olfactory nerve


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cranial nerve - olfactory nerve

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olfactory nerve

  1. 1. OLFACTORY NERVES Version-1 Dop:9.7.14
  2. 2. Olfactory epithelium
  3. 3. Olfactory mucosa
  4. 4. Olfactory Nerves (Cranial Nerve I) Olfactory bipolar receptor nerve cells (In olfactory mucous membrane in upper part of the nasal cavity above the level of the superior concha) peripheral process that passes to the surface ( about 20 fiber bundles) Short cilia /olfactory hairs
  5. 5. Tufted cells • Mitral cell granular cells synaptic glomeruli • Enter cribriform plate of the ethmoid to join the olfactory bulb. • fine central processes(unmyelinated) form the olfactory nerve fibers • Olfactory bipolar receptor nerve cells
  6. 6. • Mitral cell Tufted cells medial striae of opp olfac bulb olfactory tract medial olfactory striae paraolfactory area, subcallosal gyrus, inferior part of the cingulate gyrus lateral olfactory striae Primary olfactory cortex secondary olf area (uncus, anterior hippocampal gyrus) entorhinal area (area 28) piriform cortex, and amygdaloid nucleus) (Medial temporal lobe)
  7. 7. • FIRST ORDER NEURON: – From olfactory epithelium to glomerulus • SECOND ORDER NEURON: – The olfactory bulb. where the second neurons of the olfactory pathway (mitral and tufted cells) are located. – Is considered an extension of the telancephalon. – The axons of these Second order neurons pass centrally as the olfactory tract. • THIRD ORDER NEURON: – The prepiriform area (area 28) is considered the primary olfactory cortex which contains the third order neurons.
  8. 8. OLFACTORY BULB – is an elongated oval structure that lies just above the cribriform plate. – It is continuous posteriorly with the olfactory tract through which it is connected to the base of the cerebral hemisphere. – The olfactory bulb and tract are not parts of PNS but Instead constitute the extension of the telencephalon this contains CNS-specific cell types (oligodendrocytes and microglia). – The olfactory bulb and tract share with the telencephalon share a common meningeal covering.
  9. 9. OLFACTORY TRACT When traced posteriorly the olfactory tract divides into – Medial – Lateral olfactory striae – Intermediate striais sometimes present. • The point of bifurcation is expanded and forms the olfactory trigone.
  10. 10. • Some of the axons of the olfactory tract run in the lateral olfactory stria to the olfactory centers amygdala, semilunar gyrus, and ambient gyrus. • Other axons of the olfactory tract run in the medial olfactory stria to the nuclei in the septal (subcallosal) area which is part of the limbic system and to the olfactory tubercle a small elevation in the anterior perforated substance. • other axons of the olfactory tract teminate in the anterior olfactory nucleus where the fibres cross to the opposite side branch off and relayed. • This nucleus is located in the olfactory trigone, which lies between the two olfactory striae in front of the anterior perforated substance.
  11. 11. ANTERIOR PERFORATED SUBSTANCE: • The olfactory striae are intimately related to a mass of grey matter called the anterior perforated substance. • The medial and lateral striae form the anteromedial and anterolateral boundaries of this substance. • The intermediate stria extends into the anterior perforated substance and ends in a slight elevation (in the anterior part of the substance) called the olfactory tubercle. • Posterolaterally, the anterior perforated substance is related to the uncus . • while posteromedially it is bounded by a bundle of fibres called the diagonal band (of Broca)
  12. 12. The uncus • Is a part of the cerebral hemisphere that lies on the tentorial surface a little behind and medial to the temporal pole. • It represents the anterior end of the parahippocampal gyrus and is separated from the temporal pole by the rhinal sulcus. • The uncus is subdivided into three parts. From anterior to posterior side these are – The uncinate gyrus, – the tail of the dentate gyrus(band of Giacomini) – The intralimbic gyrus Entorhinal area (area 28): – The anterior part of the parahippocampal gyrus, including the uncus,
  13. 13. • When traced backwards the lateral olfactory stria reaches the limen insulae (in the depth of the stem of the lateral sulcus). • Here it bends sharply to the medial side and becomes continuous with a small area of grey matter lying anterior to the uncus and called the gyrus semilunaris (or periamygdaloid area). • The gyrus semilunaris is closely related to the amygdaloid complex which lies deep (i.e., superior) to it. • The lateral olfactory stria is covered by a thin layer of grey matter called the lateral olfactory gyrus. • When traced backwards this gyrus becomes continuous with a part of the cortex called the gyrus ambiens. • The gyrus ambiens lies lateral to the gyrus semilunaris. • Posteriorly, it becomes continuous with the entorhinal area. The lateral olfactory gyrus and the gyrus ambiens collectively form the prepiriform region(or area).
  14. 14. Entorhinal area
