olfactory nerves

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Information about olfactory nerves

Published on February 6, 2014

Author: GghNeurology

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OLFACTORY NERVE ( S ) Dr.R.Lalitha MBBS,DNB (Gen.Med ): OLFACTORY NERVE ( S ) Dr.R.Lalitha MBBS,DNB ( Gen.Med ) Introduction: Introduction Olfaction is one of the Special senses Olfactory nerve ( s ) subserving this function represent the most finely developed parts of the sensory nervous system and The most neglected of clinical evaluation The sense of smell: The sense of smell is greatly underappreciated, despite the fact that it monitors the intake of airborne agents into the human respiratory system and determines to a large degree the flavor and palatability of foods and beverages In addition to enhancing quality of life:  In addition to enhancing quality of life This primary sensory system warns of spoiled foods, leaking natural gas, polluted air and smoke, and mediates basic elements of communication (e.g., mother–infant interactions). It is now : It is now apparent that smell dysfunction is known to be among the earliest ‘‘preclinical’’ signs of neurodegenerative disorders such as Alzheimer’s disease (AD) and sporadic Parkinson’s disease Historical Aspects: Historical Aspects The morphology of the olfactory organ in fish has fascinated scientists for a long time. At the turn of the 18th to the 19th century, investigators were already attracted by the formation of olfactory lamellae in fish. From the second half of the 19th century, extensive comparative investigations followed (comprehensive historical review: Kleerekoper , 1969, 1982). Schultze (1863) was the pioneer of studies on the histology of the olfactory epithelium of vertebrates, including fish M.Schultzes: M.Schultzes His work contains numerous methodological details, vigorous defense against theoretical and experimental criticism, and precise descriptions of the olfactory mucosa throughout the vertebrate kingdom, and left little for further investigations for many years; practically all of his careful observations proved to be correct. Taste & Smell: Taste & Smell Respond to chemical stimuli Interdependent clinically Loss of either of these modalities is rarely a serious handicap Can have serious consequences if noxious odours are missed May signify a number of systemic, neurodegenerative and intracranial disorders A Bird’s eye view: A Bird’s eye view Anatomy and physiology of this primary sensory system Means of assessing its function and Major diseases and disorders with which it is intimately associated ANATOMY & PHYSIOLOGY: ANATOMY & PHYSIOLOGY Olfactory nerve: Olfactory nerve is a pure sensory nerve. Olfactory information is processed in the primitive areas of the brain. It is a phylogenetically ancient sensation. In lower mammals olfactory cortex constitute a large part of cerebral hemispheres Olfaction has bilateral representation in the cerebral corex The olfactory nerves: The olfactory nerves develop from the cells of the ectoderm which lines the olfactory pits these cells undergo proliferation and give rise to what are termed the olfactory cells of the nose. The axons of the olfactory cells grow into the overlying olfactory bulb and form the olfactory nerves. The olfactory nerves or nerves of smell: The olfactory nerves or nerves of smell are distributed to the mucous membrane of the olfactory region of the nasal cavity comprising the superior nasal concha , and the corresponding part of the nasal septum. Olfactory Mucosa: Olfactory Mucosa The olfactory epithelium: The olfactory epithelium consists of 3 cell types: basal, supporting, and olfactory receptor cells. Basal cells are stem cells that give rise to the olfactory receptors. The continuous turnover and new supply of these neurons are unique to the olfactory system. In no other location in the mature nervous system do less differentiated stem cells replace neurons. Supporting cells are scattered among the receptor cells and have numerous microvilli and secretory granules, which empty their contents onto the mucosal surface. Olfactory receptors : Olfactory receptors The receptor cells are actually bipolar neurons, each possessing a thin dendritic rod that contains specialized cilia extending from the olfactory vesicle and a long central process that forms the fila olfactoria . The cilia provide the transduction surface for odorous stimuli. Olfactory receptors: Olfactory receptors First order neurons: First order neurons originate from the central or deep processes of the olfactory cells of the nasal mucous membrane. form a plexiform net-work in the mucous membrane, and are then collected into about twenty branches, which pierce the cribriform plate of the ethmoid bone to enter the cranial cavity . Olfactory Nerves: Olfactory Nerves Olfactory Nerve and the Cribriform Plate : Olfactory Nerve and the Cribriform Plate Conduction velocities are extremely slow, and support is provided in bundles by a single Schwann cell. the trigeminal nerve (cranial nerve V) sends fibers to the olfactory epithelium to detect caustic chemicals, such as ammonia. the cribriform plate of the