first-order Epithelium neurons of the olfactory pathway. Each olfactory receptor is a bipolar neuron with an exposed knob-shaped dendrite and an axon projecting through the cribriform plate and ending in the olfactory bulb. The parts of the olfactory receptors that respond to inhaled chemicals are the olfactory hairs; cilia that project from the dendrite. (Recall that transduction is the conversion of stimulus energy into a graded potential in a sensory receptor.) 8 9  Chemicals that have an odor and can therefore stimulate the olfactory hairs are called odorants. Olfactory receptors respond to the chemical stimulation of an odorant molecule by producing a generator potential, thus initiating the olfactory response. 10 11  Olfactory Epithelium Supporting cells: columnar epithelial cells of the mucous membrane lining the nose. These provide physical and metabolic support, and electrical insulation of the olfactory receptors. Also help detoxify chemicals coming into contact with the olfactory epithelium Basal cells are stem cells located between the bases of the supporting cells – continually undergo cell division to produce new olfactory receptors and 12  Olfactory Epithelium Olfactory (Bowman’s) glands produce mucus – moistens the surface and dissolves odorants so that transduction can occur – innervated by CN VII (same as supporting cells innervation) – impulses in this nerve can stimulate the lacrimal glands in the eyes and nasal mucous – result in tears and a runny nose after inhaling substances such as pepper and ammonia 13  Olfactory Olfactory receptors Epithelium  bipolar neurons with cilia or olfactory hairs Supporting cells  columnar epithelium Basal cells = stem cells  replace epithelium monthly Olfactory glands  produce mucus Both epithelium & glands innervated by cranial nerve VII. 14  Olfaction Genetic evidence suggests there are hundreds of primary scents. Ability to recognize 10,000 different odours depends on patterns of activity in the brain that arise from activation of many different combinations of olfactory receptors In olfactory reception, a generator potential develops and triggers one or more nerve impulses. 15  Olfaction Odorant binds to an olfactory transmembrane receptor protein (GPCR) in the plasma membrane of an olfactory hair – coupled to a G protein – activates Adenylate cyclase – produces cAMP – opens sodium ion channels – inflow of sodium – depolarizing generator potential – generation of nn impulse and propagation along axon of olfactory receptor 16 Olfactory transduction Binding of an odorant molecule to an olfactory receptor protein activates a G protein and adenylate cyclase, resulting in the production of cAMP. Cyclic AMP opens sodium ion (Na+) channels and Na+ ions enter the olfactory receptor. The resulting depolarization may generate an action potential, which propagates 17 along the axon of the olfactory receptor. Olfaction Odorants bind to receptors Na+ channels open Depolarization occurs Nerve impulse is triggered Q) In which part of an olfactory receptor does olfactory transduction occur? Answer: ? 18  Olfaction Adaptation to odours occurs quickly and the threshold of smell is low: only a few molecules of certain substances need be present in air to be smelled. E.g. methyl mercaptan - smell of rotten eggs - similar molecule to ethyl mercaptan which is detected at 1/25 billionth of a milligram per ml of air – added to odourless petroleum gas that is used for cooking to provide olfactory warning of gas leaks 19  Olfaction Adaptation is rapid - by 50% in the first second or so, but adapt slowly thereafter. Complete insensitivity to certain strong odours about a minute after exposure (involves CNS input as well) Olfactory receptors convey nerve impulses through olfactory nerves: to olfactory bulbs, olfactory tracts, and the cerebral cortex and limbic system. Unmyelinated axons of olfactory receptors extend through ~20 olfactory foramina in the cribriform plate of the ethmoid bone. 20  Olfaction 40 or so bundles of axons collectively form the R and L olfactory nn – terminate in the brain in paired masses of gray matter called the olfactory bulb (located below the frontal lobes of the cerebrum and lateral to the crista galli of the ethmoid bone) – axon terminals form synapses with dendrites and cell bodies of olfactory bulb neurons – extend posteriorly and form the olfactory tract – project to the primary olfactory area of the cerebral cortex. There is debate on where the olfactory cortex is located. It is generally accepted to be on the medial aspect of the temporal lobe and 21  Olfactory sensations are the only sensations that reach the cerebral cortex without first synapsing in the thalamus Collateral axons of the olfactory tract project to the limbic system and hypothalamus – account for emotional and memory-evoked responses to odours, e.g. sexual excitement upon smelling a perfume; nausea upon smelling a food that once made you violently ill; odour- evoked memory of a childhood experience From the primary olfactory area – extend to frontal lobe – for odour identification and discrimination is the orbitofrontal 22 Adaptation & Threshold Adaptation = decreasing sensitivity Olfactory adaptation is rapid  50% in 1 second  complete in 1 minute Low threshold  only a few molecules need to be present  methyl mercaptan added to natural gas as warning  The trigeminal nerve innervates the posterior nasal cavity to detect noxious stimuli. 