  15. 15. • PRIMARY OLFACTORY CORTEX: – The piriform cortex is considered the primary olfactory cortex. The area most closely associated with identifying the odor. • The medial amygdala – is involved in social functions such as mating and the recognition of animals of the same species. • The entorhinal cortex – is associated with memory, e.g. to pair odors with proper memories. • The exact functions of these higher areas are a matter of scientific research and debate. • Olfactory information is processed in primitive areas of the brain. • Olfaction is the only sensation not directly processed in the thalamus
  16. 16. • However indirectly the olfactory fibres from the olfactory cortex pass to the neo cortex passes via the thalamus terminating in the basal fore brain
  17. 17. • The axons that run in the medullary striae of the thalamus terminate in the habenular nuclei. • This tract also continues to the brainstem. where it stimulate salivation in response to smell.
  18. 18. Limbic system
  19. 19. Amygdala
  20. 20. SPECIAL FEATURES OF OLFACTORY EPITHELIUM • These neurons have a limited lifespan of up to several months, but are continiously replenished from the pool of precursor cells • In the olfactory mucosa this undergo periodic mitosis. • New olfactory receptors are thus generated throughout adult life, • and their axons enter the olfactory bulb to form new synapses with existing CNS neurons. • The regenerative capacity of the olfactory mucosa gradually diminishes with advancing age. • Resulting in net loss of receptors and a slow decline in overall sensory function
  21. 21. Olfaction and coding
  22. 22. Examination of olfactory nerve • Examine each nostril separately while occluding the other. With the patient's eyes closed and one nostril occluded, • bring the test substance near the open one. Ask the patient to sniff and indicate whether she smells something and, • if so, to identify it. Repeat for the other nostril and compare the two sides
  23. 23. Inference • The perception of odor is more important than accurate identification. • Perceiving the presence of an odor indicates continuity of the olfactory pathways. • Identification of the odor indicates intact cortical function as • Since there is bilateral innervation, a lesion central to the decussation of the olfactory pathways never causes loss of smell, • A lesion of the olfactory cortex does not produce anosmia. • The appreciation of the presence of a smell, even without recognition, excludes anosmia.
  24. 24. Causes of Persistent Loss of Smell • Smoking • Chronic rhinitis • Deviated nasal septum • Nasal polyps • Intranasal tumors (e.g., epidermoid carcinoma) • Postviral • General anesthesia • Dental trauma
  25. 25. Causes of Persistent Loss of Smell • Chemical burns of the olfactory epithelium • Normal aging • Pregnancy • Congenital anosmia • Chemotherapeutic agents • Cadmium toxicity • Antibiotics • Antihistamines
  26. 26. Causes of Persistent Loss of Smell • Olfactory groove meningioma • Frontal lobe tumor, especially glioma • Sellar/parasellar tumor • Neuro-olfactory tumor (esthesioneuroblastoma) • Korsakoff's syndrome • Vitamin deficiency (B6, B12, A) Zinc or copper deficiency
  27. 27. Causes of Persistent Loss of Smell • Craniocerebral trauma, • Surgery- Temporal lobectomy • Alzheimer's disease • Parkinson's disease • Multiple sclerosis • Meningitis • Arhinencephaly • Olfactory dysgenesis Kallmann's syndrome (hereditary hypogonadism with anosmia) • Propylthiouracil • Familial dysautonornia
  28. 28. Causes of Persistent Loss of Smell • Refsum's syndrome • Psychiatric conditions (depression, conversion disorder, schizophrenia) • Cocaine • Levodopa • Amphetamines • Chronic sinus disease • Radiation therapy
  29. 29. Terms and Definitions Related to Olfactory Abnormalities • Anosmia -No sense of smell • Hyposmia -A decrease in the sense of smell • Hyperosmia- An overly acute sense of smell • Dysosmia -Impairment or defect in the sense of smell • Parosmia -Perversion or distortion of smell • Phantosmia -Perception of an odor that is not real Presbyosmia Decrease in the sense of smell due to aging • Cacosmia- Inappropriately disagreeable odors • Coprosmia -Cacosmia with a fecal scent • Olfactory agnosia -Inability to identify or interpret detected odors
  30. 30. • sphenoidal ridge meningioma • consists of unilateral optic atrophy or papilledema • exophthalmos, • ipsilateral anosmia • The Foster Kennedy syndrome – classically due to a large tumour involving the orbitofrontal region, such as an olfactory groove meningioma. • anosmia (direct compression) • unilateral ipsilateral optic atrophy (direct compression) • contralateral papilledema(d/t inc ICP) • Pseudo-Foster Kennedy syndrome. – This ophthalmologic picture, without the anosmia, is more often due to anterior optic nerve ischemia, sometimes termed the
  31. 31. • Kallmann's syndrome – a hereditary disorder, usually X-linked, • hypogonadism • anosmia,( due to hypoplasia or aplasia of the olfactory bulbs and tracts.) • Uncinate fits • Complex partial or temporal lobe seizures preceded by an olfactory or gustatory aura, usually disagreeable, and often accompanied, as the patient loses awareness, by smacking of the lips or chewing movements. Such attacks are typically due to a seizure focus involving medial temporal lobe structures. There is never objective loss of smell interictally.