ethmoid bone, separated at the midline by the crista galli , contains multiple small foramina through which the olfactory nerve fibers, or fila olfactoria , traverse. Fracture of the cribriform plate in traumatic settings can disrupt these fine fibers and lead to olfactory dysfunction. Olfactory Bulb : lies inferior to the basal frontal lobe. The olfactory bulb is a highly organized structure composed of several distinct layers and synaptic specializations. The layers (from outside toward the center of the bulb) are differentiated as follows: Glomerular layer External plexiform layer Mitral cell layer Internal plexiform layer Granule cell layer . Olfactory Bulb Contd…….: Contd ……. Each mitral cell is contacted by at least 1000 olfactory nerve fibers. The external plexiform layer contains the passing dendrites of mitral cells and a few tufted cells, which are similar in size to mitral cells. Some of the granule cell dendrites in the plexiform layer contact mitral cell dendrites through a specialized dendrodendritic synapse, which also is termed a reciprocal synapse (vesicles seen within presynaptic and postsynaptic membranes ) Contd….: Contd …. Tufted cells also receive granule cell input, through dendrodendritic and dendrosomatic contact. Pyramidal mitral cells are the largest neurons in the bulb and are located in a narrow band between the external and internal plexiform layers In the brain: In the brain Mitral cell axons project to the olfactory cortex via the olfactory tract. Form into two groups, a lateral and a medial group, and end in the glomeruli of the olfactory bulb Each branch receives tubular sheaths from the dura mater and pia mater, the former being lost in the periosteum of the nose, the latter in the neurolemma of the nerve. Olfactory Tract and Central Pathways Second order neurons : Olfactory Tract and Central Pathways Second order neurons Medial fibers of the tract contact the anterior olfactory nucleus and the septal area. Some fibers project to the contralateral olfactory bulb via the anterior commissure . Lateral fibers contact third-order neurons in the primary olfactory cortex ( prepyriform and entorhinal areas) directly. Third-order neurons send projections to the dorsomedial nucleus of the thalamus, the basal forebrain, and the limbic system. Olfactory Bulb: Olfactory Bulb Physiological importance: Physiological importance The thalamic connections are thought to serve as a conscious mechanism for odor perception, the amygdala and the entorhinal area are limbic system components and may be involved in the affective components of olfaction. Central Projections : Central Projections The pyriform lobe includes the olfactory tract, the uncus , and the anterior part of the parahippocampal gyrus . The prepyriform and the periamygdaloid areas of the temporal lobe represent the primary olfactory cortex. Contd..: Contd.. The entorhinal area is known as the secondary olfactory cortex and is included in the pyriform lobe. The olfactory system is the only sensory system that has direct cortical projections without a thalamic relay nucleus. Contd…: Contd … The region of anterior perforated substance contains cells that receive direct mitral cell collaterals and input from the anterior olfactory nucleus, amygdaloid nucleus, and temporal cortex. This area ultimately projects to the stria medullaris and the medial forebrain bundle. Contd…: Contd … The dorsomedial nucleus of the thalamus receives some olfactory fibers that ultimately reach the orbitofrontal cortex. The anterior olfactory nucleus receives collateral fibers from the olfactory tract and projects to the contralateral olfactory bulb and anterior olfactory nucleus via the anterior commissure . Contd…: Contd … Using the uncinate fasciculus, the entorhinal area sends projections to the hippocampal formation, anterior insula , and frontal cortex Types of Olfactory Dysfunction ( Clinical ): Types of Olfactory Dysfunction ( Clinical ) Anosmia - Absence of smell sensation Hyposmia - Decreased sensation Dysosmia - Distortion of smell sensation Cacosmia - Sensation of a bad or foul smell Parosmia - Sensation of smell in the absence of appropriate stimulus Types of Olfactory dysfunctin ( Pathological ): Types of Olfactory dysfunctin ( Pathological ) Transductory Sensory Neural History: History Head injury Drugs Cigarette smoking and Alcoholism Occupation Seizures Dental procedures and prosthesesRadiation therapy URTI Pending litigation Systemic review: Systemic review Headache Vision changes Nose bleeds Menstrual history Memory disturbances Psychiatric disturbances TESTS OF OLFACTORY FUNCTION : TESTS OF OLFACTORY FUNCTION Accurate olfactory assessment is essential to (1) establish the validity of a patient’s complaint (2) characterize thespecific nature of the problem (3) reliably monitorchanges in function over time, including those resulting from medical interventions or treatments (4) Detect malingering and (5) establish compensation for permanentdisability . The problem with olfactory dysfunction: The problem with olfactory dysfunction Several patients who present with complaints of anosmia or hyposmia actually have normal function relative to their