23  Olfactory The olfactory bulb transmits smell Tract information from the nose to the brain, and is thus necessary for a proper sense of smell. Within the olfactory bulb, the glomerular layer receives direct input from olfactory nerves, made up of the axons from approximately ten million olfactory receptor neurons in the olfactory mucosa The ends of the axons cluster in spherical structures known as glomeruli such that each glomerulus receives input primarily from olfactory receptor neurons that express the same olfactory receptor. 24 25  Olfactory Tract Glomeruli are also permeated by dendrites from neurons called mitral cells, which in turn output to the olfactory cortex. Numerous interneuron types exist in the olfactory bulb including periglomerular cells, which synapse within and between glomeruli, and granule cells which synapse with mitral cells. 26  Olfactory Tract As a neural circuit, the olfactory bulb has one source of sensory input (axons from olfactory receptor neurons of the olfactory epithelium), and one output (mitral cell axons). As a result, it is generally assumed that it functions as a filter, as opposed to an associative circuit that has many inputs and many outputs. However, its exact function is as of yet, 27  Olfactory The basal dendrites of mitral cells are Tract connected to interneurons known as granule cells, which by some theories produce lateral inhibition between mitral cells.  It is not clear what the functional role of lateral inhibition would be, though it may be involved in boosting the signal-to-noise ratio of odor signals by silencing the basal firing rate of surrounding non- activated neurons. The synapse between mitral and granule cells is of a rare class of synapses that are "dendro- dendritic" which means that both sides of the synapse are dendrites that release 28  Olfactory Tract In this specific case, mitral cells release the excitatory neurotransmitter glutamate, and granule cells release the inhibitory neurotransmitter Gamma-aminobutyric acid (GABA). As a result of its bi-directionality, the dendro-dendritic synapse can cause mitral cells to inhibit themselves (auto- inhibition), as well as neighboring mitral cells (lateral inhibition). 29  Olfactory Axons from olfactory receptors form the Tract olfactory nerves (Cranial nerve I) that synapse in the olfactory bulb  pass through 40 foramina in the cribriform plate Second-order neurons within the olfactory bulb form the olfactory tract that synapses on the primary olfactory area of the (mainly) temporal lobe  conscious awareness of smell begins Other collaterals lead to the limbic system 30 Simplified diagram of cortical regions thought to be involved in the processing of olfactory information as it passes from the olfactory epithelium to the brain. 31  Pathologies Many people in the United States experience some type of olfactory dysfunction, causes of which include head trauma, upper respiratory infections, tumors of the anterior cranial fossa, and exposure to toxic chemicals Hyposmia, a reduced ability to smell, affects as many as 4 million people in the US, with incidence increasing with age  Hyposmia also can be caused by neurological changes, such as a head injury, Alzheimer 32 disease, or Parkinson disease; certain drugs Selected Medications that Reportedly Alter Smell and Taste Antibiotics Antihistamines Anti- Antipsychotics Ampicillin and inflammatory Clozapine (Clozaril) Azithromycin decongestants agents Trifluoperazine (Zithromax) Chlorpheniramine Auranofin (Ridaura) (Stelazine) Ciprofloxacin Loratadine Colchicine Antithyroid (Cipro) (Claritin) Dexamethasone agents Clarithromycin Pseudoephedrine (Decadron) Methimazole (Biaxin) Antihypertensives Gold (Myochrysine) (Tapazole) Griseofulvin and cardiac Hydrocortisone Propylthiouracil (Grisactin) medications Penicillamine Lipid-lowering Metronidazole Acetazolamide (Cuprimine) agents (Flagyl) (Diamox) Antimanic drug Fluvastatin (Lescol) Ofloxacin (Floxin) Amiloride Lithium Lovastatin Tetracycline (Midamor) Antineoplastics (Mevacor) Anticonvulsants Betaxolol (Betoptic) Cisplatin (Platinol) Pravastatin Carbamazepine Captopril (Capoten) Doxorubicin (Pravachol) (Tegretol) Diltiazem (Adriamycin) Muscle relaxants Phenytoin (Dilantin) (Cardizem) Methotrexate Baclofen (Lioresal) Antidepressants Enalapril (Vasotec) (Rheumatrex) Dantrolene Amitriptyline Hydrochlorothiazid Vincristine (Dantrium) (Elavil) e (Esidix) and (Oncovin) Clomipramine combinations Antiparkinsonian (Anafranil) Nifedipine agents Desipramine (Procardia) Levodopa (Norpramin) Nitroglycerin Doxepin (Sinequan) Propranolol 33 Imipramine (Inderal)  Pathologies The following terms are used to describe the degree of smell aberration:  Anosmia - Absence of smell sensation  Hyposmia - Decreased sensation  Dysosmia - Distortion of smell sensation  Parosmia - Perception of smell in the absence of appropriate stimulus  Cacosmia - Perception of a bad or foul smell  Phantosmia - Perception of smell in absence 34 of odorant  Pathologies The classic description of partial complex epilepsy with a temporal focus includes an aura of foul-smelling odors (termed uncinate fits – affecting the temporal lobe superficial to the hippocampal uncus) that occur at seizure onset. Olfactory dysfunction is associated with early Parkinson disease and with other neurodegenerative disorders such as Alzheimer disease and Huntingtons Can help with diagnosis 35  Questions 1. Receptors for smell are located in the (superior? inferior?) portion of the nasal cavity. Receptors consist of _________ -polar neurons which are located between _________ cells. The distal end of each olfactory receptor cell consists of dendrites that have cilia known as _________. 36  Questions 2. What is unique about the function of basal stem cells? 3. What is the function of olfactory (Bowman's) glands in the nose? 4. Because smell is a _________ sense, protein receptors in olfactory hair membranes respond to different chemical molecules, leading to a _________ and _________. 37  Questions 5. Adaptation to smell occurs (slowly? rapidly?) at first and then happens at a much slower rate. 6. Upon stimulation of olfactory hairs, impulses pass to cell bodies and axons; the axons of these olfactory cells form cranial nerves (I? II? III?), the olfactory nerves. These pass from the nasal cavity to the cranium to terminate in the olfactory _________ located just inferior to the _________ lobes of the cerebrum. 38  Questions 7. Cell bodies in the olfactory bulb then send axons through the olfactory (nerves? tracts?) along two pathways. One route extends to the limbic system. What is the effect of projection of sensations of smell to this area? 8. The other olfactory pathway terminates in the _________ lobe. Pathways for smell are unique in that they (do? do not?) involve neurons of the thalamus en route to the cerebral cortex. The (right? left?) hemisphere is more actively involved in olfaction. 39  Gustation Taste, like olfaction, is a chemical sense  To be detected, molecules must be dissolved in a medium  Primary tastes include sour, sweet, bitter, salty, and umami (meaty or savory)  All other flavours are a combination of 2 or more of the 5 primary tastes, in addition to other somatic sensations  Odours of food can pass upward from the mouth into the nasal cavity. This is called retronasal olfaction. Olfaction is more sensitive than taste, so in reality much of 40  Taste How many types of taste receptors are there?Receptors  Sweet  Activated by sugars, alcohol, ketones, aldehydes, etc  Salt  Activated by metal ions, most commonly Na+  Sour  Activated by free hydrogen ions, and therefore by acids  Stronger acid means lower pH and more sour taste  This isn’t really held to be true anymore 41  Bitter 41  Gustation Do taste receptors detect “tastes” that help us determine what our body needs? Do we crave specific foods based on what our body needs?  Sweet tastes indicate energy-rich foods  Salty foods indicate electrolyte-rich foods  Bitter and sour are considered aversive tastes  Many bitter and sour foods are either toxic or “gone bad”  Umami indicates food high in 42 amino acids for building proteins 42  Gustation Classically, the 5 taste sensations have been localized to different regions of the tongue http://staff.bcc.edu/ Faculty_Websites/ CPENDLYSHOK/ TasteSmellHear.ppt 43 43  Gustation The classic distribution of taste receptors in humans is… wrong Is a more uniform distribution http://rover.vetmed.lsu.edu/student/2010/files/NEURO_TasteSmell.ppt 44 44 45 Taste Buds 10,000 taste buds found on tongue, soft palate, epiglottis & pharynx Taste buds have been found all the way down in the lungs # of taste buds decline with age Found in elevations on the tongue called papillae Three types of papillae contain taste buds:  Circumvallate papillae (12 very large structures form an inverted V-shaped row at the back of the tongue – each houses 100-300 taste buds  Fungiform papillae – mushroom shaped elevations scattered over entire surface