peers. Others are unaware of their dysfunction. In the case of PD, for example, 90%of patients have a demonstrable olfactory loss, yet lessthan 15% are aware of the problem until being tested. Detection of olfactory dysfunction: Detection of olfactory dysfunction begins with sampling of a series of common odors, which can be performed at the bedside with odiferous substances such as coffee, lemon, and peppermint. Tests, including those developed at the Connecticut Chemosensory Clinical Research Center (CCCRC), have aided examiners in the identification of abnormalities in odor detection and discrimination. The University of Pennsylvania Smell Identification Test (UPSIT) is another useful tool; it consists of 40 items for evaluation of olfactory and trigeminal nerve function in the nasal cavity. Olfactory function: Olfactory function can be classified, on an absolute basis, into one of six categories: normosmia , mild microsmia , moderate microsmia , severe microsmia , anosmia , and probable malingering. Threshold olfactory tests: Threshold olfactory tests typically employ a dilution series of a stimulus in an odorless diluent , such as light mineral oil. In most clinical applications, the stimuli are presented via small sniff or squeeze bottles, or felt-tipped pen-like devices, using a series of ascending or descending concentration trials. As with odor identification tests, forced-choice response between odorant and blank trials is required Problems with testing smell: Problems with testing smell Subjective in nature Potential for litigation Strong odours may stimulate Trigeminal nerves Investigations: Investigations Routine Biochemistry Serum B 12 levels Thyroid function tests Syphilis serology Imaging Electroolfactogram: Electroolfactogram When a jet of scented vapour isdirected to the sensory epithelium as by sniffing,a slow negative potential shift called EOG can be reorded from an electrode placed on the mucosa. The conductance changes that underlie thisreceptor potential are induced by molecules of odorous substances dissolved in the mucus overlying the receptors Investigations: Investigations Thorough evaluation of patients who have anosmia includes imaging of anterior cranial structures. Preliminary imaging studies include CT Scan and MRI Brain Advanced Imaging Modalities: Advanced Imaging Modalities Positron emission tomography (PET) scanning and functional magnetic resonance imaging (MRI) are promising modalities to assist in making the diagnosis of different types of hyposmia (central vs peripheral), as well as in delineation of the role of limbic structures as sites of odor recognition, memory, and integration of multisensory inputs Disturbances of Olfactory function: Disturbances of Olfactory function Qualitative defects Quantitative defects Olfactory hallucinations Olfactory Agnosias Causes of Anosmia: Causes of Anosmia Tumors Vascular Infections and Inflammations Systemic illness Trauma Drugs Degenerative Surgeries developmental ANOSMIA-Causes: ANOSMIA-Causes Most common causes: -Upper respiratory tract infection -trauma - nasal and sinus disease and -idiopathic Local causes: Local causes Adenoid hypertrophy Large inferior turbinates Nasal polyps Rhinitis Intranasal tumors ( Esthesioneuroblastoma ) NEUROLOGIC CAUSES: NEUROLOGIC CAUSES Meningiomas of olfactory groove Gliomas of frontal lobe Foster Kennedy syndrome Alzheimer’s disease Parkinson’s disease Multiple sclerosis Head trauma Olfactory dysfunction is associated with: Olfactory dysfunction is associated with early Parkinson disease and with other neurodegenerative disorders, such as Alzheimer disease and Huntington chorea. An association also exists between abnormal olfactory identification and obsessive-compulsive disorder. CONGENITAL CAUSES: CONGENITAL CAUSES Kallmann’s syndrome Down’s syndrome Turner’s syndrome Cleft palate Olfactory dysfunction: Olfactory dysfunction is a hallmark of certain syndromes, such as Kallmann syndrome ( ie , hypogonadism with anosmia ) and Foster Kennedy syndrome ( ie , papilledema , unilateral anosmia , and optic atrophy usually associated with an olfactory groove meningioma Contd….: Contd …. The classic description of partial complex epilepsy with a mesial temporal focus includes an aura of foul-smelling odors (termed uncinate fits) that occur before seizure onset, emphasizing presumed origination at the uncus . IATROGENIC CAUSES: IATROGENIC CAUSES Rhinoplasty Ethmoidectomy Laryngectomy Submucous resection of nasal septum Radiotherapy Injuries: Injuries Head trauma leading to fracture of the cribriform plate may cause cerebrospinal fluid (CSF) rhinorrhea and a potential for meningitis. Paranasal sinus endoscopy may lead to violation of the cribriform plate and potential infectious complications. Olfactory structures also can be injured during craniotomies involving the anterior cranial base or from subarachnoid hemorrhage, which may disrupt the fine fibers of the olfactory nerve. SYSTEMIC CAUSES: SYSTEMIC CAUSES Smoking Chronic intranasal Cocaine use Exposure to Cadmium and Toluene Deficiency of Vit B12,B 6 or A Zinc or Copper deficiency Chemotherapeutic agents PSYCHIATRIC CAUSES: PSYCHIATRIC CAUSES Conversion disorder Hysterical anosmia Complex olfactory dysfunction Olfactory Agnosias: Complex olfactory dysfunction Olfactory Agnosias Dysfunction of olfactory discrimination despite identification of odours Korsakoff’s syndrome Thalamic and prefrontal cortical lesions PowerPoint Presentation: HYPEROSMIA -Migraine -Hyperemesis gravidarum CACOSMIA/PAROSMIA -Psychiatric diseases -Head trauma -seizure involving medial temporal lobe. Olfactory hallucinations: Olfactory hallucinations They are always of central origin An odour perceived which no one else can detect ( phantosmia ) MC cause – Temporal lobe epilepsy Other causes : Depression,schizophrenia,AD Centre : Amygdala May be associated with gustatory hallucinations as well Clinician’s role: Clinician’s role The clinician should always counsel patients with anosmia regarding sensory loss, including potential risks associated with the lack of smell sensation ( eg , inability to detect dangers such as smoke, spoiled foods, toxins). Disorders of Taste: Disorders of Taste Dysgeusia is a distortion of the sense of taste. Dysgeusia is also often associated with ageusia , which is the complete lack of taste, and hypogeusia , which is the decrease in taste sensitivity. Gustatory testing : Gustatory testing In order to further classify the extent of dysgeusia and clinically measure the sense of taste, gustatory testing may be performed. Gustatory testing is performed either as a whole-mouth procedure or as a regional test. In both techniques, natural or electrical stimuli can be used. In regional testing, 20 to 50 µL of liquid stimulus is presented to the anterior and posterior tongue using a pipette, soaked filter-paper disks, or cotton swabs. In whole mouth testing, small quantities (2-10 mL ) of solution are administered, and the patient is asked to swish the solution around in the mouth. Threshold tests: Threshold tests for sucrose (sweet), citric acid (sour), sodium chloride (salty), and quinine or caffeine (bitter) are frequently performed with natural stimuli. One of the most frequently used techniques is the "three-drop test. In this test, three drops of liquid are presented to the subject. One of the drops is of the taste stimulus, and the other two drops are pure water.Threshold is defined as the concentration at which the patient identifies the taste correctly three times in a row. Suprathreshold tests: Suprathreshold tests which provide intensities of taste stimuli above threshold levels, are used to assess the patient's ability to differentiate between different intensities of taste and to estimate the magnitude of suprathreshold loss of taste. From these tests, ratings of pleasantness can be obtained using either the direct scaling or magnitude matching method and may be of value in the diagnosis of dysgeusia . EGM: EGM In addition to techniques based on the administration of chemicals to the tongue, electrogustometry (EGM ) is frequently used. It is based on the induction of gustatory sensations by means of an anodal electrical direct current . Patients usually report sour or metallic sensations similar to those associated with touching both poles of a live battery to the tongue. Although electrogustometry is widely used, there seems to be a poor correlation between electrically and chemically induced sensations. Causes of Taste disoders: Causes of Taste disoders Chemotherapy Disoredered taste buds Drugs Zinc defciency Systemic causes Chemotherapy : Chemotherapy A major cause of dysgeusia is antineoplastic chemotherapy . Chemotherapy often induces damage to the oral cavity, resulting in oral mucositis , oral infection, and salivary gland dysfunction. Healthy individuals normally have a diverse range of microbial organisms residing in their oral cavities; however, chemotherapy can permit these typically non- pathogenic agents to cause serious infection, which may result in a decrease in saliva . In addition, patients who undergo radiation therapy also lose salivary tissues. Saliva is an important component of the taste mechanism. Examples of chemotherapy treatments that can lead to dysgeusia are cyclophosphamide , cisplatin , and etoposide . The exact mechanism of chemotherapy-induced dysgeusia is unknown. Drugs: Drugs H1- antihistamines , such as azelastine and emedastine . Approximately 250 drugs affect taste. amiloride the creation of new taste buds and saliva can be impeded by antiproliferative drugs. Saliva can have traces of the drug, giving rise to a metallic flavor in the mouth; examples include lithium carbonate and tetracyclines . Drugs containing sulfhydryl groups, including penicillamine and captopril , may react with zinc and cause deficiency. Metronidazole and chlorhexidine have been found to interact with metal ions that associate with the cell membrane . inhibiting angiotensin converting enzyme , eprosartan for example, have been linked to dysgeusia . Systemic causes: Systemic causes Xerostomia Injury to the 9 th nerve Lead poisoning Gastric reflux Diabetes mellitus Aging Idiopathic Damabe to the peripheral nerves Burning mouth syndrome( Menopausal women ) PowerPoint Presentation:  Thank YOU

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