of tongue with 5 taste buds each 46  Taste Buds Each taste bud is an oval body consisting of three kinds of epithelial cells: Gustatory receptor cells – specialized sensory receptors with hairs (cilia) projecting to the external surface through a taste pore Supporting cells surround the ~20 gustatory receptor cells in each taste bud Basal cells are stem cells found at the periphery of the taste bud near the connective tissue layer Differentiate into 47  Taste Buds The entire surface of tongue is also covered with filiform papillae – pointed, conical structures that act as tactile receptors, but don’t contain taste buds Increase friction between tongue and food, making it easier to manipulate food Allow for general sensory sensation of food (touch, pain, etc) Gustatory receptor cells synapse with dendrites of first order neurons that form the first part of the gustatory pathway – dendrites of each first order neuron 48  Gustation Chemicals that stimulate gustatory receptor cells are known as tastants. These are dissolved in saliva, which washes them into the papillae to make contact with the gustatory hair plasma membrane. This results in receptor potentials developing in gustatory hairs, causing the release of neurotransmitter onto the 1st order neurons Which neurotransmitter is used here? Thresholds for tastes vary among the 5 primary tastes 49  most sensitive to bitter (many natural toxic  Gustation Receptor potentials arises differently for different tastants:  1) Na+ in salty food enter gustatory receptor cells via Na+ channels in the plasma membrane – accumulation of the ions cause depolarization – leads to release of neurotransmitter  2) H+ in sour tastants may flow into gustatory receptor cells via H+ channels – influence opening and closing of other types of ion channels (K+)  This is no longer thought to be true, but we don’t have a more valid explanation yet  3) For sweet, bitter and umami tastes – bind to G- protein receptors (GPCR) in plasma membrane – 50  Gustation If all tastants cause the release of neurotransmitter from many gustatory receptor cells, why do foods taste different?  Different tastes arise from activation of different combinations of groups of taste neurons  Each individual gustatory receptor cell responds to more than one of the 5 primary tastes and may respond more strongly to some tastants than to others (think of taste 51  Threshold Threshold for taste varies for each primary taste  Threshold for bitter (e.g. quinine) is lowest  Possibly because poisonous substances are often bitter – low threshold (high sensitivity) to have protective function  Threshold for sour substances (e.g. lemon) measured by HCl – is higher  Thresholds for salty (NaCl) and sweet (sucrose) are similar and higher threshold than those for bitter or sour substances 52  Threshold How much of a molecule do you need in order to be able to taste it?  Variable  Bitter substances tend to be lowest (poisons are often bitter)  quinine (bitter) = 0.0000004 M;  sucrose (sweet) = 0.02 M Can you adapt to taste?  With continuous stimulation, adaptation can occur in 1-5 min  Taste adaptation due to changes that occur in the taste receptors, in olfactory receptors and in neurons of the gustatory pathway in the CNS 53 53  Gustation Although taste is technically defined as activation of the taste buds, when “tasting” something other inputs are also important  Temperature, texture, pain  Sight, colour, sound, expectation, memory, satiety  Smell  Smell plays a large role in “taste”  Up to 80% of what you “taste” is actually what you smell  What happens when you have a cold? 54 54  Loss of smell can lead to depression  Gustatory First-order gustatory fibers found in cranial nerves  V Pathway  VII (facial) serves anterior 2/3 of tongue  IX (glossopharyngeal) serves posterior 1/3 of tongue  X (vagus) serves palate, epiglottis, esophagus Signals travel to thalamus and then cortex, and limbic system & hypothalamus Taste fibers extend from the thalamus to the primary gustatory area on the insular lobe of 55  Gustatory Once taste buds are activated and one Pathway or more of the 3 cranial nerves carrying taste sensation (what are they?) make it to the gustatory nucleus (where is it?), what are some of the possible responses?  Info goes to thalamus (VPN)  Routed to limbic system and hypothalamus for appreciation of taste and link to ???  Routed to cortex for ???  Info goes to other cranial nerve nuclei for salivation reflexes 56 56  Gustatory Pathway Where is the primary gustatory cortex located? Gustatory nucleus found in rostral part of the solitary nucleus/nucleus solitarius/nucleus of the tractus solitarius in the medulla http://www.med.howard.edu/physio.biophys/MILLIS%20HOME%20PAGE_files/Biomed/15PPT_lect/15-01_pptlect.ppt 57 Human Anatomy and Physiology 6th ed Marieb